Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 12, 2016
Valid: August 13-17, 2016 (Saturday-Wednesday)

Summary:

Tropical heat and humidity will persist across the Mid-Atlantic through the weekend. The presence of ample regional heat and moisture will support the development of diurnal convection and precipitation over the weekend, though this precipitation is looking more scattered in this morning’s weather forecast models. Periods of afternoon sunshine are possible both Saturday and Sunday before clouds and thunderstorms form, which may support some ozone formation along the I-95 Corridor. However, a fast and clean transport remaining in place aloft should temper rising ozone, keeping the chance for an ozone exceedance Slight through the weekend. A weak cold front is expected to move into the northern Mid-Atlantic on Monday morning and push southward towards the Mason-Dixon Line during the afternoon. Clouds and precipitation are expected to form along and ahead of this front, but locations along I-95 may remain clear though much of the afternoon, allowing for some localized ozone formation. As a result, the chance for an ozone exceedance will rise to Marginal on Monday. This front should stall across the central Mid-Atlantic on Tuesday, but it may move northward as a warm front. The placement of this weak front, combined with the approach an organized line of intense vorticity aloft, will trigger clouds and precipitation across most of the Mid-Atlantic on Tuesday. Once again, there is the possibility for clear skies along I-95, which will keep the chance for an ozone exceedance Marginal for another day. On Wednesday, the potent vorticity streaming aloft will pass directly overhead, acting as a trigger for another round of more widespread cloudiness and precipitation. The chance for an ozone exceedance will drop to Slight on Wednesday as a result.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with differences starting on Monday regarding the placement of a slow-moving cold front. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough will progress eastward across southern MB/ON and the northern Plains on Saturday, pulling its associated cold front into the Ohio River Valley. Concurrently, a semi-permanent mid-level ridge centered over the Bermuda High out on the western Atlantic will remain in place on Saturday persist through the medium range. On Sunday, the upper level trough will flatten slightly across the Great Lakes region. The shortwave disturbance which has brought torrential rain to much of the southeast in the past week will get absorbed into the base of the trough, with its energy streaming northeastward into the Mid-Atlantic beginning Sunday evening. At the surface, the cold front will slowly move into the Mid-Atlantic from the northwest and reorient itself east-west, before pushing southward across PA. The upper level trough will pivot northeastward towards New England/Canadian Maritimes on Monday, returning the northern Mid-Atlantic to mostly zonal flow aloft. The GFS and EC differ with their placement of this front, with the GFS placing the front roughly south of the Mason-Dixon Line by 00Z Tuesday. The EC, on the other hand, keeps the front further northward across central PA at this time. WPC stalls the cold front across the central Mid-Atlantic on Tuesday, in the vicinity of the MDL, and keeps it in place through Wednesday. This appears to be a compromise between the GFS and EC solutions, with the GFS keeping the front farther south and the EC farther north. In any case, the front is very diffuse in the 850 mb model guidance. On Tuesday, an upper level shortwave developing across the northern Plains and progressing eastward towards the Great Lakes will invigorate the flattened trough from the weekend. The new upper level trough will slowly move further eastward on Wednesday, with its axis reaching the Ohio River Valley by 00Z Thursday. Additionally, a strong and organized line of vorticity ahead of the trough will push southeastward across the northern and central Mid-Atlantic, triggering precipitation on Wednesday.

Saturday will be yet another oppressively hot and humid day for much of the Mid-Atlantic. The NWS has placed southeastern PA, and essentially the entire states of NJ, DE, and MD under an Excessive Heat Watch for Saturday. Additionally, northern DE, PHL, western NJ and the Newark area remain under an Excessive Heat Warning, which has been extended to Sunday evening. Temperatures will remain well above average, with tropical dew points in the mid-70s °F, resulting in heat indices climbing as high as 110 °F in some locations. This ample heat and moisture across the forecast region will support afternoon convection and thunderstorms, as has been the case the past several days. The GFS brings precipitation to the I-95 Corridor by 18Z, while the NAM and EC do so around 00Z Sunday. The 00Z NMM ARW, 03Z SREF, and 06Z 4 km NAM appear to side with the slower NAM/EC solution. Periods of intense afternoon sunshine before these clouds and precipitation arrive could support some ozone formation along the I-95 Corridor. However, a clean transport pattern will remain aloft. 06Z GFS back trajectories for the I-95 Corridor are fast and southerly, from the NC/SC border, which should act to temper ozone formation across the Mid-Atlantic enough to keep daily average ozone in the Moderate range along the I-95 Corridor. The 06Z air quality models all develop Moderate ozone along and just north of I-95, but they do not resolve any USG ozone across the forecast region. Due to the clean transport pattern in place, the chance for an ozone exceedance will remain Slight on Saturday.

Another round of diurnal thunderstorms is expected across the Mid-Atlantic on Sunday, enhanced by the development of a surface trough ahead of an approaching cold front. As on Saturday, however, the exact placement and extent of this precipitation remains a question, mainly due to the models struggling to resolve the track of the weak front. The NAM keeps the I-95 Corridor dry through 00Z Monday, while the GFS and EC bring scattered precipitation to I-95 by this time. The 03Z SREF still shows a high probability of precipitation near and along the I-95 Corridor Sunday afternoon, however. It is certainly plausible for this approaching front to slow down, as the operational NAM is suggesting, given that the air mass it will be encountering will be especially hot and humid. A second day of at least partly sunny afternoon skies will promote rising ozone once again along the I-95 Corridor. That said, 06Z GFS back trajectories are southwesterly and even faster that on Saturday, originating from northern GA. This clean transport pattern aloft, combined with sustained surface winds, should once again have a moderating effect on rising ozone across the forecast region. The 06Z air quality models are split, likely due to the uncertainty in the precipitation forecast. The BAMS models bring the northern and central Mid-Atlantic down into the Good range on Sunday, while the NCDENR continues to develop a narrow strip line of Moderate ozone along and east of the I-95 Corridor. How high ozone rises on Saturday will give us a sense of which solution is more likely, as well as how fast local ozone production can counteract clean flow aloft. Regardless, the continued clean transport pattern and the possibility of late day thunderstorms will keep the chance for an ozone exceedance Slight on Sunday.

