Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June, 1 2017
Valid: June 2-6, 2017 (Friday-Tuesday)

Summary:

Questions about the degree of modification of the current air mass in place over the region and the degree to which the current cold front brings in a cleaner air mass in its wake will dominate the air quality forecast for the first part of the medium range period, increasing the chance for an exceedance to Marginal. Unsettled weather will return for the end of the period, keeping the exceedance risk at Slight. The trend for a dry and clear Friday in the southern Mid-Atlantic continues in today model runs, whereas the central and northern Mid-Atlantic (NMA) can expect clear skies possibly giving way to afternoon thunderstorms. This morning’s hi-res models are not in consensus on Friday’s chances for convection, with a trend toward drier weather. As a result, ozone may reach the Moderate range at more widespread locations, particularly along the I-95 Corridor and the SMA. Saturday appears to have the best chance for more widespread Moderate ozone across the region as a mid-level ridge builds overhead. The main forecast questions for Friday and Saturday will be chances for isolated to scattered convection, if any, and air mass characteristics in the wake today’s weak cold front. High pressure on Saturday will give way to a surface low approaching the region from the west, bringing more unsettled weather to primarily the NMA on Sunday. There is however, very high uncertainty on the timing and placement of precipitation for Sunday, Monday and Tuesday, given questions about the strength and location of the next upper level trough, which will move over the region beginning Sunday.

 

Discussion:

The weather models are in strong agreement in the placement of major synoptic features on Friday but they continue to diverge regarding the reinvigorating of the recent semi-permanent upper level trough over the eastern U.S. and its associated surface low. This translates into low confidence in the timing and strength the next storm system that will impact the Mid-Atlantic in the Sunday-Tuesday time frame. The models consulted for this discussion were the 06Z GFS, 06Z NAM and the 00Z ECMWF. An upper level trough is currently residing over much of eastern Canada, extending southward to the Southeast U.S., with its closed center of circulation over ON. The trough will slowly progress eastward over the Northeast U.S. and Canadian Maritimes by 00Z Saturday. A shortwave will spin off and move over the northern Mid-Atlantic (NMA) on Friday afternoon, possibility triggering scattered thunderstorms. On Saturday, a weak mid-level ridge will move over Mid-Atlantic region, lasting through 00Z Sunday. At the same time, the westward extension of the Bermuda High at mid-levels will continue to build over the southern Mid-Atlantic (SMA).

Upstream, a series of stronger shortwaves will reinvigorate the upper level trough over the upper Great Plains/Great Lakes beginning on Saturday. There is still a lot of uncertainty between the models in the timing and movement of the trough between 00Z Sunday and 18Z Monday. The EC continues to be the fastest and strongest solution, while the GFS and NAM are on the slow end of the guidance. WPC prefers the 12Z UKMET, to which we do not have access, as a blend of the faster EC and slower GFS/NAM solutions. All of the models have a strong shortwave dropping down from the Canadian Prairies into the northern Plains on Saturday. The EC still phases this shortwave with a shortwave spinning around the western side of the eastern U.S. trough, reinvigorating an upper level trough and strengthen it as it dips southward over the Ohio River Valley/Mississippi River Valley. In contrast, the slower GFS and NAM do not phase these two shortwaves, but they do reinvigorate the eastern trough on Monday. The GFS and EC come into close consensus on placement of the upper level trough/closed low over the eastern Great Lakes on Monday, but they differ in its strength. The GFS is overall weaker with a closed low centered over southern ON and a weak trough extending southwestward to TX, whereas the EC keeps the closed low much stronger over southern ON and more organized, with its trough extending into the Southeast U.S. by 00Z Tuesday. On Tuesday, the GFS and EC come into a general consensus that the center of the upper level circulation will progress over the NMA, with the mid-level low in the vicinity of PA/NJ at 12Z. As the center of the circulation passes over the NMA, both models also agree that the system will begin to weaken and expand into the northeast by 00Z Wednesday.

Friday continues to look dry and sunny for the much of the SMA, and with today’s models runs, mostly sunny for the NMA as well. Today’s model runs hint at the possibility of increasing clouds and thunderstorms across the NMA on Friday, due to overhead shortwaves, but the hi-res model in particular are not in consensus. The 00Z NMM/ARW show essentially no thunderstorm development on Friday afternoon, while the 06Z 3km NAM, 12 km NAM, and 13 km GFS all show an area of scattered thunderstorms moving over TTN/PHL/ILG around 21Z Friday to 00Z Saturday. The 03Z SREF has a low probability of precipitation over the NMA during this same period. The concern for Friday is, given the lower chances for more widespread afternoon thunderstorms, how modified will the air mass in place be? The cold front currently moving through the region is stalled to the west of I-95. If this front does not clear the northern I-95 Corridor by Friday, there is certainly a chance for rising ozone. Yesterday, a couple of the Philadelphia area monitors had a big jump in ozone for about 4 hours in the afternoon, suggesting the current air mass is sufficiently modified. Therefore, there is a Marginal chance for an exceedance on Friday, with a focus on the I-95 Corridor and possibly the SMA, where there will be clear skies and potentially stagnant air due to the extension of the Bermuda High. The forecast questions for the NMA will focus on the air mass characteristics, associated location of today’s cold front, as well as chances for afternoon convection. Similar to yesterday, this morning’s 06Z air quality models did not develop much in the way of Moderate ozone, but they are all generally Good for today. The CMAQ models in particular have a strong seasonal bias to under-predict ozone early in the season, so they may well be under-predicting for tomorrow and Saturday, when more widespread Moderate ozone is possible.

Saturday will likely be the day with the best chance for widespread Moderate ozone across the region. As the upper level low pulls northeast early Saturday morning, the mid-level ridge will build over the entire region, promoting mostly clear skies. Again depending on air mass characteristics, rising ozone is likely, possibly with some carry-over from Friday. Flow aloft in the NMA will be northwesterly and relatively fast, with plenty of vertical mixing. NWS and WPC take today’s cold front all the way to the south of the region on Saturday, so if that verifies, the air mass may be relatively clean, which would help to limit any rising ozone. But given a full day of strong early June sun, Moderate ozone seems likely, especially across the SMA and along the I-95 Corridor.

On Sunday, surface low pressure will develop in the Great Lakes, bringing unsettled weather back much of the Mid-Atlantic. A warm front will lift northward, reaching the central Mid-Atlantic around 12Z Sunday and continuing to slowly move northward, resulting in widespread clouds and rain showers. The intensity and placement of rain showers vary between the NAM/GFS and EC, as discussed above. The GFS and NAM place a much weaker low pressure center farther to the north than the EC, resulting in a weaker front and less widespread precipitation, particularly across the SMA. In contrast, the EC has a rain out for essentially the entire region on Sunday. The WPC seems to put more weight on the EC. Given potential for precipitation and clouds across the entire Mid-Atlantic, therefore the exceedance risk will fall back to Slight.

The uncertainty for the end of the period continues in today’s model runs. Both the GFS and EC have a slow progression of the surface low and associated cold front on Monday and Tuesday. In general, widespread clouds and rain seem likely for Monday, with wrap-around showers and a closed low overhead on Tuesday. The chances for an exceedance will remain Slight.

-Enlow/Huff