Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 8, 2016
Valid: August 9-13, 2016 (Tuesday-Saturday)

Summary:

Despite the presence of persistent upper and mid-level ridging patterns, the chances for an ozone exceedance will remain relatively low throughout the medium range, due to a predominantly southerly transport pattern and continued chances for afternoon clouds and precipitation throughout most of the forecast period. High pressure will drift off the Mid-Atlantic coast during Tuesday afternoon, promoting onshore flow at the surface and aloft across the forecast region. This maritime transport, as well as the presence of a clean regional air mass, will limit rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal on Tuesday. This high will move further eastward on Wednesday, with continued southerly flow causing temperatures and humidity to rise across the Mid-Atlantic. This will mark the beginning of a heat wave that is expected to affect much of the forecast region through the weekend, with temperatures rising well above average, with heat indices topping off above 100 °F through Sunday. Despite the oppressive heat, a back door cold front will arrive across the northern Mid-Atlantic on Wednesday, stalling across the NY/PA border through Friday. This front will act a focus for diurnal convection and thunderstorms across the northern and central Mid-Atlantic Wednesday through Friday, which combined with clean regional transport, will keep the chance for an ozone exceedance Slight all three days. On Saturday, an approaching low pressure system will pull another cold front towards the western Mid-Atlantic. Pre-frontal clouds and precipitation are expected to form, but there is uncertainty regarding how far east this precipitation will reach by the end of Saturday. Given this uncertainty, and the possibility of afternoon sunshine along the I-95 Corridor, the chance for an ozone exceedance will rise to Marginal on Saturday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A weak upper level ridge will build over the Mid-Atlantic on Tuesday, with its axis over southern QC by 18Z. Concurrently, a mid-level ridge also connected with the Bermuda High will remain over the Mid-Atlantic through the forecast period, though it will be slightly suppressed on Saturday with the approach of an upper level trough. On Wednesday, the upper level ridge will be shunted northeastward toward the Canadian Maritimes ahead of an upper level trough traversing northern ON/QC, whose associated backdoor cold front will move into the Great Lakes region by 12Z. Most of the northern and central Mid-Atlantic will return to zonal flow aloft, while the southern Mid-Atlantic will remain influenced by a broad and disorganized area of shortwave energy across the Gulf Coast that will linger across the Southeast U.S. through Friday. This energy is fostering a surface that is already bringing flooding rain to the eastern Gulf Coast states, and it will linger through into at least Friday. Some of this low’s shortwave energy may shear off and stream into the northern and central Mid-Atlantic on Wednesday. The zonal flow pattern aloft will remain in place across the northern and central Mid-Atlantic on Thursday, allowing the approaching cold front to stall roughly across the NY/PA border beginning roughly 12Z Thursday, and become quasi-stationary through Friday. On Friday, the zonal flow aloft will persist across most of the Mid-Atlantic, while an upper level trough extending down across the Plains shifts eastward, pulling its associated cold front into the central Plains by 12Z. The GFS and EC differ slightly on Saturday with the strength of the upper level trough and its embedded shortwave energy. The GFS shows a broader trough, extending all the way southward into the south central U.S. On the other hand, the EC shows a more compact trough, with its center across southern MB/ON and the northern Plains by 00Z Sunday. Though the upper level trough will remain largely to the west of the Mid-Atlantic through Saturday, the shortwave disturbance is expected to get absorbed into its periphery, with some of its associated energy streaming into the northern Mid-Atlantic by 00Z Sunday. Additionally, the approach of the trough will push the stationary front across the NY/PA border northward into southern QC/New England on Saturday.

Surface high pressure will drift across the northern Mid-Atlantic during Tuesday morning, before shifting offshore by the late morning/early afternoon. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating in response to the close proximity of the high in the morning, which will allow for a temporary period of light winds across the northern and central Mid-Atlantic. However, as the high moves eastward, its circulation will turn transport south/southeasterly across much of the forecast region, both at the surface and aloft. Sustained onshore winds will usher clean maritime air into the Mid-Atlantic, tempering ozone formation along the I-95 Corridor. The focus for rising ozone will therefore shift to locations north and west of I-95, where the transport of highway emissions could support ozone production. This will be aided by the presence of mostly sunny skies and seasonable temperatures, afforded by the nearby high. The 06Z NOAA and NCDENR models are responding to these factors, both developing a bullseye of USG ozone to the northwest of PHL, over east central PA near Allentown. Additionally, the NOAA model resolves USG ozone across part of northern NJ. The BAMS models, however, are more conservative, only bringing these areas up into the Moderate range. The main forecast question will be how quickly the regional air mass can modify, despite the presence of sustained onshore flow for much of the day. At this point, the air mass in place across the region looks relatively clean. Ozone concentrations were very low across much of the northern Mid-Atlantic through the weekend, although widespread NOx titration this morning could foretell ozone formation later this afternoon, which will demonstrate how easily this air mass can modify. Given the strength of afternoon onshore flow, the recent tendency of the air quality models to over-forecast ozone, and the likely relatively clean air mass in place to start the day, the chance for an ozone exceedance on Tuesday will be Marginal for areas north and west of the I-95 Corridor.

