Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, August 2, 2014
Valid: August 3-7, 2014 (Sunday-Thursday)

Summary:

Clouds and rain from the coastal front will keep air quality in the Good range for most locations on Sunday, with the exception of Moderate PM2.5 at inland areas of the northern Mid-Atlantic. Very light and recirculating flow associated with the return of surface high pressure will promote deteriorating air quality on Monday-Tuesday in the northern Mid-Atlantic, with a chance for upper Moderate to USG ozone along the I-95 Corridor, depending on cloud cover. A cold front will move through the region Wednesday-Thursday, pushing the chance for Moderate air quality to the southern Mid-Atlantic on Wednesday and bringing a return to generally Good conditions across the region on Thursday.

Discussion:

The weather forecast models are in general agreement this morning on the overall synoptic features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were used for this analysis. The period begins with a broad upper level longwave trough over the eastern U.S. A large shortwave will move over the northern Mid-Atlantic (NMA) on Sunday. It will act as a “kicker” to push the coastal front/inverted trough away from the NMA coastline, and also to promote a transition to more zonal flow aloft for Tuesday. The NAM has a large shortwave/low approaching the Southeast coast late Sunday into Monday, and brings it up the Mid-Atlantic coast on Tuesday. This solution is considered an outlier, per the WPC, since it has no support from the other models or their ensembles at this time. Weak longwave troughing will remain over the eastern U.S. on Monday, allowing weak surface high pressure to build back in over the NMA. The axis of the persistent upper level trough over eastern Canada will push back southward Tuesday through Thursday, re-amplifying the longwave trough over the Mid-Atlantic, assisted by several shortwaves dropping down over the Great Lakes. The GFS and ECMWF differ on minor aspects of the progression and amplification of the longwave trough, but they have the same trend and the same general location of the trough through the end of the period. By Thursday, the axis of the longwave trough will be along the East Coast. Here, the models diverge slightly, with the GFS forming a closed low over southeastern QC/New England/Canadian Maritimes. The ECMWF has the axis of the upper level trough in the same place as the GFS, but it is not as intense, and it does not close off through. This re-amplifying trough will pull a cold front through the Mid-Atlantic late Tuesday through Thursday.

The mesoscale models are still having some trouble with the location of the coastal front and track of waves of low pressure along it for Sunday. Current guidance indicates that there will be another wave moving along the front from south to north overnight and into Sunday morning, similar to what occurred overnight on Saturday. Rain will follow the track of the wave, with the heaviest rain along the coast, closest to the frontal boundary. Scattered showers and thunderstorms will develop in the afternoon, moving from west to east, following the track of the large upper level shortwave across the region. It will be overcast along the coast, particularly in the morning, with partly cloudy skies inland. Surface winds will be light and variable, with veering back trajectories from northeast to southwest. The proximity of the coastal front will keep air quality in the Good range for most of the region, particularly the southern Mid-Atlantic (SMA) and coastal NMA. PM2.5 concentrations are lingering in the Moderate range this morning in the NMA, however, despite the periods of heavy rain overnight and sustained onshore flow. Given the weak surface winds on Saturday, PM2.5 will likely remain in the Moderate range at inland locations on Sunday, in areas that do not experience heavy rainfall.

Monday and Tuesday continue to be the days of most interest in the medium range period as surface high pressure builds back on over the NMA and temperatures rebound to near normal values. The re-developing upper level trough will keep temperatures from rising too high, limiting them to the upper 80s °F. There is still uncertainty in the clouds/precipitation forecast for Monday in the NMA, depending on how fast the coast front retreats offshore, but a trend toward clearer and drier conditions continues. Current guidance suggests it will dry out in the NMA on Monday, but there is still a chance for afternoon showers or thunderstorms, especially along the coast. Very light and recirculating surface winds on both Monday and Tuesday, with convergence inland along I-95, will promote rapid afternoon ozone formation, given sufficient sunlight. Back trajectories will be slow, localized, and recirculating both days, especially Tuesday. Given the better chance for mostly sunny skies and less precipitation on Tuesday, it may be the day with higher pollutant concentrations. The main forecast questions will be the amount of sun/showers on Monday and the impact of the approaching cold front from the northwest on Tuesday. The 09Z SREF indicates a high probability of precipitation along the northwestern part of the region on Tuesday afternoon associated with the advancing front. The BAMS and NC air quality models are not in consensus regarding the chances for upper Moderate to USG ozone in the NMA for Monday and Tuesday; they develop mid-upper Moderate ozone along 1-95 on Monday, but the BAMS models indicate upper Moderate/USG from New York City to Richmond on Tuesday while the NC CMAQ returns ozone to the Good range across the region. Thus, with the very light winds aloft and at the surface, Monday and Tuesday are the days to watch for rising ozone in the NMA, given sufficient afternoon sunshine. PM2.5 will also be in the Moderate range in response to the stagnation and ample humidity. The NAAPS model is indicating a plume of smoke over the NMA on Monday and Tuesday, which may impact PM2.5 as well. Conditions in the SMA should remain in the Good range for most locations, given the continued proximity of the coastal front through Wednesday.

The cold front will move into the Mid-Atlantic from the Great Lakes late Tuesday into Wednesday, reaching roughly the Mason-Dixon Line by 12Z Wednesday. Rain and clouds associated with the advancing front will most likely return air quality to the Good range in the NMA, while the SMA will have the best chance for mid-upper Moderate ozone and rising PM2.5 concentrations into the Moderate range.

The weather forecast models diverge on the precipitation forecast for Thursday, with the GFS keeping the region mostly dry as surface high pressure builds in behind the cold front, while the ECMWF develops a band of clouds and showers across the central and southern parts of the region, following the track of the cold front. With the upper level trough axis overhead, and showers likely in the SMA and northwesterly flow in the NMA, Good to low Moderate air quality is expected.

-Huff