Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 25, 2017
Valid: July 26-30, 2017 (Wednesday-Sunday)

Summary:

A combination of unsettled conditions to finish the work week and a cool air mass moving into the region over the weekend will keep the risk of an exceedance in the Mid-Atlantic minimal throughout the medium range period. Wednesday will be a pleasant day across most of the Mid-Atlantic as high pressure moving over NY/NJ will promote partly skies. Despite the clean air in place, the air quality models develop an isolated area of USG ozone in the Washington D.C. area on Wednesday. This feature is consistent through the BAMS and NC models and has been appearing in the air quality models for the last few days. It is likely in response to converging surface winds at inland locations, enhanced by emissions from I-95. As a result, risk of an exceedance will be Marginal on Wednesday with the lone focus on the MD/VA border near Washington D.C. Warmer and more humid conditions will return to the Mid-Atlantic on Thursday as a warm front lifts across the NMA and CMA. The timing of the precipitation is the biggest forecast question for Thursday. The operational weather models show that precipitation associated with an approaching cold front/surface wave is not expected to heavily impact the Mid-Atlantic during the daylight hours, while the 03Z SREF suggest that precipitation will reach I-81 by 21Z and approach I-95 by 00Z. In addition, the SPC has most of the region in a Slight risk of severe weather for fast-moving thunderstorms. Onshore flow, breezy winds and partly cloudy skies in the SMA will keep ozone formation to a minimal. Risk of an exceedance will be Marginal, with a focus on areas west of I-95, due to the uncertainty in the timing of precipitation. The approaching front/surface wave will push through the NMA and CMA on Friday before stalling along the southern edge of the SMA Friday evening. Widespread heavy rain and mostly cloudy skies are expected through the day across the NMA and CMA. Despite periods of morning and afternoon sunshine, onshore flow will keep ozone accumulation to a minimal in the SMA. A seasonably cool and much less humid Canadian air mass will move into the Mid-Atlantic for the weekend. The wild card for the weekend will be the potential for smoke from ON wildfires to reach the Mid-Atlantic. There is a large expanse of dilute smoke aloft over Canada, enhanced locally by fires burning in northern ON near Hudson Bay. As the Canadian air mass moves into the Mid-Atlantic, back trajectories shift northerly into ON, pulling air from the vicinity of the wildfires. Given below normal temperatures expected in the NMA and CMA on Saturday and Sunday, ozone exceedances are unlikely even if the air is smoky, but a fast rise in particle concentrations is possible. Risk of an exceedance will remain Slight for the weekend.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models have come into closer consensus today in regards to timing and strength of upper level features. The models are in general agreement with the synoptic setup through 00Z Saturday, when the GFS and EC diverge with strength and southward extent of shortwaves associated with an upper level closed low over QC and an open trough over the eastern US. By 12Z Wednesday, the upper level trough moving over the Mid-Atlantic today will have moved over the Atlantic allowing weak upper level ridging to build over the eastern US. This ridging will be slightly amplified over southern ON/QC and the northeast US as it is wedged between the departing upper level low and a strengthening closed low over western ON. The center of this next upper level low is expected to pass to the north of the Great Lakes on Thursday, pulling an associated cold front eastward, but a series of small shortwaves will move across the Midwest and into the Mid-Atlantic, bringing a short period of zonal flow to the Mid-Atlantic on Thursday. By 12Z Thursday, a stronger shortwave will drop down over MN/WI, digging out a weak through over the Midwest through 00Z Friday. By 06Z Friday, this strong shortwave will phase with the series of shortwaves over the NMA, creating a strong trough over most of the eastern US. The axis of this trough will move into the western Mid-Atlantic by 18Z Friday, but the length of time it remains over the Mid-Atlantic is in question as the GFS and EC diverge soon after. The disagreement between the GFS and EC starting at 06Z Saturday impacts how the upper level low progress throughout Saturday and Sunday. The EC keeps the parent upper level closed circulation over QC and moves it into the Canadian Maritimes by 00Z Sunday, with the strong shortwave in the trough axis moving over the Mid-Atlantic between 18Z Friday and 00Z Sunday before moving offshore. The EC keeps these two upper level lows separate. By 12Z Sunday the EC develops shortwaves across the Southeast, keeping weak upper level toughing over the eastern US through 00Z Monday. In comparison, the GFS has a similar outcome but the parent upper level closed low is much broader and drops into southeastern ON by 06Z Saturday, phasing the shortwave in the axis with the shortwave energy in the closed circulation by 18Z Saturday. In this way, the GFS phases the two upper level lows. This results in one large upper level trough/closed low over the northeast US and southeast Canada by 00Z Sunday, which is stronger than the one the EC develops.
The Dailies:

Wednesday will be a pleasant day across most of the Mid-Atlantic as high pressure moving over NY/NJ will promote a mix of sun and clouds. Northerly flow aloft will tap into the cooler air mass left behind by the recent cold front, limiting temperatures to the low-to-mid 80s° F for most of the region. Light onshore surface winds along the NMA and CMA coast and southerly surface winds inland will slowly increase humidity throughout the day. Monday’s cold front will remain stalled across the SMA in the morning hours before weak high pressure pushes the boundary off the coast by 00Z Thursday. This will result in the chance for scattered showers across the eastern SMA with partly sunny skies. The 06Z air quality models are responding to these conditions as expected with widespread Good ozone across the NMA and CMA. Despite the NOAA model being consistently low, it develops an isolated area of USG ozone in the Washington D.C. area. This feature is consistent through the BAMS and NC models and has been appearing in the air quality models for the last few days. It is likely in response to converging surface winds at inland locations, enhanced by emissions from I-95. The NC and BAMS models develop more widespread Moderate ozone across the SMA possibly due to near normal temperatures. Aside from features highlighted in the air quality models, wildfires continue to plague the Great Plains, the Pacific Northwest and central/western Canada, resulting in a large plume of dilute smoke over the Midwest and extending into the Ohio River Valley. If this smoke pushes eastward, it could impact air quality across the NMA and CMA. At this time, however, the smoke does not appear to be mixing to the surface to our west, given this morning’s current PM2.5 concentrations. Risk of an exceedance will be Marginal on Wednesday with the lone focus on the MD/VA border near Washington D.C.

Warmer and more humid conditions will return to the Mid-Atlantic on Thursday. The strong shortwave dropping down into the Midwest will interact with an existing frontal boundary and develop a wave of low pressure in the Ohio River Valley by 12Z Thursday. The warm front portion of this next storm system is expected to lift into the NMA and CMA around 12Z Thursday. The GFS and EC show a slightly different picture than the NAM, as the NAM places the wave more southward at 850mb, resulting in slightly different timing. The WPC is putting more weight into the similar GFS and EC solutions with placement and timing of this feature. The operational weather models show that precipitation associated with this system is not expected to heavily impact the Mid-Atlantic during the daylight hours; however, the 03Z SREF shows the a medium to high probability of precipitation along the western Mid-Atlantic increasing at 18Z Thursday and pushing into the center NMA and CMA by 21Z Thursday and to the I-95 Corridor by 00Z Friday. The timing of the precipitation is the biggest forecast question for Thursday as the eastward push could have an impact on air quality. The SPC has all of the region except NC in a Slight risk for severe weather on Thursday, for fast-moving bands of thunderstorms. The 06Z BAMS brings the SMA mostly into the Good range while the CMA and NMA have widespread Moderate ozone with a few isolated locations of USG ozone. The model develops the USG ozone near PIT and in the Susquehanna Valley of PA, again likely due to converging winds inland and emissions from I-95. A combination of southerly surface winds, recirculating flow aloft, and near average temperatures will set the stage for potential ozone formation. The hi-res models are showing partly to mostly cloudy skies, however, which may be enough to limit ozone formation. If precipitation is not as widespread as the SREF suggests, then increasing ozone may be possible across the NMA and CMA. Onshore flow, breezy winds and partly cloudy skies in the SMA will keep ozone formation minimal. Risk of an exceedance will remain Marginal on Thursday, with a focus on areas west of I-95, due to the uncertainty in the timing of precipitation.

The approaching wave and its associated cold front will push through the NMA and CMA on Friday before stalling along the southern edge of the SMA Friday evening. Widespread heavy rain and mostly cloudy skies are expected through the day across the NMA and CMA. Precipitation will begin around 06Z Friday, pushing eastward covering the entire NMA and CMA by 12Z Friday before pushing into the SMA by 18Z. Persistent rain and cloud cover will keep temperatures in the NMA and CMA in the upper 70s-low 80s °F, while a short period of sunshine before the rain enters the SMA will allow temperatures to rise into the mid-80s °F. Despite the period of sunshine, onshore flow will keep ozone accumulation to a minimal in the SMA as shown in the 06Z air quality models where ozone only reaches the low Moderate range in urban areas. The models clean out the rest of the region as widespread mid-Good range ozone blankets the NMA and CMA. Unsettled conditions and onshore flow in the SMA will bring risk of an exceedance down to Slight on Friday.

High pressure building over the Midwest and into the Ohio River Valley will bring more settled conditions to the Mid-Atlantic on Saturday. Despite mostly sunny skies and warmer conditions in the NMA, breezy to gusty surface winds and a strong northerly flow aloft should keep ozone accumulation to a minimum. Temperatures will remain slightly below normal and humidity will drop significantly with a cool and dry Canadian air mass filtering into the region. The wild card for the weekend will be the potential for smoke from ON wildfires to reach the Mid-Atlantic. As mentioned previously, there is a large expanse of dilute smoke aloft over Canada, enhanced locally by fires burning in northern ON near Hudson Bay. As the Canadian air mass moves into the Mid-Atlantic, back trajectories shift northerly into ON, pulling air from the vicinity of the wildfires. Given below normal temperatures expected in the NMA and CMA on Saturday, ozone exceedances are unlikely even if the air is smoky, but a fast rise in particle concentrations is possible. In the SMA, mostly sunny skies will combine with light surface winds and near normal temperatures to create an ozone friendly environment, but northerly flow aloft will limit any accumulation. The 06Z BAMS models are reacting to these conditions as widespread Good ozone remains across the NMA and CMA while a strip of upper Good/low Moderate ozone develops along central NC. Risk of an exceedance will remain Slight on Saturday.

High pressure from the upper Midwest will move into the Ohio River Valley on Sunday, resulting in another day of mostly sunny skies and warming conditions. Temperatures in the NMA will be in the low-to-mid 80s °F with diminishing surface winds. In the SMA, temperatures will rise into the upper 80s/low 90s °F with light surface winds. Another day of strong northerly flow aloft should keep ozone accumulation to a minimal across the region. But once again, the potential impact of wildfire should be monitored. Risk of an exceedance will remain Slight.

-Enlow/Huff