The cold front should move into the northern Mid-Atlantic during Monday morning, before pushing southward during the afternoon. As discussed above, there is uncertainty regarding the placement of this front by the end of Monday. The GFS brings it further southward into VA by 00Z, while the EC keeps it across central/southern PA through this time. 06Z GFS back trajectories for the I-95 Corridor shift westerly and are localized at 500m, suggesting that the front will still be north of PHL by 12Z. Clouds and scattered precipitation are expected to form both along and ahead of this front. Locations behind the front will experience post-frontal clearing and a shift to northwesterly surface winds. These winds will be light, however, so the presumably cleaner and slightly cooler/drier air mass behind the front will likely be slow to build in. Locations that remain ahead of the front will remain under the influence of a potentially modified air mass. If clouds and precipitation are not as widespread as expected, converging winds along the front could cause locally enhanced ozone formation. The BAMS models are showing hints of this, with a line of Moderate ozone across the central Mid-Atlantic, with small patches of USG ozone resolved near PIT and DC. This seems slightly overdone, however, given the support for at least scattered cloud cover and precipitation along the front. Given the uncertainty associated with the track and impacts of the front, the chance for an ozone exceedance on Monday will rise to Marginal.

On Tuesday, the cold front is expected to stall across the central Mid-Atlantic, in the vicinity of the Mason-Dixon Line. The placement of this front, combined with intense upper level vorticity approaching the forecast region, should support more widespread afternoon clouds and precipitation, with both the GFS and EC developing precipitation across almost the entire Mid-Atlantic by 00Z Wednesday, although the GFS keeps the I-95 Corridor clear for much of the afternoon. 06Z GFS back trajectories for the I-95 Corridor shift northwesterly behind the front, but they are relatively slow. This presumably clean northwesterly flow both aloft and at the surface, along with ample diurnal precipitation, should limit ozone formation across most of the Mid-Atlantic. If the GFS’s afternoon sunshine along I-95 verifies, however, there may be opportunity for localized ozone formation, given high pressure building in from the north, light surface winds, and still very warm conditions. The BAMS models develop Moderate ozone all along and west of the I-95 Corridor and resolve areas of USG ozone over PHL and southern NJ. Once again, this seems entirely overdone, given the limiting factors in place expected to be in place on Tuesday. But given the uncertainty in the precipitation forecast, the chance for an ozone exceedance will remain Marginal on Tuesday, primarily for the I-95 Corridor.

The WPC keeps the stalled front across the central Mid-Atlantic on Wednesday, but as discussed above, it may be north or south of the MDL, depending on which model solution verifies. The intense vorticity streaming aloft will pass directly over the northern and central Mid-Atlantic, providing a trigger for another round of widespread clouds and precipitation throughout the forecast region during the afternoon. 06Z GFS back trajectories for the I-95 Corridor become southerly again in response to the strengthening of the Bermuda High/subtropical ridge. This will reinforce the likelihood of Good air quality across the region, given the presence of regional cloudiness and precipitation. Therefore, the chance for an ozone exceedance will drop to Slight for Wednesday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 11, 2016
Valid: August 12-16, 2016 (Friday-Tuesday)

Summary:

Above average heat and stifling humidity will continue across much of the Mid-Atlantic through the weekend, but persistent southerly flow will keep clean maritime air flowing into the region. Ample regional heat and moisture and the approach of a cold front will support the development of diurnal convection and precipitation essentially every day during the medium range, but the exact extent of this precipitation remains a question. On Friday, this afternoon precipitation is looking increasingly scattered, which will allow for periods of intense sunshine at some locations. Despite this sun, the persistent southerly transport pattern should limit any areas of ozone formation to the Moderate range at most, keeping the chance for an ozone exceedance Slight. Similar conditions will be present on Saturday, with the possibility of periods of afternoon sunshine allowing for some ozone formation, but the clean transport pattern will keep the chances for an ozone exceedance Slight once again. More widespread clouds and precipitation are expected on Sunday as the cold front enters the Ohio River Valley, further enhanced by the development of pre-frontal surface trough. As a result, the chance for an ozone exceedance will remain Slight on Sunday. The cold front will pass through the northern Mid-Atlantic on Monday morning, and stall near the Mason-Dixon Line. Another round of clouds and precipitation along the front will limit ozone formation across the forecast region. Therefore, the chance for an ozone exceedance on Monday will remain Slight. On Tuesday, the cold front will remain stalled across the central Mid-Atlantic, acting as a focus for clouds and precipitation. Although surface high pressure moving across the northern Mid-Atlantic will support the chance for ozone formation along the northern extent of the I-95 Corridor, the presumably clean post-frontal air mass in place should keep the chance for an ozone exceedance Slight on Tuesday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with differences at the end of the period regarding the placement of a slow-moving cold front. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad upper level trough extending southward across the Plains will progress eastward on Friday, with its center traversing MB/southern ON. The leading edge of this trough will be positioned just to the north of the Mid-Atlantic, across NY/New England, for the entirety of Friday. Concurrently, a semi-permanent mid-level ridge centered over the Bermuda High out on the Atlantic will remain in place on Friday persist through the medium range. The axis of the upper level trough will shift slightly eastward on Saturday towards the Great Lakes, pulling its associated cold front into the Ohio River Valley during Saturday afternoon. The NAM shows a shortwave disturbance across the Gulf Coast, which caused record flooding to much of the southern U.S., getting absorbed into the periphery of the upper level trough beginning 12Z Saturday. The GFS and EC have a similar thing happening, but not until roughly 12Z Sunday. On Sunday, the upper level trough will pivot across the Great Lakes, reorienting its cold front east-west, and bringing it right to the northern edge of the Mid-Atlantic by 00Z Monday. The upper level trough will flatten and contract northward on Monday, with its axis shifting northeastward towards the Canadian Maritimes. Its associated cold front will move southward towards the central Mid-Atlantic, reaching the vicinity of the Mason-Dixon Line by 00Z Tuesday. On Tuesday, the Mid-Atlantic will remain under mostly zonal flow aloft, allowing a line of potent vorticity, streaming off from the decaying Gulf Coast low underneath the flattened upper level trough, to approach the Mid-Atlantic. The GFS keeps this vorticity mostly to the north of the forecast region through Tuesday, while the EC brings it to the MDL by 00Z Wednesday. At the surface, the cold front will stall across the central Mid-Atlantic, in the vicinity of the MDL.

