Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Tuesday, July 15, 2014
Valid: July 16- 20, 2014 (Wednesday-Sunday)
Summary:
A cold front will linger along the East Coast on Wednesday, while surface high pressure builds into most of the Mid-Atlantic behind the front. Air quality will be in the Good range for most locations, with the chance for Moderate ozone at isolated locations that see afternoon sun and light winds. The chance for isolated to scattered Moderate ozone will continue on Thursday as surface high pressure centered over the Ohio River Valley promotes sunny skies and light winds. PM2.5 will remain in the Good range due to low humidity. Friday to Sunday, the center of surface high pressure will settle over NYC/western New England. This pattern will raise the chances for upper Moderate to USG ozone along I-95, but several factors (e.g., clouds, cooler air mass) may act to limit rising ozone concentrations. As a result, there is uncertainty in the forecast after Thursday. PM2.5 is expected to gradually rise into the Moderate range beginning Friday in response to increasing atmospheric moisture.
Discussion:
The weather forecast models are in close agreement for the medium range period until Sunday. An upper level longwave trough will pass through the Mid-Atlantic on Thursday and pull away north and east on Friday afternoon. A series of shortwaves, developing over the Southern Plains overnight Thursday into Friday, will move eastward into the Mid-Atlantic on Friday and Saturday. The GFS and the ECMWF differ on the location on the shortwaves yet they both agree on the progression of a weak upper level trough created from these shortwaves through Saturday. The models become out of phase with each other, in regard to the trough and the placement of the shortwaves, starting Sunday morning. The ECMWF keeps a series of shortwaves passing through the Mid-Atlantic on Sunday, while the GFS doesn’t. If the ECMWF were to verify, there will be cloudy skies and scattered rain showers throughout the day for Sunday, which would result in a cleaner air quality forecast.
A cold front will travel through the Mid-Atlantic and arrive at the coast by Wednesday morning. It will stall on the coast for the remainder of the day, creating clouds and convection for locations closest to the shoreline, especially in the southern Mid-Atlantic (SMA). High pressure, centered over the central Plains, will build in to inland areas of the Mid-Atlantic, creating partly sunny skies and light northerly surface winds. The arrival of cooler air aloft will promote vertical mixing that should keep ozone in the Good range for most locations. However, the NOAA ozone model is predicting scattered Moderate ozone along the I-95 Corridor. Increased sunshine throughout the day and light winds will allow a chance for Moderate ozone on Wednesday. Areas near the coast, especially the SMA, will have Good ozone due to cloud cover and a chance for rain showers in the first half of the day. Low humidity will limit PM2.5 to the Good range.
By Thursday, the cold front will move off the coast and high pressure will dominate over the region. The day will be mostly sunny with light winds, allowing ozone to reach the Moderate range for scattered locations. PM2.5 will remain in the Good range as northwesterly flow will continue to bring in dry air.
The center of surface high pressure will station itself over NYC on Friday, resulting in very light, easterly surface flow. This pattern has resulted in isolated USG ozone along the I-95 Corridor several times so far this summer. Slightly cooler temperatures on Friday, compared to the past USG ozone cases this summer, may keep ozone from rising beyond the Moderate range. In addition, the 12Z version of the 12 km NAM suggests partly to mostly cloudy skies in the afternoon, which may also act to limit rising ozone concentrations. We will monitor this situation in the coming days to see how conditions evolve. Moisture levels will begin to rise on Friday allowing the chance for PM2.5 to reach the Good/Moderate threshold.
The models are having trouble with the fate of the remnants of today’s cold front, which will stall along the Gulf Coast states and Southeast coast through at least Saturday. After that, it is not clear if the front will continue to linger in the Southeast or begin to move north as a warm front. The most recent WPC analysis keeps the front along the Southeast (e.g., NC/SC coast) through Sunday. As noted above, there is uncertainty in Sunday’s precipitation forecast due to discrepancies between the weather forecast models, with the GFS keeps things dry while the ECMWF develops widespread rain and clouds on Sunday. All of these factors lead to uncertainty in the air quality forecast for Saturday and Sunday. On Saturday, lingering surface high pressure over NYC will continue calm and mostly to partly sunny weather for the northern Mid-Atlantic (NMA). As a result, the chances for upper Moderate to USG ozone will continue, but remain very uncertain. Cloud cover and rain showers will primarily impact the far western side of the Mid-Atlantic on Saturday as shortwaves aloft pass through the region, which will keep air quality in the Good range in those areas. PM2.5 will rise into the Moderate for most locations due to increased moisture and light winds. For Sunday, air quality will depend on the precipitation forecast and location of the possible warm front.
-Catena/Huff