Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 14, 2015
Valid: August 15-19, 2015 (Saturday – Wednesday)

Summary:

A mid-level ridge will build into the Mid-Atlantic for the medium range period, acting as a block for approaching frontal boundaries, and allowing above average temperatures and humidity to persist through the end of the period and beyond. A High chance of USG ozone exists for Saturday, dependent on how fast the current air mass modifies (i.e., today’s persistence for tomorrow) and the fate of weak frontal boundaries (e.g., bay/sea breeze fronts, weak back door surface trough). Short and recirculating back trajectories and consistent air quality model guidance lend confidence to a possible exceedance forecast in the usual area of concern along the I-95 Corridor. An Appreciable chance for USG ozone will continue on Sunday and Monday, with the main questions centering on lower Sunday emissions and a possible shift to sustained onshore flow aloft at 500 m. The forecast for Tuesday and Wednesday is uncertain, but the global models are showing widespread light precipitation across the region both days due to weak 500 mb troughing/shortwaves. In addition, the southerly 500 m AGL flow will continue. These factors may be enough to limit ozone to the Good to low Moderate range. Looking ahead to the end of next week, the Bermuda High will extend westward beginning next Thursday, centered over the Southeastern US. Although the influence of the Bermuda High will keep temperatures and humidity above average in the Mid-Atlantic, the southern location of the high should allow southerly transport aloft to dominate. Thus, although warm, stagnant, and sunny weather will continue through at least next weekend, the risk for USG ozone should remain Marginal due to southerly flow aloft. Particles will increase as humidity rises and the air mass over the Mid-Atlantic modifies over the next few days, peaking in the upper Moderate range on Tuesday in the Pittsburgh metropolitan area.

This is the last medium range discussion for the 2015 summer season. The discussions will return in 2016 and run from mid-May to mid-August.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but exhibit differences regarding precipitation on Tuesday and Wednesday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. The large trough that had settled over the eastern seaboard for the past couple of days has almost completely lifted out of the region, and a mid-level ridge building into the Mid-Atlantic is promoting abundantly clear skies and calm conditions. A center of surface high pressure will persist over WV throughout the period to varying degrees as the mid-level ridge (850 mb) slowly moves eastward. Whereas in previous days’ guidance, a weak back door cold front was expected to impact the northern Mid-Atlantic to some degree on Saturday-Sunday, and a stronger cold front was expected to move into the region on Monday-Tuesday and stall near the Mason-Dixon Line (MDL), this morning’s GFS and EC runs now show these fronts staying to the north and west of the Mid-Atlantic. The mid-level ridge now appears to dominate the Mid-Atlantic through the end of the medium range period and beyond, into at least next weekend, as the Bermuda High extends westward, centered over the Southeastern US.

At 500 mb, however, the weak trough/upper level shear axis is still evident in the EC and GFS guidance, which complicates the forecast. The shortwave energy dropping down on Saturday from the northern stream flow is still there today, but not quite as strong as in yesterday’s analyses. This feature is associated with a weak back door cold front at the surface. This front is even weaker today than in previous model runs, and now looks to only bring a chance of clouds and scattered precipitation to areas west and north of roughly I-81 on Saturday. There is still a question as to whether this boundary will act as a line of converging winds in the NMA on Saturday and Sunday, as discussed below. On Monday, the weak shortwave energy aloft will remain along a shear axis running approximately along the ridge of the Appalachian Mountains from the Mid-Atlantic to the Texas Gulf Coast. The persistent western US upper level ridge will try to build eastward at the beginning of next week, but the weak trough/shear axis will inhibit it. Small shortwaves dropping down from the northern stream flow on Monday will phase, to certain degree, with the shear axis energy, and dig out a very shallow trough over the Mid-Atlantic on Tuesday-Wednesday. These shortwaves will bring scattered precipitation to the Mid-Atlantic on Tuesday-Wednesday, as discussed below. At the same time, a much more substantial shortwave will drop down over the Plains and pull a cold front as far east as the Great Lakes before pulling northward. Consequently, this next cold front will not make it to the Mid-Atlantic next week, but instead the mid-level ridge will continue to provide warm and humid weather to the region.

