Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 8, 2016
Valid: June 9-13, 2016 (Thursday-Monday)

Summary:

Friday and Saturday are the days of interest during this period, when there will be a Marginal chance for an ozone exceedance, mainly across the central and southern portions of the region due to a mid-level ridge over the Southeastern U.S. and the waning influence of an eastern upper level trough. On Thursday, the upper level trough will keep a clean northwesterly flow over the region, for generally Good air quality. A broad upper level ridge centered over the western U.S. will push the eastern trough away from the Mid-Atlantic on Friday, allowing the southeastern mid-level ridge to build northward. A warm front will move into the central part of the region, but it appears weaker than in previous guidance, which will allow for rising ozone roughly south of the Mason-Dixon Line. A cold front will move into the region on Saturday, but locations in the central and southern Mid-Atlantic will remain sunny, with rising temperatures and light winds. As a result, the chances for an ozone exceedance are Marginal on Friday and Saturday. The cold front will push southward through the region on Sunday, reaching SC by Monday morning. Another round of cool and dry air will arrive behind the front, along with blustery northwesterly winds, which will drop the chances for an ozone exceedance back to Slight for the end of the period.

Discussion:

The weather forecast models are in excellent agreement with the synoptic pattern through the medium range. The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF. The two main synoptic features that will impact the Mid-Atlantic during the period continue to be an upper level trough over the northeastern U.S. and a mid-level ridge that will build over the Southeast U.S. beginning tomorrow. The broad upper level ridge currently in place over the western U.S. will move eastward across the central Plains on Thursday, causing the upper level trough currently over much the eastern U.S. to lift toward the Canadian Maritimes. At the same time, the mid-level ridge will be developing over the southeastern U.S., and will remain in place through Monday. Throughout Friday, the ridge over the central Plains will broaden across the majority of the country and push the northeastern trough eastward, with the result that the Mid-Atlantic will be on the eastern edge of the upper level ridge. This is different from previous guidance, which kept the edge of the upper level trough over the northern Mid-Atlantic on Friday. The GFS and EC are in much better agreement compared to yesterday’s analyses regarding the development of an upper level closed low starting on Saturday, which will be initiated by a shortwave dropping southeastward from southern ON down across NY/northern PA late Friday. This feature closes off by Sunday afternoon in both models, with its center in the vicinity of ME. The GFS places the center slightly further south than the EC, but regardless the low itself remains to the northeast of the forecast region, with only some shortwave energy circulating down into the northern Mid-Atlantic on Monday. The closed low/upper level trough will pull a cold front through the Mid-Atlantic on late Saturday into Sunday, with the front clearing the region by 12Z Monday.

Conditions at the surface will continue to de dominated by the upper level trough and mid-level ridge developing over the southeastern U.S. on Thursday. Thursday will likely have the lowest pollutant concentrations of the medium range period. Surface high pressure will develop over the Ohio River Valley on Thursday in response to the mid-level ridge, and will move into the southern Mid-Atlantic (western NC) on Friday. Today’s 06Z GFS back trajectories are even faster than yesterday’s, with mostly northwesterly flow originating from interior ON. The combination of these features, paired with the cool pool aloft, will create sunny, breezy, and still relatively cool conditions across the Mid-Atlantic throughout Thursday. Generally Good air quality is expected across the forecast region, with only a Slight chance of an ozone exceedance in the southern Mid-Atlantic, where winds will begin to subside in response to the developing mid-level ridge. The air quality models reflect this, with only pockets of isolated Moderate ozone across NC for Thursday.

On Friday, the mid-level ridge across the southeastern U.S. will build a bit northward, bringing the central and southern portions of the region under more westerly flow aloft. There will be upper level warm air advection for locations roughly south of the Mason-Dixon Line, and all of the deterministic models show a warm front entering the central Mid-Atlantic from the northwest by Friday afternoon. The guidance today appears to be slightly slower with its track of this warm front and therefore keep much of its precipitation on the western edge of the forecast region, the exception being the GFS which is slightly wetter as generally expected. This trend is likely related to the change in the extent of the upper level ridge, which now appears to be reaching over the Mid-Atlantic. The 03Z SREF and 06Z Hi-Res models support the GFS, with scattered showers training into central portions of the Mid-Atlantic Friday evening and into Saturday morning, but less clouds and precipitation than in yesterday’s guidance. Surface winds will diminish as surface high pressure moves into the southern part of the region, but it will still be relatively cool, with temperatures in the upper 70s to low 80s °F. As a result, ozone levels will likely be on the rise in the central and southern portions of the region, with some uncertainty regarding any clouds and showers associated with the arriving warm front. Therefore, the chance for an ozone exceedance rises to Marginal for those locations. The northern Mid-Atlantic will still be under the influence of the departing upper level trough, with more sustained surface winds and very fast northerly back trajectories.

Friday’s warm front will lift through the northern Mid-Atlantic on Saturday morning, allowing temperatures and humidity to rebound. A cold front will move into the region from the west on Saturday, triggering clouds and scattered showers from west to east across the northern Mid-Atlantic. The GFS shows more widespread precipitation associated with this front than the EC, with the EC showing a more compact and intense band of precipitation taking form across PA, while the 03Z SREF has a medium probability of precipitation roughly west of I-81 through 18Z Saturday. There are some questions regarding the exact track of this cold front and its associated precipitation, given that it is still four days out. However, all of the deterministic models show the cold front moving into the region on Saturday, and the Mt Holly NWS Office and SPC think that convection associated with the front’s approach could trigger severe weather. The air quality models are having trouble resolving the impacts of the front, with the BAMS models showing areas of Moderate ozone across the entire Mid-Atlantic, while the NC model keeps Moderate ozone limited to central and southern portions of the forecast region. Overall, regional conditions will be more favorable for ozone production, particularly south of roughly the MDL, and as a result the chance for an ozone exceedance will remain Marginal for Saturday.

On Sunday, the cold front will be moving through the region, reaching the MDL by approximately 12Z and then continuing southward through the remainder of the day. A chance for scattered showers will precede the front across the central and southern Mid-Atlantic. Very blustery northwesterly winds will build in behind the front, due to the strong closed low located over New England. It will be mostly sunny, with cooler and drier air arriving behind the front, which will extend into Monday. The chance for an ozone exceedance will therefore drop back to Slight for Sunday and Monday.

-Brown/Huff