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