Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 29, 2016
Valid: July 30-August 3, 2016 (Saturday-Wednesday)

Summary:

Clouds and precipitation will reign supreme through the weekend as a wave of low pressure slowly progresses across the northern and central Mid-Atlantic, but at least some sunshine will return by next week as the wave departs and a broad upper ridge arrives on Tuesday. The wave of low pressure will initiate over the Midwest on Saturday, with the wave and its associated precipitation riding into the Mid-Atlantic along a stalled frontal boundary by the late afternoon. Widespread clouds and rain along and ahead of this wave will limit ozone formation across most of the forecast region, keeping the chance for an ozone exceedance Slight on Saturday. This wave will be slow to depart on Sunday, only reaching the eastern U.S. coast by 00Z Monday. Another round of clouds and precipitation, aided by diurnal heating and convection, should suppress ozone production for another day across the Mid-Atlantic. The chance for an ozone exceedance will remain Slight on Sunday as a result. On Monday, the wave will finally depart off shore, allowing the stalled front to push southward towards NC. Despite the presumably clean air mass in place from the weekend, partly sunny skies and a shift to westerly transport may allow ozone to rise slightly along the I-95 Corridor. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday. Tuesday will be a mostly sunny day for the Mid-Atlantic as high pressure builds into the region, but a shift to onshore surface winds in the afternoon should temper rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal. Wednesday is a day to watch, as the GFS brings surface high pressure directly over eastern PA, with upper and mid-level ridges building into the region. These features will promote ample sunshine and light and recirculating winds, which historically have led to isolated USG ozone along the I-95 Corridor. The EC, however, brings a potent shortwave into the southern Mid-Atlantic on Wednesday, which inhibits the northward surge of the mid-level ridge, for a cleaner air quality solution. Given the potential for high ozone, the chance for an ozone exceedance will rise to Appreciable on Wednesday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences remaining regarding the extent of precipitation developing in association with a second of low pressure that will slowly ride a stalled front across the Mid-Atlantic through the weekend. In addition, the GFS and EC diverge on Wednesday regarding shortwave activity under an upper level ridge building over the region. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad but not particularly strong upper level trough currently over the Plains will progress eastward, reaching the Great Lakes/Ohio River Valley by 00Z Sunday. A second wave of low pressure will initiate over the Midwest, with the wave and its associated precipitation moving into the Mid-Atlantic during the afternoon along a stalled front draped across northern VA/southern MD. The semi-permanent mid-level subtropical ridge will remain centered across the southeastern U.S. through Monday. The wave of low pressure will continue eastward on Sunday, but will be slow to depart. This morning’s guidance is even slower than yesterday in its track of this wave, with it only reaching the eastern U.S. coast by roughly 00Z Monday. On Monday, the wave of low pressure will finally move offshore, but the Mid-Atlantic will remain under the influence of the broad trough aloft. The departing low will allow the stalled front to finally sweep southward toward NC on Monday. The upper level trough will finally relinquish its control of the Mid-Atlantic on Tuesday, with the leading edge of an approaching broad upper level ridge pushing in the western Mid-Atlantic by 18Z Tuesday. This will allow the mid-level ridge to build back northward into the northern and central Mid-Atlantic. The deterministic models diverge on Wednesday. Both the GFS and EC show the Mid-Atlantic remaining under the leading edge of the upper level ridge throughout the day, but the EC brings a shortwave embedded within the underside of the ridge impacting the central Mid-Atlantic by 18Z, while the GFS does not have this feature.

There is growing support for widespread clouds and convection across the Mid-Atlantic on Saturday in association with the second wave of low pressure expected to impact the region during the afternoon. All of the deterministic models bring widespread and relatively heavy precipitation across the northern and central Mid-Atlantic by 18Z. This is further by the current Hi-Res guidance, with the 00Z NMM/ARW, 03Z SREF, both the 06Z 4 km and 12 km NAM, and the 06Z 13 km GFS all showing a similar precipitation pattern. Even in areas that may not be directly affected by precipitation, widespread cloud cover will be in place. The 06Z air quality models are split regarding how much this afternoon precipitation and convection will affect air quality across the Mid-Atlantic on Wednesday. The NOAA and BAMS MAQSIP-RT models keep the eastern portions of the northern and central Mid-Atlantic in the Moderate range, with the NOAA model developing USG ozone in northern NJ, and the MAQSIP-RT models developing USG ozone across southeastern PA and parts of northern DE. The NCDENR and BAMS CMAQ models, however, reflect the wet and cloudy trend, with the northern and central Mid-Atlantic under upper Good and low Moderate ozone. The latter of these solutions seem more likely, given the consensus in this morning’s weather forecast models in developing substantial cloud cover and precipitation by the afternoon, as well as the recent tendency for the air quality models to over-forecast ozone. As a result, the chance for an ozone exceedance on Saturday will be Slight.

