Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 17, 2015
Valid: July 18 – 22, 2015 (Saturday – Wednesday)

Summary:

As heat and humidity increase through the weekend, ozone and particle concentrations will build accordingly. There is an appreciable chance of USG ozone as early as Sunday although Monday is the more likely day in areas not affected by convection advancing from the west. After the front’s passage overnight Monday into Tuesday morning, precursors will be scarce enough to limit ozone production on Tuesday. However, a possible building ridge to our west as well as continuing clear skies and light winds will allow concentrations to rebound during the second half of next week.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. This morning, a small ridge with its axis through the Mid-Atlantic is promoting clear skies across the region. However, a cluster of shortwaves aloft over the Great Lakes are generating clouds and showers in their vicinity, which will spread over the Northeast overnight. These upper level waves will promote convection throughout the Mid-Atlantic tomorrow. The weather models show a trough in Canada digging towards northern ND around 12Z Saturday, with a ridge building just ahead of it over the Great Lakes. This ridge will become shallower and weaker as it moves eastward into the Mid-Atlantic, allowing the heat and humidity to develop Saturday and continue Sunday. The Canadian trough is expected to continue to strengthen and grow in size as it moves eastward across the US/Canada border, reaching the vicinity of New England by 12Z Monday. With its axis dipping into the northeastern US, this trough will pull a defined cold front across the eastern seaboard starting on Monday. Despite model discrepancies regarding the pattern of vorticity maxima aloft, the models are in good consensus regarding the general timing and strength of the cold front. As was discussed yesterday, the models have yet to pick up on the slowing tendency that the heat and humidity in the Northeast will have on the eastward progression of the front. Although the WPC surface analysis reflects this pattern and shows the front not reaching the Delmarva until 12Z Tuesday, the global weather models show the frontal impacts reaching that area about 12 hours earlier. Regardless of this discrepancy, the front is expected to be strong enough to promote widespread clouds, rain, and possibly strong convection along its boundary as it sweeps through the region Monday into Tuesday. By Tuesday the models agree that a secondary upper level trough will form behind the first Canadian one, allowing a break from the clouds and showers. The secondary trough will follow the path of the first – moving eastward to the north of New England – and pull a similar cold front through the region as well. However, this front will be weaker and will be moving through a less hot and humid air mass, making it simply a focus for scattered afternoon convection as it passes through on Wednesday afternoon. Surface high pressure will build in behind the front, helping to clear skies quickly for Thursday.

Saturday will be a day of building heat and increasing humidity as temperatures reach into the low 90s F across the region with dew points in the 70s F. A weak warm front moving northeastward through the southern and central Mid-Atlantic (SMA, CMA), as well as a small shortwave moving overheard, will promote scattered clouds and showers throughout the region tomorrow. Back trajectories for 1500m show subsidence of air originating in KY, while 1000m and 500m trajectories for PHL showed air looping onshore near Cape Hatteras. This origin of the boundary layer air tomorrow, as well as the possibility for periods of broken cloud cover will result in mostly Good ozone with areas along and downwind (northeast) of the I-95 Corridor reaching into the Moderate range. Scattered areas of Moderate particles are possible, especially in the SMA where the humidity will be highest.

Sunday will see the continued buildup of ozone and PM2.5 as surface winds diminish, temperatures rise into the mid-90s F, and the sun shines for much of the day. Back trajectories from the Ohio River Valley – an area rich in ozone precursors – are predicted for Sunday in the northern Mid-Atlantic (NMA). The weather pattern Sunday is conducive to high ozone concentrations, however, recent history shows that USG range ozone on Sunday is rare. For the period 2008-2014, the chance of USG ozone on Sunday is three times less likely than other weekdays. As a result, the most likely air quality outcome on Sunday is high Moderate, however, there is an appreciable chance of USG ozone. Particles will continue to rise into the Moderate range from Saturday into Sunday due to the calm southwest surface winds, high temperatures and humidity, and westerly trajectories.

