Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 27, 2017
Valid: June 28-July 2, 2017 (Wednesday-Sunday)

Summary:

Risk for an ozone exceedance will rise to Appreciable late this week due to a period of clear skies and warming temperatures before lowering to Slight as a cold front approaches over the weekend. Cool and dry conditions across the region on Wednesday will keep risk of an exceedance Slight despite clear skies. High pressure over the SMA will allow for temperatures to slowly increase towards normal on Thursday. Clear skies and slow, recirculating back trajectories will allow for ozone to accumulate along the I-95 Corridor. Risk of an exceedance will rise to Appreciable. The biggest concern for an ozone exceedance is on Friday as clear skies and near average temperatures continue across the region. A combination of southwesterly flow and the formation of a lee trough over the eastern NMA will allow for additional ozone accumulation to occur, keeping risk of an exceedance Appreciable. Unsettled conditions will return to the Mid-Atlantic over the weekend as a cold front approaches the western NMA on Saturday, slowly approaching the CMA Atlantic coast by 12Z Monday. A period of unsettled conditions in the NMA/CMA and onshore flow in the SMA will bring risk for an exceedance back to Slight for both Saturday and Sunday.

 
Weather Model Discussion:
There is strong consensus between the weather models through the end of the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The upper level trough that is currently over the Mid-Atlantic will depart to the east on Wednesday. At the same time, mid-level ridging will be quick to build into the region as it closely follows the upper level ridge, building into the Mid-Atlantic by 12Z Wednesday and lasting until 00Z Saturday. However, shortwaves sweeping across the Great Lakes will flatten the upper level ridge, resulting in zonal flow over the region on Thursday. At the same time, these shortwaves will interact with a frontal boundary across the Great Lakes, developing a surface low pressure by 18Z Thursday. By 12Z Friday, another series of shortwaves moving across the Great Plains will dig out a trough over the upper Midwest. This trough will slowly spin across the upper Midwest, with its axis moving over the upper Mississippi River Valley by 12Z Saturday. At the same time, the shortwaves spinning around the trough will interact with another frontal boundary, developing a surface low pressure over IW/IL by 12Z Friday, moving northeast into the Great Lakes/southern ON by 12Z Saturday. At mid-levels, the westward extension of the Bermuda High will begin to build back into the SMA and Gulf Coast states, pushing the low pressure system to the north. The clash between these two features will be a major factor in the air quality forecast as it will affect the strength and speed of the cold front associated with the low pressure system noted above. The upper level trough driving this system, will continue to progress eastward, moving over the Great Lakes and partially into the Mid-Atlantic by 00Z Sunday. By 12Z Sunday, the trough will begin to contract northward, weakening as it moves over the Mid-Atlantic by 00Z Monday.

The Dailies:

Wednesday will be another pleasant day across the Mid-Atlantic as high pressure builds into the region for one last day. Temperatures will remain below normal as the cool Canadian air mass lingers below the upper level trough. Despite the cool temperatures, widespread clear skies and light winds will promote ozone formation across the Mid-Atlantic. Back trajectories in the NMA are northwesterly, but trajectories in the SMA are significantly slower as the center of high pressure moves over the SMA/CMA throughout the day. The 06Z air quality models are picking up on clear skies and subsidence across the Mid-Atlantic as ozone rises into the upper Good range with scattered low Moderate ozone despite the below average temperatures. There are a few differences between the models, as the NC air quality model develops more scattered Moderate ozone, epically across the SMA, where the NOAA model keeps ozone in the upper Good range with only a few isolated areas of Moderate. The BAMS models seem to be a good compromise as they are in the middle of the NC and NOAA models. In addition to the regional conditions, there is some concern with the transport of smoke from the Midwest. The latest NOAA Fire Analysis shows a large plume of smoke over central Canada, extending into the Midwest and Ohio River Valley. At this time, it appears that most of this smoke is not mixing to the surface, or is very dilute as concentrations of ozone and particles are not excessive in these locations. Despite the risk of dilute smoke and clear skies, the below average temperatures will result in a Slight chance for an exceedance for Wednesday.

Summer returns on Thursday as temperatures rebound to near normal across the Mid-Atlantic. Mostly sunny skies are expected with the exception of the NMA in the afternoon/evening hours. A warm front associated with a low pressure system over the Great Lakes will bring some unsettled weather to the northern NMA. The 03Z SREF shows precipitation is most likely north of I-80 between 18Z Thursday and 00Z Friday. The rest of the Mid-Atlantic is expected to remain dry and clear with light southerly flow. Onshore flow in the SMA will be enough to keep ozone levels in the upper Good/low Moderate range. Recirculating, slow back trajectories in the eastern NMA will combine with clear skies to promote ozone formation. The 06Z air quality models are all highlighting the I-95 Corridor and locations upwind as an area of concern for an ozone exceedance. All of the models show an area of mid-Moderate ozone and an isolated area of upper Moderate/USG in NJ/NY. As a result of these factors, risk for an exceedance will increase to Appreciable for Thursday, with a focus on the I-95 Corridor.

The biggest concern for an ozone exceedance is on Friday as clear skies and near average temperatures continue. High pressure building over the SMA will provide clear skies to combine with near average temperature and promote ozone formation across the region. Southwesterly/southerly back trajectories in the NMA will keep some locations in the low/mid Moderate range. A lee trough is expected to form east of the I-95 Corridor early Friday and then slowly drift westward. If this feature verifies, it will act as an area of convergence along the I-95 Corridor late in the day, allowing ozone to accumulate. This is appearing in the 06Z air quality models as a swath of upper Moderate ozone develops along the I-95 corridor with a few areas of USG in northern NJ. This will bring keep risk for an exceedance Appreciable for the NMA. A key area of uncertainty for Friday will be the magnitude of transported ozone as back trajectories become more southerly, perhaps entraining cleaner maritime air. Air quality in the SMA is expected to remain Good as the nearby high pressure brings an onshore flow to the region. Despite near average temperatures and clear skies, the onshore flow will keep ozone in the Good range across the SMA on Friday.

A combination of an approaching cold front and afternoon/evening thunderstorms will bring unsettled conditions to the Mid-Atlantic on Saturday and Sunday. Near average temperatures and increasing humidity on Saturday will lead to widespread showers and thunderstorms as a cold front approaches from the west. The front is expected to slowly push into the NMA late Saturday and into Sunday, eventually stalling along the eastern/central CMA by 12Z Monday. As mentioned, this slow moving front will bring widespread unsettled conditions to the CMA and NMA, bringing risk for an exceedance back to Slight.

-Enlow/Ryan