Daily Archives: July 28, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday July 28, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 28, 2017
Valid: July 29-August 2, 2017 (Saturday-Wednesday)

Summary:

The big change for today is that the weather models are trending back toward a progressive solution for the weekend shortwave/surface wave that threatened to bring several days of flooding rains to the region. As a result, mid-level ridging will be able to build eastward into the Mid-Atlantic sooner than expected. Consequently, an unseasonably cool weekend will keep the risk of an exceedance Slight before ozone friendly synoptic conditions set up for the rest of the medium range period, increasing the risk to Marginal on Monday and Appreciable on Tuesday and Wednesday. Periods of heavy rain are still expected across the region Friday night through Saturday morning, but skies will clear by Saturday afternoon at most locations. Breezy northeasterly winds and a cool air mass filtering into the Mid-Atlantic will result in a Slight risk for an exceedance on Saturday. Sunday will be cool and considerably less humid as the Canadian air mass pushes into the entire region. Lower Sunday emissions, northerly flow aloft and below average temperatures will keep the risk of an exceedance remain Slight on Sunday. As the mid-level low pulls away, mid-level high pressure will build into the Mid-Atlantic on Monday, making it a likely transition day. Mostly sunny skies and light/calm surface winds will allow temperatures to rise into the 80s °F across the region. A final day of northeasterly flow aloft and about 2 km of vertical mixing should be able to keep any ozone accumulation minimal, however. Any accumulations are mostly likely in the western Mid-Atlantic, where the effects of the building ridge will be felt first. The risk of an exceedance will rise to Marginal. Tuesday and Wednesday are days to watch for potentially high ozone. Surface high pressure will move into Ohio River Valley on Tuesday and the western VA on Wednesday, resulting in an ozone friendly setup across the Mid-Atlantic. Temperatures will rise into upper 80s to low 90s °F on Tuesday and the low-to-mid 90s °F under mostly sunny skies and relatively strong subsidence, preventing any synoptic scale precipitation. The main forecast question will be how quickly ozone rises upwind, as ozone in the residual layer will be a key component for any exceedances on Tuesday and Wednesday. Risk of an exceedance will rise to Appreciable for Tuesday and Wednesday, with the usual focus on the I-95 Corridor and urban centers across the region.
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The big change for today is that the weather models are trending back toward a progressive solution for the weekend shortwave/surface wave that threatened to bring several days of flooding rains to the region. The EC is the most progressive, taking the low offshore fastest on Sunday. The GFS is the slowest, keeping the 500 mb shortwave over eastern VA/NC on Sunday, while the NAM splits the difference. At mid-levels and the surface, the solutions are somewhat closer, with the mid-level low moving offshore by Sunday and precipitation tapering off on Saturday afternoon for most of the region. The GFS and EC diverge again slightly by the end of the period regarding a closed low in ON that will push a cold front toward the region on Wednesday.

The upper level trough that will bring periods of torrential rain to most of the Mid-Atlantic Friday night through Sunday morning will be centered over the NMA and CMA by 12Z Saturday, with the southern extent of the trough reaching into the Southeast U.S. The shortwave energy within the trough axis will drop slightly southward over VA, spreading out and extending to the northeast into New England by 18Z Saturday, as the trough slowly progresses eastward to the coast. By 12Z Sunday, the models diverge with the eastward progression of the shortwave energy within the trough axis, as described above. The EC is the fastest to move the trough east, advancing the energy off the coast, whereas the GFS and NAM keep the energy along the coastline through 18Z Sunday. All of the models are trending away from lingering the upper level disturbance over the Mid-Atlantic as we saw in yesterday’s model runs, but the EC is the most progressive. By 00Z Monday, the GFS is the lone model with shortwave energy associated with the weakening upper level trough over the Mid-Atlantic, as it lingers over the CMA and SMA through 06Z Tuesday. By 12Z Monday, the NAM and EC still have weak troughing lingering over the Mid-Atlantic, as shortwaves across the Southern Plains and Southeast prevent upper level ridging from building eastward, but mid-level ridging does start to encroach into the Mid-Atlantic. The broad mid-level ridge will fully build over the region on Tuesday, centered roughly over the Tennessee River Valley, and keep pushing eastward on Wednesday. By 12Z Wednesday, a strong closed low moving eastward over the Hudson Bay will begin to push a cold front toward the northeastern U.S., slightly suppressing the mid-level ridge southward, but the front will remain well north of the Mid-Atlantic. There are slight differences between the way the EC and GFS handle the Canadian low and mid-level ridge, but the models are in general agreement on their strength and placement.

