Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 7, 2014
Valid: August 8-12, 2014 (Friday-Tuesday)

Summary:

There is major uncertainty in the air quality forecast beyond Friday given discrepancies in numerical model guidance regarding the development of waves of low pressure that will bring widespread clouds and periods of heavy precipitation to the WV/VA/NC/MD area Saturday-Monday. Depending on the modification of the air mass and the track of the surface lows, Moderate air quality is most likely on Friday and Saturday, mainly in the northern part of the region. Otherwise, the surface lows and a transition to onshore flow in the north on Sunday will promote mostly Good air quality.

Discussion:

The weather forecast models are in consensus on the overall synoptic pattern during the medium range period, but they do not agree on the mesoscale features. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The period will begin with an upper level closed low over New England that will retreat to the Canadian Maritimes while the western ridge builds eastward over the Mid-Atlantic. The axis of the upper level ridge will be over the Mid-Atlantic/East Coast by Monday, at which point a longwave trough will redevelop over the Great Lakes and begin pushing eastward on Tuesday. At the surface, high pressure centered over southern ON/QC and the northern Mid-Atlantic (NMA) on Friday and Saturday will slide to New England and the Gulf of Maine on Sunday and Monday. The frontal boundary making its way south of the region today will stall across the Southeast/Mississippi River Valley (MRV) through the end of the period.

The weather forecast models agree that a series of shortwaves moving from west to east in the upper level flow will trigger waves of low pressure somewhere along the stalled frontal boundary draped across the MRV/Southeast on Friday-Sunday, but there is no consensus yet as to exactly where the waves will track and where the areas of heaviest precipitation will be over the weekend. The operational NAM and its mesoscale models have consistently been the farthest north with the shortwaves and surface waves. For example, the NAM brings the surface wave across the central Mid-Atlantic on Saturday, through WV/VA/MD, to Washington DC by 00Z Sunday and the Delmarva by 12Z Sunday. The WPC considers the NAM’s solution to be too far north and thus an outlier, but it has had quite good run to run consistency so it needs to be monitored for the potential impact on air quality in the NMA. The WPC prefers the ECMWF, which is the farthest south with the wave, bringing it over southern VA/NC on Saturday and taking it to the NC/SC coast on Sunday morning. The GFS solution is located somewhat between the NAM and ECMWF, but the axis of heaviest precipitation is farther west and not as strong as the NAM or ECMWF, so the WPC considers it an outlier as well.

Cloud cover and heavy precipitation along the actual track of the surface waves will keep air quality well in the Good range for Saturday-Monday. Given the uncertainty with the model guidance, however, the demarcation line between areas that will experience clouds and precipitation is not clear. For now, we will follow the WPC and rely primarily on the ECMWF.

On Friday, the center of surface high pressure will be overhead, promoting mostly sunny skies and light surface winds. A relatively clean air mass will be in place, with transport aloft from interior ON. Light surface winds may allow isolated pockets of Moderate ozone along I-95 and western NC, depending on how clean the air mass is. The cleaner and drier Canadian air mass has been slow to filter into the region behind the cold front, so PM2.5 concentrations are still in the Moderate range this morning in many areas, especially western PA and VA/NC. There is dry air aloft, and presumably the transition of the air mass will continue to progress, and PM2.5 concentrations will continue to fall today and remain generally in the Good range tomorrow.

Per the ECMWF, clouds and rain will be restricted to the southern Mid-Atlantic (SMA) on Saturday, primarily western and central VA/NC, where air quality will be in the Good range. Areas roughly from northern VA northward will be mostly sunny, with very light and recirculating surface winds. The conditions, in conjunction with slow and localized back trajectories, will push ozone into the Moderate range along I-95, with Moderate PM2.5 possible as well, especially in the Susquehanna Valley. The main forecast questions will be the degree of modification of the air mass and the northern extent of clouds associated with the surface wave. If the farther north NAM solution for the surface wave verifies, there will be little chance of Moderate ozone in the NMA on Saturday.

Flow turns onshore – both aloft and at the surface – in the NMA on Sunday and continues through the end of the period. As a result, temperatures will drop slightly but atmospheric humidity will increase. Isolated areas of Moderate air quality are possible inland on Sunday and Monday, depending on the strength of the onshore winds. Good air quality will be the rule in the areas of the SMA that are impacted by the surface waves.

On Tuesday, a weak coastal low moving up from NC may keep precipitation across the eastern part of the region. The GFS favors the development of the low, while the ECMWF pushes the area of low pressure out into the Atlantic. Continued onshore flow in the NMA should restrict air quality to the Good to low Moderate range.

-Huff