Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 8, 2014
Valid: August 9-13, 2014 (Saturday-Wednesday)

Summary:

Smoke transported from Canada and nearby surface high pressure will promote mid-upper Moderate air quality in the northern Mid-Atlantic over the weekend. Areas roughly south of northern VA will experience Good air quality from clouds and periods of heavy rain associated with a series of surface waves moving from west to east. Mostly Good air quality is expected across the region Monday-Wednesday as sustained onshore flow, a slow-moving coastal low, and an approaching cold front keep the weather unsettled.

Discussion:

The weather forecast models are in reasonable agreement on the overall synoptic pattern for the medium range period, although there are some minor discrepancies late in the period. The models are coming into consensus today on a more southerly solution for the series of surface waves that will impact mainly the southern Mid-Atlantic (SMA) today through Sunday. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. Aloft, a closed low over the Canadian Maritimes today will continue to retreat eastward, allowing the broad ridge over the Plains/Great Lakes to build over the Mid-Atlantic on Sunday. By Tuesday, a deepening upper level trough will displace the ridge and push a cold front through the Mid-Atlantic on Wednesday. Guidance suggests that this will be a strong trough that will cut off over ON/QC/NY on Thursday.

The main focus will be on the series of shortwaves that will trigger waves of low pressure along a stalled frontal boundary draped across the Mississippi River Valley (MRV) and Southeast and bring periods of heavy rain to the SMA through Sunday. There is still disagreement among the models regarding the location and progression of the waves, but they are trending toward the more southern solution of the ECMWF. The WPC once again prefers the ECMWF over the NAM and GFS this morning. The NAM is still the farthest north of the operational models with the track of the waves and extent of precipitation, but it is farther south than it was yesterday. The GFS places the heaviest precipitation north of the ECMWF on Saturday and is slower to move the wave toward the coast on Sunday. The WPC follows the ECMWF and places the axis of heaviest precipitation over southern-central VA and central NC on Saturday and coastal NC on Sunday, following the slow eastward progression of the wave. On Monday, the GFS and ECMWF develop a weak coastal low off of the NC coast. The models agree – although with some differences – that this low will slowly move up the Mid-Atlantic coast and interact with the approaching cold front on Tuesday-Wednesday.

The bottom line for air quality is some continued uncertainty regarding the northern extent of clouds and precipitation over the weekend, especially on Saturday. For now, it appears that areas roughly north of northern VA will remain clear, but this could change depending on the actual track of the surface waves. For central VA southward, especially NC, clouds and periods of heavy rain associated with the passage of the surface waves will keep air quality well inside the Good range over the weekend.

The northern Mid-Atlantic (NMA), where conditions will be dominated by surface high pressure over the weekend, is the area of most interest for air quality. The center of surface high pressure will be overhead or just to the north on Saturday, moving to New England on Sunday, and then continuing on to the Canadian Maritimes on Monday and Tuesday. Temperatures over the weekend will be seasonably warm, with mostly sunny skies and relatively low humidity. With high pressure nearby, surface winds will be very light and recirculating. Back trajectories on Saturday are still northwesterly from ON, but they turn very slow and localized on Sunday. Thus, rising ozone is a concern along the I-95 Corridor for both weekend days. The 06Z BAMS and NC ozone models all develop a bull’s-eye of USG ozone in the vicinity of Washington, DC on Saturday, likely in response to the stagnating winds. The 06Z NOAA ozone model is much less aggressive and keeps ozone in the mid-upper Moderate range in the DC/BAL area. Certainly, localized mid-high Moderate ozone is possible, given sufficient sunlight, a modified air mass, and calm or converging winds. The main forecast questions will be the extent of any afternoon cloud cover from the disturbances to the south and the degree of modification of the air mass.

PM2.5 concentrations remain in the Moderate range across the region this morning, despite the arrival of a relatively clean air mass behind the recent frontal passage. This is due to a smoke plume being pulled over the Mid-Atlantic and Great Lakes by the closed low over New England. The smoke plume was very striking in yesterday afternoon’s visible satellite imagery, and it is still evident this morning. None of the smoke models picked up on the surface transport (e.g., NOAA or NAAPS), although they do show the plume aloft. The influence of the smoke on PM2.5 in the NMA seems likely to continue through the weekend, keeping conditions in the Moderate range.

As the surface high retreats eastward on Monday, winds aloft and at the surface will turn onshore in the NMA, which will increase humidity and decrease temperatures slightly. Air quality along the coast should fall into the Good range, while Moderate conditions will continue at inland locations, particularly for PM2.5. The skies across most of the SMA should clear as the surface wave exits to the coast, but the with the recent periods of heavy rain, the air mass should be quite clean, allowing Good air quality to continue.

Tuesday and Wednesday will be unsettled, as the next cold front approaches from the northwest and the weak coastal low moves up the coast. Sustained onshore flow and widespread clouds and showers will keep air quality in the Good range on Tuesday. The front will move through the region on Wednesday, promoting another day of Good air quality.

-Huff