Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 20, 2017
Valid: July 21-25, 2017 (Friday-Tuesday)

Summary:

Despite a few rounds of thunderstorms across the NMA, the current mid-summer heatwave will last into early next week, keeping the chances for an exceedance Appreciable through Saturday. A weak cold front will push into the NMA on Friday but will bring little in the way of precipitation, instead possibly acting as a line of convergence. The plume of smoke transported from BC still remains upwind, so slow westerly back trajectories and sufficient atmospheric vertical mixing (~2km) could bring some of this smoke into the region, enhancing ozone formation. Strong high pressure building over western NC will promote mostly sunny skies will allow temperatures to rise into the upper 90s °F across the SMA. As a result, the focus for an exceedance will be on central NC and the I-95 Corridor on Friday. The primary forecast question in the NMA and CMA for Saturday will be the timing and strength of the precipitation and cloud cover. The front will move back northward as a warm front on Saturday, removing one trigger for precipitation, especially along I-95. A MCS will move into the NMA from the west, however, bringing precipitation as far east as I-81. Above average temperatures, mostly sunny skies and slow westerly back trajectories with possible smoke will keep the focus for an exceedance on central NC and the I-95 Corridor on Saturday. Sunday looks mostly unsettled, with a higher chance for more widespread showers and thunderstorms in the NMA and CMA compared to Friday and Saturday. Mostly clear skies will persist in the SMA as temperatures rise near 100 °F. Risk of an exceedance will lower to Marginal with the lone focus on central NC. There is uncertainty between the models with the strength and orientation of the next cold front pushing into the region on Monday. Despite the uncertainty in the precipitation forecast, temperatures across the region are expected to drop to near normal under mostly cloudy skies and northwesterly flow in the NMA. Risk of an exceedance will be Marginal for Monday and Slight for Tuesday, when most of the region should be post-frontal.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, the 06Z NAM and the 00Z ECMWF. With the exception of the strength of the next longwave trough to impact the Mid-Atlantic later in the period, the weather models have come into general agreement with the synoptic setup. The mighty, long lasting upper level ridge that has been centered over the southern Great Plaines will begin to weaken as it moves over the lower Mississippi River Valley by 12Z Friday. The ridge will remain strong and dominate at mid-levels with the ridge axis extending over the upper Midwest, encompassing most of the eastern CONUS at 12Z Friday. An upper level trough centered over the Hudson Bay will move eastward slowly on Friday, pushing a weak front into the Mid-Atlantic in the afternoon. By 00Z Saturday, small shortwaves will develop over the upper Midwest, and drop into the Ohio River Valley and NMA between 12Z and 18Z Saturday. The NAM is too strong with these features, developing a wave along Friday’s front, which moves northward as a warm front on Saturday. We are discounting the stronger wave at this time, with zonal flow over the NMA and CMA at mid and upper levels through 12Z Sunday. As this is occurring, a strong shortwave that has been moving across southern Canada will drop over the Great Lakes, digging out a weak longwave trough over the upper Midwest by 12Z Sunday. The weather models are not exactly in agreement with the strength and shape of the feature initially, however, the models come back into agreement with the positions at upper levels by the end of the period. By 18Z Sunday, the GFS and NAM are similar as they show a stronger and more organized open trough over the western Great Lakes/upper Midwest, while the EC is weaker with a broader trough over the entire Great Lakes region. By 12Z Monday the GFS and EC have the trough in the same location with similar shape but continue to disagree with the strength of the trough. The GFS phases its trough with shortwave energy dropping down from a larger trough over northern QC, keeping the longwave through over southern Canada/northeast U.S by 00Z Tuesday. The EC has a similar final outcome, with a trough over the same region, but it is much weaker as less potent shortwaves spin through the axis. Both models have the trough moving into the NMA between 12Z and 18Z Tuesday, slowly pushing over the central Mid-Atlantic by 00Z Wednesday. The differences in the GFS and EC regarding this feature translate into differences in the strength and speed of the next significant cold front, arriving late Monday.
The Dailies:

The heat wave will continue on Friday as temperatures remain in the low-to-mid 90s °F for most of the Mid-Atlantic. A weak cold front will push into the NMA from the north, reaching the MDL around 18Z Friday to 00Z Saturday and either stalling or pushing slightly farther south overnight. This front is not expected to trigger much in the way of precipitation. The 03Z SREF is showing a low to medium probability of precipitation in occurring in VA/WV, keeping the rest of the NMA and CMA dry. The GFS is the lone model developing scattered showers between 15Z and 18Z across the eastern parts of the NMA and CMA. The plume of smoke transported from BC still remains over the Midwest and Ohio River Valley. Slow westerly back trajectories (eastern OH at 36 hrs) and sufficient atmospheric vertical mixing (~2km) could bring some smoke into the eastern NMA. The main factor in the NMA will be the progression of the weak front. Since it does not appear to be a trigger for showers or thunderstorms, it may act as a line of convergence and enhance ozone formation. Alternatively, if it speeds up, locations behind the front may see cleaner air for the afternoon. In the SMA, strong high pressure building over western NC will promote mostly sunny skies will allow temperatures to rise into the upper 90s °F. Slow westerly aloft and light southwesterly flow at the surface will be favorable for ozone accumulation. The air quality models are reacting to the weak front in the NMA and clear conditions in the SMA as they develop widespread mid-Moderate ozone with scattered Good ozone in the NMA and CMA. The NOAA model is the most conservative with widespread Good and scattered Moderate ozone, but is in agreement with the NC and BAMS models that the primary focus is in the SMA along central NC and southeastern VA. For the NMA, it is important to remember the models do not factor in regional smoke and if smoke is transported into the region, could increase both ozone and particle concentrations. Certainly, weather conditions seem favorable for another day of ozone exceedances along I-95, but the air quality models are not as aggressive as they have been the past few days, perhaps due to the arrival of the weak front. As a result of these factors, risk of an exceedance will be Appreciable with a focus on central NC and the I-95 Corridor.

Unsettled conditions will move into the Mid-Atlantic on Saturday as a mesoscale convective system (MCS) develops in the upper Midwest and sweeps into the NMA as early as 12Z. Friday’s front will move northward back into NY State as a warm front, so it will not be a focus for precipitation. It is important to remember that the NAM is a bit of an outlier, developing a strong wave moving from west to east along the front in NY. There is slight disagreement with the timing of the precipitation but all models have unsettled conditions well into the western portions of the NMA and CMA by 00Z Sunday, associated with the MCS. The primary forecast question in the NMA and CMA for Saturday will be the timing and strength of the precipitation and cloud cover. The 03Z SREF shows a high probability of precipitation reaching as far east as I-81 in the NMA. Temperatures will reach the upper 80s/low 90s °F with breezy winds increasing to gusty in the afternoon and evening hours, depending on precipitation. Locations in the CMA and SMA can expect mostly sunny skies and another day of above average temperatures as some locations in the SMA will near 100 °F. Continued slow westerly flow and light surface winds will once again be favorable for ozone accumulation. The 06Z air quality models continue to increase ozone into the upper Moderate in central NC as favorable conditions persist. In the NMA, the air quality models agree with the weather models that precipitation will not reach the I-95 Corridor as they develop scattered USG ozone in MD and along the interstate. Risk of an exceedance will stay Appreciable with continued focus on central NC and the I-95 Corridor.

Widespread unsettled conditions are expected on Sunday. The models all agree on a higher chance for precipitation compared to Friday and Saturday. The Mid-Atlantic will be in the warm section, resulting in mostly cloudy skies across the NMA with scattered thunderstorms across the NMA and CMA. The 03Z SREF shows thunderstorms moving into the NMA by 12Z Sunday, pushing southward and eastward into the CMA and the I-95 Corridor by 18Z Sunday. Unsettled conditions throughout the day should limit ozone formation across the region. Temperatures will continue to near 100 °F in the SMA as mostly clear skies persist. The 06Z air quality models keep mid-upper Moderate ozone in the SMA while responding to the widespread unsettled conditions in the NMA and CMA by lowering widespread ozone into the lower Moderate/upper Good range. Risk of an exceedance will lower to Marginal with the lone focus on central NC.

Unsettled conditions will continue across the NMA and CMA and move into the SMA on Monday and Tuesday as a weak cold front pushes through the Mid-Atlantic. There is uncertainty between the models with the strength and orientation of the front. The GFS develops a much stronger, northeast to southwest oriented front while the EC has a much weaker, east to west oriented front. The GFS solution is favored by the WPC and would bring the front through the NMA and CMA, stalling along the NC/VA border by 12Z Tuesday. This would result in widespread thunderstorms across the NMA and CMA starting near 18Z Monday pushing eastward off the Atlantic coast by 12Z Tuesday. The EC outcome would bring more scattered showers to the NMA on Monday, with more widespread thunderstorms across the CMA and SMA through Monday and Tuesday. Despite the uncertainty in the precipitation forecast, temperatures across the region are expected to drop to near normal under mostly cloudy skies and northwesterly flow in the NMA. Risk of an exceedance will be Marginal for Monday and Slight for Tuesday, which should be post-frontal for the entire region.

-Enlow/Huff