Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 5, 2017
Valid: July 6-10, 2017 (Thursday-Monday)

Summary:

Widespread unsettled conditions on Thursday will promote generally Good air quality, but the risk of an exceedance will be Marginal with a focus on western PA, where the BAMS air quality models have USG ozone, presumably due to mostly sunny skies and slow southeasterly flow. Unsettled conditions will linger on Friday as a stalled frontal boundary across the CMA slowly dissipates and a cold front approaches the NMA from the northwest. There is disagreement between the air quality models again on Friday as the BAMS models are much higher with ozone than the NC model across the CMA and I-95 Corridor. More weight is being put on the NC model output as cloud cover, slightly below average temperatures and onshore flow should be enough to keep ozone in the upper Good/low Moderate range on Friday. Risk of an exceedance will remain Marginal, however, due to the uncertainty among the air quality models. Slightly cooler and drier air behind a weak cold front on Saturday will drop the chance for an exceedance to Slight for the weekend. There is uncertainty in the forecast for Monday as the weather models diverge with the southward progression of another weak cold front that may push into the NMA. At this time, it appears that high pressure will continue to build over the Mid-Atlantic on Monday, resulting in mostly sunny skies yet again. A combination of near normal temperatures, calm surface winds, and clear skies will allow for ozone to accumulate across the region, bringing the risk for an exceedance up to Marginal for Monday.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are generally in agreement until 18Z Saturday. The NAM is slightly slower and stronger than the GFS and EC for its entire model run, but WPC recommends a blend of the NAM, GFS, and EC through Saturday. Upper level zonal flow that is currently over the Mid-Atlantic will be disrupted by shortwaves moving over eastern MO/southern IL by 12Z Thursday. These shortwaves will dig out a weak trough over the Ohio River Valley that will phase with a larger, upper level trough over eastern Canada/northeastern U.S. by 00Z Friday. The shortwaves from MO/IL will move over the Mid-Atlantic between 12Z and 18Z Friday as they spin through the upper level trough axis. By 00Z Saturday, a cluster of shortwaves will drop down across the Upper Midwest/western Great Lakes, further deepening the trough over the Mid-Atlantic. This set of shortwaves, along with the trough axis, will move over the Mid-Atlantic between 12Z and 18Z Saturday. The EC and GFS diverge beginning on Saturday afternoon. Both the GFS and EC keep the upper level trough lingering over the Mid-Atlantic through the end of the medium range period, but the EC brings the center of circulation, as well as the entire trough, further south than the GFS. The GFS keeps the center of the circulation over Hudson Bay, while the EC brings it over James Bay. The result is that the GFS has a weaker trough over the Mid-Atlantic through 00Z Tuesday. The difference between the two outcomes hinges on the fate of a weak cold front; the EC brings a weak cold front into the NMA sometime between 18Z Sunday and 00Z Tuesday, while the GFS has high pressure building over the Mid-Atlantic on Monday as the cold front dissipates over the US/Canadian border. WPC seems to be utilizing a blend, with a slight nod to the EC’s weak cold front.
The Dailies:

A wave of low pressure moving through the Ohio River Valley will bring scattered unsettled conditions to much of the Mid-Atlantic on Thursday. By 12Z Thursday, overhead shortwaves will interact with a frontal boundary draped over the CMA, developing a wave of low pressure in the Ohio River Valley. Precipitation will begin around 12Z, moving west to east across the NMA and CMA throughout the day. The 03Z SREF shows that precipitation is most likely between I-80 and the VA/NC border lasting through 00Z Friday. Widespread cloud cover and precipitation in the NMA and CMA will keep temperatures below average. In the SMA, a combination of mostly sunny skies, near normal temperatures and slow westerly flow, will allow for some ozone formation. The 06Z air quality models are picking up on the widespread precipitation and cloud cover across the CMA and NMA as Good ozone is shown across the region. In western PA, however, the BAMS air quality models show ozone reaching into the upper Moderate range, with a bull’s-eye of USG ozone in the PIT metro area. This is possibly due to calm/light converging winds at the surface under mostly sunny skies to the west of I-79. This feature is not really present on the NOAA and NC air quality models, as the NOAA keeps ozone in the upper Good range and the NC model only develops low Moderate ozone near ERI. Our instinct is to discount this USG ozone from the BAMS models, but they have been correct earlier this season in identifying areas of possible ozone exceedances that we didn’t think were likely. Therefore, the risk for an exceedance will be Marginal on Thursday with a focus on western PA.

Unsettled conditions will linger on Friday as the frontal boundary across the CMA slowly dissipates and a cold front approaches the NMA from the northwest. In the NMA, mostly cloudy skies in the morning hours will clear by 15Z, allowing for temperatures to reach near normal in many locations. Clouds and precipitation will begin to move into the NMA from the northwest beginning around 15Z Friday. The 03Z SREF shows that precipitation will stay to the west of I-99, tapering off soon after 00Z Saturday. Mostly sunny skies across the rest of the Mid-Atlantic will allow temperatures will rise slightly above normal, especially in the SMA. Despite clear skies and above normal temperatures, a strong onshore, southwesterly flow in the SMA will keep ozone levels in the mid/upper Good range. In the NMA, locations not impacted by the precipitation will see partly cloudy skies after 15Z, allowing temperatures to reach near normal. The 06Z BAMS air quality models are developing low Moderate ozone across the NMA and an isolated USG in along the Bay in MD, while the NC air quality model keeps ozone in the upper Good range across the Mid-Atlantic. More weight is being put on the NC model output as cloud cover, slightly below average temperatures and onshore flow should be enough to keep ozone in the upper Good/low Moderate range on Friday. Risk of an exceedance will remain Marginal, however, due to the uncertainty among the air quality models.

A weak cold front will push through the NMA and CMA by 12Z Saturday. Temperatures behind the front will drop back to slightly below normal, as a swift northwesterly flow brings a slightly cooler and drier air mass into the Mid-Atlantic from the Upper Midwest. High pressure will begin to build over the NMA and CMA after the front passes, resulting in mostly sunny skies. An air mass change and northwesterly flow will keep ozone in the Good range across the NMA and CMA. In the SMA, the front is expected to weaken substantially as it advances to the Atlantic coast by 12Z Sunday. Despite partly cloudy skies, near normal temperatures and northwesterly flow will allow for ozone to accumulate into the low Moderate range. Risk for an exceedance will drop to Slight on Saturday.

High pressure will continue to build over the Mid-Atlantic on Sunday, allowing for mostly sunny skies to persist. Temperatures across the NMA will remain just below normal as flow shifts to northerly, while northwesterly flow across the CMA and SMA will keep temperatures near normal under a clear July sky. Northwesterly/northerly flow will continue to bring a relatively dry air mass into the region as dew points drop into the low 50’s/upper 40’s °F. The 06Z air quality models develop scattered low Moderate ozone across CMA and SMA, most likely in response to clear skies and light surface winds, while the NMA remains in the Good range. Risk for an exceedance will remain Slight on Sunday.

There is uncertainty in the forecast for Monday as the GFS and EC disagree with the southward progression of another weak cold front that may push into the NMA. At this time, it appears that high pressure will continue to build over the Mid-Atlantic on Monday, resulting in mostly sunny skies yet again. A combination of near normal temperatures, calm surface winds, and clear skies will allow for ozone to accumulate across the region, bringing the risk for an exceedance up to Marginal for Monday.

-Enlow/Huff