Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 25, 2016
Valid: July 26-30, 2016 (Tuesday-Saturday)

Summary:

The weather forecast through the medium range looks unsettled for most of the region, due to the arrival of a cool front that will stall near the Mason-Dixon Line on Tuesday morning and remain in place, oscillating slightly around the MDL, through the remainder of the forecast period. This front will act as a focus for upper level shortwave energy each day, with the strongest shortwave developing across the Plains on Thursday and progressing eastward into the northern and central Mid-Atlantic on Friday and Saturday. On Tuesday, the cold front will pass through most of the northern Mid-Atlantic by 12Z, with northwesterly winds behind the front ushering drier and cleaner air into the northern extent of the forecast region, where it should be sunny. Convection and precipitation are expected to develop along and south of the front, keeping the chance for an ozone exceedance Marginal on Tuesday. The stalled front will remain in place near the MDL on Wednesday, triggering another round of diurnal convection and precipitation. Northwesterly flow at the surface and aloft behind the front should limit ozone formation along the northern I-95 Corridor. Once again, the chance for an ozone exceedance on Wednesday will remain Marginal for locations along and ahead of the front. The front may lift slightly northward on Thursday, allowing clouds and precipitation associated with the front to move into the northern Mid-Atlantic. Recirculating surface winds and near-front convergence may allow ozone to rise along the I-95 Corridor, if it is not as cloudy as expected. Therefore, the chance for an ozone exceedance on Thursday remain Marginal. There is uncertainty in the forecast for both Friday and Saturday, due to questions about the location of the front and the extent of upper level shortwave energy developing across the Midwest. But the GFS and EC models both develop relatively strong waves of low pressure (albeit in different places) that will ride along the stalled front, still over the MDL, and bring periods of heavy precipitation to parts of the northern and possibly central Mid-Atlantic. Uncertainty in the exact placement of the shortwave energy aloft and its associated precipitation will keep the chances for an ozone exceedance for Friday and Saturday Marginal.

Discussion:

The weather forecast models are in relatively close agreement with the main features of the synoptic pattern during the medium range, with some differences at the end of the period regarding the strength of upper level shortwave energy developing along a stalled front across the Mid-Atlantic. The models consulted for this analysis were the 06Z NAM and 00Z GFS and ECMWF. A shallow upper level trough currently over southern ON/northern Plains will move eastward into QC Tuesday morning, pulling a cool front into the northern and central Mid-Atlantic, before lifting northeastward toward the Canadian Maritimes by 00Z Wednesday and allowing the front to stall near the Mason-Dixon line. Concurrently, the semi-permanent subtropical ridge will be centered over the southeastern U.S. and hooked up with the Bermuda High on Tuesday, and will remain in place through the end of the period. On Wednesday, another upper level shortwave trough will circulate over northern ON/QC, but will remain far enough north of the forecast region for the Mid-Atlantic to return to mostly zonal flow aloft. This will allow weak shortwave energy along the periphery of the upper level trough to track across the northern and central Mid-Atlantic, in the vicinity of the stalled front, throughout the day. The stalled front will remain across the central Mid-Atlantic, acting as a focus for this shortwave energy aloft. A similar zonal flow pattern will remain in place aloft on Thursday, with an upper level shortwave beginning to develop across the Plains. The NAM is an outlier with this feature, as its shortwave is too strong compared to the GFS and EC solutions. The GFS and EC start to diverge on Friday with their treatment of this shortwave, but they develop the same general pattern. The GFS has the shortwave and its associated energy spreading out and progressing quickly eastward into the northern and central Mid-Atlantic by 18Z Friday. The EC, however, slows the progression of this shortwave, amplifying it across the Midwest with its eastern edge only reaching the western Mid-Atlantic by 00Z Saturday. Both the GFS and EC solutions suppress the mid-level ridge/Bermuda High, though this is more substantial in the EC solution due to its more potent shortwave energy. These differences will carry over into Saturday, with the GFS showing weak shortwave energy spread out across the northern and central Mid-Atlantic, while the EC lifts its more organized trough northeastward into the Great Lakes by 00Z Sunday. Given the overall similarity of the GFS and EC solutions, we expect them to begin to converge on a common solution in the next several model runs.

