Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 27, 2016
Valid: July 28-August 1, 2016 (Thursday-Monday)

Summary:

The weather forecast models are trending toward a common solution with the timing of several waves of low pressure that will ride along a quasi-stationary frontal boundary draped across the Mid-Atlantic for the entire period, but there is still some uncertainty, particularly on Friday. Thursday remains a day to watch closely, since there is a chance for afternoon clearing along the I-95 Corridor. But the trend in the weather models is for unsettled conditions in the afternoon, with scattered convection firing up in the vicinity of the stalled frontal boundary, which will be located near northern VA/southern MD. A complicating factor is the advancement of a slightly stronger back door cold front, which may merge with the stalled front as early as Thursday evening, further promoting clouds and precipitation. Thus, even though confidence is not high, there is growing support for enough afternoon convection to keep the chances for an ozone exceedance Marginal on Thursday. For the rest of the period, a series of waves will ride along the semi-permanent front, which will remain across the central Mid-Atlantic. The heaviest precipitation is expected on Friday, although the timing is still uncertain, with the NAM/GFS bringing the main wave through in the morning and the EC lagging behind by ~12 hours. The GFS and EC come into closer consensus with the main synoptic features on Saturday, keeping the weather unsettled for the rest of the medium range. Temperatures will drop down to around average for this time of year, and the combination of daily afternoon diurnal/frontally driven convection and a relatively clean air mass will keep the chances for an ozone exceedance Slight from Friday to Monday.

Discussion:

The weather forecast models are still struggling to come into consensus, but they are trending toward a common solution regarding the track and several waves of low pressure aloft that will ride a stalled front remaining across the central Mid-Atlantic on Thursday, possibly lifting northward on Friday, and then settling back southward into early next week. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad, upper level shortwave trough will dip down across the Midwest on Thursday, absorbing shortwave energy near MS which will then stream northeastward into the Ohio River Valley and Mid-Atlantic region beginning around 18Z. The NAM may be a little overdone with the strength of several shortwave impulses embedded within this trough, but not enough for it to be considered an outlier. The mid-level ridge/Bermuda High centered over the southeastern U.S. will be suppressed by this upper level trough. At the same time, a slightly stronger back door cold front will approach form the north, possibly merging with the stalled frontal boundary in the vicinity of south/central VA. The two weak fronts are challenging to analyze at 850 mb, but the trend certainly seems to be for another, slightly stronger, front to move into the region from the north in the late Thursday to Friday time period. On Friday, potent shortwave energy aloft will move directly over the northern and central Mid-Atlantic, initiating a wave of low pressure that will traverse the stalled front that is expected to lift into the northern Mid-Atlantic. The NAM/GFS and EC still differ on the timing of this wave, with the EC about 12 hours slower than the NAM/GFS, translating into the period of heaviest rain being either in the morning (NAM/GFS) or evening (EC). This wave of low pressure will move eastward and offshore by Saturday morning, returning most of the Mid-Atlantic to zonal flow aloft. Another shortwave embedded within the upper level trough will develop across the Midwest throughout Saturday. The NAM shows a stronger shortwave developing by 18Z, while both the GFS and EC show a broad and weak shortwave. This shortwave will slowly progress eastward on Sunday, with its axis reaching the western Mid-Atlantic by 00Z Monday, which will cause another wave of low pressure to ride into the northern and central Mid-Atlantic. By this time, it appears that the GFS and EC are in relatively close agreement with the strength and placement of their synoptic features. On Monday, the back end of the broad upper level trough will still be in place across the Mid-Atlantic, but a broad ridge behind it will be nipping at its heels. Concurrently, the mid-level ridge/Bermuda High will build back northward toward the northern Mid-Atlantic.

There is still uncertainty in Thursday’s forecast, with continued questions about the exact north-south placement of a stalled front, which will act as focus for a wave of low pressure which will create substantial diurnal convection and precipitation. The other major question is the timing of waves of low pressure that will move along the front, triggering heavier, more widespread precipitation. The NAM seems to place this front the farthest north, near the Mason-Dixon Line, resulting in heavy precipitation across the central Mid-Atlantic and portions of the northern Mid-Atlantic (namely the I-95 Corridor) by 00Z Friday. The GFS and EC keep the front slightly further south across the central Mid-Atlantic, with the GFS keeping the heaviest precipitation south of the MDL through 00Z Friday. The EC brings some light precipitation to the southern Mid-Atlantic and western portions of the central Mid-Atlantic by 00Z Fri, but keeps the Delmarva area and the I-95 Corridor mostly dry, due to its slower progression of shortwave energy aloft. 06Z GFS back trajectories for the I-95 Corridor are localized, suggesting that the front will be nearby PHL by 12Z. This morning’s Hi-Res guidance resolves heavy precipitation in locations where the front ends up settling, with the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM all showing precipitation in the vicinity of the MDL during Friday afternoon. As a side note, the SPC has placed most of the central Mid-Atlantic under a MARGINAL risk for severe convective active tomorrow, with eastern VA under a SLIGHT risk, citing ample regional instability and moderate to strong deep layer shear. The 06Z air quality models do not seem to be responding strongly to this expected precipitation, however. The NOAA, BAMS MAQSIP-RT, and NCDENR models all resolve areas of USG ozone near PHL, with the NOAA and BAMS models also developing larger patches of USG ozone across northern and central NJ. Furthermore, the BAMS MAQSIP-RT model shows USG ozone across central MD. The BAMS CMAQ model, however, shows a solution much more indicative of widespread precipitation, with the only area of USG ozone confined to northern NJ, and the rest of the northern and central Mid-Atlantic under upper Good and low Moderate conditions. Areas that remain far enough north of the front and its associated precipitation will experience mostly sunny conditions and continued heat, which would explain some of the models showing USG ozone along the far northern extent of the I-95 Corridor. However, with growing support for substantial convection and precipitation, the chance for an ozone exceedance on Thursday will remain Marginal.

