Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 11, 2016
Valid: July 12-16, 2016 (Tuesday-Saturday)

Summary:

Upper and mid-level ridges in place over the Mid-Atlantic will dominate the first half of the period, but the approach of an upper level trough and its associated weak cold front lends some uncertainty to the end of the medium range. Tuesday is a potential day of interest for the northern Mid-Atlantic. Rising temperatures and light south/southeasterly surface winds will allow for an Appreciable chance for isolated USG ozone north and west of the I-95 Corridor in PA and NJ. On Wednesday, the chances for an ozone exceedance drop to Marginal due to increasing support for diurnal convection, mainly west of I-95. Back trajectories turn onshore (southerly) on Thursday, which, along with more widespread regional convection, will limit the chances for an ozone exceedance to Slight. The GFS and EC differ slightly on the timing of the weak cold front on Friday. The EC is slower, keeping the I-95 Corridor clear through Friday afternoon. If this solution verifies, hot and sunny conditions will allow the chances for an ozone exceedance to rise to Appreciable. The slow-moving cold front will stall somewhere over the central Mid-Atlantic through the weekend, promoting scattered clouds and showers, and dropping the chances for an ozone exceedance to Marginal for Saturday.

Discussion:

The weather forecast models are in fair agreement through most of the medium range, with some slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An amplifying upper level ridge currently over the Northern Plains will push eastward on Tuesday, completely enveloping the Mid-Atlantic by 00Z Wednesday. Shortwave energy developing beneath this ridge will stream into the central and southern Mid-Atlantic throughout Tuesday. Concurrently, a mid-level ridge centered over the Southeastern U.S., hooked up with Bermuda High, will extend into the Mid-Atlantic through Wednesday. The upper-level ridge will begin to depart on Wednesday as a closed low aloft slowly progresses eastward across southern MB/ON. Lobes of vorticity will continue to move across the central and southern Mid-Atlantic beneath the ridge on Wednesday. The Canadian closed low will open up into a broad trough on Thursday with its southern extent swinging down into the Great Lakes region, suppressing the mid-level ridge southward and pulling a cold front near the Ohio River Valley by 12Z. On Friday, the upper level trough gradually pushes southeastward, pulling the cold front into the northern Mid-Atlantic by 12Z. The GFS shows a more organized circulation within the trough compared to the EC, which translates into a slightly faster and more compelling cold front. This trough will begin to lift slowly northeastward on Saturday, but the majority of the Mid-Atlantic will remain under its influence through the end of the medium range, with the cold front stalling somewhere across the central Mid-Atlantic through the weekend.

Tuesday is a potential day of interest for the Mid-Atlantic. Surface high pressure currently over the northern Mid-Atlantic will move just off the NJ coast by 12Z Tuesday. The high should be close enough to promote clear skies and light southerly surface winds throughout much of the northern and coastal central Mid-Atlantic. Ample atmospheric moisture will drive diurnal convection across most of the southern and inland portions of the central mid-Atlantic, however. As a result, the main areas of concern for ozone formation are north and west of the northern branch of the I-95 Corridor. 06Z GFS back trajectories for the I-95 Corridor are northerly, but relatively short at 500 m AGL, originating in southern NY before recirculating in the vicinity of DE. This is reflected within the 06Z air quality guidance, with the NOAA, BAMS, and NCDENR models all developing isolated patches of USG near PHL/TTN and across central and northern NJ. Additionally, the BAMS MAQSIP-RT also shows patchy USG ozone across western and south central PA, as well as northern MD. The main questions will be 1) how quickly the very clean air mass in place can modify in response to local ozone production, and 2) the impact of light onshore south/southeasterly surface winds, which tend to blow emissions from I-95 westward. The chance for an isolated ozone exceedance will borderline Appreciable on Tuesday for locations north and west of the I-95 Corridor as a result.

On Wednesday, high pressure will move further offshore, but will remain close enough to the eastern U.S. coast to encourage continued southerly and southeasterly surface winds. Back trajectories for the I-95 Corridor shift onshore as a result, although they are somewhat recirculating in the morning. Increasing regional moisture and intense afternoon sunshine will support the development of diurnal convection and thunderstorms throughout the Mid-Atlantic, which are shown in the GFS and EC, as well as the 06Z 4 km NAM and 03Z SREF. As a result, ozone formation will be limited along the I-95 Corridor in comparison to Tuesday. The current runs of the air quality models limit ozone across most of the region to the Good range, with some scattered Moderate in the northern Mid-Atlantic. The NCDENR models shows a very small area of USG ozone stretching down into northern NJ. There should be enough factors in place to temper rising ozone across the forecast region, but another day of intense sunshine will likely promote some ozone production. Therefore, the chance for an ozone exceedance will drop slightly to Marginal on Wednesday.

The approaching weak cold front will remain to the west of the Mid-Atlantic on Thursday, allowing for another day of abundant sunshine and rising ozone. Even more widespread diurnal clouds and thunderstorms are expected to form in the late afternoon due to rapidly rising moisture and intense heat. Back trajectories for the I-95 Corridor turn fully onshore from the south, which should keep the I-95 Corridor clean. The BAMS models bring the northern Mid-Atlantic and the eastern edge of the central Mid-Atlantic up into the Moderate range on Thursday, however. Given the chances for widespread afternoon convection and the onshore flow aloft, this model guidance seems overdone. The chance for an ozone exceedance will drop to Slight on Thursday as a result.

On Friday, there is uncertainty in the air quality forecast due to questions about the speed of the weak cold front. It should reach northwestern PA by 12Z and push southeastward across the region during the day. Widespread clouds and precipitation are expected along and ahead of this front. The main forecast question will be how quickly this front, and its associated precipitation, progress across the Mid-Atlantic. The GFS is slightly faster with the track of the front, showing organized precipitation across the northern and central Mid-Atlantic through 00Z Saturday, whereas the EC keeps the I-95 Corridor clear for the entirety of Friday. If an EC-like solution verifies, another day of intense afternoon sunshine across the northern Mid-Atlantic could allow ozone levels to rise rapidly along the I-95 Corridor. 06Z GFS back trajectories for the I-95 Corridor shift westerly, which could contribute to rising ozone depending on how high ozone concentrations are upstream. The current BAMS models are siding with the EC, developing USG ozone along the I-95 Corridor, with particular widespread exceedances shown in the BAMS-CMAQ (which even develops Unhealthy ozone across DC). Given the uncertainty in the exact placement of the front, the chance for an ozone exceedance will rise to Appreciable on Friday for locations along I-95, assuming they will remain clear through the afternoon.

The cold front should push southward toward southern VA by 12Z Saturday. However, there will not be a substantial air mass change behind the front, since it will be a very weak front. The main impact will be a slight decrease in dew points. Clear skies and calm winds across the northern and southern Mid-Atlantic will promote ozone formation along the I-95 Corridor. The only exception to this is if the EC solution verifies, which would mean that the cold front does not arrive until Saturday, promoting widespread clouds and convection across the northern and central Mid-Atlantic. The chance for an ozone exceedance will drop slightly to Marginal on Sunday as a result.

-Brown/Huff