Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 23, 2016
Valid: June 24-28, 2016 (Friday-Tuesday)

Summary:

Though the weekend and Monday remain days of interest for the Mid-Atlantic, the synoptic setup reflected in today’s guidance is continuing to look less supportive of a widespread ozone event. It will still be increasingly warm, with low humidity and light surface winds, but persistent sustained onshore transport will potentially keep most locations out of the USG range. On Friday, the forecast models are now showing a second shortwave aloft moving through the Mid-Atlantic along the quasi-stationary front across the region, possibly triggering another round of clouds, showers, and thunderstorms, similar to today’s. The models are not in consensus as to geographic extent of the precipitation, so close consideration of the 12Z weather model guidance is recommended. Overall, the trend today appears to be for less sun across the northern Mid-Atlantic, which would result in less ozone formation on Friday. The main change in today’s guidance for the weekend is that surface high pressure remains farther north and east, particularly on Sunday. As a result, the transport pattern aloft is onshore, not westerly. The risk of ozone exceedances certainly remains for the weekend, due to warm conditions and full sun, with locations along and west of I-95 having the best chances for USG ozone. Monday will continue to be very warm, but the forecast models are consistent in developing an organized line of precipitation ahead of a slow-moving and weak cold front. This precipitation, along with continued onshore transport, will likely limit rising ozone, but the chances for an exceedance remain along I-95. As a result, the chances for an ozone exceedance are Appreciable for Saturday-Monday. On Tuesday, the models bring the cold front through the region early in the day, but due to the possibility that the front may slow or stall, the chances for an ozone exceedance will drop but only to Marginal for Tuesday.

Discussion:

The weather forecast models are all back in close agreement regarding the synoptic pattern through the medium range, except for the impacts of a shortwave moving through the region tomorrow. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The wave of low pressure moving through the Mid-Atlantic today along the quasi-stationary front draped across the region will push eastward towards the Mid-Atlantic coast early Friday. However, another shot of shortwave energy will move through PA/NJ on Friday, which is causing all of the models some consternation with the precipitation forecast. The shortwave aloft will keep the stalled front farther north, across southern VA, compared to yesterday’s guidance. The EC is most aggressive with developing another round of precipitation across most of the northern and central Mid-Atlantic on Friday, while the GFS and NAM keep the bulk of the precipitation in the vicinity of the front, along the central and southern parts of the region. The Hi-Res models are also split, with the 00Z NMM/ARW showing widespread showers and some convection across much of PA and NJ, as well as DE, MD, and VA, while the 06Z 4 km NAM keep the precipitation further south. The shortwave energy will be slow to depart on Saturday, allowing a minor upper level trough to remain over the northeastern Mid-Atlantic/New England through Saturday morning, which will slow the advancement of the upper level ridge building eastward from the Great Lakes. The eastern edge of the upper level ridge will push into the western Mid-Atlantic around 12Z Saturday, and fully build over the region by late Saturday afternoon, with the ridge axis over the Mid-Atlantic by 00Z Sunday. Concurrently, a mid-level ridge will hook up with the Bermuda High and extend over the entire eastern U.S. Both features will remain in place through Sunday. In today’s guidance, the EC does not keep Thursday’s and Friday’s shortwave energy off of the Mid-Atlantic coast over the weekend as it did in yesterday’s runs, but instead moves it north and east to the NS coast. A potent upper level shortwave will move eastward across southern SK and ON on Monday, which will bring a weak cold front to northeastern OH by 00Z Tuesday. The GFS and EC are in remarkably close agreement with the placement of this front. They are still developing an organized line of precipitation ahead of the front, stretching north/south across the entire region, reaching the I-95 Corridor by 00Z Tuesday. The main differences today are that the precipitation is not quite as strong, but it does move a bit farther east. On Tuesday, the sluggish cold front will reach roughly to I-81 by 12Z and slowly progress eastward, generating additional precipitation.

