Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 19, 2014
Valid: June 20-24, 2014 (Friday-Tuesday)

Summary:

Friday will have showers in the southern Mid-Atlantic and sunshine in the northern Mid-Atlantic as a cold front moves southward through NC. Air quality will be limited to the Good range with the chance for some areas of Moderate ozone in the northern half of the Mid-Atlantic. Saturday will see a chance for widespread scattered rain and thunderstorms as the cold front moves back northward as a warm front. Cloud cover and mixing will keep air quality in the Good range. As the front switches direction once again and follows its path back southward to SC on Sunday, rain and cloud cover will keep air quality in the Good range for the southern part of the region, while ozone and PM2.5 will reach the Moderate range at scattered locations in the northern Mid-Atlantic due to sun and stagnant air. There is some uncertainty in the forecast for Monday and Tuesday but air quality is expected to reach the Moderate range for both days.

Discussion:

The weather forecast models are in relatively close agreement for the medium range period, with slight differences beginning on Sunday that amplify by Tuesday. The 06Z NAM starts to diverge from the 06Z GFS, the 00Z ECMWF, and the 00Z CMC beginning 18Z Saturday at 500 mb so we will leave it as an outlier. By Tuesday, the GFS, ECMWF, and CMC have completely different solutions for a developing longwave trough over the Great Lakes/Mid-Atlantic that will pull the next cold front through the Mid-Atlantic. The GFS is the fastest and farthest east with the developing upper level trough, followed by the CMC, and then the ECMWF. These differences result in uncertainty in the precipitation forecast for Tuesday, as the GFS generates widespread showers and thunderstorms from west to east across the Mid-Atlantic, ahead of the approaching front. There is also a question of the timing of the frontal passage itself; WPC is leaning toward the slower solutions, with the cold front holding back to the west over the Great Lakes and slowly moving into the northern Mid-Atlantic on Wednesday and Thursday.

A cold front will slowly move south through NC on Friday, generating rain and thunderstorms in its wake across the southern Mid-Atlantic. Surface high pressure will move down into the Northeastern U.S. from interior ON, resulting in sunny and calm weather for the northern Mid-Atlantic. Northerly clean and dry flow aloft in the northern and central Mid-Atlantic and rainout of pollutants in the southern Mid-Atlantic will limit PM2.5 to the Good range for most locations. Ozone will also be in the Good range for most areas, with some scattered pockets of Moderate ozone possible in the northern Mid-Atlantic due to sunny skies and light winds.

On Saturday, the cold front will retrace its tracks northward as a warm front, reaching the vicinity of central/southern VA. There will be a chance of widespread scattered rain and thunderstorms with cloud cover that will last throughout the day mainly in the central and southern Mid-Atlantic. A wave of low pressure will form along the front. The forecast models differ slightly with the timing and strength of the wave, which introduces some uncertainty in the extent of the precipitation forecast. Regardless, air quality will likely remain in the Good range for Saturday, with the chance for isolated Moderate ozone in the northern part of the region, where skies will be mostly clear.

Sunday, the warm front will reverse its direction once again as a cold front and move southward to SC. The northern Mid-Atlantic is expected to stay dry while there will be a chance for showers in the lower half of the region. There will be some recirculation of upper level flow, resulting in accumulation of PM2.5 along the I-95 Corridor. As a result, PM2.5 concentrations will increase, but the absence of high atmospheric humidity should be enough to keep PM2.5 in the upper Good range. There is the chance for isolated Moderate ozone in the northern Mid-Atlantic due to increased sunshine and stagnant flow, while showers and cloud cover in the southern part of the region will keep ozone in the Good range.

Humidity will begin to rise on Monday and Tuesday as the surface flow turns southerly. Monday will likely be mostly dry and sunny, while chances for rain increase on Tuesday ahead of the next cold front, which will approach from the northwest. The possibility for Moderate ozone will remain on Monday and Tuesday, mainly along the I-95 Corridor. PM2.5 concentrations will continue to rise steadily in response to increasing humidity, likely reaching the Moderate range at scattered locations. If the GFS’s faster track for the cold front and associated precipitation verify for Tuesday, air quality conditions will return to the Good range. However, we favor the slower solutions of the ECMWF and CMC, which would keep Moderate air quality in the forecast for early next week.

-Catena/Huff