Friday will be another scorching day for much of the Mid-Atlantic, with above average temperatures, dew points in the 70°F, and heat indices climbing above 100 °F. The NWS has placed portions of the I-95 Corridor, including northern DE, PHL, and western NJ under an Excessive Heat Warning through Saturday evening. Ample regional heat and moisture will support the formation of afternoon convection and thunderstorms. The GFS and EC both continue to develop widespread precipitation across the northern and central Mid-Atlantic beginning 18Z, with this rain reaching the I-95 Corridor by 00Z Saturday. The NAM, however, shows more scattered precipitation, which is also supported by the 06Z 4 km and 12 km NAM, as well as the 00Z NMM/ARW. The 03Z SREF sides with the GFS/EC solution, with high probabilities of precipitation across the northern and central Mid-Atlantic by the afternoon. 06Z GFS back trajectories for the I-95 Corridor remain relatively fast and southerly, bringing a persistent flow of clean maritime air into the forecast region. Surface winds will be southwesterly and relatively light, which will keep highway emissions along I-95. If a NAM-like solution verifies, periods of intense afternoon sunshine may allow for some ozone formation along the I-95 Corridor. The 06Z air quality models reflect this possibility, with all of the models developing a strip of Moderate ozone along I-95, more so than they did in yesterday’s model runs. The main forecast question will be how quickly this relatively clean air mass can modify, but the continued presence of a clean regional transport pattern suggests that any ozone that forms will not reach beyond the Moderate range. As a result, the chance for an ozone exceedance will remain Slight on Friday.

Another round of diurnal clouds and precipitation is expected on Saturday, ahead of the cold front arriving across the Ohio River Valley during the afternoon. The GFS and EC bring the leading edge of this precipitation to the I-95 Corridor by 00Z Sunday, while the NAM keeps this precipitation just to the north and west of I-95. 06Z GFS back trajectories for the I-95 Corridor remain fast and southerly, again promoting a persistent flow of clean maritime air. Surface winds will also remain southwesterly, but will slightly stronger and more persistent than on Friday. Even if heavy precipitation does not reach I-95 by Saturday afternoon, there will likely be enough nearby cloud cover to temper ozone, which can be seen in the 06Z 15-panel NAM. As on Friday, however, the potential for period of sunshine lends support to the possibility of some ozone formation along the I-95 Corridor. The air quality models are split, with the BAMS models bringing most of the Mid-Atlantic back down into Good range, while the NCDENR model continues to shows a line of Moderate ozone right along I-95. How high ozone rises on Friday will give a sense of how quickly local ozone production can counteract the clean regional air mass and transport pattern on Saturday. Given the continued strong support for a clean transport pattern aloft, the chance for an ozone exceedance will remain Slight.

More widespread clouds and precipitation are expected on Sunday as the front progresses to just north and west of the Mid-Atlantic. This will be further enhanced by the development of a pre-frontal surface trough expected to form, roughly aligned with the I-95 Corridor. The GFS and EC both develop precipitation beginning 18Z and reaching I-95 by 00Z Monday. 06Z GFS back trajectories for the I-95 Corridor turn more west and south, coming from the Gulf Coast, and they remain fast. The BAMS models keeps the entire Mid-Atlantic in the Good range for ozone, while the NCDENR model develops a few patches of Moderate ozone along the northern extent of the I-95 Corridor. One question will be how high ozone levels climbed both Friday and Saturday. If Moderate conditions were observed both of those days, then any periods of sunshine or converging winds along the surface trough before precipitation arrive could lead to spikes in ozone concentrations. However, given the support for more widespread precipitation, and continued clean transport aloft, local ozone formation should be even more limited than it was the two preceding days. As a result, the chance for an ozone exceedance will remain Slight on Sunday.

