Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 22, 2014
Valid: June 23-27, 2014 (Monday-Friday)

Summary:

Strong June sun/rising humidity vs. onshore flow will be the competing factors Monday and Tuesday, keeping ozone and PM2.5 in the Moderate range at inland locations along and west of I-95. An approaching cold front will trigger rain and thunderstorms along the western Mid-Atlantic Tuesday afternoon, spreading into the eastern Mid-Atlantic on Wednesday, which will return air quality to the Good range. There is uncertainty in how far south the advancing cold front will reach on Thursday, with the possibility for more rain on Friday associated with a wave of low pressure.

Discussion:

The weather forecast models have come back into closer agreement today regarding the track of the next cold front, which will impact the Mid-Atlantic beginning on Tuesday, but they are still having trouble handling the development of the front’s associated low pressure system. We continue to prefer the 00Z ECMWF solution, which has shown the best run-to-run consistency over the past few days. WPC and the PHL NWS Office also prefer the ECMWF.

At 500 mb, weak ridging over the eastern Great Lakes/Mid-Atlantic on Monday will give way to the development of a weak trough over the Plains/western Great Lakes on Tuesday. At the surface, an area of low pressure will intensify over northern MI by 12Z Tuesday. This low will slide to the northeast and pull a cold front into the Mid-Atlantic on Wednesday. The models agree on the track of the low but differ in its speed; the 12Z NAM and 00Z ECMWF bring the low to the St Lawrence River by 12Z Wednesday, but the GFS is about 6 hours slower. Despite these differences, the operational models all bring precipitation to the western Mid-Atlantic on Tuesday afternoon/evening. At the surface, both the GFS and ECMWF move the cold front to roughly the Mason-Dixon Line by 12Z Thursday. At this point, the models again diverge slightly. Both develop a wave of low pressure that rides along the front early Friday into Saturday, but the ECMWF keeps the front along the Mason-Dixon Line, while the GFS moves the front and track of the wave farther north. These differences impact the location of precipitation in the Mid-Atlantic on Friday.

On Monday, surface high pressure will slide east to the Gulf of Maine as a warm front approaches the western Mid-Atlantic. Light winds and periods of stagnation overnight and through the morning will give way to steady south/southeasterly winds in the afternoon. It will be mostly sunny with seasonable temperatures. Humidity will begin to increase but dew points will remain relatively low until Tuesday. Onshore winds will push pollutants inland, keeping Good air quality along the coast, with scattered Moderate ozone and PM2.5 inland, particularly along the I-95 Corridor.

As the cold front approaches the western Mid-Atlantic on Tuesday, it will generate rain and clouds in the afternoon. All of the operational models and the SREF show rain and clouds along the western Mid-Atlantic beginning 18Z Tuesday. The 12Z 4km NAM keeps a line of convection and clouds to the west of the Mid-Atlantic through 00Z Wednesday, however, which adds some uncertainty to the precipitation forecast. For now, we assume that precipitation will arrive in the afternoon and keep ozone in the Good range along the western part of the region. PM2.5 may also fall into the Good range, depending on the strength and extent of rain and thunderstorms. Along the eastern Mid-Atlantic, where it will be mostly sunny and humid, steady onshore winds will moderate rising ozone and PM2.5, keeping concentrations in the low-to-mid Moderate range.

The cold front will slowly move into the northern part of the Mid-Atlantic on Wednesday and reach roughly the Mason-Dixon Line on Thursday morning. The front will trigger rain and thunderstorms across the region, particularly the eastern Mid-Atlantic, on Wednesday afternoon. As a result, ozone will be in the Good range for most locations. Some of the storms may have be severe and include periods of heavy rainfall, which will return PM2.5 to the Good range for most locations as well.

With flow aloft turning mostly zonal late in the period, it is not clear how far south the cold front will travel. The ECMWF and GFS both back off on precipitation for Thursday, as surface high pressure builds into the region, centered over the northern Great Lakes. Given strong June sun and relatively light winds, ozone has the potential to reach the Moderate range at locations along the I-95 Corridor. Dew points are not expected to fall dramatically behind the front, so PM2.5 may rebound to the Moderate range as well at scattered locations.

Both the GFS and ECMWF show a wave of low pressure impacting the Mid-Atlantic on Friday-Sunday. The GFS has the wetter solution for Friday for the Mid-Atlantic. It is too far out to track this system, but we expect Good/Moderate air quality for the end of the period.

-Huff