Daily Archives: July 17, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday July 17, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 17, 2017
Valid: July 18-22, 2017 (Tuesday-Saturday)

Summary:

A broad ridge will build over the southern Great Plains and slowly move eastward resulting in an elevated risk for exceedance throughout the medium range period. Mostly sunny skies by Tuesday afternoon will combine with light/calm winds and slightly above average temperatures to create conditions that will yield a High risk of an exceedance. As the frontal boundary dissipates, it will give way to high pressure building over the western SMA and Great Lakes on Wednesday. Another day of mostly sunny skies, above average temperatures and light surface winds will be favorable for ozone formation. The main forecast question for Wednesday is how fast the air mass will begin to modify as the high pressure builds over the western SMA. A combination of these factors will bring risk of an exceedance down to Appreciable. A back door cold front will approach the NMA on Thursday. Mostly sunny skies ahead of the front will give way to increasing clouds throughout the late afternoon and evening hours. Mostly sunny skies and continued southerly flow in the SMA will allow for temperatures to rise into the upper 90s °F, while flow aloft in the NMA and CMA will shift to westerly. Possibly the biggest forecast questions on Thursday will be the southward and eastward extent of the smoke plume over the Midwest and southern Canada and the influence of cloud cover associated with mesoscale convective systems well to our west. A combination of the atmospheric conditions and uncertainty pertaining to the upwind air mass and cloud cover will result in Appreciable risk for an exceedance on Thursday. A back door cold front will enter the NMA, pushing to and stalling along the MDL by 12Z Friday. Mostly sunny skies with periods of clouds with allow temperatures to rise into the mid-upper 80s F in locations to the north of the front and into the mid-upper 90s °F for the rest of the region. Risk of an exceedance will decrease to Marginal due to the possibility of precipitation in the NMA and CMA on Friday. Uncertainty with the placement of shortwaves between the EC and GFS on Saturday brings uncertainty to the precipitation forecast in the CMA and NMA. In the SMA, Saturday will be the hottest day of the week as temperatures near/exceed the 100 °F mark. Mostly sunny skies and above average temperatures will be enough to overcome northwesterly flow aloft, allowing for some ozone formation to take place. Risk of an exceedance will remain Marginal on Saturday due to the uncertainty in the forecast.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in consensus with the synoptic setup through the end of the medium range period. A strong upper level ridge will build over the southern Great Plains and its axis will slowly drift eastward to the southeastern US by Friday. This is the classic “ring of fire” weather pattern where mesoscale convective systems (MCSs) are likely to develop each evening over the Midwest, crest the ridge overnight and then dissipate in the morning hours as the remnants move ESE down the eastern slope of the ridge toward the mid-Atlantic.

For air quality concerns, the key forecast issue will be the location, strength and timing of the MCSs that are expected to form upstream. We do not expect convection with these systems to affect the mid-Atlantic, however, as we observed last week, cloud cover associated with the remnants of the MCSs can be long-lived and impact ozone formation locally. Local weather conditions will be conducive to ozone formation with high temperatures and light winds.

The forecast models are in good agreement on the large scale pattern but their forecasts for daily mesoscale disturbances differ. As a result, there is high uncertainty in the day-to-day ozone forecasts through the medium range period. Our best sense of the daily forecasts are given below.
The Dailies:

A dissipating frontal boundary and high pressure building over the SMA and Great Lakes will bring a mixed bag of conditions on Tuesday. A frontal boundary draped across the NMA will bring mostly cloudy skies in the morning hours but nearby high pressure will result in mostly sunny skies by the afternoon. Southerly flow aloft will bring in an increasingly humid air mass to locations south and east of the frontal boundary and will combine with mostly sunny skies and near average temperatures creating a chance for isolated afternoon/evening thunderstorms in the CMA and NMA. Locations to the north and west of the boundary will have northerly/westerly flow aloft, possibly transporting a smoky air mass into the western NMA. The most recent NOAA Fire Analysis shows areas of high density smoke over the Upper Midwest and southwestern ON. Although back trajectories for PIT do not quite reach into these regions, the possibility for dilute smoke transport remains. The 06Z air quality models are picking up on these conditions as all of the models develop isolated/scattered USG ozone along the I-95 corridor, Washington metro area, PIT, and a few location in the SMA. USG ozone may be developing in the air quality models due to light/calm surface winds under mostly sunny skies throughout the evening and afternoon hours. As a result of these conditions, risk of an exceedance will be High for Tuesday.

