Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 11, 2017
Valid: July 12-16, 2017 (Wednesday-Sunday)

Summary:

Heat wave conditions on Wednesday and Thursday will keep an Appreciable to High risk of an ozone exceedance in the forecast, despite chances for daily afternoon/evening thunderstorms. The risk is slightly lower on Wednesday, given a downward trend in the air quality models from yesterday’s runs. In today’s 06Z BAMS and NC model runs, only upper Moderate ozone is predicted, down from USG ozone in yesterday’s runs. Bermuda High pressure centered over the SMA on Wednesday will keep the majority of the Mid-Atlantic under mostly sunny skies. A combination of a back door cold front to the north and overhead shortwaves will result in partly cloudy skies and a few isolated showers across the NMA. The I-95 Corridor in the NMA and CMA and the I-81 Corridor in the SMA are the areas of interest for Wednesday. The main forecast questions focus on chances for afternoon/evening thunderstorms, air mass modification (the air in place continued to be quite clean yesterday), and transport of ozone and precursors in the residual layer. Thursday will be the warmest day across the Mid-Atlantic, with temperatures climbing into the upper 90s °F with some locations reaching 100 °F. The air quality models highlight Thursday as the best chance for USG ozone, although afternoon thunderstorms are more likely on Thursday in the NMA, but the weather models keep the precipitation west of I-95 through 00Z Friday. The forecast questions will be the same as for Wednesday. The heat wave will end on Friday for the NMA and parts of the CMA, as a wave riding along the back door cold front pushes the front southward into the Mid-Atlantic. More widespread showers are expected across much of the region, with periods of heavy rain possible, lowering the risk of an exceedance to Marginal. Temperatures will return to normal on Saturday across the region as the weak cold front pushes through the rest of the Mid-Atlantic Friday evening and overnight. The front is expected to stall across the eastern SMA by 12Z Saturday, triggering showers and thunderstorms in its vicinity on Saturday and Sunday afternoons. With the arrival of a presumably clean air mass, ozone production should be limited, despite clearing skies across much of the region. The risk for an exceedance will remain Marginal for the weekend.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in quite close agreement with the synoptic setup throughout the medium range period, with some small discrepancies on day 4-5. By 12Z Wednesday, the westward extension of the Bermuda High will continue to build into the Southeast U.S. and SMA, with the southern extent of an upper level trough over eastern Canada. This pattern will result in zonal flow over the Mid-Atlantic at upper levels, which will persist through Thursday. A back door cold front will be caught between these synoptic two features, draped in the vicinity of the U.S./Canada border, around northern NY State. At the same time, a shortwave over MB/ON/MN will begin to dig out a weak trough by 12Z Thursday. The trough will progress eastward, briefly developing a closed circulation over northern Lake Superior by 18Z Thursday. As the trough continues to move over the Great Lakes, a line of shortwaves will train over the NMA between 12Z Thursday and 12Z Friday. By this time, the upper level through will phase with an upper level low over Hudson Bay, forming a much larger but still relatively weak trough over eastern Canada, stretching down into the Ohio River Valley by 00Z Saturday. Consequently, the upper level pattern re-amplifies starting Friday, with ridging over the west and troughing over the east. The initial shortwaves that formed the trough will rotate through the trough axis, reinforcing it as it moves over the NMA between 06Z and 12Z Saturday. At this time, the trough will remain in place, as both the GFS and EC develop shortwaves over the SMA and Southeast U.S. that keep weak troughiness over the Mid-Atlantic through 06Z Sunday. By 12Z Sunday, another round of shortwaves will develop over the Great Lakes, reinvigorating the trough over eastern Canada and the northeastern U.S. Although the GFS and EC show similar outcomes at upper levels, there less agreement at mid-levels, as the GFS is much more bullish with the development of the reinvigorated trough on Sunday. This difference between the models will result in some uncertainty for Sunday’s precipitation forecast.
The Dailies:

High pressure building over the SMA on Wednesday will keep the majority of the Mid-Atlantic under mostly sunny skies. Westerly flow in the NMA and southwesterly flow in the SMA will maintain a humid air mass and above average temperatures across the entire region. These conditions will promote isolated afternoon/evening thunderstorms across the NMA, triggered by shortwaves moving through aloft. The 03Z SREF shows precipitation is most likely in the northeast NMA between 15Z and 21Z Wednesday. The 06Z air quality models are picking up on the above average temperatures and periods of sunshine across the region as they develop widespread Moderate ozone, with a swath of mid-to-upper Moderate ozone along the entire I-95 Corridor from the NMA through the SMA. The I-95 corridor is most likely highlighted because precipitation is not expected to push that far south and east, and surface winds are expected to be rather light. The Moderate ozone in the air quality models is a step down from yesterday’s runs, when areas of USG ozone were present along I-95. We are missing the NOAA model, which did not run this morning, but the BAMS and NC models now do not have any USG ozone in the Mid-Atlantic on Wednesday. The models may be picking up on the convection, scatted cloud cover, and still relatively clean air mass. The main forecast questions will be how quickly the air mass modifies, transport of ozone and precursors in the residual layer, and chances for afternoon/evening thunderstorms. Given the trend in the air quality models, the risk of an exceedance will drop to Appreciable on Wednesday.

Thursday will be the warmest day across the Mid-Atlantic as Bermuda High pressure continues to build over the SMA. This high pressure will again result in mostly sunny skies across the SMA and CMA while the slowly approaching back door cold front and overhead shortwaves will bring some clouds to the NMA. Clear skies and continued light westerly/southwesterly flow will allow temperatures to climb into the upper 90s °F with some locations reaching 100 °F. Once again, above average temperatures, a humid air mass and mostly sunny skies will allow for afternoon/evening thunderstorms to occur across the NMA, triggered by the approaching front and a train of shortwaves moving overhead. Precipitation is more likely on Thursday compared to Wednesday, but the operational and hi-res guidance keeps it to the west of I-81 through 18Z and west of I-95 through 00Z Friday. The 06Z BAMS and NC air quality models are highlighting Thursday as the most likely day for USG ozone. The BAMS-CMAQ has a strip of USG ozone along I-95 from Washington D.C. stretching northeastward to MA, while the BAMS-MAQSIP keeps most of I-95 in the Mid-Atlantic in the upper Moderate. The NC models are also mixed, with the CMAQ-D being the most aggressive, developing a strip of USG ozone from the Chesapeake Bay to southern NJ. Given the many ozone-conducive conditions in place, including hot weather, mostly clear skies along I-95, and westerly back trajectories from IN/OH, the risk of an exceedance remains High on Thursday. Forecast questions will be similar to Wednesday, focusing on air mass characteristics and chances for afternoon clouds/convection.

The heat wave will end on Friday for the NMA and parts of the CMA, as a wave riding along the back door cold front pushes the front southward into the Mid-Atlantic. More widespread showers are expected across much of the region, with periods of heavy rain possible. Temperatures across the NMA will return to near normal due to cloud cover and a shift to northeasterly flow behind the front, while temperatures in the SMA will remain above normal under the influence of the Bermuda High. Showers and thunderstorms will move into the NMA overnight, and will continue to push eastward and southward throughout the day, making it into the SMA between 18Z Friday and 00Z Saturday. Despite widespread showers and thunderstorms the 06Z air quality models are developing scattered Moderate ozone across the region. This is likely caused by recirculating, westerly flow aloft from the Midwest and breaks in the rain/clouds. Risk of an exceedance will lower to Marginal on Friday.

Temperatures will return to normal on Saturday across the region as the weak cold front pushes through the rest of the Mid-Atlantic Friday evening and overnight. The front is expected to stall across the eastern SMA by 12Z Saturday, triggering showers and thunderstorms in its vicinity after 18Z Saturday. In the NMA, high pressure moving into the Ohio River Valley will promote mostly sunny skies, but presumably the air mass in place will be clean, limiting ozone production. The 06Z air quality models are picking up on these two features as the SMA lowers to widespread upper Good ozone. In the NMA, under mostly clear skies, average temperatures, and light surface winds, the air quality models develop mostly Good to low Moderate ozone along the I-95 Corridor. Risk of an exceedance will remain Marginal for Saturday.

The next weak cold front will approach the region from the northwest on Sunday. The GFS is faster to bring pre-frontal precipitation into the NMA on Sunday afternoon, while the EC keeps skies clear in the NMA. Despite this uncertainty, mostly sunny skies are expected across the NMA and CMA through the afternoon hours. In the SMA, shortwaves will develop overhead and interact with Saturday’s stalled cold front bringing widespread unsettled conditions. Clear skies and average temperatures will promote some ozone formation along the I-95 Corridor, presumably limited by lower Sunday emissions and a relatively clean air mass still in place. Due to the uncertainty surrounding the precipitation forecast for Sunday, risk for an exceedance will remain Marginal.

-Enlow/Huff