Mid-Atlantic Medium Range Air Quality Discussion, Issued Tuesday, July 31, 2018

 

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 31, 2018
Valid: August 1-5 (Wednesday-Sunday)

Summary

An upper level pattern similar to last week’s will keep a Slight risk of an ozone exceedance through Friday, before the westward building of the Bermuda high will gradually increase ozone conducive conditions in western portions of the region, increasing the risk to Marginal over the weekend. Widespread, and sometimes heavy precipitation will continue to impact the Mid-Atlantic throughout the rest of the work week. The presence of the Bermuda high centered in the western Atlantic Ocean and a stalled upper level trough over the Mississippi River Valley will allow a conveyor belt of moisture to surge northward into the Mid-Atlantic, resulting in a stretch of unsettled days. Strong southerly flow aloft, cloudy skies, and widespread precipitation will keep ozone formation minimal across the entire region through Friday. Saturday and Sunday will be transition days as the synoptic pattern begins to shift towards an ozone friendly pattern, rising the risk of an exceedance to Marginal over the weekend, with the focus on the western part of the Mid-Atlantic. The trend is toward a classic Bermuda High ozone-conducive pattern, but the greatest risk looks to be mid-next week, beyond the medium range period. Over the weekend, weak upper level ridging will keep a surface trough along the vicinity of the I-95 Corridor and lingering shortwave energy aloft, which will limit ozone formation in the eastern part of the region. A mix of sun and clouds, light/calm surface winds, near average temperatures, and flow aloft weakening as it shifts southwesterly/westerly will be favorable for ozone formation in western PA. The primary forecast questions will be how quickly the air mass will modify under mid-level ridging, the coverage of precipitation and cloud cover, and air mass characteristics across the Ohio River Valley (source region for PIT back trajectories) and western PA. The risk of an exceedance will rise to Marginal, with a focus on western PA. Conditions favorable for ozone formation will continue to develop on Sunday across the western Mid-Atlantic as the mid-level ridge continues to build westward. A shift to westerly flow aloft, mostly sunny skies, and light surface winds will be favorable for ozone formation in western parts of the region. The development and coverage of afternoon precipitation will be the primary forecast questions on Sunday. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast and air mass characteristics, with a focus on the western parts of the NMA and CMA.

 

NWP Model Discussion

Although the NAM continues to be a slight outlier compared to the GFS and ECMWF, the weather models are in close agreement on the main synoptic features throughout the medium range period. The upper level trough currently over the Mississippi River Valley will remain in place and gradually weaken through the end of the work week, as it is sandwiched between two ridges of high pressure: the first over the Four Corners region to the west and the other a Bermuda high over the western Atlantic. By 12Z Wednesday, embedded shortwave energy will be lifting northeastward, out of the base of the trough, across the Ohio River Valley and into the Great Lakes region. Weak shortwave energy lingering throughout the base of the trough will keep this feature in place, albeit weakened, through 00Z Friday. On Friday, the westward building of the Bermuda high and eastward building of the four corners ridge will begin to pinch off the longwave trough, gradually filling it in by 18Z Saturday. The weakening of the Mississippi River Valley trough will allow the Bermuda high to build westward over the eastern U.S. at mid-levels as early as Friday. Beginning on Saturday and continuing on Sunday, the center of the ridge will push into the Southeastern U.S. By 00Z Monday, the majority of the eastern CONUS will be under the Bermuda high. The upper level ridge will be relatively weak, however, with a few shortwave perturbations remaining across the Mid-Atlantic over the weekend, keeping the potential for unsettled conditions across parts of the region.

The Dailies

Day 1 (Wednesday): Widespread unsettled conditions will impact the entire Mid-Atlantic on Wednesday, as the region will be in the warm sector of a weak low pressure system lifting northeastward through the Ohio River Valley and Great Lakes region. Strong southerly flow will pull a tropical air mass (precipitable water values near or greater than 2 inches) into the Mid-Atlantic, promoting widespread, locally heavy precipitation and mostly cloudy skies. The WPC has placed the entire region under Marginal risk of excessive rainfall with a strip of Slight risk of excessive rainfall from the Gulf Coast stretching northeastward along the spine of the Appalachians into northeastern PA. The NC and BAMS-CMAQ air quality models are in agreement that Good ozone will blanket the region under unsettled conditions. The BAMS-MAQSIP appears to be unreasonably aggressive as it develops scattered Moderate with pockets of mid-to-upper Moderate ozone throughout PA. Given the flow pattern and widespread nature of unsettled conditions, ozone levels reaching the Moderate range seems extremely unlikely in the Mid-Atlantic. This risk of an ozone exceedance will remain Slight on Wednesday.