The weekend continues to be of interest for USG ozone conditions. Obviously, persistent sunny skies, very light to calm winds, and temperatures in the low 90s °F set the stage for rising ozone across the region. The main forecast questions continue to be 1) the degree to which the air mass currently in place modifies, 2) the impacts of weak frontal boundaries, including sea/bay breeze fronts and a weak back door surface trough, and 3) air mass transport aloft, which appears to turn more southerly at 500 m AGL beginning on Sunday. Saturday is the primary day of concern, given recirculating back trajectories and higher Saturday emissions. Historically, fewer emissions of ozone precursors on Sundays limit rising ozone to the upper Moderate range, all other things being equal. The air quality models have been resolving upper Moderate ozone concentrations along and surrounding the I-95 Corridor for the past several days, lending confidence to the expectation that this previously very clean air mass will modify substantially by Saturday. The CMAQ-based air quality models (e.g., NOAA, BAMS, NCDENR) do have a high bias by this point in the summer, however, which should be taken under consideration. The BAMS, NCDENR, and NOAA air quality models agree in resolving USG ozone in the Baltimore metropolitan area and along the southern NJ coast on Saturday. Upper Moderate ozone is resolved in a wide area surrounding this sector of the I-95 Corridor, but there is less consensus regarding the extent of these conditions. Due to the very light southerly/southwesterly surface winds, Moderate ozone will most likely be limited to areas along and just to the east of I-95, where emissions will be highest. Moderate ozone is also likely along the eastern coast of MD and VA, where the high-res mesoscale weather models are resolving areas of surface divergence, and therefore subsidence and limited atmospheric mixing. A complicating factor is the weak back door trough, which is no longer a cold front. The 09Z SREF shows a medium to high probability of precipitation on Saturday afternoon for areas west and north of I-81, similar to yesterday’s analyses. The hi-res weather models are showing only increased cloud cover in these areas, which adds uncertainty to the likelihood of clouds or scattered showers making their way to the I-95 Corridor. The 06Z 12 km NAM and 13 km GFS do show a line of convergence over southeastern PA and central NJ, however, associated with the trough. As we saw a couple of weeks ago in the Philadelphia metro area, a weak semi-stationary boundary can push ozone well into the USG range on local scale. Trying to forecast where this trough will settle is extremely difficult, however. We will have to make our best assessment based on the 12Z model runs. The same thing can be said about the bay and sea breezes, which may be an issue on both weekend days, given the weak synoptic winds. Particles are expected to reach the Moderate range at scattered locations due to a slight increase in the humidity compared to today and the continuing calm conditions.

Another day of above average temperatures, clear skies, calm surface winds, and short, looping, southwesterly back trajectories leave room for concern regarding building ozone on Sunday. The NCDENR air quality model shifts its USG concerns to the DC metropolitan area for Sunday, while the BAMS guidance looks almost identical to Saturday’s. Regardless, upper Moderate conditions are expected to extend along and east of the I-95 Corridor as very light southwesterly surface winds persist throughout the region due to the surface high pressure centered over WV. A question for Sunday is the fact that back trajectories at 500 m AGL turn due south, inland from the Atlantic coast. We have seen previously this summer that southerly flow aloft can completely suppress ozone to the Good range. Although this transport signal is somewhat weak, it may be sufficient to keep ozone out of the USG range. On the other hand, Saturday’s weak surface trough in the vicinity of PHL may linger, and sea/bay breezes are a threat as well. Particles are expected to continue to increase into the Moderate range at scattered locations as calm conditions and another uptick in humidity allow the buildup of PM2.5.

Monday will be another day of temperatures in the low to mid 90s °F, increasing dew points, light southwesterly surface winds, and surface high pressure throughout the Mid-Atlantic. The main difference is that lower level (500 m AGL) back trajectories are fully south along the Atlantic coast of NC. This is most likely due to the fact that the mid-level ridge will continue to build eastward, placing the Mid-Atlantic on its western edge by early next week. The NC air quality models appear to be responding to this change in upper level flow, as they begin a downward trend in ozone on Monday, continuing into Tuesday, with only isolated Moderate ozone at coastal locations. The BAMS models keep upper Moderate ozone all along the I-95 Corridor, however.

