Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 26, 2015
Valid: May 27-31, 2015 (Wednesday-Sunday)

Summary:

Despite having a warm and humid summer-like air mass over the Mid-Atlantic region through the end of the medium range period, the chances for Code Orange/Red air quality have decreased substantially, with only a marginal chance on Thursday-Friday. Onshore transport of maritime air aloft essentially cleaned out the region yesterday, and that trend is expected to continue through Wednesday. The forecast for Thursday will hinge on the location of clouds and convection associated with the a weak cold front that is expected to stall in the vicinity of the I-95 Corridor; upper level transport from the Ohio River Valley and diminishing winds will keep the chances for rising ozone to the northern part of the Mid-Atlantic. A substantial cold front will move into the region on Saturday and stall near the Mason-Dixon Line on Sunday. Clouds and organized convection ahead of the cold front will limit chances for rising ozone over the weekend, as will a return on onshore transport aloft.

Discussion:

The weather forecast models are in relatively close agreement on the main features of the medium range period. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The overall synoptic setup continues to be dominated by an upper level ridge extending over the western Atlantic; the Mid-Atlantic will remain on the western edge of the ridge through the end of the medium range period. On Wednesday, shortwave energy over the Plains/western Great Lakes will move northeastward into southern ON and reach southern QC by 12Z Thursday. This feature will pull a weak cold front through the northern Mid-Atlantic (NMA) on Wednesday and Thursday. There is considerable uncertainty regarding the strength, speed, and position of this front, considering the relatively weak forcing aloft and the fact that the front will be moving into a warm air mass. WFC transitions it from a cold front (today) to a surface trough (Wed) to a stationary front (Thu). The weather models are consistent in bringing the front across the NMA on Wednesday, reaching the I-95 Corridor by roughly 18-21Z. At this point, it will stall in the vicinity of I-95 through Thursday before dissipating on Friday morning. The main impact for air quality is that the models develop widespread clouds and convection across the region on Wednesday afternoon. The GFS and EC have a slightly stronger shortwave aloft, which translates into more precipitation at the surface that reaches farther east, to the I-95 Corridor. This stalled front acts as a focus for cloud and convection again on Thursday, generally centered slightly farther south, over the central part of the Mid-Atlantic. On Friday, the upper level ridge strengthens and a surface high moves west to east across NY and New England, leading to clearing skies and stagnating winds across the Mid-Atlantic. Aloft on Friday, a large short shortwave/vorticity maximum will drop down from west-central Canada and reach southwestern ON by Saturday morning. This vort max will amplify the semi-permanent upper level trough over eastern Canada and pull a substantial cold front into the NMA in the Saturday-Sunday time frame. The usual discrepancies between the global models remain, with the GFS being the fastest to move the vort max eastward and the EC about 12 hours behind. This translates into some question about how quickly clouds and convection ahead of the front will reach to the eastern Mid-Atlantic on Saturday, as discussed below. The front will reach the vicinity of the Mason-Dixon line on Sunday morning and either stall or sag very slowly southward.

The chances for ozone reaching the USG range in the Mid-Atlantic this week have decreased substantially. The onshore/maritime transport aloft effectively cleaned out the region yesterday, somewhat faster than I expected, given there was still strong subsidence aloft. Even though the ridge will persist all week, with a warm air mass in place, the combination of onshore flow aloft on Wednesday (and then returning for the weekend) and much higher chances for widespread clouds and convention associated with the weak frontal boundary on Thursday and Friday look to suppress ozone to the Good to Moderate range. Particles are relatively low today, but as winds subside on Thursday and Friday, concentrations should increase, especially with surface convergence in the vicinity of the weak cold front.

Wednesday will be similar to Tuesday, except with more chances for afternoon clouds and convection as the weak front moves through the NMA. It will be warm and humid, with breezy south/southwesterly winds. Continued onshore back trajectories over the NC coastline should keep ozone and particles in the Good to Moderate range.

Back trajectories will turn more westerly on Thursday, from the southern Ohio River Valley (ORV). Winds will also diminish, particularly in the central and southern parts of the region. This should drive up particle concentrations into the Moderate range, given the relatively high humidity. Ozone will of course depend on available afternoon sunlight, which will in turn depend on where the weak cold front stalls and acts as a focus for clouds and convection. At this point, Good ozone is expected along the I-95 Corridor but ozone may reach the Moderate range at locations to the west, such as southwestern PA. The BAMS air quality models predict scattered Moderate ozone along the I-95 Corridor and some isolated Moderate PM2.5 in western PA/MD.

A strengthening ridge on Friday will help to keep skies clear and winds light. This will lead to more widespread Moderate ozone, possibly limited by onshore surface flow to the lower Moderate or Good range along the coast. The BAMS air quality models develop a more widespread area of higher Moderate to low USG ozone over eastern PA/northern NJ, likely in response to subsidence and afternoon sun.

The forecast for Saturday and Sunday is uncertain due to questions about the speed and progression the cold front. The GFS, being fastest with the front, brings precipitation to the I-95 Corridor on Saturday afternoon, while the EC is slower. Back trajectories are onshore both days, however, which may be sufficient to keep air quality in the Good to low Moderate range, regardless of any widespread clouds and convection that may develop ahead of the cold front on Saturday and in the vicinity of the stalled/slowing front on Sunday.

-Huff