An extended period of hot and humid weather will begin on Wednesday and last through the medium range period (and possibly beyond). High pressure will shift even further offshore on Wednesday, turning the predominant transport pattern southerly across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are due south and relatively fast, originating from off the SC coast. Southerly and southwesterly surface winds will funnel heat and moisture into the forecast region. This will support the formation of afternoon clouds and precipitation, especially with the approach of the back door cold front which may provide sufficient pre-frontal lift to initiate convection across the northern and central Mid-Atlantic. All of the deterministic models show widespread and moderately heavy precipitation across the northern and central Mid-Atlantic by 18Z Wednesday. This precipitation pattern is also supported by this morning’s Hi-Res guidance, including the 03Z SREF, the 06Z 4 km and 12 km NAM models, and the 06Z 13 km GFS. A combination of onshore transport and the development of afternoon precipitation should be sufficient to limit ozone formation across most of the Mid-Atlantic. The 06Z air quality models clearly reflect this cleaning trend, with the BAMS and NCDENR models bringing the entire forecast region down well into the Good range for ozone. As a result, the chance for an ozone exceedance on Wednesday will fall to Slight.

On Thursday, a back door cold front will arrive across the NY/PA border by 12Z, where it is expected to stall. 06Z GFS back trajectories for the I-95 Corridor remain fast and generally southerly. High pressure settling over western VA around 12Z Thursday will promote southerly surface winds across the forecast region, allowing temperatures to climb above average throughout much of the Mid-Atlantic. Ample regional humidity will again support the development of diurnal precipitation, with the stalled front acting as a focus for afternoon convection and thunderstorms. The deterministic models are not as convincing in comparison to Wednesday, however, regarding the extent and placement of this precipitation. The GFS has the most precipitation, as per its usual wet bias this time of the year, with the heaviest rain focused across the northern Mid-Atlantic. The EC shows a similar placement as the GFS, but develops more scattered precipitation. The NAM brings the front slightly further south, with the heaviest of its associated precipitation located across the central Mid-Atlantic beginning 18Z Thursday. Depending on the exact placement of the pre-frontal clouds and thunderstorms, portions of the I-95 Corridor may receive sufficient afternoon sunshine to support some ozone formation. But the continued onshore transport should limit any ozone to the Good to low Moderate range, and the front should be close enough to the I-95 Corridor to generate sufficient cloud cover. Additionally, the regional air mass is expected to be relatively clean after widespread precipitation on Wednesday. The BAMS air quality models support the influence of the transport pattern and the nearby front, keeping the entire Mid-Atlantic in the low Good range. Therefore, the chance for an ozone exceedance on Thursday will remain Slight.

High pressure will slide southward on Friday, settling over western NC by 12Z. 06Z GFS back trajectories for the I-95 Corridor becoming more southwesterly, but they are still fast and should be clean. Temperatures will continue to climb, reaching the low-to-mid 90s °F across much of the Mid-Atlantic. The stalled front across the northern Mid-Atlantic, likely in the vicinity of the NY/PA border, will act as a focus for afternoon clouds and thunderstorms, with both the GFS and EC developing widespread rain across the Mid-Atlantic by 00Z Saturday. Additionally, the regional air mass in place should still be fairly clean after two consecutive days of widespread clouds and thunderstorms, which will keep ozone from spiking in the early afternoon, before convection fires up across the Mid-Atlantic. The BAMS models keep the region mostly in the Good range, with the MAQSIP-RT model showing a very small area of low Moderate ozone west of PHL. Regardless, there should be enough diurnal precipitation to keep the chances for an ozone exceedance Slight another day.

On Saturday, the quasi-stationary front across the northern Mid-Atlantic will lift northward, ahead of an approaching low pressure system located over the Great Lakes by 12Z. The cold front associated with this trough will approach the western Mid-Atlantic during the afternoon, potentially providing another round of clouds and thunderstorms to the northern and central Mid-Atlantic. The GFS shows widespread precipitation reaching the I-95 Corridor, while the EC keeps the I-95 Corridor essentially dry through 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor remain south/southwesterly from the Gulf Coast, and temperatures will be well above average across most of the forecast region. If an EC-like solution verifies, intense afternoon sunshine and southwesterly surface winds ahead of the front may allow for some ozone formation along the I-95 Corridor. Given the uncertainty in the precipitation forecast, the chance for an ozone exceedance will rise to Marginal on Saturday.

-Brown/Huff