Tuesday and Wednesday have considerable uncertainty, with continued Moderate ozone very likely, but less chance for USG ozone. The next cold front that was expected to stall near the MDL on Tuesday-Wednesday will now stay well to the north and west of the region, allowing above average temperatures and summer-time humidity to continue. However, the weak shortwave energy/troughing aloft will keep the chances for scattered clouds and rain showers in the forecast. Both the EC and GFS show scattered light precipitation across the region on Tuesday and Wednesday, which may be enough to again limit ozone to the Moderate range. In addition, back trajectories at 500 m AGL will continue to be due south, from coastal NC.

Looking ahead, the Bermuda High will extend westward beginning next Thursday, centered over the Southeastern US. Although the influence of the Bermuda High will keep temperatures and humidity above average in the Mid-Atlantic, the southern location of the high should allow southerly transport aloft to dominate. Thus, although warm, stagnant, and sunny weather will continue next week, the risk for USG ozone should remain Marginal beyond the end of the medium range period, due to southerly flow aloft.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 10, 2015
Valid: August 11-15, 2015 (Tuesday – Saturday)

Summary:

Good air quality is expected for the beginning of the medium range period as a clean and dry air mass moves in from Canada behind tomorrow’s cold front. By the end of the period, however, a return to above average temperatures, mostly sunny skies, and light surface winds will promote rising ozone, although levels are not expected to exceed the Moderate range. On Tuesday, widespread clouds and showers will inhibit ozone production. For Wednesday and Thursday, the air mass change will promote Good ozone despite sunny skies and light surface winds. Friday and Saturday are days of Marginal interest due to rising temperatures and a slowly modifying air mass, although several factors will be present that should limit any ozone increases, including onshore back trajectories at lower levels and the approach of a backdoor cold front on Saturday. Particles will persist in generally the Good range through Friday as humidity falls behind the front.

Discussion:

The weather models are in very close consensus throughout the medium range period, having closed the timing discrepancy regarding this weekend’s frontal passage. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. An upper level closed low over the James Bay is currently digging a trough into the Great Lakes region, which will continue to strengthen, move eastward, and dip into the Carolinas by 12Z Tuesday. A cluster of strong shortwaves in the base of the trough will develop a surface low that will skirt just north of the Mid-Atlantic on Tuesday, pulling a strong cold front through the entire region. Clouds and showers will impact the Mid-Atlantic starting Monday night (associated with the leading warm front) and continuing through Tuesday morning, with another round of pre-frontal convection ahead of the cold front on Tuesday afternoon. Periods of heavy rain are expected in the central and northern Mid-Atlantic (CMA, NMA) on Tuesday morning. A bought of cleaner and drier air will filter into the CMA and NMA and persist through Thursday. The upper level trough will slowly lift north- and eastward Wednesday through Friday, allowing an area of surface high pressure to build eastward into the Mid-Atlantic. The close proximity of surface high pressure will dominate and keep skies clear until the next cold front approaches on Saturday. The weather models are in much closer agreement today regarding this next front. Due to the weak nature of the backdoor front, it will begin to dissipate and result in only cloud cover and scattered rain showers on Saturday for the NMA and northern parts of the CMA.

Tuesday will be a warm and humid day with breezy south/southwesterly surface winds (turning westerly behind the cold front), cloudy skies, and plenty of rain for the Mid-Atlantic. Southerly looping onshore back trajectories will combine with these factors to make for a day of mostly Good air quality. The BAMS and NOAA air quality models are resolving a swath of Moderate ozone in eastern DC/BAL metropolitan areas. They are likely responding to some pockets of clearing shown in the high-res mesoscale weather models during the afternoon hours, between the two rounds of precipitation. This guidance is likely over-predicting, seeing as the models have been doing so for the past few days and that periods of morning rain will rid the atmosphere of precursors. Since PM2.5 takes longer to respond to air mass changes, areas that receive only light rain in the western Mid-Atlantic (WMA) will see scattered locations reach the top of the Good range.

Temperatures will rebound quickly on Wednesday back into the mid-80s °F, although dew points will drop slowly as the Canadian air mass filters in behind the front. The arrival of the cooler and drier air mass is slightly slower in today’s analyses, with the full impact of the air mass change not occurring until Thursday. As a result, isolated Moderate ozone is possible, particularly along the coastline, where the cleaner air mass will reach last. Particles should remain in the Good range as well in response to the decreasing humidity.

