Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 19, 2014
Valid: July 20- 24, 2014 (Sunday-Thursday)

Summary:

Light onshore flow and a mix of sun and clouds will keep air quality in the Good to low Moderate range for Sunday and Monday. Tuesday-Thursday will be very warm and humid as a mid-level ridge builds over the region. The possibility for scattered upper Moderate to USG ozone remains along I-95 primarily for Wednesday, but chances are lower today given the more rapid development of an upper level trough that will pull the next cold front into the region on Thursday.

Discussion:

The air quality picture for the medium range period is much less clear today, given some discrepancies in the weather forecast models and a move away from the definitive arrival of upper level ridging late in the period, as seen in yesterday’s analysis. The 00Z NAM, GFS, and ECMWF were used for today’s analysis. The period begins on Sunday with weak troughing aloft over the eastern U.S. as the center of surface high pressure slides northeast to the Canadian Maritime coastal waters. On Monday, the NAM and GFS lift the weak upper level trough north and east and transition to weak ridging over the eastern U.S. by 12Z Tuesday. In contrast, the ECMWF keeps the trough over the Mid-Atlantic through about 18Z Tuesday, anchored by small shortwaves that rotate through the base of the trough. These discrepancies are reflected at the mid-levels, as the GFS is faster than the ECMWF to establish a mid-level ridge over the Mid-Atlantic on Tuesday. The models come back into relatively close agreement on Wednesday regarding an upper-level trough that will develop over the Great Lakes and then amplify and slide eastward into the Mid-Atlantic on Thursday, pulling a cold front through the northern part of the region.

On Sunday, air quality will remain in the Good range along the eastern Mid-Atlantic due to light onshore flow and along the western Mid-Atlantic due to persistent clouds and rain showers. The NOAA and BAMS air quality models develop scattered Moderate ozone along/north/west of I-95, which seems possible if the onshore surface winds remain light and sufficient afternoon sun can break through the clouds. The air quality models also predict Moderate PM2.5 at inland locations, roughly between I-79 and I-95, likely in response to the stagnant conditions away from the coast. Smoke from the numerous wildfires in western and central Canada remains aloft over the northern Mid-Atlantic this morning, and it does not appear to be affecting surface air quality. The NAAPS model keeps the smoke well to the north of the Mid-Atlantic through the medium range period, but we will continue to monitor the smoke plumes in case this situation changes.

Temperatures and dew points begin an upward trend on Monday as surface high pressure moves back overhead and mid-level ridging tries to establish itself over the region. The persistent stationary front along the Southeast and NC coastline will act as a trigger for clouds and scattered rain, which will keep air quality in the Good range across the southern Mid-Atlantic through Wednesday. A mix of sun and clouds in the northern Mid-Atlantic, with continued light onshore flow, will keep air quality along the Good/Moderate threshold.

Tuesday and Wednesday are the days of most interest in the medium range period, but given the evolution of the mid/upper level forecast conditions, the chances for USG ozone are less than they appeared to be yesterday. To that end, the BAMS air quality models have backed off from yesterday’s predictions of upper Moderate/USG ozone along the I-95 Corridor for Tuesday. This morning’s runs keep ozone mostly in the Good range on Tuesday and bring concentration up only to the mid-Moderate range on Wednesday. This change is likely due to the weather forecast models coming into agreement on the developing upper level trough over the Great Lakes on Wednesday that will flatten the mid-level ridge over the Mid-Atlantic.

Surface winds and flow aloft will shift to the south/southwest on Tuesday, with some recirculation at lower levels (e.g., 500 m AGL). There is uncertainty in the precipitation forecast given that the ECMWF keeps the weak trough over the region through mid-day, which would result in more rain and clouds and a cleaner forecast. At this point, there is a chance for scattered Moderate ozone along the I-95 Corridor on Tuesday, given mostly sunny skies, light winds, and high temperatures rising into the upper-80s °F.

Wednesday has the greatest chance for widespread Moderate ozone across the northern Mid-Atlantic, with a chance remaining for isolated USG ozone along I-95. It will be the hottest and most humid day of the period, with high temperatures reaching the mid-90s °F as the mid-level ridge extends over the region. The weather forecast models develop some isolated instability showers and thunderstorms in the afternoon, given the warm and humid conditions and the approach of the next cold front from the northwest, which could limit rising ozone levels. PM2.5 will reach the Moderate range across the northern Mid-Atlantic in the humid and increasingly modified air mass.

The cold front will move across the northern Mid-Atlantic on Thursday, triggering widespread clouds, showers, and thunderstorms that will return air quality to the Good range for most locations.

-Huff