Monthly Archives: August 2016

Surprise Ozone Exceedances in Wilmington on August 27

Ozone unexpectedly exceeded the daily NAAQS of 70 ppbv on Saturday at two monitors in Wilmington, DE: MLK (73 ppbv) and Bellefonte (71 ppbv). The only other monitor along the I-95 Corridor that exceeded was the nearby Fair Hill, MD monitor (75 ppbv). Figure 1 shows the daily ozone AQIs for the region (click on individual figures to enlarge).

The exceedances were a surprise because the region was post-frontal (Figure 2), with what had seemed on Friday to be a relatively clean air mass in place. I think that was true, given the essentially isolated nature of the exceedances. The NOAA model guidance for Saturday (Figure 3) had indicated the possibility for ozone exceedances along the Fair Hill-Wilmington-Chester stretch of I-95, but the NOAA model had been over-forecasting consistently for the past three weeks, and the other air quality models predicted Moderate ozone, so I undercut the guidance. That was the right call for Chester, where daily ozone only reached 66 ppbv (right in line with my forecast of 65 ppbv), but not in Wilmington.

So what went wrong? There was a strong ridge in place (Figure 4), with the center directly overhead, which promoted stagnation. It was sunny all day (Figure 5), with temperatures reaching 91 °F at KILG, and slightly lower humidity than in recent days (dew points were hovering around 60 °F at KILG). Surface winds were light all day and recirculated from the north in the morning (Figure 6a) to the east/southeast (Figure 6b) in the afternoon. I think the two Wilmington monitors (and the Fair Hill monitor) were in a “sweet spot” of surface convergence that kept I-95 highway emissions localized enough to promote abundant sustained afternoon ozone production. The exceeding monitors had fairly steady high hourly ozone from 2 PM all the way through 7 PM, whereas hourly ozone values at neighboring monitors were either slower to rise or quicker to drop off.

I’m not sure what I could have realistically done differently with the forecast. In my many years of forecasting for the Mid-Atlantic, I can’t recall ever seeing a situation like we had on Saturday – in which two Wilmington monitors exceeded but not any in Philadelphia, under afternoon onshore flow (albeit light) and absent any strong mesoscale convergence feature like a weak frontal boundary or sea breeze. Part of it is inexperience with the new ozone standard – Saturday would not have been an exceedance day under the previous daily NAAQS of 75 ppbv. I will also have to remember that light northerly flow aloft with recirculating surface winds, like we had on Saturday, can be an issue for northern Delaware, particularly with a mid- or upper-level ridge overhead.

peak_o3_ny_pa_nj_20160827
Figure 1.  Daily peak ozone AQI values for Saturday, August 27.

WPC_18Z_20160827
Figure 2.  WPC surface analysis for 18 UTC on August 27, showing a stalling cold front well south of the Delaware forecast region.

12Z_NOAA_O3_20160827
Figure 3.  NOAA-EPA model guidance for August 27.

12Z_eta500_vrt_20160827
Figure 4.  500 mb analysis for 12 UTC on August 27.

add_BWI_vis
Figure 5.  GOES visible satellite imagery for the Mid-Atlantic at 16UTC on August 27.

sfc_bwi_amsfc_bwi_mid
Figure 6.  METARS surface analysis for the Mid-Atlantic for 1255 UTC (a) and 1625 UTC (b)

 

Checking in on the 2016 Ozone Season: the Recent Downward Trend in Observed Ozone Continues

Now that we are almost 80% of the way through the 2016 ozone season, it seems like a good time to check in and see how things are progressing. Back on May 20, I wrote a post on the outlook for the 2016 season. We were very interested to see how this year’s ozone season would stack up to the previous three seasons, during which we saw a sudden and very dramatic decrease in observed ozone levels compared to the period 2003-2012. Based on this recent downward trend, and taking the new ozone NAAQS into account, we predicted about 18-22 exceedance days in Philadelphia and 6-8 days in Delaware for the 2016 ozone season. Exceedance days occur when observed 8-hour average ozone is ≥ 71 ppbv.

Table 1 (click on table to enlarge) shows that we are well behind our predicted average for 2016 in Philadelphia (10 exceedance days so far) but just about on pace in Delaware (5 days so far). The red rectangles in Figure 1 indicate the days for which we correctly forecasted the ozone exceedances. We are doing really well in Delaware, having correctly identified all of the exceedance days so far this season and issuing only three false alarms. (To be fair, I should acknowledge that officially, there have been two additional exceedance days in Delaware, but they were what Bill and I consider technicalities, in the sense that they were validated at one monitor with only 6 continuous hours of data. While these two days technically count as exceedances from a regulatory standpoint, I contend that they don’t count from a health/forecasting perspective.)

