Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 4, 2015
Valid: June 5-9, 2015 (Friday – Tuesday)

Summary:

A progressive weather pattern will only allow for a slight chance of USG ozone through the medium range period. A cold front will move through the region Friday evening into Saturday, keeping clouds and scattered precipitation in the forecast for much of the region. A slightly cooler and drier Canadian air mass will briefly build into the Mid-Atlantic on Sunday. The next cold front and chance for widespread precipitation will approach on Monday and move through the region on Tuesday. The leading pollutant during much of the medium range period will likely be PM2.5, which may reach the Moderate range at scattered locations on Saturday and linger in the Moderate range through Tuesday, depending on the intensity of precipitation and air mass characteristics at the end of the period.

Discussion:

The models are in close agreement during the beginning of the period, but show some differences beginning on Sunday. The 06Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. Currently, the 500 mb pattern features a shortwave over SC and a developing ridge over the central Midwestern US. A shallow trough is also beginning to emerge from the northern stream over the Hudson Bay.

The overall synoptic pattern is a very progressive one through the end of the medium range period. All of the weather models agree that the dip in the Canadian stream will intensify significantly on Friday due to a strengthening vort max that rotates through the base of the upper level trough over eastern Canada, and it will continue to move east, reaching the Canadian Maritimes on Saturday. This feature will pull a surface cold front through the Mid-Atlantic overnight Friday into Saturday. The models agree that surface high pressure will move over NE on Sunday, leading to some clearing. The influence of high pressure will be short-lived, however, as another dip in the northern flow will develop over south-central Canada on Sunday. The CMC does not generate a substantial upper level trough, keeping a much more zonal flow aloft overall, which makes it an outlier by Monday. There are timing differences between the GFS and the EC in the exact movement and phasing of the shortwaves that strengthen the developing upper level trough over the Great Lakes, but their surface features come into close agreement by the time the accompanying surface low impacts the Mid-Atlantic overnight Monday. By 00Z Tuesday, the models agree that the upper level trough will continue to move eastward, into QC, but the EC continues to have a much stronger trough than the GFS, therefore tightening the accompanying surface low. The GFS and EC both bring widespread precipitation into the Mid-Atlantic from west to east on Monday, but the EC’s stronger upper level trough translates into more intense precipitation. All of the models continue to move the Canadian upper level trough eastward on Tuesday. There is close agreement between the models that the surface front will continue to swing southward through the Mid-Atlantic during the day, bringing heavy precipitation primarily to areas south of the Mason-Dixon Line.

On Friday, the oscillating stationary front along the Carolina coast will promote clouds and precipitation along the east coast as it begins to weaken. The influence of surface high pressure centered off of the coast of NS will allow for some clearing north of the Mason-Dixon Line. A cold front approaching the region from the northwest will reach Lake Erie by 18Z Friday. This front will increase the chances for late afternoon and evening clouds and convection along the western part of the region. Light and converging winds, coupled with rising atmospheric humidity ahead of this front, will allow for increasing PM2.5 concentrations at scattered locations. The 12Z run of the 4-km NAM shows that Friday afternoon convection will be scattered instead of an organized line of storms. As a result, widespread heavy precipitation is not expected. The areas that have the best chance for Moderate PM2.5 will be in western and southern PA, in locations where precipitation is less intense. Onshore winds will keep skies mostly overcast throughout the day across the region, with a few pockets of afternoon sunshine possible along the I-95 Corridor and central Mid-Atlantic (CMA), which will limit ozone formation for yet another day.

The cold front approaching from the northwest will reach approximately to central MD at 12Z Saturday. There is not a substantial difference in air mass characteristics behind the front. As a result, the change in air mass will be gradual on Saturday, despite the relatively early arrival of the front. Stagnating surface winds overnight and recirculating back trajectories will allow particle concentrations to continue to increase. Given the slow change in air mass and relatively light winds in the afternoon behind the front, more widespread Moderate PM2.5 concentrations are likely. Cloudy skies in the morning and the chance for afternoon rain showers south of Mason-Dixon Line should once again limit any increases in ozone.

By 12Z Sunday, the cold front will reach SC. The Canadian air mass behind Saturday’s cold front will briefly build into the region, bringing slightly cooler and drier conditions. There looks to be a mix of sun and clouds throughout the day, with a chance for rain showers along western part of the region in the afternoon as the next cold front approaches. Back trajectories show transport aloft from interior ON, south of James Bay, which will help to support generally Good air quality across the region, along with light onshore surface winds. The exception could be particles, which may linger in the Moderate range at scattered locations, depending on how clean the air mass is. Furthermore, the cold front will begin to move back north as a warm front on Sunday afternoon/evening, bringing a potentially modified air mass back into the region.

By 12Z Monday, the surface warm front will have reached central PA. The differences between the EC and GFS regarding the strength of the upper level and surface lows on Monday translate into uncertainty regarding the intensity of precipitation for the end of the period. The trend is for precipitation to arrive from west to east on Monday, but as mentioned above, the EC has more intense precipitation. The leading pollutant will likely continue to be particles, as widespread cloud cover should limit rising ozone concentrations. The EC solution would clean out particles faster on Monday. On Tuesday, the cold front approaching from the northwest will reach the Mason-Dixon Line by approximately 12Z. The EC has less precipitation on Tuesday, but both models generally show clearing from north to south, following the track of the front. The arrival of a new air mass in the NMA and clouds and precipitation across the southern Mid-Atlantic (SMA) will, yet again, limit ozone formation. Particles may continue to linger in the Moderate range, depending on the strength and location of precipitation, and the speed at which the frontal boundary moves through the region.

-Huff/Eherts/DeBoe