The cold front will move through the northern Mid-Atlantic during the morning and early afternoon on Monday, approaching the Mason-Dixon Line during the latter half of the day. The GFS and EC differ with their placement of precipitation along this front. The GFS shows precipitation further south, across the central and southern Mid-Atlantic by 18Z. On the other hand, the EC places this precipitation further northward across the northern and central Mid-Atlantic, indicating a slightly slower front. 06Z GFS back trajectories for the I-95 Corridor are westerly, which suggest that the front will not have cleared the I-95 Corridor by 12Z. The effects of the air mass change behind the front will not be felt immediately, meaning that locations behind the front will still experience sweltering heat and humidity. Post-frontal surface winds will be northwesterly, but relatively light. Therefore, locations just behind the cold front may experience enough sunshine and near-front convergence to support some ozone formation. The BAMS air quality models shows hints of this, with a line of Moderate ozone roughly aligned with the MDL in the vicinity of the front. All of this said, widespread clouds and precipitation on Saturday are expected to help to keep the regional air mass clean, which may make it difficult for the air mass to modify quickly on Sunday. Additionally, there should be ample cloud cover near the front to temper ozone formation, despite the factors discussed above. Therefore, the chance for an ozone exceedance will remain Slight on Monday.

On Tuesday, the cold front is expected to stall in the vicinity of the Mason-Dixon Line, providing a focus for another round of diurnal convection and precipitation. Again, the GFS and EC differ with their placement of this precipitation. The GFS has this precipitation focused across the central and southern Mid-Atlantic, while the EC develops it across the northern and central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are northwesterly from MI. Despite post-frontal conditions across the northern Mid-Atlantic, surface high pressure moving across the Great Lakes into PA and NJ. will promote clearing skies and diminishing winds at some locations, despite its close proximity to the cold front. This will allow for the chance of ozone formation along the northern extent of the I-95 Corridor. But with a presumably clean post-frontal air mass in place, any ozone that forms should remain inside the Moderate range. As a result, the chance for an ozone exceedance will remain Slight on Tuesday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 10, 2016
Valid: August 11-15, 2016 (Thursday-Monday)

Summary:

Oppressive heat and humidity will reign across the Mid-Atlantic beginning on Thursday and lasting through the weekend, with temperatures climbing above average and dewpoints in the 70s °F. Furthermore, heat indices are expected to reach up above 100 °F beginning on Friday. Ample regional moisture and instability, paired with the slow approach of a cold front, will support diurnal clouds and convection essentially every day during the forecast period. This will be further aided by the development of a prefrontal surface trough on Saturday and Sunday, which will act as a focus for robust afternoon convection. There is a small chance for some rising ozone in locations that experience periods without precipitation during the weekend, mainly in the vicinity of the I-95 Corridor. However, widespread cloud cover and the presence of a thoroughly cleaned out air mass should suppress most local ozone production. Once the front moves into the northern Mid-Atlantic and progresses southward towards the central Mid-Atlantic on Monday, the clean southerly/southwesterly transport pattern from the weekend will be swapped out for an equally clean northwesterly transport pattern behind the front. The sum total of these limiting factors will keep the chances for an ozone exceedance Slight each day of the medium range.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The Mid-Atlantic will remain under mostly zonal flow aloft on Thursday, allowing some shortwave energy to stream across northern and central portions of the forecast region. Concurrently, the semi-permanent mid-level ridge centered over the southeastern U.S. will persist over much of the Mid-Atlantic and remain in place for the entirety of the forecast period, though it will be slightly suppressed by an upper level trough arriving over the weekend. On Friday, this upper level trough will shift eastward over the northern Plains, with its center traversing southern MB/ON through 00Z Saturday. The leading edge of the trough will remain to the north of the Mid-Atlantic region, across New England, for the entire day on Friday. The axis of the upper level trough will shift slightly eastward on Saturday towards the Great Lakes, pulling its associated cold front into the Ohio River Valley during Saturday afternoon. Both the NAM and EC show the shortwave disturbance that has brought torrential rain to the southern U.S. in the past week getting absorbed into the periphery of the nearby trough by 00Z Sunday. The GFS holds onto this disturbance until Sunday, before also bringing it up into the flow around the base of the upper level trough. On Sunday, the upper level trough will flatten slightly and pivot directly over southern ON/Great Lakes, with its associated front remaining just to the north and west of the Mid-Atlantic through most of the day. The GFS and EC differ slightly on Monday, with the GFS completely flattening the upper level trough across southern QC/New England, while the EC maintains the structure of the trough with its axis pivoting and lifting northeastward across southern QC around 12Z. Both solutions, however, bring the trough’s cold front into the northern Mid-Atlantic during the morning, progressing it southward to roughly the Mason-Dixon Line by 00Z Tuesday.

The effects of an impending heat wave will be felt beginning on Thursday, and are expected to last through at least the weekend. The NWS has issued an Excessive Heat Watch for northern DE, PHL, and western NJ, effective today through Saturday evening. On Thursday, southwesterly surface winds will funnel heat and moisture into the Mid-Atlantic, allowing temperatures to climb above average, with dewpoints in the 70s °F. The GFS and EC develop widespread precipitation across the northern Mid-Atlantic by 18Z, while the NAM continues to keep the heaviest precipitation further south across the central Mid-Atlantic at this time. The 00Z ARW model and 03Z SREF support the GFS/EC solution. Despite this difference between the deterministic models, the NAM still does show precipitation along the I-95 Corridor by 18Z. 06Z GFS back trajectories for the I-95 Corridor are southerly and relatively fast, which will continue to transport clean air into the forecast region. If a NAM solution verifies, there may be some periods without rain across the northern Mid-Atlantic which, combined with intense heat and slight lighter surface winds (in comparison to those that are expected today), may support some rising ozone. The 06Z air quality models show some hints of this, with the BAMS and NCDENR models resolving some patches of low Moderate ozone along the I-95 Corridor across DC, northern DE, PHL, and west central NJ. The NOAA model, on the other hand, keeps the entire Mid-Atlantic well into the Good range on Thursday. Though either of these two solutions are possible, there will be enough moderating factors in place to suppress any substantial ozone production across the forecast region. As a result, the chance for an ozone exceedance will be Slight on Thursday.