Monday is another day of interest due to continued heat, humidity, light southwest surface winds, and westerly back trajectories. Although these factors will be prevalent throughout the weekend as well, the addition of commuter traffic along the I-95 Corridor as well as an anticipated lee trough through NJ/DE will give ozone a push towards USG in areas surrounding the Bay. Moderate or USG concentrations of both ozone and PM2.5 will be limited to the eastern Mid-Atlantic as a cold front approaches from the west, promoting widespread pre-frontal clouds and rain. Although the global weather models are anticipating the passage of the cold front through the Delmarva around 18Z Monday, non-computer forecasts – such as this one and the WPC surface analysis – delay the frontal passage by about 12 hours due to the anomalous heat and humidity ahead of the front. Therefore, PM2.5 concentrations will likely reach into the Moderate range at locations along the I-95 Corridor on Monday ahead of the frontal passage.

Decreasing temperatures and humidity on Tuesday as dew points drop into the mid-60s F behind the front. Back trajectories from northern MI will provide a cleaner background atmosphere, but abundant sunshine will allow ozone to rebound into the Moderate range in the usual interstate locations. The early morning passage of the front will bring widespread showers to the area, limiting particles to the Good range. The air quality models are showing a quick rebound of particles into the Moderate range across the CMA and NMA on Tuesday, likely due to the fact that the global weather models analyze the cold front as moving off the coast by Monday night.

Wednesday’s temperatures will remain in the high 80s F despite the passage of a second, weaker cold front during the afternoon hours. This front will promote only scattered clouds and convection in its vicinity during the late afternoon, with widespread sunny skies and light winds allowing ozone and particle concentrations to rebound into the Moderate range.

– Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 16, 2015
Valid: July 16 – 21, 2015 (Thursday – Tuesday)

Summary:

This weekend will be one of building ozone and particle concentrations across the Mid-Atlantic with Monday the day of most interest. Temperatures over the weekend will reach into the 90s F and dewpoints will hover in the low 70s F, creating an atmosphere prime for particle formation. Subsidence resulting from a succession of small ridges aloft will not be strong enough to suppress afternoon convection, particularly with very high humidity and an unstable mid-layer. This may allow periods of evening rain and thunderstorms to prevent ozone from reaching USG concentrations. On Monday, the focus for convection will stay to the west as a cold front approaches, allowing full sun, heat, humidity, light winds, and weekday traffic to push ozone to its peak for the medium range period. The rainy Monday night passage of the cold front will allow cleaner northern air to filter into the region, depleting the atmosphere of ozone and particle precursors despite a sunny Tuesday.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, the large ridge that has been dominating weather in the southern US is peaking in western ON, with its axis reaching southward through the western Great Lakes region. Shortwaves embedded in the flow over NE/IA will continue eastward towards the Mid-Atlantic over the next few days. By 00Z Friday the ridge axis will move eastward into the Mid-Atlantic, with the closed high aloft remaining over eastern TX. The embedded shortwaves will reach the eastern Great Lakes region by 12Z Friday as the ridge axis reaches through the region northwest to southeast. By 00Z Saturday, although there will be mini ridges aloft to both our east and west, the cluster of shortwaves will be powerful enough to develop scattered periods of clouds and showers across the Mid-Atlantic as they move overhead. The model solutions for Saturday also show a large shortwave spinning eastward across southcentral Canada. On Sunday the main feature aloft impacting Mid-Atlantic weather will be a ridge that builds over the Great Lakes ahead of the Canadian trough, although there are discrepancies among the GFS and EC regarding the amplitude of the ridge. However, both models show periods of clouds and convection moving across the region during the afternoon hours. Around 12Z Sunday the Canadian trough is expected to dig into the Great Lakes with some slight structural differences between the GFS and the EC. Overall however, both show very similar synoptic scale patterns and bring a cold front southeastward through the Mid-Atlantic starting on Monday afternoon. On Tuesday the flow over the region will begin to shift towards a zonal pattern, but a developing ridge is visible over the Great Lakes again, hinting at another period of possible deteriorating air quality.