High pressure will continue to build over the western SMA on Wednesday, resulting in mostly sunny skies across the Mid-Atlantic. Persistent southerly flow aloft in the NMA and CMA will continue a warming trend as temperatures rise a few degrees. In the SMA, flow aloft will begin to recirculate as the high pressure moves overhead. Another day of mostly sunny skies and light surface winds will be favorable for ozone formation. The 06Z air quality models are picking up on the persistent ozone favorable conditions as the trend of increasing ozone continues. The BAMS models are both developing a swath of ozone along the I-95 corridor, stretching through the D.C. area and into VA. This outcome should be taken with a grain of salt, as the BAMS models have been over predicting ozone for the last several days. The main forecast question for Wednesday is how fast the air mass will begin to modify as the high pressure builds over the western SMA. A combination of these factors will bring risk of an exceedance down to Appreciable.

A back door cold front will approach the NMA on Thursday. Mostly sunny skies ahead of the front will give way to increasing clouds throughout the late afternoon and evening hours. Precipitation ahead of this front is expected to move into the NMA 18Z Tuesday but not push further south than I-80 until early Friday morning. Mostly sunny skies and continued southerly flow in the SMA will allow for temperatures to rise into the upper 90s °F, while flow aloft in the NMA and CMA will shift to westerly. The two biggest forecast question on Thursday will be: (1) The location of the smoke plume currently over southern Canada, and, (2) the possibility of upper level clouds advected from upstream MCS development. Light westerly surface winds will be favorable for ozone accumulation as high pressure remains to the west of the region. The 06Z BAMS models appear to level off ozone as USG appears to be confined to the Chesapeake Bay and the eastern NMA where this I often an artifact of the model. The models develop widespread Moderate ozone across the rest of the region with isolated areas of Good and a few isolated areas of USG in the NMA and SMA. Risk of an exceedance will remain Appreciable

A back door cold front will enter the NMA, pushing to and stalling along the MDL by 12Z Friday. This will result in an increase in clouds and the chance for a few showers in the NMA and CMA during the morning and afternoon hours. Mostly sunny skies with periods of clouds with allow temperatures to rise into the mid-upper 80s F in locations to the north of the front and into the mid-upper 90s °F for the rest of the region. The 06Z BAMS models are responding to the slowly changing air mass behind the front as they drop ozone into the Good range across most of the NMA, while locations to the south of the MDL remain in the mid-Moderate range. Risk of an exceedance will decrease to Marginal on Friday.

The forecast models have considerable uncertainty with the placement of the slow moving back door front and shortwaves aloft on Saturday. The EC has stronger shortwaves moving into the region and therefore develops a stronger surface low pressure near the stalled frontal boundary over the MDL. This outcome would bring more widespread heavy rain across the NMA and CMA than the more conservative GFS. The GFS keeps only scattered showers across the CMA as it moves very weak shortwaves overhead. Temperatures in the NMA and CMA will be dependent on the possible precipitation as the GFS is showing temperatures along the I-95 corridor reaching into the 90s °F with no precipitation and cloud cover where the NAM and EC show temperatures limited to the mid-80s °F with more clouds and rain. In the SMA, Saturday will be the hottest day of the week as temperatures near/exceed the 100 °F mark. Mostly sunny skies and above average temperatures will be enough to overcome northwesterly flow aloft, allowing for some ozone formation to take place. Risk of an exceedance will remain Marginal on Saturday due to the uncertainty in the forecast.

-Enlow/Ryan