Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 22, 2015
Valid: May 23-27, 2015 (Saturday-Wednesday)

 

Summary:

There is an appreciable chance for Code Orange/Red air quality in the Mid-Atlantic region beginning on Monday, but uncertainty still remains. A brief surge of cool, dry, and clean Canadian air on Saturday will keep air quality in the Good to low Moderate range across the region. Sunday is a transition to a potential period of deteriorating air quality. An upper level ridge will build over the region, with steadily increasing temperatures and humidity through the end of the period. It now appears that any clouds and rain associated with a weak frontal boundary across the northern Mid-Atlantic on Monday will stay to the north of the region, increasing the chances for rising ozone. However, the upper level ridge will be located east of the preferred position, which could introduce maritime air transport, particularly on Tuesday and Wednesday. This transport pattern may suppress ozone formation along the I-95 Corridor late in the period, despite temperatures in the 90s °F and mostly sunny afternoon skies.

Discussion:

The weather forecast models are in agreement on the overall synoptic features for most of the medium range period, but they diverge slightly on Tuesday and Wednesday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. Saturday will be the transition day to a new upper level pattern. A closed low rotating through the semi-permanent upper level trough over eastern Canada will reach northern New England by 12Z and the Canadian Maritimes by 00Z Sunday. As this energy moves eastward, an upper level ridge will build over the Plains, which will extend eastward over the Mid-Atlantic on Sunday. The ridge will link up with the Bermuda High in the Atlantic Ocean and remain in place over the Mid-Atlantic through the end of the medium range period, although the ridge will continue to gradually shift eastward. A series of shortwaves are still expected to drop down from northern Canada and rotate around the eastern Canadian trough axis through the end of the period, but the trend to keep them farther north continues today, which has implications for Monday’s precipitation forecast. One of the Canadian shortwaves is still expected to push another, weaker back door cold front into the northern Mid-Atlantic (NMA) late Sunday into Monday, but this weak front looks to stall slightly farther north in the NMA compared to yesterday’s model runs. As a result, the 09Z SREF, 06Z 12 km NAM and the other operational models all keep most clouds and all of the rain north of the Mid-Atlantic on Monday. Aloft, the upper level ridge will shift slightly north and east on Monday compared to Sunday’s position, with the ridge axis settling over the Mid-Atlantic. By Tuesday and Wednesday, the global models shift the ridge farther east, placing the Mid-Atlantic on the western periphery of the ridge. This will locate the ridge east of the preferred position for high ozone across the Mid-Atlantic. There are slight differences between the global models in regard to the ridge placement; the GFS is the fastest to move the ridge eastward and the EC is the slowest, but they all put the Mid-Atlantic on the far western periphery of the ridge by Tuesday, which will impact the upper level transport pattern, as discussed below.

On Saturday, a brief surge of cool, dry, and clean Canadian air will arrive in the wake of Friday’s back door cold front. The front will reach to roughly southern NC by 12Z Saturday. Surface high pressure will move across the Mid-Atlantic during the day and shift offshore in the evening. Full sun, stagnating winds, and Memorial Day weekend emissions will allow ozone to rebound, but with a clean and cool air mass in place, generally Good to low Moderate conditions are expected. Particles should also remain generally in the Good range in the clean, very dry, and unmodified air mass.

The warming trend will begin on Sunday as the ridge builds overhead and return flow around the surface high in the Atlantic brings warmer and more humid air into the Mid-Atlantic. Mostly sunny skies, light winds, and recirculating transport aloft will push ozone into the Moderate range at locations along the I-95 Corridor, with particles lagging in the still relatively dry air mass.

Memorial Day will be warm and humid. The weak back door cold front will stall in the vicinity of central PA/NJ on Monday morning and then move back north as a warm front during the day. The SREF and operational models have been trending toward keeping rain and clouds associated with this front to the north, which suggests that there will be sufficient sun on Monday for ozone to rise to the upper Moderate range across most of the region. An Appalachian Lee trough is expected, which will act as a line of convergence across MD/VA/NC. However, since the upper level ridge will be situated east of the preferred position, low level back trajectories (@500 m AGL) are southeasterly, indicating the possibility of some maritime air transport. Farther aloft (@1000 m and 1500 m AGL), back trajectories on Monday are westerly from the Ohio River Valley, a source region for ozone precursors. The forecast question for Monday will focus on the possible impact of onshore transport at low levels compared to rising temperatures and the presence of an Appalachian lee trough.

Tuesday and Wednesday will be hot and humid, with mostly sunny skies but the chance for isolated afternoon convection driven by diurnal heating. By this point, the Mid-Atlantic will be situated along the far western edge of the upper level ridge, and as a result, back trajectories all shift to the Southeastern US, with a continued maritime component, particularly at low levels. We’ve seen this pattern in the past suppress ozone formation along the I-95 Corridor, even with temperatures in the 90s °F and clear skies. Of note is that all of the operational air quality forecast models (BAMS-MAQSIP and -CMAC, NCDENR) keep ozone fairly low through the medium range period, despite the hot and humid weather, with some scattered Moderate conditions on Monday but Good ozone and particulate matter across the board on Tuesday. My guess is that the models are responding to the transport pattern, but CMAQ-based models in particular have a bias toward under-forecasting early in the season, which makes them unreliable.

-Huff