Surface high pressure centered over IL will continue to expand eastward on Thursday, promoting clear skies and calmer conditions across the east coast. This is the day that the Canadian air mass fully makes itself felt over the region. Northwesterly back trajectories from interior ON and the coolest and driest weather of the week will promote Good air quality across the board despite sunny skies and light northwesterly winds.

Friday will be another day of calm and sunny conditions as the surface high pressure continues to build eastward, with the center settling over WV. Temperatures will quickly rebound into the upper 80s °F, but humidity will remain relatively low. Ozone will increase into the Moderate range in some locations as the air mass slowly modifies – specifically, downwind of the I-95 Corridor. On Friday, light southwesterly surface winds place these areas of interest in DC, BAL, southwest PA, NJ, DE, and along the eastern shore of MD. Although it is too far out to forecast with confidence, the lack of strong synoptic forcing on Friday will increase the chances for a possible sea breeze. This feature would greatly increase the chances of high ozone concentrations along and east of I-95.

Saturday will be a pre-frontal day for northern part of the region as a weakening backdoor cold front reaches PA in the morning. Temperatures will continue their upward trend from Friday, with highs approaching 90 °F. There are a few features that should limit any rising ozone to the Moderate range. Back trajectories at 500 m AGL hook onshore, which we have seen this summer tends to very effectively cap rising ozone. In addition, the weak backdoor cold front will move into the NMA. Scattered afternoon clouds and showers will decrease chances for Moderate ozone in the CMA and NMA. In NC and parts of VA, continuing high pressure and mostly clear skies may allow ozone to reach the Moderate range, depending on air mass transport aloft. Increased humidity ahead of the frontal boundary may allow for scattered Moderate PM2.5 concentrations in PA and NJ.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 7, 2015
Valid: August 8-12, 2015 (Saturday – Wednesday)

Summary:

Overall Good air quality is expected to prevail throughout the medium range period in the Mid-Atlantic due to a relatively progressive pattern and only intermittent periods of calm, sunny conditions. The recent washout in the central and southern Mid-Atlantic, along with breezy northeasterly winds, will limit ozone mostly to the Good range on Saturday. Because of the anticipated sunny skies and lighter winds inland, the exception will be in areas of increased precursors directly downwind of the I-95 Corridor. Sunday will be another dry day, but possible cloud cover in the northern Mid-Atlantic and lower Sunday emissions will keep ozone mainly in the Good to low Moderate range once again. Throughout the entire weekend, particles are only expected to reach into the Moderate range at inland locations where winds will be lightest and humidity will be higher. Monday will see the arrival of the next disturbance, bringing afternoon clouds and showers eastward across the region, with Good ozone expected for most locations. Due to the light nature of the precipitation, as well as southerly shifting surface winds, PM2.5 will likely continue in the Moderate range in the western Mid-Atlantic. Immediately following Monday’s disturbance – a relatively weak surface low – an extensive cold front will sweep west to east across the eastern seaboard, bringing widespread clouds and showers to the region on Tuesday. Model discrepancies lend uncertainty to the exact precipitation forecast, but regardless, the passage of the cold front will continue the trend of Good ozone. PM2.5 will thrive on the increased humidity ahead of and along the front, depending on the strength of the precipitation. Skies will clear quickly behind the frontal passage on Wednesday morning, but a cooler, drier, and clean Canadian air mass is expected to arrive behind the front, which will return air quality to the Good range across the region.

Discussion:

The weather models are in generally close consensus throughout the beginning of the medium range period, but begin to differ on Monday. However, the discrepancies mainly occur aloft, and they do not have a large impact on the air quality forecast or its confidence. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, an upper level trough axis along the eastern seaboard with an associated area of shortwave energy over the Carolinas is the source of widespread showers and cloud cover. While the showers are limited to coastal areas south of the Mason-Dixon Line (MDL) this morning, the clouds spread from the Atlantic coast to the Appalachians. As the trough continues moving eastward today, the area of shortwave energy will organize enough to give some structure to its resulting surface low that will move northeastward and away from the coast by 12Z Saturday. This will allow an upper level ridge currently centered over the Plains to briefly build into the Mid-Atlantic for Sunday, which will be a clear and very calm day throughout the region. By Monday morning, a cluster of shortwave energy over the Great Lakes is resolved by all of the global weather models, and develops a surface low, but there are discrepancies regarding its strength and exact extent of associated precipitation across the Mid-Atlantic. Since the timing of this system is virtually identical for the GFS and the EC, the confidence of the air quality forecast is not impacted substantially by this difference in shortwave characteristics, with the bulk of precipitation likely remaining west of I-95 through the late afternoon. The models have come into closer consensus on the precipitation forecast for Monday and Tuesday, with the previously drier EC actually surpassing the GFS on the extent of the rain. The EC has completely backed off on yesterday’s solution of forming a nor’easter like coastal low and now has a traditional cold frontal passage on Tuesday, similar to the GFS. Both models analyze a large upper level trough digging southward into the northeastern US in a winter-like fashion, with the southernmost edge of the trough reaching as far south as southern GA on Tuesday. All in all, the models show a very similar solution for the end of the period, with a widespread area of pre-frontal rain moving eastward across the entire Mid-Atlantic from 18Z Monday to 06Z Wednesday. This looks to be a substantial cold front, with both the EC and GFS showing a pool of cooler Canadian air filtering into the Mid-Atlantic on Wednesday behind the frontal passage.

The clouds and showers currently impacting the central and southern Mid-Atlantic (CMA, SMA) will linger along the NC coastline through Saturday morning. Except for these showers and some scattered, topographically-driven showers in western MD and WV, Saturday will be a mostly sunny day throughout the region. Back trajectories from eastern New England, breezy northeasterly winds along the coast, and the recent washout will limit ozone to the Good range for most of the Mid-Atlantic. The air quality models haven’t been handling the impacts of the surface wave well, with substantial over-forecasting of ozone yesterday and (one hopes) today along I-95. This lends much less confidence to their guidance for tomorrow, which features scattered Moderate ozone downwind of I-95 and in the Pittsburgh metro area. Nevertheless, the I-95 Corridor will act as the usual area of increased precursor buildup on Saturday. Combined with the abundant sunshine and lighter surface winds inland, there is a chance for isolate Moderate ozone along and downwind of the interstate. Particles may see some buildup in these areas as well, but it will be limited to the low Moderate range due to the low humidity.

Sunday will be a calm and mostly sunny day as surface high pressure prevails over the Tennessee River Valley (TRV). However, some of the high-res mesoscale weather models are resolving some pop-up afternoon showers along the eastern edge of the Appalachians in the CMA and northern Mid-Atlantic (NMA), as well as widespread cloud cover. Depending on the optical thickness of these clouds and the characteristics of the northerly air building into the region on the western flank of the coastal low, ozone will likely be limited to the mostly Good range. The exception, according to the air quality forecast models, is a small pocket of possible low Moderate ozone in DC/along the southern coast of MD. We suspect that, given the full sun and very light to calm surface winds, area of Moderate ozone may be more scattered than suggested by the air quality model guidance. The calm winds will increase PM2.5 across the region, with the highest concentrations in the western Mid-Atlantic where humidity will be the highest.

Increasing clouds throughout the day on Monday ahead of an approaching surface low will limit ozone to the Good range for most of the region. Swaths of low Moderate ozone are possible east of I-95, where the clouds will reach last, but potentially breezy southerly surface flow may limit ozone to the Good range in these locations as well. The southerly will raise dew points and promote Moderate PM2.5 from the I-95 Corridor to the edge of the Appalachians, where only light rain will fall on Monday.

The surface low that sourced Monday’s rain will move across southern PA overnight Monday into Tuesday morning, reaching the PHL metropolitan area by 12Z Tuesday. At the same time, a low pressure center will pass well to our north – along the northern NY/Canada border – and drop an extensive cold front through the region immediately following the passage of the surface low. Although there are discrepancies between the global weather models as to the exact extent of the resulting precipitation, the combination of these two rain-promoting disturbances will keep clouds and at least scattered showers over the region through Tuesday night. The lack of sunlight will inhibit ozone production, keeping it in the Good range across the board. Particles may linger in the Moderate range in scattered locations until a cooler and drier Canadian air mass builds in on Wednesday. Areas that see heavier rain will observe PM2.5 concentrations dropping into the Good range.