As an upper level ridge currently over the Great Lakes moves eastward towards the Mid-Atlantic, an area of surface high pressure will spread across the eastern seaboard tomorrow. This feature will keep surface winds relatively light, although an approaching surface wave will keep scattered to broken cloud cover over much of the northern Mid-Atlantic (NMA) on Friday. Southerly and southeasterly surface winds may cause Moderate ozone and particle buildup to the west and northwest of the I-95 Corridor. However, cleaner, drier back trajectories from northern New England will keep ozone and particle concentrations in the Good range throughout the rest of the region. Lines of showers and convection are expected to sweep across the NMA during the evening and overnight hours as a weak warm front approaches, limiting daily pollutant concentrations.

As the surface wave skirts New England on Friday evening, it will pull a surface trough/weak warm front eastward through the region and bring a warmer, very humid air mass into the Northeast on Saturday. This is one factor in making Saturday the first day of deteriorating air quality across the region, in addition to western back trajectories, mostly sunny skies, and light surface winds. As the weak warm front continues through the Mid-Atlantic during the day, it will act as a focus for building clouds and scattered rain showers, especially throughout the morning hours. Clouds will dissipate as the afternoon wears on, helping temperatures reach into the 90s F across the southern and central Mid-Atlantic (SMA, CMA). The atmosphere will be very humid and potentially unstable, so that clearing skies may provide the trigger for another round of convection in the late afternoon (per the SREF). The chance of convection and the clean background state of the atmosphere on Saturday will limit ozone to the Moderate range. Just downwind of the I-95 Corridor and in the SMA where the humidity is highest, Moderate particle concentrations are likely. Elsewhere, expect Good air quality to persist.

Sunday will be a hot and humid day with light southwesterly surface winds. With back trajectories from western WV, a lee trough extending southward through NJ, DE, and VA, and mostly sunny skies throughout much of the day, Sunday will be a day of Moderate ozone throughout the NMA. Locations along the I-95 Corridor are of particular interest due to the nearness of the anticipated lee trough. However, the lack of weekday commuter emissions and the likelihood of scattered afternoon convection will limit the possibility for USG ozone concentrations. The continued humidity and light winds in the CMA and SMA will keep particles in the Moderate range.

While high temperatures and dew points will persist on Monday, an approaching cold front will bring a line of clouds, showers, and convection through the Northeast. The models are likely bringing the front through the area too quickly, since this pattern has a history of slowing as it enters a hot and humid air mass. This means that the front’s impacts likely will not reach the I-95 Corridor until Monday night, even though the global weather models show the precipitation reaching the coast by 18Z Monday. A slow frontal boundary will allow ozone and particles to continue to build throughout the eastern Mid-Atlantic on Monday, making it the day of most interest in the medium range period. The likelihood for USG ozone is appreciable due to westerly back trajectories, light southerly winds, workday traffic, and the continued presence of a lee trough extending through DE and eastern VA. These factors combine to make the areas of most interest along and east of the I-95 Corridor, where any frontal impacts will not arrive until after sunset.

The post-frontal air mass in the Mid-Atlantic on Tuesday will bring temperatures closer to average, drop dew points into the 60s F, and bring air aloft originating from over Lake Superior. These will allow air quality to be limited to the Good range on Monday despite mostly sunny skies and calm winds across the eastern US.

– Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 14, 2015
Valid: July 15 – 19, 2015 (Wednesday – Sunday)

Summary:

Friday will be the main day of interest during this period as it will be the second day of full sun and calm winds in the Mid-Atlantic. Pre-frontal precipitation and brisk, northerly, post-frontal surface winds will keep air quality Good across the Mid-Atlantic tomorrow. While an area of high pressure will extend southward into the region on Thursday and keep skies clear and winds light, clean air filtering in behind Wednesday’s front will limit ozone and particles to the upper Good range. By Friday, however, light southeast surface winds and clear skies will allow the air mass over the Mid-Atlantic to modify sufficiently, allowing for the formation and buildup of ozone along and northwest of the I-95 corridor. Less humid conditions behind the front will likely limit any particle buildup to isolated locations just northwest of the interstate, with concentrations reaching into the low Moderate range. Saturday will be unsettled as a surface wave moving across NY allows clouds and showers to form over much of the region, limiting ozone to the Good range. Sunday’s forecast is questionable, as a more humid air mass is expected to take over and increase particle concentrations and ozone precursors in the atmosphere. Model discrepancies regarding precipitation chances will decide whether Sunday will be another day of inhibited production or rebounding ozone concentrations.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a shortwave over the Great Lakes is digging out a trough that will reach the Mid-Atlantic by 12Z Wednesday. The weather in the rest of the country is being dominated by a large upper level ridge with its axis through the Great Plains. The arrival of the trough over the region on Wednesday will be evidenced at the surface by the passage of a cold front from the northwest, preceded by a period of clouds and showers on Wednesday. By 00z Thursday the front will reach the Delmarva, but unlike past Mid-Atlantic fronts this summer, will continue to dip southward. Thursday morning will see the exit of the shortwave and trough aloft off to sea, allowing a period of zonal flow to take hold over the Northeast as the cold front reaches the Carolinas. At the surface, the center of an area of high pressure will build near the QC/ON border, keeping abundantly clear skies throughout the Mid-Atlantic. This high pressure center will move eastward to ME by 12Z Friday with its influence keeping skies clear in the Northeast for one more day. Shortwaves embedded in the zonal flow over the Great Lakes on Friday morning provide the first source of disagreement between the global weather models, which translates to forecast differences for the remainder of the period. While the EC solution shows a single, relatively disorganized shortwave over Lake Huron, the GFS has three shortwaves along a NW-SE axis over the Lakes. These short waves will likely be accompanied by organized mesoscale convective systems so that, by Saturday morning, these forecast differences translate to differing precipitation forecasts for the Mid-Atlantic. While both models show unsettled periods of clouds and rain in the Mid-Atlantic on Sunday, the weather models have yet to come into consensus on the exact timing and location of the precipitation.
Wednesday will see the arrival of a large shortwave aloft, reflected at the surface via a cold front that will sweep southeastward through the NMA. Clouds and scattered rain showers will move through the area ahead of the front, limiting ozone production along the front’s path. The front will reach the Delmarva by 00Z Thursday, allowing the frontal impacts to reach the majority of the Mid-Atlantic and inhibit ozone production on Wednesday. Particle concentrations will also stay in the Good range, mostly due to the brisk northerly winds and cleaner air behind the front, with back trajectories on Wednesday originating from the currently clean state of IA. Areas that remain south of the frontal boundary throughout the day – namely along the coast south of the Bay – may see daily PM2.5 concentrations reach just into the Moderate range as calm winds and humid conditions ahead of the front promote particle buildup.

Thursday will be a much more settled day weather-wise as the front continues through the Carolinas, leaving abundantly clear skies in its wake throughout the Mid-Atlantic. An area of surface high pressure centered in western QC will sprawl southward through the central Mid-Atlantic (CMA), continuing to promote clear skies and decreasing surface winds. These winds will slowly shift northerly to easterly throughout the day, containing any ozone buildup to southern portions of the I-95 Corridor. The air quality models are reflecting this logic, showing isolated pockets of low Moderate ozone at locations along and south of the DC/BAL axis. The rest of the region is expected to remain in the Good range on Thursday when it comes to both particles and ozone due to the cleaner, drier state of the new air mass filtering in behind Wednesday’s frontal passage.

Friday is the first day of interest for the period as the center of the area of high pressure reaches ME, continuing to inhibit clouds and surface winds in the Mid-Atlantic. These conditions will allow the air mass to begin to modify in favor of ozone formation, with light southeast winds confining the area of Moderate ozone to along and northwest of the I-95 Corridor. The 45-km BAMS-CMAQ air quality model resolves a pocket of USG ozone just west of the BAL metro area on Friday. Although relatively little credibility can be given to a nation-wide air quality model for Day 4 forecasts, consensus among weather models for weekday clear skies and calm winds keep the possibility for isolated pockets of USG ozone alive. Particles will build up in a very similar pattern as ozone, with the 45-km BAMS models showing concentrations reaching just into the Moderate range to the west of the interstate.

