Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 19, 2017
Valid: June 20-24, 2017 (Tuesday-Saturday)

Summary:

Most of the Mid-Atlantic will see clear and dry skies on Tuesday, as weak high pressure builds behind Monday’s passing cold front. The cold front is expected to reach the Atlantic coast by 12Z Tuesday and stall in the SMA, and just off the Atlantic coast in the CMA and NMA. Precipitation across the SMA and a fresh air mass in the NMA and CMA will keep risk for an exceedance Slight for Tuesday. Similar to Tuesday, Wednesday will be mostly dry across the Mid-Atlantic as mid-level ridging builds into the region. The frontal boundary will remain across the SMA in the morning but dissipate as the ridging moves overhead, confining precipitation to coastal locations. Fast westerly flow and scattered evening showers will keep risk for an exceedance Slight on Wednesday. Mid-level ridging will continue into Thursday resulting in more widespread clear skies. Slow westerly flow and average temperatures under clear skies will favor ozone formation, allowing scattered areas of Moderate to develop. This will bring risk for an exceedance up to Marginal for Thursday, focusing on locations in the NMA and CMA. There is a lot of uncertainty late in the medium range period as the GFS and ECMWF begin to diverge. There are two causes for this divergence, the first is the track of the remnants of a tropical system in the Gulf of Mexico and the second is the strength of an upper level trough and its associated cold front dropping down from the upper Midwest on Friday. Despite the uncertainty, widespread unsettled conditions are expected, bringing risk for an exceedance back down to Slight on Friday and Saturday.

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 strong consensus with placement of key atmospheric features until 12Z Wednesday, where the GFS diverges from the NAM and EC with the track and strength of an area of tropical low pressure currently over the Yucatan Peninsula that is expected to move into the southern/central Gulf of Mexico on Tuesday. At this time the National Hurricane Center (NHC) is giving this system an 80% chance of developing a tropical cyclone in the next 48 hours. The weather models are in agreement that the system will located off the Texas coast around 12Z Wednesday, but the GFS quickly brings the system eastward and up the Mississippi River Valley, over AR/LS/MS by 00Z Friday, whereas the EC weakens the system over the Gulf Coast of Texas by 12Z Friday. There is high uncertainty regarding the path and strength of this feature. It will be important to monitor this system as it may impact the SMA late in the medium range and into early next week.

The upper level trough currently over the upper Midwest will move into the Mid-Atlantic by 12Z Tuesday. The axis of this trough is expected to reach the Mid-Atlantic between 06Z and 12Z Wednesday, as it begins to pull northeastward. The trough will exit the Mid-Atlantic, moving over New England by 00Z Thursday, with weak ridging at upper and mid-levels, building in from the west by 12Z Thursday. At this time another upper level trough/closed low will be moving across central Canada, with its center over MB by 12Z Thursday. There is some disagreement between the models regarding the positioning and shape of this trough through the end of the medium range as the GFS progresses the trough quicker yet weaker than the EC and NAM. The center of circulation will be over ON by 00Z Friday but the trough will be rather weak as there will be zonal flow over much of the CMA, NMA and southern Great Lakes.

The Dailies:

After significant rain overnight, the Mid-Atlantic will see dry and mostly sunny skies on Tuesday. The cold front driving this precipitation is expected to pass through the NMA and CMA on Monday but stall over the eastern SMA by 12Z Tuesday. The stalling of this front will bring unsettled weather for most of the SMA coast between 12Z Tuesday and 12Z Wednesday. For locations west of the front, weak high pressure will build overhead between 12Z and 18Z, resulting in mostly sunny skies and near normal temperatures and westerly flow. The 06Z air quality models are suggesting that unsettled conditions in the SMA and a fresh air mass across the CMA and NMA will keep ozone in the Good range as all models show widespread mid-upper Good ozone for Tuesday. As a result, risk for an exceedance will be Slight for Tuesday.

Wednesday is similar to Tuesday as the frontal boundary will continue to reside along the Atlantic coast. Precipitation will be less widespread, focusing on coastal locations in NC, as the boundary is weakened by the westward extension of the Bermuda high by 00Z Thursday. Weak ridging at both mid and upper levels will bring mostly sunny skies to the rest of the region. The 06Z air quality models are picking up on this clearing as well as westerly flow across the Mid-Atlantic as they show ozone level creeping into the low Moderate range across the CMA and interior SMA. The air quality models may show mid Good ozone in the NMA due to the potential for a few afternoon/evening showers. The 03Z SREF shows the chance for showers across the NMA between 15Z Wednesday and 00Z Thursday. Despite mostly sunny skies and continued westerly flow, near normal temperatures will keep risk of an exceedance to Slight for Wednesday.

Mostly sunny skies will continue across the Mid-Atlantic on Thursday as mid-level ridging persists over the region. There is some uncertainty with the southward extend of precipitation associated with a warm front approaching from the west. The EC is showing precipitation across northern and central PA around 00Z Friday whereas the GFS keeps all precipitation scattered and over the Great Lakes. If conditions remain clear as we are expecting, slow westerly flow with clear skies and near average temperatures will allow ozone to creep into the Moderate range across the NMA as shown in the 06Z air quality models. In the SMA, the possibility of an approaching tropical/subtropical system from the southwest, possibly starting as early as 00Z Friday, will keep the forecast for this area highly uncertain through the end of the forecast period. Despite the uncertainty of specific timing in the SMA, southerly onshore flow will keep ozone in check despite possible clearing. Risk for an exceedance will increase to Marginal for Thursday, focusing on locations in the NMA and CMA.

As noted above, the medium range models begin to diverge significantly beginning Friday continuing into Saturday. There are two causes for this divergence, the first is the track of the remnants of the Gulf of Mexico tropical system and the second is the strength of an upper level trough and its associated cold front dropping down from the upper Midwest. The uncertainty around the tropical system makes it difficult to choose between the GFS and EC at this lead time. The EC has a strong cold front arriving as early as Saturday while the GFS is much slower allowing tropical moisture to pool ahead of a Sunday cold front. The latest WPC forecast aligns better with the GFS. The net result for air quality is probably not significant. Under either solution, clouds and rain are likely during the weekend with only a Slight chance of an exceedance.

-Enlow/Ryan