Daily Archives: July 10, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday July 10, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 10, 2017
Valid: July 11-15, 2017 (Tuesday-Saturday)

Summary:

Above average temperatures and mostly sunny skies throughout the week will result in multiple days of elevated risk for an ozone exceedance. A weak back door cold front will remain well north of the region through Thursday, allowing the influence of the westward extension of the Bermuda High to dominate weather conditions. Although hot and humid weather will allow for daily chances of scattered afternoon convection, aided by shortwaves aloft and the presence of a lee trough, widespread and organized precipitation is not expected. On Tuesday, a potential line of thunderstorms will arrive too late to impact locations along the I-95 Corridor from TTN to northern VA, but breezy afternoon winds should provide some atmospheric ventilation. Mostly sunny skies and temperatures in the 90s °F will promote ozone formation, resulting in an Appreciable risk of an exceedance on Tuesday, with a focus on the I-95 Corridor. Winds will diminish on Wednesday, becoming light and recirculating, with sea/bay breezes likely. Risk of an exceedance will rise to High on Tuesday with a focus on the I-95 Corridor and locations along the eastern coast of the NMA and CMA. Thursday will be the hottest day of the week, with slightly higher chances of afternoon thunderstorms compared to Wednesday. Risk of an exceedance will remain High on Thursday with focus again on the I-95 Corridor. The back door cold front will sag southward on Friday, with a wave of low pressure moving along it. Precipitation is expected to be widespread across the NMA and more scattered across the CMA and SMA. Despite the weather models showing widespread/scattered precipitation, the air quality models are not in agreement, with BAMS keeping ozone in the upper Moderate to USG range at scattered locations in the NMA. Due to the uncertainty between the air quality models, risk for an exceedance will remain Appreciable on Friday. There is some uncertainty on Saturday regarding the southern push of the back door front as well as how quickly the next, stronger cold front approaches from the northwest. Most of the region should be post-frontal, however, with a seasonable and slightly less humid air mass filtering in. As a result, risk of an exceedance will lower to Marginal on Saturday.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models have final come into consensus with key synoptic features through the end of the medium range period, although there is still uncertainty regarding the exact track of a weak back door cold front that will affect the region for most of the period. By 12Z Tuesday, an upper level closed low will be centered over southern Hudson Bay, with the associated trough axis over the eastern NMA/CMA. At the same time, the westward extension of the Bermuda High will build into the SMA and Southeastern U.S. at mid-levels. By 12Z Wednesday, the upper level low over Hudson Bay will progress eastward, moving into QC, allowing for upper level zonal flow to build over the Mid-Atlantic. The zonal flow will be disturbed by a shortwave moving over the NMA and the building of the westward extension of the Bermuda high at mid-levels throughout the day on Wednesday. Another series of shortwaves be spinning over southern ON/northern MN by 00Z Thursday with an associated surface low pressure moving through the upper Midwest/Great Lakes between 00Z and 12Z Thursday. While zonal flow continues across the CMA and SMA, this series of shortwaves will dig out a weak longwave trough over the Great Lakes and NMA by 12Z Friday. There is some disagreement between the GFS and EC with the placement of the associated upper level low in northern Canada, with the GFS building a stronger trough that extends farther south into the Mid-Atlantic. The trough axis will continue to deepen as far south as VA, moving across the NMA by 12Z Saturday, setting up an amplified pattern over the CONUS once again, with ridging over the west and troughing over the east. One last round of shortwaves will develop over western ON by 12Z Saturday and will spin across the Great Lakes, reinforcing the trough axis over the Mid-Atlantic once again through 00Z Sunday.
The Dailies:

Tuesday will be the first day of a presumably 3-day heat wave across the region, with very hot and humid conditions. I-76 will be the dividing line for unsettled conditions and mostly sunny skies during the day on Tuesday. Locations to the north of I-76 will experience scattered showers early Tuesday morning giving way to partly sunny skies by 12Z. This clearing, combined with dew points in the low 70s° F will allow for scattered thunderstorms to develop between 18Z and 21Z Tuesday. The 06Z 3 km NAM shows the thunderstorms developing along I-80 and pushing southward to I-76 by 00Z Wednesday and diminishing quickly thereafter. Locations south of I-76 will be under the influence of Bermuda High pressure, resulting in mostly sunny skies throughout the day, allowing temperatures to exceed average values. The 06Z air quality models are acknowledging precipitation in the NMA and clearing across the rest of the region, but they have widely divergent ozone predictions. The NOAA model only has a strip of Moderate ozone along I-95 from TTN to BAL, while the BAMS models have upper Moderate ozone all the way along I-95, from NYC to NC. All of the NC models did not update this morning, but the CMAQ-D is siding closer to the BAMS models. This uptick in ozone comes with no surprise as the thunderstorms in the NMA may not push through the I-95 corridor, and when combined with the forecasted lee through in the area, will create an area of converge along the eastern NMA. Weak southwesterly flow aloft, clear skies and above average temperatures will promote ozone formation in locations south of I-76, resulting in an Appreciable risk of an exceedance on Tuesday. Ample atmospheric ventilation, including breezy afternoon winds, may take the edge off rising ozone. The main forecast questions will center on location and timing of convection, as well as how quickly the air mass modifies.