Skies will be clear across the eastern seaboard by 12Z Wednesday behind the passage of Tuesday’s cold front. Dew points will decrease throughout the day as northwesterly surface winds filter in cleaner, drier air behind the front. Based on the 850 mb model guidance, Good air quality is expected across the entire Mid-Atlantic on Wednesday. However, if the air does hold some precursors, the full sun and light winds may be sufficient to allow Moderate ozone downwind – to the southeast – of the I-95 Corridor. Overall, however, Tuesday’s washout will be enough to limit concentrations to the bottom half of the Moderate range.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 5, 2015
Valid: August 6-10, 2015 (Thursday – Monday)

Summary:

Generally Good to low Moderate air quality is expected throughout the medium range period due to two substantial disturbances. The first will arrive in the form of a surface wave along a frontal boundary stalled over northern NC, which will bring widespread clouds and rain to the Mid-Atlantic south of the Mason-Dixon Line from Thursday through Saturday afternoon. North of these impacts, the clean post-frontal air mass in place will limit ozone production. Following this large system, we will see two days of at least partial sun, with Moderate ozone possible around the southern I-95 Corridor on Sunday. Then the next cold front will move through the region on Monday. Although there is some uncertainty in the precipitation forecast this far in advance, widespread pre-frontal clouds and convection are expected, which will drop particles and ozone into the Good range, while Moderate air quality may continue east of I-95.

Discussion:

The weather models are in generally good consensus throughout the beginning of the medium range period, but begin to differ on Friday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large closed low aloft just east of James Bay has an associated trough reaching into the central Mid-Atlantic (CMA). An area of shortwave energy aloft over NE/KS will move eastward with the flow and begin to dig out its own mini trough. The global and mesoscale models have come into much closer consensus today regarding the fate of this area of shortwave energy, but some discrepancies remain. In particular, the global weather models begin to show slight differences on Friday regarding the exact shape and speed of this shortwave. However, resulting model discrepancies do not impact the air quality forecast until Monday. This shortwave feature will begin to impact the Mid-Atlantic on Thursday afternoon as a wave of surface low pressure, bringing widespread clouds to areas south of the Mason-Dixon Line (MDL) and associated rain showers to I-95. The models now agree in containing the major impacts of this disturbance to areas south of the PA/MD border; as a result, PA and north/central NJ will see little to no precipitation. Saturday will be a transition day following this rainy system as it moves northeastward up the Atlantic coast. The persistent upper level ridge in the western US will flatten and spread towards the Mid-Atlantic on Sunday, promoting another day of mostly sunny skies. But the eastward extension of the upper level ridge will not spread any farther, as the semi-persistent eastern Canadian upper level trough re-develops for early next week. On Sunday, the models agree that an upper level low will reach the MB/ON border, reflected at the surface as an area of low pressure in southcentral ON. This low will have an associated southwestward stretching frontal boundary that will reach from the Great Lakes to northern KS on Sunday. The WPC surface analysis shows this front moving southeastward through the Mid-Atlantic throughout the day on Monday, which proves a favoring towards the GFS global model solution, which shows an extensive area of pre-frontal precipitation moving through the entire Mid-Atlantic starting on Monday night.

Extensive cloud cover will spread over the entire Mid-Atlantic from west to east tomorrow, due to both a stalled frontal boundary along the VA/NC border and an approaching surface low riding along the stalled front. Rain showers are expected to spread west to east across the CMA and southern Mid-Atlantic (SMA) throughout the day, reaching the coast by overnight Thursday into Friday. Model consensus has grown closer over the past couple of days, resulting in increased confidence that the rain will stay to the south of the MDL. Back trajectories from the Upper Peninsula of MI, along with the widespread clouds and afternoon rain, will limit ozone to the Good range for most areas. The exception will be where there is surface convergence resulting from the frontal boundary near coastal VA and NC, where the clouds may not reach until after nightfall. Good PM2.5 is anticipated across the region, due to either cleaner, drier air filtering in to the north of the front or the widespread evening washout.

