Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 21, 2017
Valid: June 22-26, 2017 (Thursday-Monday)

 

A wet and soggy weekend lies ahead as a tropical system will meet a cold front over the Mid-Atlantic starting Thursday. As this feature moves into the region, precipitation and clouds will be confined to the west of I-81 in the NMA, allowing near normal temperatures and clear skies to the east. The CMA and SMA will endure clouds and precipitation throughout the day as the two systems meet to the west of the region. Slow westerly back trajectories combined with conditions favorable for ozone formation in the eastern NMA, will bring risk of an exceedance to Appreciable for Thursday. After the lone threat for an exceedance on Thursday, widespread cloud cover and precipitation will prevent any further threat of an exceedance through the end of the medium range period.

 

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. There continues to be strong consensus between the weather models early in the medium range, with divergence between the models beginning after 12Z Friday. Much like yesterday’s model runs, the GFS continues to deepen an upper level trough over the upper Midwest, much quicker than the EC. Both models eventually have the same general outcome by 12Z Monday, with an upper level trough over the Ohio River Valley and Mid-Atlantic, but reach this outcome in two separate ways. The NAM outcome appears to be a blend of the GFS and EC as features slightly vary, but conditions appear similar at the surface.

An upper level trough/closed low will be over southern Canada by 12Z Thursday with its center circulating over western MB. At the same time, Tropical Storm Cindy will have made landfall between 06Z and 12Z Thursday along the Gulf Coast of TX/LA with its remnants reaching AR by Friday. With these two systems in place to the west of the Mid-Atlantic, weak ridging/zonal flow will move over the Mid-Atlantic as the westward expansion of the Bermuda High, between 12Z Thursday and 00Z Friday. The remnants of Tropical Storm Cindy will move along the Mississippi River Valley, phasing with the Canadian trough over much of the northern U.S. by 00Z Saturday.

The weather models begin to diverge when dealing with the Canadian trough by 18Z Friday. The upper level trough will be over the upper Midwest and northern Plains by 18Z Friday with the center of circulation over central ON. The disagreement is in the development of a short wave disturbance in the trough over the upper Plains by 06Z Saturday. The NAM and the GFS have a stronger disturbance, placed further south, than the model consensus. The EC also shows this center of circulation but displaced further north. In either event, all models bring the trough into the Mid-Atlantic by 18Z Monday. The GFS has the trough axis over the Mid-Atlantic by 00Z Tuesday extending southward, where the EC places the axis over the Great Lakes, extending along the Mississippi River Valley to the southwest, as a much stronger trough.
The Dailies:

Most of the Mid-Atlantic will experience widespread unsettled conditions on Thursday. Tropical moisture from Tropical storm Cindy will surge into parts of the western SMA due to strong southerly flow. This will result in increasing cloud cover and precipitation through the day, starting at 12Z Thursday. Strong southerly/southwesterly flow and widespread clouds and precipitation will keep ozone and particles in the Good range across the SMA on Thursday. The eastern Mid-Atlantic will be more conducive to ozone formation Thursday. A frontal boundary along the MDL will push northward as a warm front, placing the Mid-Atlantic in the warm sector of an approaching storm system. Precipitation is expected to be minimal and limited to locations west of I-81 as weak high pressure keeps skies mostly sunny. The 06Z air quality models are mostly in agreement that precipitation and clouds will remain in the western NMA leading to widespread Moderate ozone with a few isolated areas of USG along the I-95 Corridor. Higher ozone in that location will be due to clear skies, above average temperatures, and slow westerly back trajectories. In addition the 3km-NAM is showing converging/calm winds over MD/DE/PA/NJ between 12Z and 18Z Thursday. As a result, risk of an exceedance will be Appreciable for Thursday with a focus on the eastern NMA and the I-95 Corridor.

Friday will be a washout across the Mid-Atlantic as a combination of tropical moisture and an approaching cold front from the west will bring clouds and widespread, possibly heavy, precipitation. The remnants of Tropical Storm Cindy will phase with the upper level trough by 12Z Friday, brining heavy precipitation to the region. The 03Z SREF and GEFS show precipitation is most likely across the entire Mid-Atlantic starting around 09Z Friday and lasting through 12Z Saturday when precipitation will clear from northwest to southeast. Regardless of any uncertainty between the weather models, the most recent air quality models are in agreement that widespread unsettled conditions will bring ozone and particles well into the Good range. Risk for an exceedance will be Slight for Friday.

Precipitation from Friday will linger across the Mid-Atlantic on Saturday morning. The NMA can expect to dry from west to east starting at 12Z, with only a few isolated showers remaining north of the MDL by 18Z Saturday. The cold front from Friday will be stalled across the CMA, oriented southwest to northeast, by 12Z Saturday and slowly push south easterly, over southern VA/NC by 12Z Sunday. This front combined with the remaining moisture from Tropical Storm Cindy will bring widespread, and in some places, heavy precipitation across the CMA and SMA. The precipitation will move west to east, lasting through 00Z Sunday as the system moves off the Atlantic coast. Despite the clearing in the NMA, slightly below average temperatures and swift southerly flow will keep ozone in the upper Good range across the Mid-Atlantic. Risk of an exceedance will remain Slight.

Due to differences between the GFS and EC in the progression of the upper level trough, uncertainty remains for Sunday and Monday. The EC is significantly cooler than the GFS on both Sunday and Monday. As a result of this difference, the air quality forecast for both days remains in flux, although the most probable case is a Slight chance for poor air quality due to ozone.

-Enlow/Ryan