Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, July 26, 2015
Valid: July 27-31, 2015 (Monday – Friday)

Summary:

A slowly-moving upper level ridge will promote weather conditions conducive to deteriorating air quality in the Mid-Atlantic Tuesday-Thursday, with Marginal to Appreciable chances for USG ozone. Scattered clouds, showers, and thunderstorms across much of the region on Monday will limit ozone to the Good to Moderate range. Temperatures and humidity will increase on Tuesday and continue through Thursday as the ridge aloft approaches the Mid-Atlantic. The main forecast question continues to be the impact of air mass transport aloft, which will be northwesterly (typically cleaner) on Tuesday and Wednesday. The next substantial cold front will move through the region on Thursday. Model guidance is showing less organized pre-frontal convection on Thursday afternoon, which may allow for another day of upper Moderate to USG ozone along the I-95 Corridor. A post-frontal return to seasonable temperatures and humidity on Friday will end the threat for USG ozone.

Discussion:

Today, the weather models continue to be in consensus on the synoptic features during the medium range period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The big story is the amplifying upper level ridge over the central US that will gradually move toward the Mid-Atlantic mid-week. On Monday, the broad upper level ridge draped over the center of the CONUS will begin to amplify, with the ridge axis located along the western Great Lakes. The ridge will be sandwiched between a strong upper level trough over the Pacific Northwest and a weak trough over the Mid-Atlantic/New England. The axis of the upper level ridge will move eastward on Tuesday and Wednesday, keeping a northwesterly flow aloft over the Mid-Atlantic. At the same time, the Pacific Northwest trough will move northeastward, cutting off over the southern Canadian Prairies/northern US Plains on Tuesday. By Thursday, this closed low will have moved over Hudson Bay, displacing the upper level ridge axis to the east and temporarily bringing a more zonal flow over the Mid-Atlantic. The closed low will pull a stronger cold front (compared to today’s) into the region on Thursday.

A weak cold front moving into PA today will dissipate on Monday. The mesoscale models agree that the remnants of the front will act as a focus for scattered showers and thunderstorms for much of the day, particularly in the afternoon. The 09Z SREF shows a high probability of precipitation along the entire I-95 Corridor from NYC to DC, including much of VA and western NC, beginning in the late morning and peaking in the late afternoon. Back trajectories ending at PHL are fully onshore from the southeast. This clean transport pattern, in conjunction with increasing cloud cover and the scattered precipitation, will limit ozone in most areas to the Good range. Some isolated Moderate ozone is possible at locations along the I-95 Corridor that see less clouds/convection than expected. This morning’s 06Z runs of the air quality models continue their trend from yesterday of showing little consensus on either the ozone or PM2.5 forecasts this week. For tomorrow, the models range from widespread Good ozone (NCDENR) to scattered Moderate ozone along I-95 (NOAA) to upper Moderate/isolated USG in DC/BAL (BAMS). Particle concentrations today are well into the mid-Moderate range to the west (OH, IN) but in the Good range to the north, northwest, and northeast. In the Mid-Atlantic, southerly flow is keeping PM2.5 concentrations in the Good range this morning at locations along the coast, while stagnation and rising humidity inland is promoting low Moderate concentrations. Humidity will increase a bit more tomorrow, and light surface winds will continue, suggesting that particles will increase higher in the Moderate range inland but locations along the immediate coast (e.g., DE) will remain in the Good range.

Tuesday and Wednesday remain days of interest during the period. With the upper level ridge situated just to the west of the Mid-Atlantic, temperatures and humidity will be on the rise, with temperatures in the low to mid 90s °F through Thursday. Subsidence will promote mostly sunny skies and light surface winds. The main forecast question for rising ozone continues to be air mass transport aloft. The location and amplitude of the ridge axis will keep northwesterly transport aloft over the Mid-Atlantic both days. 36-hour 06Z GFS back trajectories ending at PHL show veering transport to the north/northwest on Tuesday and fully northwest transport on Wednesday. The trajectories aren’t fast but they aren’t localized either. This transport pattern is reflected in the dew point forecasts on Tuesday and Wednesday – although dew points will rise, they will remain in the mid-to-upper 60s °F, which is a step down in terms of the atmospheric humidity typically associated with this type of synoptic pattern. The northerly flow aloft may be sufficient to limit ozone to the upper Moderate range mid-week, but given the heat, mostly sunny skies, and light surface winds, USG ozone continues to be a possibility, particularly on Wednesday. The air quality models are not in consensus on the magnitude of ozone increases during these two days, with the BAMS in general showing much higher levels of ozone, but the models do agree that the highest ozone will be at the usual locations along the I-95 Corridor. Particle concentrations will also likely continue to rise, with widespread Moderate levels expected, although the magnitude may be limited by the transport pattern aloft as well, similar to ozone.

The next substantial cold front will move through the Mid-Atlantic on Thursday. There continue to be the usual minor model discrepancies associated with the timing of the front, but it should reach the I-95 Corridor in the afternoon. Today’s model runs are showing less organized pre-frontal convection, however, which keeps the chances for another day of upper Moderate to USG ozone in the forecast for locations along the I-95 Corridor. The air quality models are also split on Thursday’s forecast, with the guidance ranging from Good ozone (BAMS-CMAQ) to isolated USG (NCDENR).

The cold front will stall along the southeastern VA/eastern NC on Friday, promoting clouds and rain along coastal locations in the central and southern Mid-Atlantic. Elsewhere, temperatures will drop to near seasonable levels, and less humid air will move into the region on northwesterly winds. The air mass change will not be substantial, but it should be enough to drop ozone back into the Good to low Moderate range across locations behind the cold front.

-Huff