The wave of low pressure from Saturday will be slow to depart on Sunday, sluggishly progressing eastward toward the eastern U.S. coast, and triggering clouds and scattered precipitation throughout much of the morning and early afternoon. A second pulse of precipitation will be provided by the development of diurnal convection and thunderstorms. The NAM is a little less intense with the precipitation it develops, likely due to the fact that it shifts the most potent shortwave energy aloft to the north of the Mid-Atlantic as early as 06Z Monday. The GFS and EC, however, show widespread precipitation across the northern and central Mid-Atlantic for most of the day. The 06Z Hi-Res guidance largely agrees with the GFS/EC solution, lending confidence to its precipitation forecast. 06Z GFS back trajectories for the I-95 Corridor are onshore, which combined with clouds and rain, should limit ozone formation across the forecast region. The 06Z air quality models largely agree with this notion, with the NCDENR model keeping the entire Mid-Atlantic in the Good range for ozone, and the BAMS CMAQ model showing a similar solution except with some low Moderate ozone across eastern MD and DE. The BAMS MAQSIP-RT model, which has been running particularly high this past week, develops Moderate ozone across most of PA, as well as MD and DE. In spite of this, ample clouds and rain, a clean onshore transport pattern, and the expected presence of a relatively clean air mass from Saturday will substantially limit ozone formation. Therefore, the chance for an ozone exceedance on Monday will remain Slight.

On Monday, the wave of low pressure from the weekend will depart offshore, allowing its associated stalled front to push southward as a cold front toward NC. There will not be much of an air mass change behind this front, however, keeping most of the forecast region under seasonable temperatures and stifling humidity. There is the possibility of diurnal convection and thunderstorms due to ample regional moisture and the continued presence of shortwave energy aloft within the persistent upper level trough, with all the deterministic models developing precipitation in the vicinity of the I-95 Corridor by 18Z Monday. The NAM is most aggressive, developing widespread afternoon precipitation along the entire east coast. 06Z back trajectories for the I-95 Corridor are westerly and short at 500m AGL, though slightly faster at 1000m and 1500m. Light northwesterly surface winds could usher some slightly cleaner air into the forecast region, but might only be sufficient to push I-95 emissions to the south and east of the highway. The BAMS air quality models bring most of the I-95 Corridor up into the Moderate range for ozone, but it does not appear that they resolve any USG ozone. The main forecast questions will be how quickly the likely clean air mass in place from the weekend will be able to modify, as well as the extent of afternoon clouds and thunderstorms. Given the presence of partly sunny skies and short westerly transport, however, it seems possible that there may be at least some ozone formation along the I-95 Corridor. The chance for an ozone exceedance will rise to Slight on Monday as a result.

On Tuesday, an approaching upper level ridge will build into the western Mid-Atlantic, allowing the mid-level ridge to surge back northward. At the surface, high pressure will settle over northwestern PA/Lake Erie. These synoptic features will support mostly sunny skies across the Mid-Atlantic, with diminishing surface winds later in the day. Once again, there is the chance for afternoon clouds and thunderstorms, but the model support for this is much less convincing in comparison to Monday. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL, but short and localized at 1000m and 1500m. These factors suggest rising ozone along the I-95 Corridor, but a shift to onshore surface winds in the afternoon may be sufficient to temper rising ozone. The BAMS models respond to this in a surprisingly convincing fashion, bringing most of the northern and central Mid-Atlantic well into the Good range for ozone. Given uncertainty in the impacts of approaching high pressure, the chance for an ozone exceedance will remain Marginal on Tuesday.

Wednesday is a day to watch, even 5 days out. The Mid-Atlantic will be under the leading edge of the upper level trough on Wednesday, with the axis over the Great Lakes. The GFS and EC do diverge on Wednesday regarding the synoptic pattern. The GFS solution has surface high pressure settling directly over eastern PA by 12Z, promoting ample sunshine and light and recirculating winds across the northern and central Mid-Atlantic. Historically, this type of synoptic setup is conducive to rising ozone along the I-95 Corridor, with isolated USG ozone possible. If the EC solution verifies, the mid-level ridge will not be nearly as strong over the Mid-Atlantic, which would promote cleaner air quality conditions. Given the possibility of high pressure directly overhead, the chance of an ozone exceedance will rise to Appreciable on Wednesday, mainly for the I-95 Corridor.

-Brown/Huff