By Saturday the pattern in the vicinity of the Mid-Atlantic once again becomes unsettled, as shortwaves embedded in the zonal flow aloft promote rising air and scattered showers throughout the region. The WPC surface analysis attributes these showers to a weak surface wave that moves from Lake Ontario through southern New England as it continues to dissipate. Although there is model disagreement regarding the exact surface impacts of the waves aloft and at the surface, both the EC and the GFS resolve clouds and showers across the NMA and CMA on Saturday and Sunday. This spells Good air quality across the region on Saturday. The 850mb solutions suggest the very weak frontal boundary will move back northward into the region on Sunday, perhaps bringing a more humid and dirty southern air mass to the Mid-Atlantic. These conditions, along with any possible periods of sun, would allow ozone to rebound into the Moderate range on Sunday. Particle concentrations would also rise as well, but only into the upper Good range due to a lack of additional pollutants (such as smoke or Saharan dust).

– Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 13, 2015
Valid: July 14 – 18, 2015 (Tuesday – Saturday)

Summary:

An oscillating frontal boundary, with embedded waves of low pressure, will keep clouds and rain in the forecast Tuesday and Wednesday with Good to low Moderate air quality expected. A cold front will cross the region Wednesday with drier, cleaner air slowly filtering in behind it. As high pressure builds in Thursday, Moderate air quality is expected, with a further increase in pollutant concentrations on Friday. Saturday’s forecast depends on the speed of a warm front moving northward and remains uncertain although clouds and some precipitation are likely.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The upper level ridge currently over the southern US will persist through the end of the medium range period. A trough over eastern Canada will interact with a series of shortwaves cresting the upper level ridge and move over the Midwest and into the Mid-Atlantic. This shallow trough will bring a cold front to the region, reaching northwestern PA Wednesday morning. By 18Z Wednesday, the cold front will have progressed through most of the region with the exception of eastern VA and NC. The cold front will reach SC Thursday morning as the axis of the southern US ridge shifts eastward, setting up just to the east of the Mississippi River. This translates to a center of surface high pressure over southern QC. The surface high pressure will move eastward on Friday and Saturday, reaching New England and eventually the Atlantic Ocean. Friday evening into Saturday morning, a wave of low pressure will form in the Midwest, and it will pull Thursday’s cold front northward as a warm front. The EC has a slower northward progression of the warm front than the GFS, resulting in the EC having more precipitation across the region on Saturday.

Today’s warm front will stall in the southern Delmarva on Tuesday while waves of low pressure form along it, promoting clouds and precipitation. The global models show a washout throughout much of the region. The higher resolution models, however, show a drier solution, especially in southern VA and NC. Locations that experience periods of clearing will see Moderate ozone on Tuesday. The air quality models are showing Moderate ozone at a few locations along I-95. This is likely in response to light and converging winds associated with the stalled frontal boundary. Otherwise, clouds and precipitation will keep ozone in the Good range. Currently, PM2.5 concentrations are hovering in the low Moderate range in NC, MD, and PA. This will likely continue on Tuesday as winds remain light and humid conditions continue. Light density Canadian wildfire smoke is still lingering around the region, but it is not expected to make a significant impact on PM2.5 concentrations.

On Wednesday, a cold front will move through the region from the northwest to southeast, reaching the NC/VA border Wednesday evening. The change in air mass, from humid to much drier air, will lag well behind the frontal boundary. This will allow PM2.5 concentrations to remain enhanced for several hours after the boundary passes. Clouds and precipitation associated with this front will limit ozone production. The heaviest precipitation will likely be located in the vicinity of a surface wave east of the I-95 Corridor. Back trajectories originate in cleaner conditions in IL and WI suggesting that ozone will be limited to the Good range.