Friday will be another scorching day for much of the Mid-Atlantic, with heat indices expected to climb up above 100 °F during the afternoon. As on Thursday, the GFS and EC develop widespread precipitation across the northern and central Mid-Atlantic beginning 18Z. The NAM shows a similar placement with its precipitation, but this precipitation is more scattered than the GFS/EC, with the heaviest rain focused across the central Mid-Atlantic. The 03Z SREF is again in agreement with the GFS/EC solution, with high probabilities of precipitation across the entire northern Mid-Atlantic and portions of the central Mid-Atlantic during Friday afternoon. 06Z GFS back trajectories for the I-95 Corridor remain fast and southerly, originating just off the coast of the Carolinas. Some ozone production is possible in the vicinity of I-95, given the intense heat, sustained southwesterly surface winds, and the possibility of few breaks of sunshine following the NAM solution. The BAMS and NCDENR models show an area of low Moderate ozone developing just to the north and west of the I-95 Corridor. However, the presence of relatively clean regional air mass and the likelihood of afternoon clouds and thunderstorms will keep the chance for an ozone exceedance Slight on Friday.

Saturday will be similar to the two preceding days, with oppressive heat and humidity across most of the Mid-Atlantic. Again, there is support for widespread diurnal clouds and precipitation due to the ample regional moisture and instability. Additionally, the development of a pre-frontal surface trough ahead of the cold front entering the Ohio River Valley will act as an additional focus for afternoon convection across the northern and central Mid-Atlantic. Surface winds remain southwesterly ahead of this front. 06Z GFS back trajectories for the I-95 Corridor also shift southwesterly to follow suit. All of the deterministic models bring precipitation eastward to I-95 by 18Z, as does the 03Z SREF. Even more substantial cloud cover and diurnal precipitation in comparison to Friday will limit ozone formation across the forecast region. The BAMS models reflect this, with the entire Mid-Atlantic falling back into the Good range for ozone. The only complicating factor could be if this cold front slows down, as they often can during this time of the year, especially as they encroach upon extremely warm and moist air masses like the one expected to be in place across the Mid-Atlantic on Saturday. If the front were to slow, breaks of sunshine could allow for isolated areas of rising ozone along and near the I-95 Corridor. However, given the continued support for clouds and precipitation, the chance for an ozone exceedance will remain Slight on Saturday.

The cold front will remain just to the north and west of the Mid-Atlantic for the majority of Sunday, supporting yet another round of clouds and precipitation across the forecast region during the afternoon. This will enhanced by the continued presence of a pre-frontal surface trough roughly aligned with the I-95 Corridor, which will promote robust diurnal convection. Both the GFS and EC show the northern and central Mid-Atlantic engulfed in precipitation by 18Z. 06Z back trajectories for the I-95 Corridor are more westerly, but this should have a limited impact on the air quality forecast given the presence of widespread clouds and rain. Accordingly, the BAMS models keep the entire Mid-Atlantic in the Good range for ozone on Sunday. As on Saturday, there could be some concern for local ozone production if the front slows down, but this seems unlikely at this point in time. Yet another day of persistent cloud cover, precipitation, and a relatively clean transport pattern will keep the chances for an ozone exceedance Slight for another day.

On Monday, the sluggishly progressing cold front will finally move into the northern Mid-Atlantic during the morning, before pushing southward to roughly the MDL by 00Z Tuesday. The EC develops an organized line of precipitation along the front, reaching the I-95 Corridor by 18Z. The GFS, however, shows only scattered precipitation across the northern and central Mid-Atlantic at this time. Even in locations that are spared of this intense frontal precipitation, there should be sufficient cloud cover to limit any rising ozone. Additionally, presumably clean, northwesterly winds both aloft and at the surface behind the front will also suppress ozone formation along I-95. 06Z GFS back trajectories for the I-95 Corridor are northwesterly, suggesting that front may actually pass to the south of I-95 by 12Z. As a result of these limiting factors, the chance for an ozone exceedance will be Slight once again to finish out the medium range.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 9, 2016
Valid: August 10-14, 2016 (Wednesday-Sunday)

Summary:

Ozone formation will be limited across the Mid-Atlantic for the entirety of the medium range, due to the presence of a nearby frontal boundary triggering clouds and precipitation each day. This will be supported by ample regional heat and moisture, with a wave of above average temperatures and stifling humidity beginning on Wednesday and lasting through at least the weekend. On Wednesday, sustained southerly flow, morning showers, and the development of diurnal thunderstorms will limit ozone formation across the forecast region, keeping the chances for an ozone exceedance Slight. A back door cold front will drop down across northern NY on Thursday, supporting another round of clouds and precipitation across the Mid-Atlantic. The chance for an ozone exceedance on Thursday will remain Slight as a result. This frontal boundary will stall just to the north and west of the Mid-Atlantic on Friday, and remain quasi-stationary through Saturday, before moving into the western Mid-Atlantic on Sunday, supporting three more days of regional convection and precipitation to close out the medium range. There is some support for rising ozone over the weekend, as temperatures will be well into the 90s °F and heat indices topping off above 100 °F, with southwesterly surface winds near the front. As the weekend approaches, high-resolution guidance will provide a clearer picture of how widespread this frontal precipitation will be, which will help determine if some isolated locations could experience local ozone production. At this point in time, given the likelihood of widespread clouds and precipitation, and the presence of a clean regional air mass, the chance for an ozone exceedance will remain Slight Friday through Sunday as well.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level ridge will depart northeastward toward the Canadian Maritimes on Wednesday, ahead of an upper level trough traversing northern ON/QC. This will return much of the northern and central Mid-Atlantic to zonal flow aloft, allowing shortwave energy shearing off of an upper level disturbance over the Gulf Coast to stream into the forecast region. This disturbance has been causing torrential rain and record flooding across much of the southern U.S., and it is expected to linger through at least Friday. Concurrently, a mid-level ridge also connected with the Bermuda High will remain over the Mid-Atlantic through the forecast period, though it will be slightly suppressed over the weekend with the approach of an upper level trough. The Mid-Atlantic will remain under mostly zonal flow aloft on Thursday, allowing a backdoor cold front to slowly drop southward into upstate NY by roughly 18Z. On Friday, an upper level trough extending down across the northern Plains will shift eastward, with its center passing through southern MB/ON. This trough will pull its associated cold front across much of the Midwest by 12Z. The leading edge of this upper level trough will be located just to the north of the forecast region through the majority of Saturday, with its associated front remaining just to the north and west of the Mid-Atlantic. Additionally, the shortwave disturbance over the southern U.S. will be absorbed into the periphery of the trough. The trough will move slightly eastward on Sunday, but will again remain largely north and west of the Mid-Atlantic, with its cold front just reaching the western edge of the Mid-Atlantic by 12Z. The GFS and EC differ slightly, with the GFS bringing vorticity lobes along the trough’s leading edge directly across the northern and central Mid-Atlantic during the afternoon, while the EC keeps this energy to the west of the forecast region through 00Z Monday.

Wednesday will mark the beginning of another pseudo heat wave that is expected to last through to at least the weekend. High pressure well off the Mid-Atlantic coast will turn the transport pattern mostly southerly both aloft and at the surface. This will funnel heat and moisture into the forecast region, allowing temperatures to climb above average, with stifling humidity as dewpoints rise into the 70s °F. Ample regional moisture and instability paired with shortwave energy aloft will support the development of clouds and precipitation throughout much of the day, with scattered showers during the morning hours and diurnal thunderstorms in the afternoon. This precipitation pattern is supported by all of the deterministic models, as well as this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. The combination of these persistent clouds and precipitation and sustained onshore flow will limit ozone formation across most of the Mid-Atlantic. The 06Z air quality models clearly reflect this clearing trend, with all of the models brining ozone down into the Good range across the Mid-Atlantic. As a result, the chance for an ozone exceedance will fall to Slight on Wednesday.

The back door cold front will arrive across northern NY during Thursday afternoon, around 18Z, but remain north of the Mid-Atlantic through the remainder of the day. Though the front has slowed down in comparison to yesterday’s guidance, it still appears that it will provide sufficient enough pre-frontal lift to trigger another round of clouds and precipitation across portions of the forecast region. The GFS and EC continue to keep the heaviest diurnal precipitation across the northern and portions of the central Mid-Atlantic. The NAM, on the other hand, shifts this precipitation southward across the central and southern Mid-Atlantic, due to a further southward placement of the cold front during Thursday afternoon. The 03Z SREF sides with the GFS/EC solution, with high probabilities of precipitation across the northern Mid-Atlantic by 18Z. Despite these discrepancies with the exact placement of this precipitation, regional cloud cover and a continued transport pattern will limit any rising ozone in location that are not directly affected by showers and thunderstorms. 06Z GFS back trajectories for the I-95 Corridor remain southerly and relatively fast. As a result of these factors, generally Good air quality is expected across most of the Mid-Atlantic again on Thursday. Accordingly, the BAMS and NCDENR models keep almost the entire forecast region in the Good range for ozone. Therefore, the chance for an ozone exceedance on Thursday will remain Slight.

On Friday, the cold front is expected to stall across the Mid-Atlantic, but the exact placement of the front will depend on which of the deterministic solutions verify on Thursday. A GFS/EC solution would place the front across southern NY, while a NAM solution would have the front further south across central PA. The north-south placement of this front, however, will have a limited on air quality, with a third consecutive day of clouds and thunderstorms expected across much of the Mid-Atlantic. This precipitation pattern is supported by the deterministic models, as well as the 03Z SREF, 06Z 12 km NAM and 13 km GFS. 06Z GFS back trajectories for the I-95 turn slightly southwesterly, but this will still be extremely clean, especially given the clouds and intense precipitation that have thoroughly cleaned out the air mass across the southern U.S. in the past week. Locations that do not receive quite as intense precipitation could experience some rising ozone, as temperatures will be well into the 90s °F and heat indices topping off above 100 °F, with southwesterly surface winds near the front. The BAMS air quality models show a hint of this, with a small line of low Moderate ozone developing along and just west of the I-95 Corridor. However, there will be enough factors in place to limit ozone formation, keeping the chances for an ozone exceedance Slight once again on Friday.

