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: Wednesday, July 15, 2015
Valid: July 16 – 20, 2015 (Thursday – Monday)

Summary:

A dry, post-frontal air mass will affect the region Thursday with Good to low Moderate air quality expected. Stagnant conditions Friday, as high pressure builds overhead, will increase both PM and ozone levels with PM responding to rising humidity. Cloud cover and precipitation is likely Saturday as small upper level disturbance pass in northwest flow aloft. The presence of a warm front and/or Appalachian lee trough may serve as the focus for afternoon convection. As was the case yesterday, the medium range model forecast diverge after Saturday. The EC has a more active pattern with a much stronger short wave affecting the region Sunday and Monday. This would argue for continued Moderate air quality but the GFS is not in agreement so there remains a marginal risk for poor air quality Sunday and Monday as temperatures will be in the 90’s F.

Discussion:

The weather models are in general consensus on the synoptic features until Saturday, when they begin to diverge. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. Shortwaves embedded within a trough over eastern Canada will pull a cold front through the Mid-Atlantic today from the northwest to the southeast. Behind this front, the southern US ridge that has been in place in recent days will strengthen on Thursday, with its axis just to the west of the Appalachian Mountains. On Friday, shortwaves over western ON will move eastward in the northern flow, developing a surface wave. This wave will begin to develop a southwesterly flow over the region as it moves a warm front northeastward on Friday. On Saturday, the weather models disagree on the fate of the shortwaves over ON. The GFS is much faster to move these shortwaves eastward than the EC or NAM. As these shortwaves reach the Canadian Maritimes on Sunday, the southern US ridge will strengthen, allowing high pressure to filter back into the region. At this time, another series of shortwaves will rotate around the base of a trough over the MB. The EC is the faster and stronger solution as it moves the trough axis over the Great Lakes by 12 Monday. In contrast, the slower GFS places the trough axis over the Dakotas by 12Z Monday. This difference in the models leads to a slightly different precipitation forecast for Monday ahead of the region’s next cold front, with the EC being the wetter solution.

A cold front will push through the Mid-Atlantic today, reaching SC by 12Z Thursday. A ridge of surface high pressure centered over the ON/QC border will build in behind the cold front, promoting clear skies and light winds, mainly in the northern Mid-Atlantic (NMA). Scattered clouds and light rain showers will remain in the southern Mid-Atlantic (SMA) in the vicinity of the cold front. The clear skies and light winds will promote rising ozone in the NMA, particularly along the I-95 Corridor. However, ozone will be limited to the low Moderate range due to a post-frontal air mass change and clean transport from ON and QC. Below average temperatures should also help to limit ozone production. PM2.5 concentrations will hover in the Good range for most of the region due to the drier air mass. The slight delay in air mass change along with scattered clouds and rain showers may push PM2.5 concentrations into the Moderate range at locations in NC.

The surface high pressure over the ON/QC border will move eastward to ME on Friday. Mostly clear skies and light winds will persist for another day. Along the coast, winds will be a little stronger, promoting an onshore flow and pushing pollutants inland. Short back trajectories indicate that stagnant conditions are probable, which would lead to a further buildup of pollutants. The air quality models agree with these conditions by showing Moderate ozone just to the west of the I-95 Corridor. Particle concentrations will be back on the rise as humidity will rebound on Friday. Moderate PM2.5 is expected along the western half of the Mid-Atlantic as dew points will rise there the quickest.

On Saturday, a surface wave will move through the Great Lakes into Canada, promoting a westerly flow along with scattered clouds and precipitation in the NMA. As a result, temperatures will rebound up into the upper 80’s F. Most of the region should remain mostly clear on Saturday. These conditions favor ozone production, and the air quality models are responding by showing Moderate ozone along the I-95 Corridor. PM2.5 concentrations will also hover in the Moderate range due to dew points rising into the upper 60’s F. The possibility of an Appalachian Lee trough on Saturday will also help to build up pollutants along the I-95 Corridor.

The precipitation forecast for Sunday and Monday is still uncertain at this time due to model discrepancies. The EC shows widespread precipitation on both Sunday and Monday while the GFS is much drier and keeps the bulk of the precipitation out of the region until Monday afternoon and evening. The EC’s solution would clean ozone and PM2.5, leading to Good air quality. The GFS solution would allow for Sunday to be a day of poor air quality as clear skies along with above average temperatures would enhance ozone production. The air quality models are showing upper Moderate ozone along the I-95 Corridor, DE, and NJ on Sunday.

– DeBoe/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