The cold front will reach SC Thursday morning. A surface wave along the front will be off the VA/NC coast on Thursday. This wave will promote clouds and onshore surface winds along the east coast. However, surface high pressure will build in behind this front, promoting clear skies and light winds at inland locations. As a result, the best chances for ozone production will be inland. Ozone concentrations will depend on the track of the surface low. The further east the low tracks, the higher the chances for Moderate ozone. PM2.5 concentrations will likely hover in the low Moderate range in response to light winds inland. Coastal locations will likely see a drop in PM2.5 concentrations in response to the onshore surface winds.

Friday looks to be the day of most interest for air quality concerns as high pressure will dominate the region. Clear skies and light winds will help ozone rise into the Moderate range, especially at locations along the I-95 Corridor. The air quality models are responding to this by showing Moderate ozone throughout MD, DE, NJ, and southeastern PA. PM2.5 will also likely linger in the Moderate range due to the light winds. On Saturday, the cold front in SC will move back northward as a warm front, promoting clouds and precipitation and limiting ozone production. The GFS and EC disagree on the speed of the front, so there is a chance for some locations to see Moderate ozone on Saturday.

-DeBoe/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 10, 2015
Valid: July 11 – 15, 2015 (Saturday – Wednesday)

Summary:

A “mini ridge” aloft will build over the northern Mid-Atlantic through Sunday, with an area of surface high pressure moving over PA, priming the atmosphere for a period of peak ozone concentrations by Sunday. A frontal boundary currently in central VA will dip southward tomorrow and bring clouds and showers to VA, southern MD, southern DE, and eastern NC, while a small area of surface high pressure just north of the front will keep skies clear in the northern Mid-Atlantic. This day of sun will allow ozone precursors to build up in the atmosphere, making Sunday – a warm and sunny day throughout the region – the main day of interest during the medium range period. By Monday, a developing upper level trough over the eastern US will return unsettled conditions to the Mid-Atlantic. On Monday a surface wave will approach from the west, lifting a frontal boundary back north into the northern Mid-Atlantic and bringing a period of clouds and rain through the region starting on Monday evening, with unsettled conditions continuing through the end of the period . The particle forecasts through the period will depend on the lingering effects of wildfire smoke and Saharan dust, with possible additional contributions from sulfate haze on Sunday. Periods of Moderate conditions are likely, especially for the southern part of the region.

Discussion:

The weather models are in general consensus on the synoptic features for most of the medium range period, but diverge somewhat on Tuesday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a zonal flow aloft over the Mid-Atlantic and a building upper- and midlevel ridge over the southeastern US are slowing the southward progress of a cold front in northern VA. The axis of this ridge at 500 mb extends from AB southeastward through the northern Plains. A small area of shortwave energy currently over the western Ohio River Valley (ORV) will make its way eastward in the flow along the surface boundary, triggering a surface wave and bringing an area of uplift and resulting precipitation to the central Mid-Atlantic (CMA) tomorrow. Aloft, this feature is followed immediately by a “mini-ridge” in the otherwise zonal flow, allowing surface high pressure to move across PA on Sunday. The axis of the large upper level ridge will stretch from SK to the Tennessee River Valley by 12Z Saturday as the now closed ridge over the Southeast retrogrades. The westward movement of the center of this ridge into the southern Plains will allow for a trough to start digging out down the east coast, as evidenced by the frontal boundary now being able to dive as far south as the SC/GA border by 12Z Sunday. At this time a bundle of small shortwaves over the Great Lakes will become apparent, amplifying the growing trough, with some model discrepancy as to the exact characteristics of the shortwaves. The bottom line for air quality is that this the third day of model solutions supporting a trend toward developing a longwave trough over the eastern US beginning on Monday. Although the global forecast models are having trouble resolving the specific pattern of the evolving trough on Monday and Tuesday, they end up in the same place by Wednesday. Even though there is considerable uncertainty regarding the precipitation forecast for the end of the period, as described below, the beginning of the week looks unsettled, with few chances for ozone production.

