Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 1, 2014
Valid: June 2-6, 2014 (Monday-Friday)

Summary:

Temperatures and humidity will increase on Monday as the axis of a weak upper level ridge briefly builds over the Mid-Atlantic region. Ozone will reach the Moderate range, with higher mixing ratios in the eastern part of the region. PM2.5 will increase into the low Moderate range at scattered locations in response to rising humidity. Widespread clouds and convection on Tuesday afternoon will limit rising ozone to the low Moderate range, except for the locations along and east of I-95, where afternoon sun will allow ozone to reach the upper Moderate range. Uncertainty remains in the air quality forecast for the remainder of the period due to questions about clouds and precipitation associated with a slowly moving and stalling frontal system that will impact the region Wednesday to Friday, but Good to Moderate conditions are expected.

Discussion:

The medium range period features a complex and unsettled weather pattern for the Mid-Atlantic region. The weather forecast models are in general agreement on the main features of the period, but differ in many of the details. An upper level ridge over the region on Monday will give way to a shortwave trough that will move across southern Canada, from the northwestern Great Lakes on Tuesday to the Canadian Maritimes on Friday (06Z GFS) or Saturday (00Z ECMWF). As the shortwave trough flattens the ridge over the Mid-Atlantic, a series of large and small shortwaves will zip across the region in the zonal flow aloft. These shortwaves will act as triggers for rain and convection across the Mid-Atlantic essentially daily from Tuesday through Friday. Since the operational models differ in the timing and location of many of these shortwaves, a great deal of uncertainty exists for the precipitation forecast. The WPC is keeping the chance for precipitation across the region during the medium range period in response to this uncertainty, with the chance for slightly higher amounts of precipitation in the northern Mid-Atlantic on Wednesday through Friday. The chance for higher precipitation in the northern part of the region is in response to a frontal system that will enter the region from the northwest on Wednesday, pulled by the weakening shortwave trough aloft over southern ON/QC. A large area of shortwave energy aloft will develop a low along this frontal system, over PA, on Thursday. This low will lift to the northeast on Friday, pulling the cold frontal boundary to approximately central VA, where it will stall though at least Saturday. This slowly moving/stalling frontal system increases the uncertainty in the air qualify forecast for Wednesday through Friday.

With the upper level ridge overhead on Monday, and the center of surface high pressure settling over NC, temperature and humidity will begin to increase. Warm air advection aloft and recirculating back trajectories will favor rising pollutant concentrations. With mostly sunny skies along the eastern Mid-Atlantic, ozone will rise into the mid-Moderate range. The weather forecast models have backed off on yesterday’s predictions of widespread clouds and afternoon thunderstorms in the western Mid-Atlantic. The 12Z run of the 4 km NAM has scattered rain showers in the morning followed by isolated thunderstorms in the afternoon. The 09Z SREF keeps the highest probability of precipitation in northwestern PA on Monday afternoon. As a result, ozone could reach the low-Moderate range in parts of the western Mid-Atlantic. PM2.5 will be on the rise into the Moderate range at scattered locations as well, in response to increasing humidity.

The uncertainty in the forecast begins on Tuesday. It will be warm and humid, with southwesterly flow both aloft and at the surface. Tuesday still appears to be the day with the best chances for widespread convection, with the entire region in the warm sector of the approaching weak low pressure system. The 4 km NAM develops a line of thunderstorms and moves it across the Mid-Atlantic from west to east in the afternoon, although the storms dissipate as they reach roughly I-95. The SREF has a high probability of precipitation across the region in the afternoon except along the east coast, roughly along and east of I-95. Thus rising ozone mixing ratios will likely be limited to the upper Good to low Moderate range for most of the region except the eastern coast, where clear skies through the late afternoon will give ozone the chance to increase into the upper Moderate range. The BAMS and NC air quality models support this idea, and have scattered Moderate conditions along the northeastern Mid-Atlantic. In the increasingly humid and modified air mass, widespread Moderate PM2.5 is likely.

The air quality forecast for Wednesday to Friday is highly uncertain, but Good to Moderate conditions are expected. Warm and humid weather will continue. With low pressure developing at the intersection of the warm and cold frontal boundaries over central PA on Wednesday, and then the slow progression of the cold front to roughly central VA on Thursday, where it will stall, widespread clouds and rain are likely in the vicinity of the front(s). With a series of shortwaves moving over the region, development of an MCS is not out of the question. Areas that experience clouds and heavy rain will observe Good air quality conditions. There will likely be a dividing line of cleaner air to the north of the frontal boundary and modified air to the south. But it is not clear to what extent the weak front will displace the warm, humid, and increasingly modified air mass that will be in place over the region. The air quality models keep the chances for Moderate ozone in the northeastern Mid-Atlantic on Wednesday and scattered-to-isolated Moderate PM2.5 in the northern and parts of the region through Thursday.

-Huff