Friday may be a cloudier and rainier day for the Mid-Atlantic than previously expected, with the forecast models now showing a second shortwave moving across the region, as discussed above. If the EC and NMM/ARW solutions verify, the high pressure that was expected to move in from the north/northwest will be impeded, with less sun than was previously anticipated for the northern Mid-Atlantic. We will have to wait for the 12Z model guidance to make a determination about tomorrow’s air quality forecast, but the chances for rising ozone appear to be less. If the NAM and GFS solutions verify, the heaviest of the precipitation will impact the southern Mid-Atlantic, where diurnal thunderstorms will likely fire up ahead of the front. The main forecast question will be how far north the lingering precipitation on Friday will reach, and thus how much clearing can be expected in the northern Mid-Atlantic. Slight cooling at the mid-levels in the northern Mid-Atlantic, as well as northwesterly back trajectories for the I-95 Corridor, should limit rapid ozone formation even if skies are sunny. Furthermore, 06Z Hi-Res models are showing strong onshore surface winds along the coast, which should limit ozone formation at locations along and east of the I-95 Corridor. The 06Z air quality models are buying into the sunnier solutions for the northern part of the region, with Moderate ozone conditions along and west of the I-95 Corridor. Notably, the BAMS MAQSIP-RT has a bulls-eye of USG ozone in northern VA, which seems unlikely given the trend toward more precipitation tomorrow. Given the questions about clearing across the northern Mid-Atlantic, the chance for an ozone exceedance on Friday will be Marginal.

Of interest this morning are the Moderate PM2.5 concentrations across NC, most likely due to transported smoke from fires in the Southwestern U.S. If the cold front does not reach to NC on Friday, this smoky air should remain over the southern Mid-Atlantic, likely keeping PM2.5 concentrations well inside the Moderate range, and depending on how much NOx is in the air mass, possibly enhancing ozone formation as well.

Though Saturday and Sunday remain days of interest for the Mid-Atlantic, the trend in today’s guidance continues toward a synoptic setup that is less supportive of a widespread ozone event. The surface high that was expected to drop southward into the northern Mid-Atlantic on Sunday now appears to remain well to the northeast, which will promote onshore flow aloft for much of the region, especially along the I-95 Corridor. On Saturday, the upper level ridge will reach the western Mid-Atlantic by 12Z Saturday and envelop the entire region by the early evening, remaining in place through Sunday. Its associated surface high pressure moves just north of the region across NY by 12Z Saturday and up to NS by 12Z Sunday, which is much further north and east than yesterday’s guidance, which placed the center of high pressure over PHL on Sunday morning. With this more northward placement, the stationary front from Friday will remain across northern NC on Saturday, triggering clouds and showers, and will push south of the region by 12Z Sunday. Clear skies, subsiding winds, and gradually rising temperatures are expected across the northern and central Mid-Atlantic on Saturday and Sunday, but onshore transport both days should be sufficient to keep the majority of the locations along and east of the I-95 Corridor out of the USG range. The 06Z air quality models keep the I-95 Corridor under Moderate ozone conditions on Saturday, but the BAMS models continues to show an area of USG ozone on Sunday across northern NJ. Given the less favorable conditions for ozone formation, it seems more likely that any ozone exceedances observed Saturday or Sunday will be isolated and driven by local ozone production associated with full sun and subsidence. We will keep the chance for an ozone exceedance Appreciable for both days, mainly for locations along and west of the I-95 Corridor.

Monday still remains a question for the Mid-Atlantic due to uncertainty regarding the precipitation forecast. Clouds and precipitation ahead of the weak and slowly progressing cold front will move eastward across the Mid-Atlantic throughout the day on Monday, reaching the I-95 Corridor by 00Z Tuesday. The fact that the GFS and EC are both showing a well-organized line of precipitation ahead of a weak front, even though it is 5 days out, lends confidence to the forecast. A key forecast question will be how much afternoon sunshine the I-95 Corridor can receive before the clouds and precipitation begin to build in. Flow aloft will continue to be onshore, as well, which may have a moderating effect despite weekday emissions. Given this uncertainty, the chance for an ozone exceedance will remain Appreciable on Monday, mainly for the I-95 Corridor.

Tuesday appears to be a much cleaner day, as the cold front will move across the Mid-Atlantic during the day. Scattered clouds and showers will likely form along and ahead of the front. Back trajectories behind the front are fast and northwesterly, promoting clean conditions across the majority of the forecast region as a new air mass builds in. However, given uncertainty with the speed of the front, despite model consensus, the chance for an ozone exceedance will drop but only to the Marginal category.

-Brown/Huff