The differences in the shortwave bundle are reflected at the surface via differing precipitation forecasts overnight Sunday and throughout the day on Monday. For example, by 12Z Monday, the NAM analyzes a large area of scattered centers of shortwave energy over the eastern Great Lakes making for scattered precipitation in the northwestern Mid-Atlantic; the GFS has only a few weak areas of shortwaves over the Great Lakes, keeping most rain to the west of the area; and the EC shows a tight shortwave circulation over the eastern Great Lakes bringing widespread precipitation to the entire northern and central Mid-Atlantic (NMA, CMA). The WPC model discussion explains a favoring towards a blend of the NAM and EC solutions, which both bring precipitation to the NMA by Monday afternoon. At 500 mb, these models show the bundle of shortwaves continuing to dig out the trough over the Mid-Atlantic. By 00Z Tuesday, the trough axis reaching from the QC/ON border southward to the Carolinas, with scattered shortwaves throughout its base promoting widespread clouds and precipitation throughout the eastern seaboard. In the west, although no longer closed, the ridge is still prominent and stretching from southern CA through the Gulf Coast with its axis along the eastern edge of the Rockies. By 12Z Tuesday, a large shortwave becomes apparent over the southern Hudson Bay and will continue to move eastward until the end of the medium range period, when it reaches the eastern border of QC. 00Z Wednesday analyses show substantial precipitation differences between models along the east coast. This is mainly due to a discrepancy in shortwave placement in the base of the east coast trough, with the EC solution placing the largest shortwave along with the heaviest precipitation in the southern Mid-Atlantic (SMA). The GFS keeps the bulk of the shortwave energy farther north, promoting precipitation in the NMA but keeping the SMA under clear skies. This causes the 18Z EC solutions to have a deeper trough along the east coast, but by this point the energy aloft in both models is diluted enough to keep surface impacts minimal. By the end of the day on Wednesday, although the models show discrepancies regarding the placement of any lingering precipitation, overall they are in very close agreement regarding large scale synoptic features. A wide ridge centered over the central Plains and the southern end of a trough with its axis mainly stretching southward through QC will share the atmosphere aloft over the CONUS by Wednesday night.

A cold front currently in northern VA will move very little over the next 48 hours as an area of surface high pressure building into the NMA and the continuing ridge in the southeastern US will sandwich the front in the CMA. Shortwave energy aloft will move eastward along the frontal boundary and bring an area of precipitation through the CMA tomorrow, from WV/western MD through VA to the eastern VA/NC coast. Convection is likely, initiating in the Richmond area before the disturbance moves off the coast of the Outer Banks overnight. This surface is a new feature in today’s model analyses, but all of the mesoscale and global models show it with fairly close consensus. The rain and cloud cover will keep ozone concentrations in the Good range throughout the CMA tomorrow. The rest of the Mid-Atlantic – areas north of the Mason-Dixon Line (MDL) and west/southwest of Cape Hatteras – will remain mostly clear throughout the day. Seeing as the disturbance will remain relatively localized to VA and the immediate surroundings, surface high pressure will be allowed to build over PA/NJ throughout the day on Saturday. The disturbance passing to the south will promote light onshore surface winds in the NMA and the synoptic pattern will keep back trajectories northerly – these factors, along with the heavy rain that moved through much of the NMA last night, will help to limit rising ozone, but scattered Moderate conditions are likely along the northern portion of the I-95 Corridor, from roughly ILG to TTN. The PM2.5 forecast is a bit trickier. Saharan dust is keeping hourly concentrations this morning in the 20s ug/m3 in NC, with areas just north of the frontal boundary in VA in the single digits. The NRL NAAPS model analyzed a plume of smoke dipping into the NMA behind this front this morning, and it is evident on satellite imagery. The smoke is prominent in visible satellite imagery throughout the northern Plains and over the Great Lakes, but has been slow to reach the surface and make its impact on surface monitors in the NMA. Hourly concentrations are rising in northern, western, and central PA this morning, which suggests that the smoke is gradually moving toward the eastern and central portions of the Mid-Atlantic. The 15km BAMS-CMAQ air quality model shows scattered areas of Moderate PM2.5 in the vicinity of the western edge of the PA/MD border tomorrow, without the consideration of possible smoke impacts. Depending on the vertical motion of this smoke plume, the NMA could see particle concentrations reach into the Moderate range on Saturday. Saharan dust should continue to impact NC on Saturday, for continued mid-Moderate PM2.5 conditions.