The stalled front will remain in the vicinity of the northern Mid-Atlantic during Saturday morning, slowly moving eastward during the afternoon towards the western edge of the forecast region. This will trigger yet another day of widespread clouds and precipitation across much of the region, which will be further supported by the potential development of a pre-frontal surface trough. The GFS and EC are in closer agreement in today’s model runs regarding the track and extent of this precipitation, with both models bringing widespread to the I-95 Corridor by 18Z Saturday. 06Z GFS back trajectories for the I-95 Corridor remain southwesterly and quite fast, originating from GA. This clean transport pattern, persistent cloud cover and precipitation, and the thoroughly rain-washed air mass in place will subdue ozone production for another day across most of the Mid-Atlantic. The BAMS model keeps most of the Mid-Atlantic in the Good range for ozone, with the MAQSIP-RT model showing some areas of low Moderate ozone across western PA and northeastern NJ. Rising ozone is certainly possible in places which experience less precipitation, but this ozone production would be controlled given regional cloud cover and precipitation. The chance for an ozone exceedance will remain Slight on Saturday as a result.

On Sunday, the slow-moving cold front will finally arrive across the Mid-Atlantic, entering the western Mid-Atlantic beginning 12Z. Substantial cloud cover and precipitation is expected to develop along and ahead of the front. The GFS and EC differ slightly in the extent of this frontal precipitation, with the GFS showing more widespread precipitation, and the EC resolving a more consolidated, intense line of rain due to intense upper level shortwave support. Regardless, both models develop precipitation across the majority of the forecast region by 18Z. 06Z back trajectories for the I-95 Corridor continue to be fast and southwesterly. The sum total of these factors will keep the chance for an ozone exceedance Slight for a fifth consecutive day.

-Brown/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 8, 2016
Valid: August 9-13, 2016 (Tuesday-Saturday)

Summary:

Despite the presence of persistent upper and mid-level ridging patterns, the chances for an ozone exceedance will remain relatively low throughout the medium range, due to a predominantly southerly transport pattern and continued chances for afternoon clouds and precipitation throughout most of the forecast period. High pressure will drift off the Mid-Atlantic coast during Tuesday afternoon, promoting onshore flow at the surface and aloft across the forecast region. This maritime transport, as well as the presence of a clean regional air mass, will limit rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal on Tuesday. This high will move further eastward on Wednesday, with continued southerly flow causing temperatures and humidity to rise across the Mid-Atlantic. This will mark the beginning of a heat wave that is expected to affect much of the forecast region through the weekend, with temperatures rising well above average, with heat indices topping off above 100 °F through Sunday. Despite the oppressive heat, a back door cold front will arrive across the northern Mid-Atlantic on Wednesday, stalling across the NY/PA border through Friday. This front will act a focus for diurnal convection and thunderstorms across the northern and central Mid-Atlantic Wednesday through Friday, which combined with clean regional transport, will keep the chance for an ozone exceedance Slight all three days. On Saturday, an approaching low pressure system will pull another cold front towards the western Mid-Atlantic. Pre-frontal clouds and precipitation are expected to form, but there is uncertainty regarding how far east this precipitation will reach by the end of Saturday. Given this uncertainty, and the possibility of afternoon sunshine along the I-95 Corridor, the chance for an ozone exceedance will rise to Marginal on Saturday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A weak upper level ridge will build over the Mid-Atlantic on Tuesday, with its axis over southern QC by 18Z. Concurrently, a mid-level ridge also connected with the Bermuda High will remain over the Mid-Atlantic through the forecast period, though it will be slightly suppressed on Saturday with the approach of an upper level trough. On Wednesday, the upper level ridge will be shunted northeastward toward the Canadian Maritimes ahead of an upper level trough traversing northern ON/QC, whose associated backdoor cold front will move into the Great Lakes region by 12Z. Most of the northern and central Mid-Atlantic will return to zonal flow aloft, while the southern Mid-Atlantic will remain influenced by a broad and disorganized area of shortwave energy across the Gulf Coast that will linger across the Southeast U.S. through Friday. This energy is fostering a surface that is already bringing flooding rain to the eastern Gulf Coast states, and it will linger through into at least Friday. Some of this low’s shortwave energy may shear off and stream into the northern and central Mid-Atlantic on Wednesday. The zonal flow pattern aloft will remain in place across the northern and central Mid-Atlantic on Thursday, allowing the approaching cold front to stall roughly across the NY/PA border beginning roughly 12Z Thursday, and become quasi-stationary through Friday. On Friday, the zonal flow aloft will persist across most of the Mid-Atlantic, while an upper level trough extending down across the Plains shifts eastward, pulling its associated cold front into the central Plains by 12Z. The GFS and EC differ slightly on Saturday with the strength of the upper level trough and its embedded shortwave energy. The GFS shows a broader trough, extending all the way southward into the south central U.S. On the other hand, the EC shows a more compact trough, with its center across southern MB/ON and the northern Plains by 00Z Sunday. Though the upper level trough will remain largely to the west of the Mid-Atlantic through Saturday, the shortwave disturbance is expected to get absorbed into its periphery, with some of its associated energy streaming into the northern Mid-Atlantic by 00Z Sunday. Additionally, the approach of the trough will push the stationary front across the NY/PA border northward into southern QC/New England on Saturday.

