Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Friday, June 20, 2014
Valid: June 21-25, 2014 (Saturday-Wednesday)
Summary:
A wave of low pressure will form along a frontal boundary across the central Mid-Atlantic on Saturday, bringing rain and thunderstorms to much of the region. Cloud cover and mixing will limit ozone and PM2.5 to the Good range. Sunday will see mostly sunny skies in the northern part of the region, allowing ozone to increase into the Moderate range at scattered locations, while ozone will be at the Good/ Moderate threshold for the southern half of the region, depending on cloud cover. PM2.5 will remain in the Good range across the region due to low humidity. Monday and Tuesday will feature sunny and pleasant conditions, which will increase ozone and PM2.5 to the Moderate range for most locations; the exception will be the western Mid-Atlantic on Tuesday, where afternoon precipitation may keep air quality in the Good range. Air quality on Wednesday will depend on the timing of the next cold front.
Discussion:
There is close agreement between the weather forecast models on the main features of the medium range period. In particular, the 06Z GFS and 00Z ECMWF are in much closer agreement today regarding the development and progress of an upper level trough and associated cold front on Tuesday and Wednesday. They agree to mainly zonal flow aloft for Saturday. Sunday, a weak upper level ridge will develop over the Great Lakes and spread slowly eastward. As the ridge moves eastward toward the Mid-Atlantic, an upper level closed low over MB will flatten the ridge through Sunday night, resulting in slight ridging aloft over the Mid-Atlantic for Monday. On Tuesday morning, shortwave energy will begin to dig out a longwave trough over the western Great Lakes. The GFS and ECMWF develop the trough and corresponding surface low in roughly the same location, but the ECMWF is slightly more amplified with the trough than the GFS. As a result, the ECMWF brings more precipitation into the western Mid-Atlantic on Tuesday afternoon compared to the GFS, but both models are consistent in developing precipitation over the Mid-Atlantic Tuesday afternoon through Wednesday.
The models show some minor variations in the evolution of smaller shortwaves associated with the longwave trough, which is to be expected at this time in the medium range forecast. For example, the models start to diverge Tuesday afternoon with the development of shortwave energy over the Midwest. The GFS and the ECMWF start to weaken the shortwave while the 00Z CMC intensifies it. The GFS spreads out the energy into smaller shortwaves, which pass through the Mid-Atlantic region starting around 18Z Wednesday. The CMC does not weaken the shortwave until Wednesday night and keeps it located west of the Mid-Atlantic, over the Great Lakes. This uncertainty in the arrival and intensity of the shortwave energy developing the longwave trough over the Mid-Atlantic affects the cold front expected to impact the region on Wednesday.
A cold front will move northward as a warm front and settle over central VA early Saturday morning. The front will stall as a stationary front and a wave of surface low pressure will move along the boundary overnight and through the day Saturday. The cold front will move southward to NC by Saturday evening. The wave and frontal boundary will spark showers and thunderstorms overnight and through Saturday evening, mainly in the central and southern region of the Mid-Atlantic, yet there is still a chance for the precipitation to reach the northern Mid-Atlantic also. Mostly cloudy skies associated with the presence of the frontal boundary will keep ozone in the Good range. The clean and dry air mass in the northern part of the Mid-Atlantic and precipitation and mixing in the southern Mid-Atlantic will limit PM2.5 to the Good range.
By Sunday, the frontal boundary will push south of the region, giving way to calm and sunny weather as a Canadian high pressure system extends its ridge over the Mid-Atlantic. Recirculation of air aloft along the I-95 Corridor will allow PM2.5 to accumulate. However, the existing air mass will likely be too dry for PM2.5 to increase into the Moderate range. There is a chance for ozone to reach the low Moderate in the northern part of the Mid-Atlantic, while ozone will be at the Good/ Moderate threshold for the southern half of the region, depending on cloud cover.
The surface ridge will still be over the region as the center of high pressure moves to the Atlantic on Monday. Sunshine and calm weather will continue. PM2.5 concentrations will reach the low Moderate range along the I-95 Corridor due to continued recirculation of air aloft. Afternoon sunshine and light winds at the surface will allow ozone to reach the Moderate range at scattered locations.
The surface ridge will be displaced eastward to the Atlantic by Tuesday with the arrival of the next cold front. The cold front will reach the western Mid-Atlantic by Wednesday morning and a pre-frontal trough will develop in the vicinity of central VA/NC. The approaching cold front and arrival of an associated warm front will increase the chances for precipitation beginning on Tuesday afternoon in the western Mid-Atlantic. PM2.5 and ozone will reach the Moderate range for most locations on Tuesday, with the exception of the western Mid-Atlantic, depending on rain and clouds. Wednesday’s forecast will depend on the timing of the cold front, with generally Good air quality expected.
-Catena/Huff