Sunday is still the main day of interest during the period. As the southeastern ridge retrogrades westward on Sunday, the frontal boundary in the CMA will dive southward through SC and allow a surface high pressure centered in southern MI to extend into the Mid-Atlantic and move over PA and then off shore during the day. Aloft, a “mini ridge” will build directly over the NMA, promoting abundantly clear skies and light and variable winds throughout the region on Sunday. This optimal synoptic set up should allow ozone to rebound well into the Moderate range, with the 45km BAMS CMAQ and MAQSIP-RT models showing widespread Moderate in the NMA and CMA, along with isolated areas of USG concentrations along the southern I-95 Corridor. A possible limiting factor will be the potential for strong onshore surface winds along the Atlantic coast, due to a weak coastal low off the northern NC coast. This low will begin as Saturday’s surface wave and will strengthen off of the coast, near HAT, on Sunday. All of the models show this low, but the GFS is the strongest. If the development of the low verifies, its circulation would limit rising ozone in DE, NJ, and PHL. Particle concentrations will again depend heavily on the possible intrusion of Canadian wildfire smoke. However, Moderate concentrations are resolved along the Appalachians and throughout the CMA in the air quality models, suggesting that the hot, increasingly humid, sunny, and calm weather conditions will be sufficient to bring PM2.5 into the Moderate range regardless. Any additional particle sources – such as wildfire smoke – would bring concentrations higher into the Moderate range. In the SMA, the Saharan dust is expected to dissipate by Sunday morning, helping particle concentrations drop closer to the seasonal average.

Model discrepancies on Monday make the forecast uncertain, but since the GFS is the model in least consensus, we will rely on a blend favoring the NAM and EC. An area of shortwave energy aloft over the eastern Great Lakes will help drag the frontal boundary northward as a warm front early on Monday, but its weak nature will keep the surface impacts minimal for the first half of the day, except for an increase in heat and humidity. The 06Z run of the 12km NAM resolves widespread cloud cover in the NMA which dissipates into the CMA. This gradient will intensify throughout Monday afternoon as a surface wave approaches from the west, increasing the strength of the frontal boundary and bringing an area of precipitation eastward through the NMA during the evening hours. In the western NMA and CMA, these daylong widespread clouds and showers will limit ozone to the Good range. However, in areas in the eastern Mid-Atlantic (EMA) where cloud cover will be sparser ahead of the evening rain, lingering ozone precursors could help daily concentrations to reach into the Moderate range on Monday, from PHL to NYC. Particles will also likely linger in the Moderate range ahead of the rain where light southerly surface winds and humid conditions will continue to promote the buildup of PM2.5.

Tuesday and Wednesday look unsettled, with the warm front stalling over southern PA and a back door cold front stalling in NY State. Tuesday will see the arrival of the surface wave along with its resultant cloud cover and rain showers mainly in the NMA and CMA. The WPC surface analysis resolves a lee trough extending southward through central VA and NC, which will likely promote clouds and scattered showers in its vicinity as well. Widespread cloud cover and precipitation, as well will keep ozone and particles in the Good range throughout the Mid-Atlantic on Tuesday. By Wednesday morning the center of the surface wave will reach Boston, allowing the Mid-Atlantic to clear briefly before the impacts of its trailing cold front are felt. By 18Z Wednesday, extensive cloud cover and scattered showers are expected to spread over the entire Mid-Atlantic and limit ozone and particles to the Good range for the end of the period.

– Eherts/Huff