Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 10, 2014
Valid: July 11- 15, 2014 (Friday-Tuesday)

Summary:

A cold front will stall over VA and NC on Friday, resulting in precipitation and cloud cover for the southern Mid-Atlantic (SMA). High pressure will move over VT and create northeasterly flow for the northern Mid-Atlantic (NMA). Air quality will be limited to the Good range for Friday for most locations, with the exception of the vicinity of the I-95 Corridor in NMA, where surface stagnation will result in isolated upper Moderate ozone. Saturday will be mostly sunny with easterly flow as the center of high pressure will extend its ridge over the Mid-Atlantic. The cold front will stall over NC/SC leaving a slight chance for rain in the SMA. Air quality will remain in the Good range with the exception of Moderate ozone in the NMA along I-95. The cold front over NC/SC will move northward as a warm front on Sunday, creating scattered rain and thunderstorms for the second half of the day in the NMA. PM2.5 will reach the Moderate range while ozone will stay in the Good in the NMA and will reach the Moderate range in the SMA. Ozone should return to the Good range for Monday and Tuesday as the cold front approaches the region and passes halfway through by Tuesday, while PM2.5 will remain in the Moderate range for Monday.
Discussion:

There is high confidence in the weather forecast models for the medium range period. A weak upper level trough will pass through the Mid-Atlantic on Friday. A closed low will form over northern MB and will move southeastward and intensify through the forecast period. The ECMWF and GFS start to diverge on Sunday night into Monday morning when the ECMWF splits off a lobe of energy and elongates the low, while the GFS keeps it more compacted. They return to agreement on the approximate location of the closed low around Tuesday morning over the Great Lakes.

The cold front that passed through the region on Thursday will stall over VA and NC for Friday. A wave of low pressure will form along the boundary, which will lead to scattered rain and thunderstorms with cloud cover for the southern Mid-Atlantic (SMA). The eastward movement of the center of high pressure to VT from the Great Lakes will result in light northeasterly/easterly flow and partly sunny skies for the northern Mid-Atlantic (NMA). With the center of high pressure moving over the NMA, surface winds will stagnate overnight and into the morning hours. Given this set-up, NOx titration is likely on Friday morning, which will result in rising ozone mixing ratios in the afternoon somewhere along, west, or north of I-95. We have seen this pattern several times so far this summer, and the result has been high Moderate to isolated (~1 monitor) USG ozone in a very small area (e.g., PHL/northern DE). The NOAA and BAMS air quality models support this, with a line of Moderate ozone north and west of I-95; the BAMS CMAQ even predicts USG ozone in northern VA. Consequently, air quality will be limited to the Good range for most locations on Friday, with the exception of the vicinity of the I-95 Corridor in the NMA, where upper Moderate ozone is likely at isolated locations.

On Saturday, the region of high pressure will stall and extend its ridge down the Mid-Atlantic. The stationary front in the SMA will oscillate around the NC/SC coast. Most of the Mid-Atlantic will stay dry with the exception of a slight chance for precipitation in areas closest to the frontal boundary. The high pressure will result in southeasterly flow and mostly to partly sunny skies for the rest of the region. Air quality will remain in the Good range for most locations with the exception of along the I-95 Corridor in the NMA, where surface stagnation will once again allow ozone to reach the Moderate range at isolated locations.

The stalled frontal boundary on the NC/SC coast will move northward on Sunday as a warm front. This will create rain and thunderstorms for the second half of the day in the NMA. High pressure over the SMA will keep the conditions dry. Warm air advection aloft will begin on Sunday, resulting in a more stagnant atmospheric layer. Mostly cloudy skies will keep ozone in the Good range for the NMA while Moderate ozone will reside in most locations in the SMA, where it will be sunnier. PM2.5 will reach the Moderate range throughout the region depending on how heavy the storms are.

By Monday, the strong longwave trough aloft will push a cold front eastward over western PA. There will be scattered rain and thunderstorms throughout the region with partly cloudy skies. Continued warm air advection should keep PM2.5 in the Moderate range. Ozone will be reduced to the Good range. Tuesday will have similar conditions as the cold front passes halfway through the Mid-Atlantic. Rain and thunderstorms will persist with cloud cover and northwesterly flow aloft. Air quality will be reduced to the Good range.

-Catena/Huff