Despite model discrepancies regarding the speed of the eastward motion of the surface low, widespread clouds and rain showers are expected to continue throughout the region on Friday – again, south of the MDL. Despite shorter back trajectories from the Lake Ontario region, the continuing cloud cover will keep ozone mainly in the Good range across the eastern seaboard. The BAMS air quality models are resolving an area of low Moderate ozone in northwestern VA, where an extended afternoon break in the clouds is resolved in the high-res mesoscale weather models. This most likely will not verify due to the fact that a substantial area of rain will have just cleared the region, and that the mesoscale models do not show the sun coming out until 21 to 00Z. PM2.5 is expected to stay in the Good range as well, except for a small area just north of the clouds in the Susquehanna River Valley of PA, which is being resolved by the BAMS/NCDENR air quality models. Both models are responding to the increased humidity just to the north of the rain and cloud cover, where a lack of heavy precipitation (such as there is to the south) may allow particle concentrations to build into the Moderate range.

There is general model consensus that the center of surface low pressure will move off the Mid-Atlantic coast overnight Friday into Saturday and make its way toward New England. It will take its time, however, and clear skies are not expected to overtake the region until about 18Z Saturday. Additionally, the air at 500 m AGL will originate from near Cape Cod. Ozone is therefore expected to stay mostly in the Good range. As the surface low shifts towards a more northeastward track and moves parallel to the Jersey coast, northeasterly surface winds will increase across the region, particularly along the coast, lending confidence to the Good air quality forecast. Particles will remain in the Good range as well, due to the sustained northeasterly winds and recent washout.

Sunday will be the first sunny day for the medium range period ahead of an approaching cold front. Clear skies, warm and seasonable conditions, and a slight increase in humidity will promote scattered afternoon cloud cover. However, this will not be sufficient to hinder scattered low Moderate ozone in the usual areas surrounding the southern I-95 Corridor. A stretch of Moderate PM2.5 is also resolved in the air quality models in DC, eastern VA, and much of NC. This is most likely due to the fact that winds will be calmest there for the longest behind Friday’s coastal low.

Monday will be pre-frontal, and following the GFS model solution, it will most likely be another washout throughout much of the region, dropping ozone and particles solidly back into the Good range. A cold front approaching from the northwest will sweep through the NMA, passing through northwest PA around 12Z. Widespread precipitation associated with this front will scrub the atmosphere of any precursors that managed to build since the last frontal passage. The cloud cover is not expected to reach the coast before nightfall, meaning that ozone will have an additional day of sun to build into the Moderate range in areas east of the I-95 Corridor. The same will go for PM2.5, as a calm morning and continuing humidity allow particles to build into the Moderate range ahead of the rain. Major model discrepancies do exist, however, lending uncertainty to this forecast. Whether or not the widespread showers verify, the atmosphere in place will be clean enough to limit max ozone to the Moderate range.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 3, 2015
Valid: August 4 – August 8, 2015 (Tuesday – Saturday)

Summary:

Generally Good air quality is expected throughout the medium range period due to the passage of a cold front on Tuesday that will linger over the region, promoting unsettled weather. A large upper level trough reaching into NC will drop a cold front southeastward through the northern Mid-Atlantic early tomorrow morning, but it will stall along the I-95 Corridor through the evening hours. Clear skies will prevail behind the frontal passage, but a pool of clean air filtering in behind the front will limit the buildup of ozone and PM2.5 on Wednesday. Some areas of Moderate particles are possible on Thursday as humidity increases ahead of a substantial surface wave of low pressure arriving from the west. Resulting widespread clouds and showers are likely overnight Thursday through Friday afternoon as the wave moves through the Mid-Atlantic along the stalled front, inhibiting ozone production and scrubbing the atmosphere of precursors. A resurgence of cooler air behind the disturbance, as well as onshore surface winds and back trajectories, will keep air quality Good throughout the weekend despite a return to sunny skies.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but begin to show slight differences by Wednesday and larger ones on Thursday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large upper level closed low is centered over the James Bay, with the associated trough stretching from ND to NC and off of the East Coast. This low aloft will inch eastward over the coming days, with a general model consensus showing it moving off the Canadian Maritimes on Friday morning. This trough will drop a cold front southeastward through the Mid-Atlantic early Tuesday morning, which the WPC surface analysis shows reaching the I-95 Corridor around 12Z. It will promptly stall over eastern MD as a surface low develops over eastern VA and moves off the coast throughout the day, allowing the front to later continue southward into the Carolinas on Wednesday morning. This frontal boundary will vary in strength and latitude throughout the coming week, keeping cooler conditions to its north, with a substantial surface wave moving along it on Thursday into Friday. By Wednesday, an area of shortwave energy aloft over MO becomes apparent, with slight differences in its shape and exact position growing into major forecast discrepancies by Thursday, with the GFS as an outlier at this point. The EC solution appears to be a good consensus of the GFS and NAM at first, where the NAM is the stronger of the two weather models in terms of the resulting surface low pressure and precipitation and the GFS is more disorganized. This surface low will move along the stalled frontal boundary and impact the Mid-Atlantic on overnight Thursday into Friday. Despite upper level discrepancies, both the EC and GFS bring an area of widespread precipitation through the Mid-Atlantic from 00Z Friday to 00Z Saturday. However, from Thursday into Friday morning the EC begins to grow towards the NAM solution as it strengthens into a closed surface low, while the GFS remains the weaker and more diluted solution. The tighter EC surface low will allow skies to clear more quickly behind it as it moves out to sea overnight Friday. This makes Saturday the biggest question mark of the medium range forecast, as there is an even split regarding the cloud cover forecast. It is also worth mentioning that the EC 850 mb solution brings a pool of cooler air as far south as NC on Friday afternoon, while the GFS keeps this cleaner mid-level air in PA.