The Dailies:

The weather models have backed off on a washout across the Mid-Atlantic during the daylight hours of Saturday, given the fact that the wave of low pressure will not linger along the Delmarva coast through the weekend. Heavy downpours are still expected throughout Friday night and into early Saturday morning but hi-res weather models are showing more scattered and isolated showers across the region on Saturday. Precipitation in the NMA and CMA is expected to lighten between 12Z and 15Z with most of the showers concentrated to the CMA. Periods of sun and clouds are expected at locations along and north of the MDL once showers vacate the area. Despite partly sunny skies, brisk counterclockwise flow around the mid-level low, with generally northeasterly back trajectories, and a cool air mass filtering into the Mid-Atlantic behind the front will keep temperatures in the mid-70s °F across the region. In the SMA, scattered showers associated with the frontal boundary will push to coastal regions throughout the morning hours. Skies are expected to become mostly sunny behind the front allowing temperatures to rise into the upper 70s °F. The 06Z air quality models are responding to the overnight washout and cool air mass filtering into the region as they develop widespread Good ozone across the NMA and CMA with a few areas of low Moderate in the SMA where showers are less likely. As a result of these factors, risk of an exceedance will be Slight on Saturday.

Sunday will be cool and considerably less humid as the Canadian air mass pushes into the entire region. High pressure that has been building over the Upper Midwest will begin to drop southward on Sunday into the western Ohio River Valley as the area of low pressure pushes off the Atlantic coast. This mid-level ridge of high pressure will reach into the western Mid-Atlantic starting 18Z Sunday, promoting subsidence and mostly sunny skies across most of the region bringing temperatures back up closer to normal, but locations in the eastern Mid-Atlantic will remain cooler under the influence of northeasterly flow aloft. The 06Z air quality models keep Good ozone levels across the region with the exception of the I-95 Corridor and a few isolated locations in the SMA, where they paint low Moderate conditions. Although mostly sunny skies, diminishing surface winds and warmer conditions will be favorable for some ozone formation, lower Sunday emissions, flow aloft and a clean Canadian air mass still in place will limit any ozone accumulations. Risk of an exceedance will remain Slight on Sunday.

Mid-level high pressure will build over the Mid-Atlantic on Monday. Mostly sunny skies and light/calm surface winds will allow temperatures to rise into the 80s °F across the region. A final day of northeasterly flow aloft and about 2 km of vertical mixing should be able to keep any ozone accumulation minimal on Monday. These conditions are appearing in the 06Z BAMS air quality models as they develop a band of low-mid Moderate ozone along I-95 across the NMA and CMA and central NC. As mentioned in previous discussions, the air quality models have been over predicting ozone magnitude across the region for the last few weeks. The rest of the region is under ozone in the Good range for Monday, which suggests that ozone will remain well inside the Moderate range. Any accumulations are mostly likely in the western Mid-Atlantic, where the effects of the building ridge will be felt first. The risk of an exceedance will rise to Marginal on Monday.

Tuesday and Wednesday are days to watch for potentially high ozone. Surface high pressure will move into Ohio River Valley on Tuesday and the western VA on Wednesday, resulting in an ozone friendly setup across the Mid-Atlantic. Temperatures will rise into upper 80s to low 90s °F on Tuesday and the low-to-mid 90s °F under mostly sunny skies and relatively strong subsidence, preventing any synoptic scale precipitation. Light/calm surface winds will combine with mostly sunny skies and stagnating flow aloft to allow ozone to accumulate across the region. Tuesday, the BAMS models continue the trend of increasing regional ozone as scattered Moderate ozone takes over the Mid-Atlantic, with widespread Moderate in the SMA. The band of Moderate ozone along the I-95 Corridor from Monday increases to upper Moderate. The main forecast question will be how quickly ozone rises upwind, as ozone in the residual layer will be a key component for any exceedances on Tuesday and Wednesday. Given the confluence of ozone favorable synoptic conditions, risk of an exceedance will rise to Appreciable for Tuesday and Wednesday, with the usual focus on the I-95 Corridor and urban centers across the region.

-Enlow/Huff