Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 29, 2015
Valid: July 30 – August 3, 2015 (Thursday – Monday)

Summary:

Clouds and precipitation associated with an approaching cold front on Thursday will end the threat for another day of USG ozone in the Mid-Atlantic. Continued model consensus on the timing of the cold front is providing above average confidence that pre-frontal clouds and convection will arrive at the I-95 Corridor by Thursday afternoon, limiting ozone to the Good to Moderate range. As a result, there is a chance that the front will slow as it moves into the very warm air mass over the region, which keeps a Marginal risk for another day of USG ozone. A new air mass will arrive on Friday, which will be noticeably drier but not much cooler. Clean back trajectories from the upper Midwest starting Friday-Sunday will keep the chances for USG to Slight despite clear skies, light surface winds, and temperatures in the low 90s ° F. Similar weather conditions will persist on Monday, but a shift to southerly transport will limit ozone to the Good/Moderate threshold.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period, but they begin to diverge on Sunday. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a ridge, with its axis over the Mid-Atlantic, and a closed low over the MB/ON border are the dominating upper level features. These features will move eastward in the northern flow on Thursday. The closed low will pull a cold front into the Mid-Atlantic from the northwest. This front will progress southeastward through the region on Thursday, reaching I-95 by 00Z Friday and the east coast of NC by 12Z Friday. Beginning on Friday, an upper level trough will develop over the northeastern US and an upper level ridge will develop over the western US; these features will persist through the end of the period. The axis of the eastern trough will be located well to the north, however, over Hudson Bay. This will leave the Mid-Atlantic on the very southern edge of the trough. As a result, the cooler air associated with the trough will remain well to the north, while the air mass over the Mid-Atlantic will be unseasonably warm. By Sunday, the EC concentrates several strong shortwaves over FL and tries to develop a weak tropical system. This leads to differences in the precipitation forecast for NC on Sunday and Monday; the EC brings tropical moisture into the southern Mid-Atlantic (SMA) while the GFS keeps the entire Mid-Atlantic dry.

A cold front will move through the region on Thursday from the northwest to the southeast. The clouds and precipitation associated with the front along with low level onshore back trajectories will end the threat of USG ozone in the Mid-Atlantic. The mesoscale and global models agree on showing a broken line of convection ahead of the boundary as it moves through the region. Clouds associated with this convection will be the main factor in limiting ozone production in the Mid-Atlantic. The coverage of the storms appears to decrease across the SMA, however, leaving eastern NC mostly clear through the entire afternoon. We would expect that anything less than a very strong front (which this one is not) would slow down as it moves into the very hot air mass over the Mid-Atlantic. However, the global and mesoscale models continue to be in consensus that the front will steadily progress through the region tomorrow, reaching the I-95 Corridor by late afternoon. As a result, there is less uncertainty than normal that the pre-frontal convection and clouds will reach I-95 by the afternoon. There may be some scattered Moderate ozone along and east of the I-95 Corridor, where there will pockets of clearing in the afternoon. Southeasterly onshore trajectories will assist in limiting ozone formation, however, even in areas that experience full afternoon sun. The BAMS air quality models are showing a strip of Moderate ozone along the I-95 Corridor in VA, MD, and NJ, while the NOAA model has some isolated pockets of Moderate ozone. In contrast, the NCDENR model keeps ozone Good across the region.

Currently, PM2.5 concentrations are hovering in the upper teens to low 20’s ug/m3 in NC, MD, DE, PA, and NJ. This is in response to humid conditions as well as remnant light density smoke from wildfires in Canada and Siberia. Moderate PM2.5 conditions will likely persist on Thursday due to the continuing high humidity, and the should increase higher into the Moderate range for locations along I-95 as dew points pool ahead of the cold front.

Behind the frontal passage on Friday, noticeably drier air will filter into the region. The exception will be along the NC coast, where the cold front will stall through Monday and keep clouds and scattered showers in the forecast. Despite the arrival of a new air mass, temperatures will continue to be above average across the region, with highs in the low 90s °F through the end of the period. On Friday, skies will be clear and winds will die down behind the front, but the new, cleaner air mass arriving on northwesterly back trajectories should limit ozone and PM2.5 concentrations to the Good range for most locations. There may be a few locations in NC along the stalled front that may see low Moderate PM2.5 concentrations. The air quality models are showing low Moderate ozone east of I-95. This may be possible, depending on how clean the new air mass is and how strong northwesterly flow is aloft and at the surface.

On Saturday and Sunday, ozone may rebound into the Moderate range at isolated locations, especially as temperatures remain into the low 90s °F under clear skies. Clean back trajectories from MN and lower weekend emissions should keep ozone generally in the Good range. PM2.5 concentrations should stay in the Good range as well in response to the lower humidity. A few locations along the stalled front may see PM2.5 rise just into the Moderate range due to the light and converging winds.

A southerly transport pattern on Monday should hold ozone and PM2.5 concentrations to the Good/Moderate threshold despite another day of clear skies and temperatures in the low 90s ° F. Some inland locations may reach the Moderate range for particles due to rising humidity. The precipitation forecast is still uncertain at this time with the EC showing widespread precipitation in the SMA associated with a weak tropical system over FL/GA, while the GFS keeps skies clear.

-DeBoe/Huff