In Philadelphia, however, we are not doing so well; we have only identified half of the exceedance days. Note that three of the days we missed were localized events (June 1, 15, and 25), with only one monitor exceeding by a few ppbv, driven by mesoscale weather conditions that are inherently difficult to predict. The exceptions are June 11, when we had high ozone transported from upwind on the first 90 °F plus day of the summer (see my previous post), and July 21, when transported smoke and local stagnation combined to push observed 8-hour ozone up to 81 ppbv, with exceedances at three monitors. The other main problem in Philadelphia is that we’ve issued a whopping seven false alarms, most of which have been epic fails (7-11 ppbv too high).

The five missed exceedance days and preponderance of false alarms in Philadelphia underscore the challenges of identifying which days will be high ozone days. Concurrent with the recent drop in observed ozone beginning in 2013, we’ve seen a shift in the main meteorological factors that lead to high ozone. Historically, high ozone days in the Mid-Atlantic were characterized by synoptic conditions including hot weather (Tmax ≥ 90 °F), westerly transport aloft from the Ohio River Valley, a strong ridge of high pressure aloft, and slowly migrating high pressure at the surface. These conditions led to more widespread, multi-day high ozone events. So even if we missed the onset day, we could reasonably forecast the remaining days in an event. Now, high ozone days are driven primarily by mesoscale features, such as weak frontal boundaries, stagnation, and bay/sea breezes. We still seem to have a few days that fit the more “classic” mold, such as June 20 and July 22, but they tend to be single day events. And by no means is hot weather a guarantee of high ozone anymore… for example, we had an extremely hot July, but only two exceedance days in Philadelphia and one in Delaware. We haven’t had an exceedance day in either forecast area in the past month, despite persistent above average temperatures during most of that period. And both of the exceedance days in July were influenced by transported wildfire smoke, which seems to be the real wild card in recent years. At least for Philadelphia and Delaware, the highest ozone we see now is associated with transported smoke, such as on May 25-26 and July 21-22. Yes, July 22 was influenced by “classic” hot conditions, but ozone reached up into the low 80s ppbv (instead of peaking in the 70s ppbv) due to the presence of smoke.

And we can’t rely on the air quality models to help us identify the exceedance days. Figures 1 and 2 (click on figures to enlarge) show the skill scores (hit rate and false alarm rate) for ozone exceedance days in 2016 for Philadelphia and Delaware, respectively. You’ll notice that across the board, the false alarm rate is higher than the hit rate for all of the numerical model guidance, and the “expert” forecasts have higher skill than any of the model guidance. Also included in Figures 1 and 2 are the skill scores for our updated statistical models, which include variables such as maximum temperature, surface wind speed in the morning, and relative humidity in the afternoon. Due to the lessening influence of synoptic weather conditions, the updated statistical models are not really helping us much this year, either, although they have some promise for helping to avoid false alarms.

The take home message is that the recent “step-down” in observed ozone seems to be here to stay, and it is not an anomaly due to fluctuating meteorological conditions over the past three summers. We can presumably look forward to a continuation of fewer ozone exceedance days in the next few years, relative to the 2003-2012 average. At the same time, correctly forecasting these days remains more of a challenge than ever, due to the weakened correlation between hot weather and high ozone, as well as the shift to predominantly mesoscale-driven ozone conducive conditions.

Ex_Table_20160826
Table 1.  Observed ozone exceedance days for the 2016 season to date in the Philadelphia and Delaware forecast areas.  The red rectangles indicate days when the exceedance was correctly forecasted.

PHL_SS_20160826
Figure 1.  Skill scores for ozone exceedances for the Philadelphia forecast area.  The best hit rate is 1.0 and the best false alarm rate is 0.0.  The numerical ozone models include “NOAA” (NOAA-EPA model), “BAMS” (Barons Meteorological models), and “NCDENR” (North Carolina model).  “STAT” refers to statistical models, and “ENS” indicates various “ensemble” averages of different combinations of the numerical and statistical guidance.

DE_SS_20160826
Figure 2.  As in Figure 1, but for the Delaware forecast area.

 

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday, August 12, 2016

Medium Range Air Quality Outlook
Mid-Atlantic Region

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

Daily_MedRange_Table_20160813

New Med Range Categories

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

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday, August 11, 2016

Medium Range Air Quality Outlook
Mid-Atlantic Region

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

Daily_MedRange_Table_20160812

New Med Range Categories

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

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday, August 10, 2016

Medium Range Air Quality Outlook
Mid-Atlantic Region

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

Daily_MedRange_Table_20160811

New Med Range Categories

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