Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 6, 2014
Valid: August 7-11, 2014 (Thursday-Monday)

Summary:

Air quality is not expected to rise above the low-mid Moderate range during the period, especially in the southern Mid-Atlantic, where widespread rain over the weekend will promote very clean conditions. The best chances for Moderate air quality will be in the northern Mid-Atlantic Friday-Sunday, as surface high pressure promotes mostly sunny skies and light surface winds.

Discussion:

The weather forecast models agree on the main synoptic features of the medium range period but are having trouble with mesoscale features triggered by a series of shortwaves beginning on Friday, which will mainly impact the southern Mid-Atlantic (SMA). The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The medium range begins with an amplifying upper level trough over New England and the Canadian Maritimes that will cut off on Friday and then slowly progress eastward through the end of the period. As the upper level trough retreats, broad ridging will spread eastward over the Mid-Atlantic. The influence of the ridge will be blunted over the SMA in particular due to a series of shortwaves that will move through the upper level flow and develop MCSs/waves along a stalled frontal boundary, producing widespread rainfall over the weekend across VA/NC. There looks to be a significant difference in the sensible weather between the northern Mid-Atlantic (NMA), where surface high pressure will dominate and keeps skies mostly clear, and the SMA, which will experience the effects of the MCSs/waves, although the location of the specific dividing line is not clear at this point.

The cold front moving through the region today will clear NC on Friday morning. This boundary will stall along the Mississippi River Valley (MRV) and Southeast through the end of the period. Cooler and much less humid air will filter in behind the front, with northwesterly flow. Good air quality is expected across the region for Friday.

Surface high pressure located over southern ON on Thursday will gradually move eastward though the end of the period, reaching NY state on the weekend and sliding off the Cape Cod coast by Monday. The proximity of the high will keep skies mostly sunny in the NMA, with slightly below average temperatures (e.g., highs in low 80s °F). Humidity will begin to increase over the weekend as the flow turns southerly, but it should remain relatively low for early August (e.g., low 60s °F). Air quality is expected to remain in the Good to low-mid Moderate range, with the highest ozone and PM2.5 concentrations along the I-95 Corridor, as usual. Friday and Saturday are probably the days when stagnating surface winds will push pollutant levels slightly higher, before the flow shifts onshore on Sunday and Monday. Depending on how the onshore flow is on Sunday, inland locations may see PM2.5 rise into the Moderate range, particularly the Susquehanna Valley and western PA/MD.

From a meteorological standpoint, the forecast for the SMA is far more interesting, but the only question for air quality will likely be how low ozone and PM2.5 concentrations fall into the Good range, particularly on Saturday and Sunday. The forecast models do seem to agree that shortwaves moving from the Ohio River Valley (ORV) and MRV on Friday through Sunday will interact with the stalled frontal boundary and trigger MCSs and/or waves of low pressure. As is typical, the models are having trouble agreeing on the location, track, and strength of these mesoscale features. The WPC prefers a blend of the GFS and ECMWF guidance, as they consider the NAM’s wave/MCS to be too far north and too strong on Friday-Saturday. The blended guidance indicates there will be widespread rain across VA and NC on Saturday and Sunday, with periods of heavy rain and possibly some imbedded thunderstorms. Where exactly the transition line to the rain will be, moving south from MD/northern VA, is not clear at this point. Furthermore, clouds ahead of the approaching meso low on Friday may push into the western Mid-Atlantic on Friday afternoon, along western VA/WV. By Monday, the axis of the upper level ridge will be along the East Coast, helping to dry things out, although the models do develop a weak coastal low off of the NC/SC coast, particularly the ECMWF. So the development of these mesoscale features bear watching and seem likely to bring several days of Good air quality to locations roughly south of Washington, DC/northern VA on Friday-Sunday, possibly continuing into Monday.

-Huff