Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 27, 2014
Valid: June 28- July 2, 2014 (Saturday-Wednesday)

Summary:

A high pressure system centered over New England on Saturday will generate onshore flow that will keep air quality in the Good range, with the exception of areas west of I-95 in the northern Mid-Atlantic, where ozone may reach the mid-to-upper Moderate range. Sustained onshore flow on Sunday and Monday will allow air quality to persist in the Good range. Temperatures and humidity increase beginning on Tuesday due to the influence of the westward extension of the Bermuda High. Diurnal heating will create the chance for scattered showers and thunderstorms on Tuesday and Wednesday, which will limit rising ozone and PM2.5 levels to the Good to Moderate range.

Discussion:

The weather forecast models are once again in similar agreement for the medium range period. However, the usual differences of the timing and location of shortwaves will create some uncertainty in the precipitation forecast. The 06Z NAM, 06Z GFS, and the 00Z ECMWF all agree on an the upper level ridge tilted over the Great Lakes at the beginning of the period. The ridge will slowly move through the Mid-Atlantic and will finally get pushed to the East Coast by the eastward movement of an upper level trough through southern ON early Tuesday morning. Starting Tuesday night, the ECMWF and the GFS start to diverge just a tad from each other. The ECMWF intensifies the upper level trough with additional shortwave energy moving through the axis of the trough. By Wednesday afternoon, the ECMWF then moves the trough eastward faster and broadens it, while the GFS amplifies the trough further south into the U.S., just west of the Mid-Atlantic region. This difference creates some uncertainty in the precipitation forecast for Wednesday.

A New England surface high pressure system on Saturday will result in mostly to partly sunny weather and onshore surface flow. The presence of a stationary front in southern SC will create the chance for rain and thunderstorms for the southern Mid-Atlantic. Flow aloft will be northeasterly, transporting cleaner air into the area. Sustained onshore winds, mixing, and scattered clouds will limit ozone and PM2.5 to the Good range for most locations. The 06Z runs of the NOAA, BAMS, and NC ozone models all indicate a band of mid-to-upper Moderate ozone along and west of I-95 in the northern Mid-Atlantic (e.g., DC, BAL, PHL, northern NJ). The models are probably responding to the onshore flow and a period of stagnation at the surface overnight into the morning. As a result, there is a likely chance for Moderate ozone west of I-95 along the urbanized northern corridor.

By Sunday, the development of a Bermuda High will continue the seasonably warm weather for mainly the northern half of the Mid-Atlantic. The southern half of the region will experience precipitation and cloud cover as a wave of low pressure forms along the remnant frontal boundary just off the coast of the Carolinas. Onshore flow will dominate, maintaining Good air quality across the region.

A slow moving shortwave will move over the Mid-Atlantic on Monday from the west and diurnal heating will trigger scattered afternoon rain and thunderstorms across the Mid-Atlantic. The northern Mid-Atlantic will have partly to mostly cloudy skies. The New England high pressure system will move eastward into the Atlantic Ocean, which will continue onshore flow, thus keeping air quality in the Good range.

Warm and humid conditions will begin on Tuesday and continue until the next cold front passes in the Thursday to Friday time frame. With the passing of the shortwave aloft, another day of diurnal heating, and the eastward movement of the approaching upper level longwave tough over the western Great Lakes, Tuesday will experience the chance for scattered rain and thunderstorms throughout the day across the region. The rise in temperatures and dew points could possibly increase PM2.5 to the Moderate range in scattered inland locations, but southerly flow aloft and at the surface should continue to keep PM2.5 concentrations in the Good range along the coast. Ozone will remain in the Good range due to cloud cover and mixing, with the chance for Moderate ozone along and west of I-95. The approach of the upper level longwave trough on Wednesday will shift flow southwesterly, which will help to promote more widespread Moderate air quality. However, given the warm and humid air mass that will be in place, there is a likely chance of scattered afternoon precipitation and cloud cover, which would limit rising ozone levels and possibly PM2.5 as well for areas that experience significant rainfall.

-Catena/Huff