Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 23, 2014
Valid: May 24-28, 2014 (Saturday-Wednesday)

Summary:

Generally Good air quality is expected on Saturday a cool and dry air mass fully builds into the region. A rising trend in temperatures and ozone will begin on Sunday, as an upper level ridge builds over the Mid-Atlantic region. PM2.5 is slower to rise as humidity remains relatively low until late Monday. Widespread Moderate and isolated USG ozone is possible on Monday. There is uncertainty in the outlook for Tuesday and Wednesday as the weather models are gathering consensus for a cleaner air quality solution. The chance for isolated USG ozone still remains for Tuesday, with widespread Moderate PM2.5 likely. Air quality will improve from north to south on Wednesday in the wake of a back door cold front.

Discussion:

The weather forecast models are in general consensus about the main features of the medium range period, but some differences exist in the details. On Saturday, the Mid-Atlantic will be between a broad upper level ridge centered over the Plains and a long wave trough slowly departing to the east. The axis of the upper level ridge will slide over the Great Lakes region on Sunday as a shortwave/vort max spins down from interior ON/QC toward southwestern QC. The upper level ridge will extend over the eastern U.S. on Monday, while a second shortwave moves eastward along southern MB and ON. The ECMWF phases the two shortwaves on Tuesday over southern QC and digs a longwave trough over New England. The GFS does not phase the shortwaves but still develops a longwave trough in roughly the same place as the ECMWF, although it is not as amplified. By Wednesday, both models have a longwave trough over the Mid-Atlantic, with the axis to the east over southeastern Canada. At the surface, a back door cold front will move south from roughly the Canada/NY border to the Central Mid-Atlantic on Wednesday.

Saturday still looks like a great start to the holiday weekend, with a mix of sun and clouds, northwesterly winds at the surface and aloft, and seasonably cool temperatures. A clean and dry Canadian air mass will fully build into the Mid-Atlantic region. Isolated showers and thunderstorms may develop in the afternoon and evening, mainly in the northern Mid-Atlantic, as small shortwaves move overhead. Generally good air quality is expected across the region. The high-res version of the NAM are picking up on a sea breeze along the NJ and DE coasts in the afternoon, which may increase ozone higher into the Good range than might otherwise be expected along the convergence zone inland, near the I-95 Corridor.

A warming trend will begin on Sunday as the upper level ridge builds eastward. There will be a period of stagnation overnight and into the morning as the center of surface high pressure moves over the region and settles of off the southeastern U.S. coast. Although temperatures will be increasing, humidity will still be relatively low. As a result, PM2.5 will likely remain in the Good range for most locations, with isolated Moderate conditions possible. Plenty of sunshine and light winds will push ozone into the Moderate range, particularly along the I-95 Corridor.

Monday will be warmer but humidity will remain relatively low until dew points begin rising later in the day. The westward extension of the Bermuda High will be in place, and the upper level ridge will extend over the Mid-Atlantic. Winds at the surface will turn westerly, but flow aloft will remain northerly through the first part of the day. The air quality models limit increases in ozone to the Moderate range along the I-95 Corridor, but more widespread Moderate conditions seem likely, with the chance for isolated USG. The BAMS and NC air quality models both increase PM2.5 into the Moderate range, but BAMS is much more aggressive, with Moderate PM2.5 across the entire region, while NC keeps Moderate PM2.5 concentrated along the I-95 Corridor. A more gradual increase in PM2.5 concentrations across the region seems probable, given the slow rise in dew points.

There is uncertainty in the outlook for Tuesday and Wednesday. The weather models seem to be coming into consensus regarding a cleaner air quality solution, with an upper level trough reforming over the Mid-Atlantic on Tuesday and a back door cold front moving through at the surface on Wednesday. Tuesday will have many of the key ingredients for poor air quality conditions, including the Bermuda High, an Appalachian lee trough, westerly back trajectories from the Ohio River Valley, and light southwesterly surface winds. The weather models are predicting widespread clouds and rain showers, however, which would limit rising ozone concentrations. Thus, Moderate ozone seems likely for Tuesday, with the chance for either USG or Good conditions in areas that see less or more clouds/rain, respectively. PM2.5 will likely be in the Moderate range across the region in the humid air mass. Wednesday’s forecast will hinge on the progression of the back door cold front. If it reaches the central Mid-Atlantic, as currently predicted, areas to the north will see improving air quality conditions, while Moderate PM2.5 and possibly Moderate ozone will linger in the southern part of the region.

-Huff