Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 18, 2017
Valid: July 19-23, 2017 (Wednesday-Sunday)

Summary:
A strong ridge at upper and mid-levels will result in favorable ozone conditions through the work week. Subsidence associated with the ridge will bring mostly clear skies with light to moderate winds. The most recent air quality models, show light surface winds, southwesterly flow aloft, mostly sunny skies, all of which will be favorable for ozone formation along the I-95 Corridor given weekday emissions. As a result, the risk of an exceedance will be High for Wednesday. A weak back door cold front will push into the NMA on Thursday and stall along the MDL. Transport will shift to the west where USG ozone is expected today and likely again tomorrow. Widespread precipitation is not expected with this weak front, resulting in mostly sunny skies and above average temperatures across the Mid-Atlantic. The risk of an exceedance will remain High with a focus on the I-95 Corridor where convergence along the stalled front is expected. The forecast models diverge beginning Friday with respect to the strength and position of MCSs expected to develop upstream. The ECMWF is most aggressive with MCS development and show cloud cover from these systems impacting the mid-Atlantic. This solution is not shared by the GFS which has a stronger ridge. As a result, with warm temperatures and westerly transport, the threat of USG ozone continues into the weekend.
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. There is a general consensus between the weather models until 12Z Friday where the GFS and EC diverge with timing and strength of a strong upper/mid-level ridge over the southern Great Plains/Mississippi River Valley and the presence of MCSs cresting the ridge to our west. At 12Z Wednesday, lingering shortwave energy will remain over the SMA while strong ridging continues to build over the southern Great Plains at upper and mid-levels. At mid-levels the ridge axis will parallel the Mississippi River Valley, as the ridge encompasses much of the eastern CONUS. By Thursday morning, a series of shortwaves will crest the ridge over the upper Midwest and drop across the Great Lakes and into the NMA between 18Z Thursday and 00Z Friday. This shortwave will bring weak troughing over the NMA but will be short lived as the mid-level level ridge axis pushes into the Ohio River Valley. The lingering shortwaves will remain over the SMA/Southeast U.S. through Friday as they slowly begin to move southwestward across GA and FL. Although the differences between the GFS and EC with respect to the strength and progression of the ridge at mid-levels through Friday and Saturday are small overall, it has important impacts on regional air quality due to the possible formation of MCS to our west. The GFS maintains a strong mid-level ridge over the Southeast U.S. and Mid-Atlantic through 06Z Sunday due to slower progression of a developing upper level trough over SK/MB Saturday through Sunday. The EC develops this upper level trough much quicker, dropping the weak trough into southwestern ON/MN by 00Z Sunday, resulting in a weaker, flattened ridge at mid-levels allowing convection and widespread cloud cover to develop. This difference in the development of the weak upper level trough will cause the GFS and EC to diverge even more on Sunday as the GFS has the axis of the mid-level ridge moving through the Mid-Atlantic between 18Z Sunday and 00Z Monday, while the EC brings the southern extent of the upper level trough into the SMA in the same time frame.

As mentioned in yesterday’s discussion, wildfires across Canada have created a large plume of thin smoke over much of Canada and northern U.S. This may cause some issues with air quality in the NMA and CMA as atmospheric mixing is modeled between 1.5 and 2.0 km throughout the medium range period. A forecast question that will remain throughout the period is how this smoke plume reacts to atmospheric conditions, how it moves, and if it continues to grow in area.

The Dailies:

After a foggy morning in the NMA and CMA, mid-level ridging building over the Mid-Atlantic will bring mostly sunny skies to the region on Wednesday. Sunny skies combined with slow southwesterly flow aloft will allow temperatures to rise well into the 90s °F across most of the region. High pressure centered over the western SMA will result in light/calm surface winds in the SMA with more breezy winds in the afternoon in the NMA and CMA. Daytime heating and a humid air mass will bring the possibility for a few isolated thunderstorms to develop across the region but the most recent hi-res models are showing no concrete signs of widespread precipitation. The 03Z SREF and 06Z GEFS support this as a few locations along the Appalachian Mountains and the northeast NMA are the areas where precipitation is most likely. The 06Z air quality models are picking up on these atmospheric conditions as they develop widespread Moderate ozone with scattered USG and Good ozone. All three of the air quality models (NOAA, both BAMS and the NC-CMAQ-D) are highlighting the I-95 Corridor with a swath of USG ozone stretching from northern VA through NJ. A combination of light surface winds, southwesterly flow aloft, mostly sunny skies, and weekday emissions will be favorable for ozone in this area. Another area highlighted in the air quality models is the western NMA where mid-moderate/USG ozone is shown. As a result, risk of an exceedance will be High for Wednesday.

A weak back door cold front will push into the NMA on Thursday and stall along the MDL. Little clouds or precipitation is expected with this weak front, resulting in mostly sunny skies across the Mid-Atlantic. Temperatures will remain high and flow aloft will begin to stagnate and shift to a weak northwesterly flow across the region. In the NMA, light winds in the morning will give way to breezy winds in the afternoon hours, while surface winds will remain light/calm throughout the day in the SMA. The 06Z air quality models react to the approaching weak cold front in the NMA as upper Moderate/USG ozone develops along the I-95 corridor and ozone falls to upper Moderate in western NMA. The models develop some High ozone along the Atlantic coast but this is most likely a model artifact. Surprisingly, the models show widespread low-mid Moderate with isolated upper Moderate across the region. Despite the decrease in regional modeled ozone, risk of an exceedance will remain High with a focus on the I-95 corridor as the weak cold front could slow, resulting in a line of convergence over the region.

Forecast uncertainty increases beginning Friday. Scattered thunderstorms are possible across the NMA as mesoscale convective system (MCS) may drop across the upper Midwest and into the NMA overnight. Despite this uncertainty, temperatures are expected to rise a few degrees from Thursday as flow aloft shifts to westerly and mostly sunny skies persist across most of the region. Surface winds will slightly increase on Friday as light morning winds increase to breezy in the afternoon. The air quality models appear to buy into the possible precipitation or increased surface winds across the region as widespread Moderate ozone persists in the models. Due to uncertainty regarding the development and position of the MCS, the risk of an exceedance will remain Appreciable on Friday.

Disagreement between the models continue on Saturday. Again, the EC is more aggressive with MCS development. The GFS, with its stronger mid-level ridge, keeps conditions mostly dry with a few scattered showers across the region. If the MCS in the EC does not move through the region, temperatures in the upper 90s °F will combine with mostly sunny skies and an increasingly humid air mass will result in possible afternoon/evening thunderstorms across the region. Upper air transport will shift to westerly in the NMA and southwesterly in the SMA. Once again the air quality models are buying into the precipitation and cloud cover as ozone decreases to low/mid-Moderate ozone with large areas of Good ozone across the NMA. Precipitation and cloud cover is the primary forecast question for Saturday. Again, uncertainty in the forecast models leads us to keep the risk of an exceedance as Appreciable.

More of the same on Sunday as the differences in the models create great uncertainty. The EC brings in more clouds and precipitation across the NMA and CMA while the GFS is more clear and dry. Uncertainty of atmospheric conditions is high on Sunday, keeping risk of an exceedance Appreciable.

-Enlow/Ryan