Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 9, 2015
Valid: July 10 – 14, 2015 (Friday – Tuesday)

Summary:

A strengthening and retrograding upper level ridge will bring a period of sunny and calm weather to the Mid-Atlantic Friday through Sunday, leading to Marginal chances of USG ozone on Saturday and Appreciable chances on Sunday, mainly for the I-95 Corridor. Rising ozone on Friday will depend on air mass characteristics, with periods of very heavy rain today effectively cleaning out the atmosphere, and drier air arriving behind a southward moving cold front. Fast air mass transport from southern ON on Saturday may again limit how far ozone reaches into the Moderate range. On Sunday, stagnant conditions with localized back trajectories suggests a greater chance for mid-to-upper Moderate ozone along I-95. Uncertainty in the precipitation forecast at the end of the medium range period keeps the chances for USG ozone in the Marginal range on Monday. Both the EC and GFS are trending toward re-establishing an upper level longwave trough over the eastern US early next week, leading to a period of unsettled weather. The NRL NAAPS model shows another surge of light smoke arriving in the Mid-Atlantic on Friday, which will linger through Sunday, keeping chances for low Moderate PM2.5 conditions across much of the region, with the highest concentrations across the southern Mid-Atlantic.

Discussion:

The weather models are in general consensus on the synoptic features but they begin to diverge on Monday. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS. Currently, a broad trough and accompanying shortwaves will move eastward from the Great Lakes today, dragging a surface wave and cold front along into the Mid-Atlantic. This front is expected to move through the northern Mid-Atlantic (NMA) Thursday evening into Friday morning, reaching the vicinity of the VA/NC border by 12Z Friday. A growing mid/upper level ridge over the southeastern US will prevent this front from advancing southward into NC until Saturday. This front will stall over the southern Mid-Atlantic (SMA) on Saturday, slowly dissipate and move off of the Atlantic coast by Sunday. The upper level ridge will strengthen and retrograde through Sunday, with its axis stretching along the western edge of the Mississippi River on Friday. By Saturday, this ridge will have strengthened to cover much of the southern and central US, with its axis roughly centered along the Plains. A shortwave will crest this upper level ridge and move over the SMA on Saturday, developing a surface wave along the stalled front along the SMA. The EC has a faster shortwave than the GFS, which leads to the EC producing clouds and precipitation about 6-12 hours earlier on Saturday in the SMA. The weather models agree on organizing a series of shortwaves and creating a small closed low at 500 mb over the Great Lakes around 06Z Monday. Although the GFS and EC progress this closed low very differently on Monday, both models are trending toward redeveloping a longwave trough over the eastern US early next week. This trend continues from yesterday, but the models are having trouble resolving the process by which the trough develops. The GFS has a more northerly track for the Great Lakes closed low, moving it from the Great Lakes to southeastern ON by 00Z Tuesday. In contrast, the EC has a more southerly track, moving the closed low from the Great Lakes to PA and NJ by 00Z Tuesday. The EC is different because a strong shortwave dropping down into northern QC suppresses any northward progression of the closed low. On Tuesday, both the models are showing signs to develop the winter-like pattern we saw recently of a ridge in the west and trough in the east. The EC has a much more amplified trough than the GFS due to the more southerly track of the closed low forcing shortwaves southward. Both of the models are developing widespread precipitation across the Mid-Atlantic on Monday and Tuesday, but in different places. The take away message is that the weather will become unsettled by the end of the period.

A surface wave currently in Indiana will move eastward today into PA, pushing the frontal boundary currently over PA southward to the vicinity of the VA/NC border by 12Z Friday. The cold front will weaken as it progresses southward through the NMA and CMA, eventually stalling Friday evening. Behind this front, high pressure will build into the NMA and CMA, leading to mostly clear skies throughout much of the region during the day on Friday. There may be a few scattered clouds and convection associated with the stalled frontal boundary in southern VA and NC. The NMA will be the clearest region on Friday. With clearing skies, near average temperatures, and light winds, isolated Moderate ozone is possible, especially in areas along the I-95 Corridor. The 06Z runs of the air quality models are predicting widespread Good ozone, most likely due to the fact that the NMA and CMA will be post-frontal, with a slightly less humid air mass moving into the region. Chances for Moderate ozone will depend on air mass characteristics, and with periods of very heavy rain likely today along much of the NMA and CMA, the atmosphere may be effectively scrubbed of ozone precursors. The Canadian wildfire smoke that impacted the region yesterday has dissipated, with mostly Good PM2.5 readings across the region this morning, with the exception the SMA, where conditions are still Moderate. The NAAPS model suggests that another pulse of light Canadian wildfire smoke will surge into the NMA and CMA on Friday behind the passing cold front. The NAAPS model also suggests that the Saharan dust in the CMA will be pushed southward due to the front, but it will linger in NC on Friday. As a result, PM2.5 concentrations will possibly rebound into the Moderate range in the NMA and CMA on Friday, with continuing Moderate conditions in the SMA.

Saturday’s weather will be similar to Friday’s as the upper level ridge continues to strengthen, resulting in dominating surface high pressure over the region. Skies will be the clear in the NMA and CMA, with very light surface winds. The air quality models (BAMS and NCDENR) are responding to this by showing Moderate ozone along the I-95 Corridor. The main limiting factor will be the northwest flow aloft from southern ON, which will continue to advect a drier air mass into the region. As a result, more widespread Moderate ozone is possible on Saturday, with scattered locations reaching the mid-Moderate range. Scattered clouds and precipitation will be in the SMA in association with the stalled frontal boundary and a strong shortwave that will move over the SMA sometime during the day. All the weather models agree on moving the shortwave through the area, but the EC is the fastest, bringing the surface wave and heaviest precipitation through in the morning. Regardless of the timing of the heaviest rain, mostly cloudy skies should limit ozone to the Good range in the SMA. The NAAPS model slowly pushes the smoke eastward on Saturday but still shows it impacting the region. The smoke will reach further south into eastern VA and NC while Saharan dust will retreat back into western NC. Low Moderate PM2.5 concentrations are possible on Saturday as a result.

Sunday looks to be the best day for ozone production as surface high pressure will continue to dominate the region and the stalled frontal boundary in the SMA will move off the coast. Skies look to be mostly clear across the region. Back trajectories are very short and localized, suggesting that the air mass will be stagnant. The air quality models are showing widespread upper Moderate ozone along the I-95 Corridor with small areas of USG ozone. Conditions will have to be monitored to see if the models continue to predict high ozone on Sunday. Traditionally, USG ozone does not occur on Sundays as emissions are at their minimum for the week. Most of the wildfire smoke should be dissipated by Sunday, but increasing humidity will promote particle formation, and the light winds associated with the high pressure will also help to keep local PM2.5 in the area, contributing to Moderate levels.

Monday and Tuesday look unsettled, with a developing trough aloft and warm and humid air promoting the chance for clouds and showers daily. The biggest uncertainty in the forecast for Monday and Tuesday is the location the precipitation. As discussed above, the EC and GFS disagree on the movement of a closed low over Great Lakes at 500 mb. Regardless of the exact track of precipitation, the forecast looks unsettled enough that ozone will be limited to the Good to low Moderate range, ending any possible regional increases in ozone over the weekend.

– DeBoe/Huff