Day 2 (Thursday): Widespread, locally heavy precipitation will continue on Thursday as the weak cold front associated with the low pressure system edges into the Mid-Atlantic. The WPC again has the entire Mid-Atlantic under Marginal risk for excessive rainfall with a strip of Slight risk of excessive rainfall from the Gulf Coast stretching northeastward along the spine of the Appalachians into northeastern PA. Similar to Wednesday, widespread precipitation, strong southerly flow, and mostly cloudy skies will keep ozone formation minimal across the region. The air quality models are mostly in agreement as they develop Good air quality throughout the entire Mid-Atlantic. The NC-NAM models are the most aggressive on Thursday as they develop scattered upper Good/Moderate ozone behind the slow moving cold front, across western PA. Another day of widespread unsettled conditions and strong southerly flow will keep a Slight risk of an ozone exceedance on Thursday.

Day 3 (Friday): Widespread unsettled conditions will persist on Friday as Thursday’s weak cold front stalls and slowly dissipates across the Mid-Atlantic. The Bermuda high will begin to build westward at mid-levels, keeping southerly flow west of the dissipating frontal boundary. This flow pattern will continue to act as a conveyor belt, bringing an abundance of tropical moisture northward into the Mid-Atlantic. Yet another day of widespread precipitation and mostly cloudy skies will prevent any significant ozone formation from taking place on Friday. The BAMS air quality models are in agreement with Good ozone blanketing most of the Mid-Atlantic with a strip of Moderate ozone creeping into western PA. This long stretch of unsettled conditions and strong southerly flow makes it hard to believe that ozone could reach into the Moderate range anywhere in the Mid-Atlantic. As a result, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Saturday-Sunday): Saturday and Sunday will be transition days as the synoptic pattern begins to shift. The Bermuda high over the western Atlantic will continue to build westward over the eastern U.S., but a surface trough along the eastern Mid-Atlantic and lingering shortwave energy across the region (under weak upper level ridging) will keep unsettled conditions throughout the weekend. On Saturday, the majority of precipitation will be concentrated to the CMA with more scattered showers throughout the rest of the region. A mix of sun and clouds, light/calm surface winds, near average temperatures, and flow aloft weakening as it shifts southwesterly/westerly will be favorable for ozone formation across interior portions of the NMA. In the eastern portions, persistent strong southerly onshore flow should continue to limit ozone formation. Across the SMA, continued onshore flow, scattered showers, and mostly cloudy skies will also keep ozone formation minimal. The BAMS air quality models respond to these conditions with widespread Good ozone throughout the entire CMA and SMA. The BAMS models also respond to ozone conducive conditions throughout the interior NMA as they develop Moderate ozone throughout the western NMA and CMA (roughly west of I-81), with isolated USG ozone in the PIT metro area. The primary forecast questions on Saturday will be how quickly the air mass will modify under mid-level ridging, the coverage of precipitation and cloud cover across the NMA, and air mass characteristics across the Ohio River Valley (source region for PIT back trajectories) and western PA. As a result, the risk of an ozone exceedance will increase to Marginal with a focus on western PA.

Conditions favorable for ozone formation will continue to develop slowly on Sunday as the mid-level ridge continues to build westward into the CONUS. A shift to westerly flow aloft, mostly sunny skies, and light surface winds will be favorable for ozone formation again in the interior NMA and CMA. A surface trough across the eastern half of the NMA and CMA will allow the opportunity for afternoon precipitation, however, limiting ozone formation, along with another day of fast southerly onshore flow aloft. The development and coverage of afternoon precipitation across the western portions of the NMA and CMA will be the primary forecast questions on Sunday. Across the SMA, continued onshore flow and a mix of sun and clouds will continue to keep ozone in check. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast and air mass characteristics, with a focus on the western parts of the NMA and CMA.

-Enlow/Huff