Daily Archives: July 7, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday July 7, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 7, 2017
Valid: July 8-12, 2017 (Saturday-Wednesday)

Summary:

A weak cold front will move into the Mid-Atlantic early Saturday morning, pushing to the I-81 Corridor in the NMA by 12Z Saturday. Clearing in the clouds by mid-day will allow temperatures in locations ahead of the front to reach near normal in the eastern NMA and above average in locations south of the MDL. The 06Z air quality models seem to think the precipitation in the SMA will either occur along the front or much slower than some of the more recent weather models, as all of the air quality models develop a strip of low Moderate ozone along the I-81 Corridor in the CMA and SMA, resulting in a Marginal risk of an exceedance. With the exception of a few clouds in PA and a few showers along the Atlantic Coast in the SMA, Sunday will be a mostly clear day across the Mid-Atlantic as high pressure begins to build overhead. Despite clear skies and light surface winds, below average temperatures in the NMA and northerly flow aloft in the SMA should keep ozone formation minimal. Risk of an exceedance will lower to Slight on Sunday. Temperatures will rise back near normal across the Mid-Atlantic on Monday as high pressure remains over the SMA. A few isolated showers are possible across the NMA but mostly sunny skies, southwesterly surface winds and average temperatures across the rest of the region will be favorable for ozone formation. As a result of these factors, risk for an exceedance will rise to Marginal on Monday, with a focus on the I-95 Corridor. Uncertainty enters the forecast for Tuesday as the southward push of a back door cold front is in question. The weather models have been flip-flopping the past two days regarding this feature and its associated upper level trough, and the trend continued again today. WPC is favoring today’s EC model solution, which brings the back door cold front to roughly I-76 by 12Z Wednesday. The approaching front may trigger cloud cover with precipitation on Tuesday afternoon/evening across north/central PA/NJ. Mostly clear skies, above average temperatures and southwesterly flow elsewhere will continue the trend of gradually increasing ozone. Risk for an exceedance will rise to Appreciable with a focus on the I-95 Corridor due to the uncertainty of the southeastward progression of the front. Uncertainty continues on Wednesday, given questions about the track of the front. At this time, the areas at greatest risk for rising ozone will be along the Baltimore-Washington, DC- northern VA corridor in the CMA. Bermuda High pressure over the Southeast U.S. will promote clear skies and southwesterly winds, allowing temperatures to near the century mark in some locations. These conditions will be favorable for ozone formation, and will keep the risk of an exceedance Appreciable on Wednesday.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. The weather models have come into a closer agreement over the medium range period, especially in regards to the upper level closed low/trough over eastern Canada/Eastern U.S. in the Mon-Wed time period. By 12Z Saturday, the axis of the eastern Canadian/eastern U.S. trough will move over the Mid-Atlantic along with a series of shortwaves spinning through the axis. These shortwaves will exit the region by 12Z Sunday allowing the westward extension of the Bermuda high to build into the SMA at mid-levels. A few weak shortwaves will develop and pass over the NMA through Sunday and part of Monday. By 12Z Monday, the GFS and EC begin to diverge with the placement of the center of circulation of the closed low. As was the case in yesterday’s model runs, the GFS and EC have switched solutions again today. The EC, along with the NAM, is now pushing the center of circulation southward over James Bay and the GFS keeps it farther north over Hudson Bay/Northern QC beginning Monday. This difference in the models will have an impact on the southward push of an associated back door cold front that will move into the Mid-Atlantic in the late Tuesday to Wednesday time period. The EC brings this front to roughly I-76 by 12Z Wednesday, while the GFS keeps it much farther north, out of the Mid-Atlantic. There is obviously a lot of uncertainty with this feature, as the models keep flip-flopping in their solutions. Despite the disagreement, both models show the westward extension of the Bermuda High continuing to build north and west on Monday, ultimately pushing the southward extension of the trough northward over the NMA by 00Z Wednesday and setting up mostly zonal flow over the region. One last round of shortwaves will move across the NMA on Wednesday, potentially riding along the back door cold front along the MDL.
The Dailies:

A weak cold front will move into the Mid-Atlantic early Saturday morning, pushing to the I-81 Corridor in the NMA by 12Z. The front will slow as it sweeps across the region, clearing the NMA and CMA but only progressing as far south as the I-81 corridor in the SMA by 00Z Sunday, where it will stall. Skies will be mostly cloudy in the morning hours, especially in the western half of the region, before clearing by mid-day. This clearing will allow temperatures in locations ahead of the front to reach near normal in the eastern NMA and above average in locations south of the MDL. Above average temperatures and a humid air mass will allow for a line of afternoon/evening thunderstorms to develop ahead of the frontal passage in the SMA. Thunderstorms are expected to fire up between 18Z and 21Z along the I-81 Corridor in the CMA and SMA, and push eastward to the Atlantic Coast by 00Z Sunday. The 06Z air quality models seem to think the precipitation in the SMA will either occur along the front or much slower than some of the more recent weather models, as all of the air quality models develop a strip of low Moderate ozone along the I-81 Corridor in the CMA and SMA. The shape and placement of the Moderate ozone suggests that it is developing just ahead of the front as it stalls in the SMA. This brings uncertainty to the forecast, however, westerly surface flow, clear skies, above average temperatures and a stalling front will be favorable for some ozone formation in the SMA. An air mass change with cooler and drier air moving into the NMA and CMA should promote generally Good air quality, despite the sunny afternoon skies. Risk of an exceedance on Saturday will be Marginal with focus on the SMA.

With the exception of a few clouds in PA and a few showers along the Atlantic Coast in the SMA, Sunday will be a mostly clear day across the Mid-Atlantic as high pressure begins to build overhead. With less humid and cooler air slowly moving into the region behind Saturday’s cold front, temperatures across the NMA will be lower than average while locations in the CMA and SMA will be slightly below/near average. Clear skies and light westerly flow will be favorable for some ozone formation across the region. The 06Z air quality models show widespread upper Good ozone with a few isolated areas of low Moderate in highly populated areas across the SMA and CMA. The isolated low Moderate in the CMA and SMA is most likely due to the near normal temperatures combined with Saturday’s stalled cold front presiding along the eastern shore. Despite clear skies and light surface winds, below average temperatures in the NMA and northerly flow aloft in the SMA will keep ozone formation minimal. Risk of an exceedance will lower to Slight on Sunday.

Temperatures will rise back near normal across the Mid-Atlantic on Monday as high pressure remains over the SMA. In the NMA, overhead shortwaves will cause a few clouds and a chance for a few isolated showers throughout the afternoon and evening. Mostly sunny skies, southwesterly surface winds and average temperatures across the rest of the region will be favorable for ozone formation. The 06Z air quality models continue to gradually increase ozone along the eastern half of the Mid-Atlantic with widespread upper Good and scattered Moderate ozone developing. Most of the Moderate ozone is developing along the I-95 Corridor in the NMA and along the still present, stalled frontal boundary in the SMA. As a result of these factors, risk for an exceedance will rise to Marginal on Monday.

Uncertainty enters the forecast for Tuesday as the southward push of a back door cold front is in question, competing with the Bermuda High to the south. At this point, even the faster EC keeps the front to the north of the Mid-Atlantic through Tuesday evening. Both the EC and GFS do develop scattered precipitation across the NMA on Tuesday afternoon and evening, however, which appears to be triggered by the approaching front from the north. Mostly sunny skies and above average temperatures are expected to the south and east of central PA while locations to north and west may be under cloud cover with precipitation. Mostly clear skies, above average temperatures and southwesterly flow across the rest of the region will continue the trend of gradually increasing ozone. The 06Z air quality models develop widespread Moderate ozone with a few isolated areas of upper Moderate ozone along the I-95 Corridor, which certainly seems possible. Risk for an exceedance will rise to Appreciable with a focus on the I-95 Corridor due to the uncertainty of the southeastward progression of the front and associated clouds/precipitation.

Uncertainty continues on Wednesday as questions about the placement of the back door cold front continue. WPC is currently giving more weight to today’s EC solution, bringing the front to roughly I-76 by 12Z Wednesday, where waves of low pressure may develop, triggered by shortwaves aloft. The shortwaves will bring widespread precipitation across the western NMA. Once again the uncertainty is in the eastward/southward progression of the precipitation. If precipitation keeps to the west of I-95, ozone levels may rise as clear skies, and above average temperatures continue. In the rest of the region, Bermuda High pressure over the Southeast U.S. will promote clear skies and southwesterly winds, allowing temperatures to near the century mark in some locations. These conditions will be favorable for ozone formation, and will keep the risk of an exceedance Appreciable on Wednesday, with a focus on the Baltimore-Washington-northern VA corridor.

-Enlow/Huff