Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 7, 2015
Valid: June 8-12, 2015 (Monday – Friday)

Summary:

There is more uncertainty today about a potential period of deteriorating air quality at the end of the week. The weather models are not in agreement on the strength of a ridge of high pressure that will extend over the eastern US beginning on Thursday. The trend today is toward a weaker ridge, which will allow a back door cold front to move into PA/NJ on Friday and slowly sag southward. A weaker ridge would also allow for a greater chance of scattered showers and thunderstorms on Friday. Thus, a marginal chance for USG ozone still exists for Wednesday, and chances still increase to appreciable for Thursday and Friday, but uncertainty has increased today. A cold front moving through the region Monday and Tuesday should limit ozone to the low Moderate range, depending on the eastward extend of clouds and convection tomorrow, with particles as the leading pollutant in most locations.

Discussion:

The forecast models are in general agreement on the main synoptic features through the medium range period, but they disagree on the strength of a ridge of high pressure that will build over the region at the end of the period. The 12Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. On Monday, a longwave trough over the western Great Lakes will amplify and gradually move eastward; its axis will be over the Mid-Atlantic region on Tuesday. This feature will pull a weak cold front through the Mid-Atlantic on Monday and Tuesday. On Tuesday, the next strong shortwave will drop down in the northern stream flow over MB/ON and steadily move eastward through Friday. This shortwave will close off over ON/QC on Thursday, but a broad mid/upper level ridge building across the eastern US will prevent the shortwave from developing a longwave trough over the Mid-Atlantic. The shortwave will bring a back door cold front into NY State on Thursday. The forecast models were in agreement yesterday that the eastern US ridge would be strong enough to keep the back door cold front north of the Mid-Atlantic through the weekend. Today, the global models all still have the ridge in place, but the EC’s is weaker than the GFS’s and the CMC’s. The EC’s weaker ridge allows several shortwaves to move up from the Gulf of Mexico into the southern Mid-Atlantic on Friday, and it also allows the back door cold front to move southward from NY into PA/NJ on Friday. This discrepancy among the models introduces considerable uncertainty about a possible period of deteriorating air quality for the second half of this week. The WPC is leaning toward the EC solution this morning, bringing the surface back door cold front to the PA/MD border by 12Z Saturday.

The amplifying longwave trough over the Great Lakes will pull yesterday’s cold front northward as a warm front on Monday, as the associated cold front moves into the Mid-Atlantic from the west in the afternoon. Temperatures and humidity across the Mid-Atlantic will rebound as southwesterly winds bring much warmer and more humid air surging back into the region. The main forecast question will be the timing and location of convection associated with the frontal boundaries. The mesoscale models are not in consensus regarding precipitation on Monday. The operational NAM and most of its ensembles, as well as the 12Z 4-km NAM, 00Z NMM, and 12Z 12-km NAM all keep clouds and convection/rain showers along and west of I-95. The 12Z 13-km GFS and 12Z ARW bring clouds and convection all the way to the Atlantic coast by late Monday afternoon. The NOAA, BAMS, and NCDENR ozone models rely on the NAM/WRF solution, and the 06Z runs all developed an area of Moderate ozone along and just to the west of I-95. Thus, scattered Moderate ozone is possible tomorrow along the I-95 Corridor, depending on the eastward extent of afternoon clouds and convection. Particle concentrations are currently well into the Moderate range at locations southwest of the region, and this air will be moving into the Mid-Atlantic tomorrow, making widespread Moderate PM2.5 likely, mainly in the western and central parts of the Mid-Atlantic.

The weak cold front will move through the eastern Mid-Atlantic on Tuesday. There is also some question about the extent of cloud cover and precipitation on Tuesday. Partly cloudy skies and scattered thunderstorms seem likely across the eastern part of the region most of the day, following the slow progression of the cold front. This should limit ozone to the Good to low Moderate range across the region, while particles may linger in the Moderate range, depending on the intensity of precipitation. The NRL NAAPS model has backed off on advecting smoke from Canadian wildfires into the northern Mid-Atlantic on Tuesday and Wednesday, but warm and humid conditions will continue, as little change in temperature and only a slight dip in dew points are expected with the frontal passage.

Wednesday looks mostly sunny and dry. Light and converging surface winds at inland locations and a quickly modifying air mass suggests scattered Moderate ozone and PM2.5 concentrations are likely, particularly along and west of I-95.

The forecast question for Thursday and Friday will focus on the strength of the ridge extending across the eastern US, which will impact the southward progression of the NY back door cold front. The trend today is to weaken the ridge and push the back door cold front farther south, into PA/NJ. This reduces the chances for rising ozone across the northern Mid-Atlantic but keeps chances alive for locations farther south, such as the Washington, DC and Baltimore metropolitan areas. With temperatures around 90 °F both Thursday and Friday and light SW surface winds, mid-to-upper Moderate ozone seems likely somewhere. If the EC solution verifies, more widespread afternoon precipitation on Friday, triggered by warm/humid conditions and shortwaves arriving aloft, will further help to limit rising ozone across the region.

-Huff