Tuesday will be a slightly above average day in terms of temperature and humidity behind an early-morning cold front passage. This front is expected to reach and stall near the I-95 Corridor by 12Z, placing an area of surface convergence along the interstate during the morning rush hour. The front has weakened since yesterday, as now there is little convection or other precipitation expected to form in the vicinity of the boundary until the overnight hours. Even then, only scattered storms are resolved in the high-res mesoscale models along the southern coast of MD around 00Z Wednesday. Surface winds are expected to be light and relatively sustained ahead of and along the front, with southwesterly winds promoting the highest ozone along the frontal boundary just east of I-95. Otherwise, the cleaner air mass filtering in behind the front will limit ozone to the Good range. Particles will follow the same general pattern, with moderate consensus among the air quality models that concentrations will reach the Good/Moderate threshold along the northern I-95 Corridor.

Wednesday will be only marginally cooler but noticeably less humid than Tuesday as the cold front dips southward into NC. Overall Good air quality is expected behind the front, where prevailing westerlies and fast back trajectories from the Great Lakes region will limit ozone production despite clear skies. Pockets of Moderate ozone are possible in southern NC below the front, however. Particles are expected to linger around the Good/Moderate threshold for another day due to light morning and overnight surface winds.

With continuing clean trajectories from the western Great Lakes and a frontal boundary draped through VA, most of the Mid-Atlantic is expected to see Good ozone on Thursday due to mostly cloudy skies. A rainy surface low will produce widespread precipitation in the Ohio River Valley and move eastward along the frontal boundary, increasing cloud cover in the Mid-Atlantic later in the day. Model discrepancies lend uncertainty to the forecast regarding precipitation on Thursday, as the ECMWF shows widespread rain throughout the northern and central Mid-Atlantic (NMA, CMA) overnight Thursday while the GFS brings only scattered showers to the CMA and southern Mid-Atlantic (SMA). Although there is uncertainty regarding the precipitation forecast, both models bring a pool of cooler air aloft over the NMA by 00Z Friday. Because of this, regardless of which rain forecast verifies, Good ozone is likely on Thursday. The WPC surface analysis appears to favor the EC solution of a closed surface low and widespread resulting rain, which would clean out the atmosphere. Given the close consensus between the NAM and EC with this system, we consider the GFS an outlier at this point and favor the stronger surface wave with the rainier forecast. Moderate particle concentrations are expected ahead of the system as humidity rises and converging winds increase, particularly along the frontal boundary in central and northern VA/the DC metropolitan area, except in locations that see heavy rainfall.

Friday and Saturday will see very similar conditions in one sense, as temperatures will hover around 80 °F and dew points around 60 °F with generally east/northeasterly winds on both days. However, Friday is likely to be a washout for the majority of the region if the EC/WPC forecast verifies – which seems most likely at this time. Skies will clear quickly behind the rain as the system moves out to sea overnight, allowing for a clear and sunny Saturday as strong high pressure centered over Hudson Bay tries to edge southward. Onshore trajectories aloft as well as light and sustained surface winds will prevail throughout the Mid-Atlantic, limiting ozone production in an already clean atmosphere.

– Eherts/Huff