Surface high pressure will drift across the northern Mid-Atlantic during Tuesday morning, before shifting offshore by the late morning/early afternoon. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating in response to the close proximity of the high in the morning, which will allow for a temporary period of light winds across the northern and central Mid-Atlantic. However, as the high moves eastward, its circulation will turn transport south/southeasterly across much of the forecast region, both at the surface and aloft. Sustained onshore winds will usher clean maritime air into the Mid-Atlantic, tempering ozone formation along the I-95 Corridor. The focus for rising ozone will therefore shift to locations north and west of I-95, where the transport of highway emissions could support ozone production. This will be aided by the presence of mostly sunny skies and seasonable temperatures, afforded by the nearby high. The 06Z NOAA and NCDENR models are responding to these factors, both developing a bullseye of USG ozone to the northwest of PHL, over east central PA near Allentown. Additionally, the NOAA model resolves USG ozone across part of northern NJ. The BAMS models, however, are more conservative, only bringing these areas up into the Moderate range. The main forecast question will be how quickly the regional air mass can modify, despite the presence of sustained onshore flow for much of the day. At this point, the air mass in place across the region looks relatively clean. Ozone concentrations were very low across much of the northern Mid-Atlantic through the weekend, although widespread NOx titration this morning could foretell ozone formation later this afternoon, which will demonstrate how easily this air mass can modify. Given the strength of afternoon onshore flow, the recent tendency of the air quality models to over-forecast ozone, and the likely relatively clean air mass in place to start the day, the chance for an ozone exceedance on Tuesday will be Marginal for areas north and west of the I-95 Corridor.

An extended period of hot and humid weather will begin on Wednesday and last through the medium range period (and possibly beyond). High pressure will shift even further offshore on Wednesday, turning the predominant transport pattern southerly across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are due south and relatively fast, originating from off the SC coast. Southerly and southwesterly surface winds will funnel heat and moisture into the forecast region. This will support the formation of afternoon clouds and precipitation, especially with the approach of the back door cold front which may provide sufficient pre-frontal lift to initiate convection across the northern and central Mid-Atlantic. All of the deterministic models show widespread and moderately heavy precipitation across the northern and central Mid-Atlantic by 18Z Wednesday. This precipitation pattern is also supported by this morning’s Hi-Res guidance, including the 03Z SREF, the 06Z 4 km and 12 km NAM models, and the 06Z 13 km GFS. A combination of onshore transport and the development of afternoon precipitation should be sufficient to limit ozone formation across most of the Mid-Atlantic. The 06Z air quality models clearly reflect this cleaning trend, with the BAMS and NCDENR models bringing the entire forecast region down well into the Good range for ozone. As a result, the chance for an ozone exceedance on Wednesday will fall to Slight.

On Thursday, a back door cold front will arrive across the NY/PA border by 12Z, where it is expected to stall. 06Z GFS back trajectories for the I-95 Corridor remain fast and generally southerly. High pressure settling over western VA around 12Z Thursday will promote southerly surface winds across the forecast region, allowing temperatures to climb above average throughout much of the Mid-Atlantic. Ample regional humidity will again support the development of diurnal precipitation, with the stalled front acting as a focus for afternoon convection and thunderstorms. The deterministic models are not as convincing in comparison to Wednesday, however, regarding the extent and placement of this precipitation. The GFS has the most precipitation, as per its usual wet bias this time of the year, with the heaviest rain focused across the northern Mid-Atlantic. The EC shows a similar placement as the GFS, but develops more scattered precipitation. The NAM brings the front slightly further south, with the heaviest of its associated precipitation located across the central Mid-Atlantic beginning 18Z Thursday. Depending on the exact placement of the pre-frontal clouds and thunderstorms, portions of the I-95 Corridor may receive sufficient afternoon sunshine to support some ozone formation. But the continued onshore transport should limit any ozone to the Good to low Moderate range, and the front should be close enough to the I-95 Corridor to generate sufficient cloud cover. Additionally, the regional air mass is expected to be relatively clean after widespread precipitation on Wednesday. The BAMS air quality models support the influence of the transport pattern and the nearby front, keeping the entire Mid-Atlantic in the low Good range. Therefore, the chance for an ozone exceedance on Thursday will remain Slight.

High pressure will slide southward on Friday, settling over western NC by 12Z. 06Z GFS back trajectories for the I-95 Corridor becoming more southwesterly, but they are still fast and should be clean. Temperatures will continue to climb, reaching the low-to-mid 90s °F across much of the Mid-Atlantic. The stalled front across the northern Mid-Atlantic, likely in the vicinity of the NY/PA border, will act as a focus for afternoon clouds and thunderstorms, with both the GFS and EC developing widespread rain across the Mid-Atlantic by 00Z Saturday. Additionally, the regional air mass in place should still be fairly clean after two consecutive days of widespread clouds and thunderstorms, which will keep ozone from spiking in the early afternoon, before convection fires up across the Mid-Atlantic. The BAMS models keep the region mostly in the Good range, with the MAQSIP-RT model showing a very small area of low Moderate ozone west of PHL. Regardless, there should be enough diurnal precipitation to keep the chances for an ozone exceedance Slight another day.

On Saturday, the quasi-stationary front across the northern Mid-Atlantic will lift northward, ahead of an approaching low pressure system located over the Great Lakes by 12Z. The cold front associated with this trough will approach the western Mid-Atlantic during the afternoon, potentially providing another round of clouds and thunderstorms to the northern and central Mid-Atlantic. The GFS shows widespread precipitation reaching the I-95 Corridor, while the EC keeps the I-95 Corridor essentially dry through 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor remain south/southwesterly from the Gulf Coast, and temperatures will be well above average across most of the forecast region. If an EC-like solution verifies, intense afternoon sunshine and southwesterly surface winds ahead of the front may allow for some ozone formation along the I-95 Corridor. Given the uncertainty in the precipitation forecast, the chance for an ozone exceedance will rise to Marginal on Saturday.

-Brown/Huff