Friday’s forecast is also uncertain, with even more substantial differences between the deterministic models with the track of Thursday’s wave of low pressure. But the end result is a wet day, the only question is whether the heaviest rain (wave) occurs in the morning (NAM/GFS) or late afternoon (EC). The NAM and GFS both keep precipitation across the eastern portions of the northern and central Mid-Atlantic through roughly 12Z, then the wave moves offshore. During the late afternoon, these models then show a second round of precipitation centered over the central Mid-Atlantic, likely driven by diurnal heating and the continued presences of the stalled front. This precipitation is more widespread and intense in the GFS due to its more organized shortwave energy aloft. The EC solution, however, is almost 12 hours behind the NAM and GFS at this time, bringing the wave of low pressure and its associated precipitation to the I-95 Corridor by 18Z Friday to 00Z Saturday. If the NAM or GFS solution verifies, there could be some breaks of afternoon sunshine along the I-95 Corridor, possibly allowing for some rising ozone. 06Z GFS back trajectories for the I-95 Corridor are west-southwesterly and short at 500m AGL, and slightly faster at 1000m and 1500m. That said, the areas from which these trajectories originate should have been sufficiently cleaned by widespread precipitation. The EC solution would entirely limit ozone formation across the entire Mid-Atlantic. The 06Z air quality models do not develop any USG ozone in the Mid-Atlantic on Friday, though the BAMS MAQSIP-RT does show upper Moderate ozone across northern NJ. Regardless of whether the NAM/GFS or EC solution verifies, it appears that there will be enough factors in place to limit ozone production across most of the forecast region. As a result, the chance for an ozone exceedance on Friday will drop to Slight.

There is slightly more agreement in Saturday’s forecast in this morning’s guidance, with a trend towards less widespread precipitation. It appears that another wave of low pressure will initiate across the Ohio River Valley ahead of another developing shortwave aloft. This wave and its associated precipitation will progress eastward along the stalled front remaining across the central Mid-Atlantic, but remain west of the I-95 Corridor through 00Z Sunday. However, with a semi-stationary front lingering over the central portion of the region, and ample humidity, diurnal clouds and precipitation are likely. In addition, the air mass in place will likely be quite clean due to two preceding days of widespread clouds and precipitation. That said, partly to mostly clear skies and seasonable temperatures may be sufficient for ozone levels to rise along the I-95 Corridor, but anything higher than the Moderate range seems unlikely. The BAMS models develop upper Moderate ozone all along I-95, with the BAMS-CMAQ model showing patches of USG in central and southeastern NJ, and near DC. The BAMS MAQSIP-RT model is even more aggressive, with widespread USG ozone across most of NJ, DEL, DC, and central VA. Though these solutions are likely overdone, it suggests the potential for ozone formation in the vicinity of the I-95 Corridor, which should be monitored. At this time, the chance for an ozone exceedance on Saturday will remain Slight.

On Sunday, the second wave of low pressure will move eastward into the northern and central Mid-Atlantic, causing widespread clouds and precipitation beginning 12Z. The EC in particular develops a band of intense precipitation reaching the I-95 Corridor by 18Z. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating, but this should have a minimal impact on air quality due to the presence of clouds and rain, as well as persistent onshore surface winds. The BAMS models do not seem to respond this precipitation, with the BAMS CMAQ showing Moderate ozone across eastern PA and northern NJ, and the BAM MAQSIP-RT model even resolving a bullseye of USG ozone across east central PA. Once again, these solutions seem unlikely given the agreement between the GFS and EC in developing precipitation across most of these locations by Sunday afternoon. It is worth noting, that this morning’s NAAPS guidance shows a band of smoke streaming across portions of the northern and central Mid-Atlantic beginning 12Z Sunday, which could cause locally enhanced ozone formation in areas that remain dry. Given the likely widespread precipitation, the chance for an ozone exceedance will remain Slight on Sunday.

The Mid-Atlantic will remain under the influence of the broad but weak upper level trough on Monday, with the possibility of diurnal convection and showers developing across much of the forecast region, with the quasi-stationary front still draped across the region. The presence of a clean air mass, combined with a shift to onshore transport both at the surface and aloft, will limit ozone formation along the I-95 Corridor. The chance for an ozone exceedance will remain Slight on Monday as a result.

-Brown/Huff