Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, July 5, 2015
Valid: July 6-10, 2015 (Monday – Friday)

Summary:

The chances for marginal to appreciable USG ozone continue for Monday and Tuesday in the Mid-Atlantic before falling back to slight for the remainder of the medium range period. On Monday, cloudy skies, scattered showers, and convection generated by a quick-moving warm front and associated surface wave will keep ozone in the Good range for most of the region. The exception is northern NJ and the NYC metro area, where afternoon clearing and southerly surface winds will create a marginal chance for USG ozone, which is supported by the air quality models. Tuesday will be hot and humid, with partly to mostly sunny skies, localized back trajectories, and southwesterly surface winds bringing the chances for USG ozone up to appreciable, mainly for locations along and just east of I-95. A cold front will move into the northern part of the region on Wednesday and stall in the vicinity of the Mason-Dixon Line (MDL) on Thursday and possibly Friday. The global models diverge on the fate of the front as well as the location of surface waves moving along the stalled boundary, which creates considerable uncertainty in the forecast for the end of the period, although USG ozone seems unlikely. Another pulse of Canadian/Alaskan wildfire smoke is moving into the Mid-Atlantic from the west today. This smoke will wane but continue to impact the region through at least Tuesday. At the end of the period, Saharan dust currently along the Gulf Coast may move up into the Mid-Atlantic as far as the MDL. As a result, continued widespread Moderate PM2.5 concentrations seem likely through the medium range period.

Discussion:

The weather models are in consensus on the key synoptic features for the beginning of the medium range period, but they diverge on Thursday and Friday. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS through Wednesday, per WPC. The axis of a relatively narrow and quick-moving upper level ridge will be over the Mid-Atlantic on Monday morning and over New England by the afternoon. At the same time, a strong shortwave that is over the Tennessee River Valley (TRV) this morning will move northeastward into the Mid-Atlantic on Monday and eventually weaken. This feature is reflected in a surface wave today along the quasi-stationary front draped across southern VA. This front, along with the wave, will rapidly move northward as a warm front on Monday, bringing clouds, showers, and scattered convection across much of the region. On Tuesday, a strong shortwave over ON will pull the next cold front toward the Mid-Atlantic from the west. The global models continue in agreement today that this front will move into the northern Mid-Atlantic (NMA) on Wednesday and stall in the vicinity of the Mason-Dixon Line on Thursday. From Wednesday through the end of the period, the global models have a zonal flow aloft at 500 mb. The EC and GFS bring a series of small shortwaves over the region in the zonal flow at the end of the period, although the models differ on the exact timing and location of the shortwaves. The EC and GFS diverge somewhat on Thursday and Friday; both models rebuild a mid-level ridge over the Southeastern US, but the GFS has a stronger ridge that extends farther north compared to the EC’s solution. The main impact on air quality late in the period is the location of the stalled front and the waves of low pressure riding along the front, triggered by shortwaves aloft. The GFS, with its stronger mid-level ridge, keeps the surface waves farther north on Thursday and Friday, along the NY/PA border. In contrast, the EC keeps the stalled front along the MDL with the associated waves centered over the region.

Monday will be mostly to partly cloudy for much of the region, with scattered showers and thunderstorms. The 09Z SREF keeps a high probability of precipitation over the entire region except for the far northern NJ/NYC metro area. The 12Z 4 km NAM shows a line of thunderstorms overnight into the morning impacting the southern I-95 Corridor (e.g., DC, BAL, ILG, PHL) and then additional scattered storms in the afternoon across the NMA. Surface winds will be calm in the morning and turn south/southeast and pick up in the afternoon. The air quality models (e.g., NOAA/EPA, BAMS, NCDENR) are consistent again today in painting upper Moderate to USG ozone in northern NJ and the metro NYC region, likely in response to onshore surface winds blowing I-95 emissions northward and at least partly sunny afternoon skies. Elsewhere, the clouds and periodic precipitation associated with the warm front and surface wave will limit ozone to the mostly Good range.

As the NRL NAAPS model predicted yesterday, another pulse of Canadian/Alaskan wildfire smoke moved down into the Great Lakes/Ohio River Valley (ORV) overnight. This smoke, along with the dissipating effects of Independence Day fireworks, pushed hourly PM2.5 concentrations well into the USG range this morning across the Chicago and Pittsburgh metro areas, among other locations. This smoke seems to be steadily moving eastward and will likely remain in the Mid-Atlantic on Monday and Tuesday, as back trajectories both days are slow and localized. The smoke may linger until the arrival of the cold front on Wednesday/Thursday, although the NAAPS model shows the impact of the smoke waning after Monday. However, the NAAPS model also shows Saharan dust moving up from the south/southwest for the end of the period, reaching as far north as the MDL – likely stopped by the stationary front. The Saharan dust has been impacting the Gulf Coast for the past several days, so its possible transport into the Mid-Atlantic beginning Wednesday will need to be monitored.

Tuesday will be hot and humid with partly to mostly sunny skies. The region will be in the warm sector, with Monday’s warm front roughly over NYC and Long Island and the next cold front still far enough to the west to have little impact on the Mid-Atlantic’s sensible weather. There is a chance for isolated thunderstorms and increasing clouds in the afternoon, but enough sun should be available for widespread Moderate ozone along the I-95 Corridor, with an appreciable chance for isolated USG ozone. Southwesterly flow will push the highest ozone to locations along and just east of I-95.

There is a greater chance for optically thick clouds and organized convection on Wednesday as the front moves into the very warm and humid air mass over the Mid-Atlantic. The mesoscale models show the front reaching southern PA/central NJ by 00Z Thursday. Scattered Moderate ozone is possible on Wednesday, particularly at locations farther south, such as southern MD and central/southern VA, which will be the last to feel the impacts of the approaching front.

The forecast for the end of the period is highly uncertain due to the uncertainty regarding the fate of the stalled front and associated waves of low pressure. The pattern is reminiscent of the mid-June pattern, when one frontal boundary after the next stalled in the vicinity of the MDL, which kept ozone mostly in the Good range for much of the region. Thus, generally Good to low/mid Moderate ozone seems likely on Thursday and Friday, with only a slight chance for USG ozone.

-Huff