Wednesday will be similar to Tuesday, but with less chance for organized afternoon/evening convection. Unsettled conditions are possible in the NMA due to a shortwave moving overhead, as well as hot and humid conditions. The nearby back door cold frontal boundary in NY and across the Great Lakes will bring some clouds to the NMA in the morning hours but skies will clear by 15Z Wednesday. Above average temperatures in the NMA will once again combine with a humid air mass and clear skies, resulting in scattered afternoon/evening thunderstorms in locations to the west of I-81. Locations to the east of I-81 will remain mostly sunny with light southwesterly/southerly winds. A lee trough will remain over the eastern NMA, resulting in converging winds between 18Z Wednesday and 00Z Thursday. High pressure will continue to build over the SMA, promoting mostly sunny skies and southwest flow aloft. Subsidence across the region will result in light/calm winds and will be favorable for ozone accumulation. In addition, sea and bay breezes appear likely. The 06Z air quality models are highlighting the I-95 corridor again on Wednesday as both BAMS models develop a strip of USG ozone along the interstate with widespread Moderate ozone across the NMA and CMA. In the SMA, the air quality models develop an area of Moderate across western NC as a result of light/calm winds and temperatures in the upper 90s °F. Risk of an exceedance will rise to High on Tuesday with a focus on the I-95 Corridor and locations along the eastern coast of the NMA and CMA.

A wave of low pressure will move along the back door cold front draped across NY State on Thursday, bringing the possibility for unsettled conditions to the NMA. With the exception of western PA, temperatures are expected to rise into or remain in the upper 90s °F for the entire region under continued southwesterly/westerly flow and mostly sunny skies. The possibility for afternoon/evening thunderstorms will continue across the NMA as dew points remain in the low 70s °F with above average temperatures and clear skies. A persistent lee trough and the nearby frontal boundary will result in another day with converging winds after 21Z, allowing ozone to accumulate. The 06Z air quality models are showing a combination of these conditions as widespread upper Moderate and isolated USG ozone remains across the eastern NMA. Despite clear skies and temperatures in the upper 90s °F across the SMA, the air quality models keep ozone in the Good/mid-Moderate range most likely to weak onshore flow as high pressure persists for one last day. Risk of an exceedance will remain High on Thursday with focus again on the I-95 Corridor.

Unsettled conditions will move across the Mid-Atlantic on Friday. High pressure building over the Ohio River Valley and the developing longwave trough aloft will push the wave of low pressure southward through the NMA and CMA along the back door cold front on Friday. Locations behind the weak cold front will see temperatures sink back to near normal and a slight decrease in humidity. Precipitation associated with this front is expected to begin around 12Z in the NMA and slowly move southward and eastward into the CMA and SMA by 00Z. Precipitation is expected to be widespread across the NMA and more scattered across the CMA and SMA. Despite the weather models showing widespread/scattered precipitation, the air quality models are not in agreement. The BAMS air quality models are developing USG/upper Moderate ozone in western PA and along the I-95 Corridor despite the possibility of precipitation, whereas the NC model is buying into the precipitation and lowering regional ozone into the upper Good/low Moderate range. Due to the uncertainty between the air quality models, risk for an exceedance will remain Appreciable on Friday.

There is uncertainty between the weather models on Saturday in regards the strength and southward extent of the upper level trough over the eastern U.S., which in turn will affect the approach of the next cold front in the Sunday-Monday time frame. There is also uncertainty as to how far south the back door cold front reaches on Saturday. The EC brings the front southward to the MDL by 00Z Saturday and to central NC by 12Z Saturday, about 24 hours quicker than the GFS. With the disagreement, the EC brings scattered showers and mostly cloudy skies across SMA on Saturday. In comparison, the GFS keeps conditions dry with mostly sunny skies across the Mid-Atlantic throughout the day, but it brings some showers to the SMA, as well as pre-frontal precipitation to the northern NMA in the evening. Given that most of the region should be post-frontal on Saturday, risk of an exceedance will lower to Marginal.
-Enlow/Huff