Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 20, 2014
Valid: May 22-26, 2014 (Thursday-Monday)

Summary:

A slowly-moving cold front will lead to Good air quality for most locations on Thursday; the exception will be the southern and eastern portions of the region, where PM2.5 may linger in the Moderate range. Generally Good air quality is expected for Friday and Saturday, as a cool and dry Canadian air mass builds into the region. Temperatures will begin to rise on Sunday as an upper level ridge extends eastward over the Mid-Atlantic, leading to scattered Moderate ozone. Monday may be a transition to poor air quality, with a chance for isolated USG ozone, but significant uncertainty exists due to differences in the upper air forecast guidance.

Discussion:

The weather forecast models are in agreement on the main features of the beginning of the medium range period but diverge beginning on Sunday. A shortwave/closed low over southern ON will dig out a weak longwave trough over the eastern U.S. on Thursday. A shortwave will rotate though the axis of the trough and amplify it on Friday. At the surface, a weak cold front will slowly move through the Mid-Atlantic on Thursday and clear the southern part of the region on Friday morning. A broad upper level ridge over the Midwest will push the weak trough eastward over the Canadian Maritimes on Saturday. By Sunday, the models begin to diverge. The GFS is faster in extending the broad upper level ridge eastward, while the trough over the Canadian Maritimes retards the progression of the ridge in the ECMWF run. On Monday, the GFS spins a vort max down from interior QC that closes off and flattens the eastern extend of the ridge over the Mid-Atlantic. The ECMWF doesn’t have this feature, and instead pushes the axis of the ridge over the eastern Great Lakes. The GFS solution is wetter on Monday, while the ECMWF solution keeps the Mid-Atlantic dry. These differences impact the chances for a transition to poor air quality on Monday.

There is uncertainty regarding the speed of the cold front through the Mid-Atlantic on Thursday and Friday. Model guidance and WPC’s analysis have the front slowing down compared to yesterday’s analysis. The front is expected to move through the western and northern Mid-Atlantic by Thursday night, but not clear the southern Mid-Atlantic until Friday morning. With a humid and modified air mass in place, the speed of the front will impact the PM2.5 forecast. The air quality models are in agreement that both ozone and PM2.5 will be in the Good range for the entire region on Thursday, but it seems likely that Moderate PM2.5 will linger along the eastern and southern parts of the region, where the impact of the frontal passage will be felt latest. Scattered rain showers and thunderstorms across the region will help to clean out the atmosphere, particularly in the northern Mid-Atlantic. Ozone will be in the Good range for most locations, due to cloud cover associated with the frontal passage.

A Canadian high pressure system will be building into the region on Friday. There is a chance for scattered showers triggered by a vort max that will rotate through the axis of the upper level trough. With the arrival of a cleaner and drier air mass, air quality will be in the Good range for most locations. The exception will be the southern Mid-Atlantic, where PM2.5 may linger in the Moderate range until the slow-moving cold front clears the region and the new air mass can build in.

The center of surface high pressure will settle over the southern Great Lakes on Saturday. With seasonably cool temperatures, low humidity, and northerly flow, it will be a nice day to begin the holiday weekend. Good air quality is expected across the region.

Temperatures will begin an increasing trend on Sunday as the eastern extent of the upper level ridge moves over the Mid-Atlantic. Northwesterly flow will continue on Sunday, which will keep dew points low. Ozone will likely climb into the Moderate range at scattered locations, while PM2.5 will lag in the upper Good range at most locations in the dry air mass. Scattered Moderate PM2.5 is possible along the I-95 Corridor.

Monday may possibly be a transition to poor air quality. The westward extension of the Bermuda high will be in place, and the air mass over the region will have begun to modify. Surface winds will shift westerly, with warm air advection aloft. Temperatures will be rising into the 80s °F across the region. Although humidity will still be relatively low, due to northwesterly flow aloft, it will begin to rise later in the day. PM2.5 will likely reach the Moderate range at scattered locations, while upper Moderate ozone is likely, with the chance for isolated USG. The extent of rising ozone and PM2.5 concentrations will depend on the evolution of the upper level pattern; the GFS’s wetter solution, with a closed low over southern QC, will lead to cleaner air quality conditions, while the ECMWF solution is more conducive to poor air quality.

-Huff