A cold front will drop southwestward across the northern Mid-Atlantic early Tuesday, orienting itself east-west by 12Z, when it is expected to reach roughly the MDL and stall essentially through the rest of the period. Widespread clouds and convection are expected to form along and ahead of the front, both during the morning across the northern and central Mid-Atlantic as it passes southward, and then across the central and southern Mid-Atlantic during the afternoon, aided by diurnal heating. This precipitation pattern is supported by the deterministic models and much of this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 12 km NAM. The I-95 Corridor should remain far enough north of the front to remain clear during the afternoon, but persistent northwesterly surface winds behind the front will usher drier and cleaner air into the northern portions of the forecast region. 06Z GFS back trajectories for the I-95 Corridor are westerly, but should be sufficiently fast and originating from an area which will presumably be cleaned later today by the passage of a cold front. That said, the continued presence of the mid-level ridge/Bermuda High will allow temperatures to climb well above average once again across much of the Mid-Atlantic. If surface winds are light enough, I-95 emissions could be pushed south and east of the highway. Additionally, if clouds and precipitation along the front are not as widespread as expected, near-front convergence could lead to locally enhanced ozone formation across the central Mid-Atlantic. But these possibilities have a low probability. The 06Z air quality models are struggling to reach a consensus with the exact impact of this front. The NOAA and NCDENR models seem to display to effects of frontal convergence and light, northwesterly surface winds, developing USG ozone across southern NJ and near DC. Furthermore, the NOAA model has USG ozone across central DE, and a patch of Unhealthy ozone in southeastern NJ. The BAMS models favor more of a cleaning trend, with the only area of USG ozone being near DC, with the rest of the central Mid-Atlantic in the upper Good and low Moderate range. Given the support for convection and precipitation, as well as the recent tendency for the air quality models to over-forecast ozone, the chance for an ozone exceedance on Tuesday will be Marginal for locations near the front, as well as those east of the I-95 Corridor.

The stalled front will remain draped across the central Mid-Atlantic, though its exact north-south placement is uncertain. The GFS and EC appear to push it further southward toward the VA/NC border, shifting its associated precipitation mostly across southern VA and NC. The NAM, however, keeps it further north just south of the MDL, promoting another day of clouds and convection across the central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are northwesterly on Wednesday, which should be sufficient to limit ozone formation in spite of ample afternoon sunshine. If the stalled front does move southward, the cleaner and drier air mass behind it will also be able to push into the central Mid-Atlantic. Once again, the main concern will be for locations along and ahead of the front, which could experience the effects of near-front convergence, although it will be humid enough, with triggers aloft, to promote afternoon convection. Current Hi-Res guidance supports the development of diurnal clouds and precipitation along this front, however, which would overwhelm or at least temper local production of ozone. The 06Z BAMS models develops patches of USG ozone near DC and in eastern VA, while the NCDNER model shows two small, isolated areas of USG ozone in south central PA, and northeastern VA. Given continued questions about the impacts of the stalled front, as well as uncertainty in its location, the chance for an ozone exceedance on Wednesday will remain Marginal.

On Thursday, zonal flow aloft and the presence of weak upper level shortwave energy aloft ahead of a strengthening upper level trough over the Plains aloft will allow the stalled front to lift slightly northward. This will permit the moist and modified air mass across the central and southern Mid-Atlantic to move northward, though the extent of its progression will depend on the north-south placement of the front. There is the potential for the front to trigger clouds and thunderstorms in the northern Mid-Atlantic, though the GFS and EC are not convincing in their development of afternoon precipitation in northern locations, keeping scattered showers across the central and southern Mid-Atlantic. The NAM shows more widespread precipitation, which also reaches farther north across the central Mid-Atlantic and portions of the northern Mid-Atlantic, due to its more northward placement of the front on Wednesday. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating, especially at 500m AGL, suggesting that the front will be very close to PHL around 12Z. Depending on the extent of afternoon clouds and precipitation, this recirculation and possible convergence along the front could allow ozone level to rise along the I-95 Corridor, but again, this seems unlikely given the upper level support and moist conditions. The BAMS models appears to agree with the NAM’s frontal placement, developing USG ozone across central and southern NJ, DC, BAL, northern BAL, and both south central and southwestern PA. Though the exact location of the front and its associated precipitation are uncertain, there seems to be some model support for rising ozone in the vicinity of the front. As a result, the chance for an ozone exceedance will remain Marginal on Thursday.

There is some uncertainty in Friday’s forecast due to differences between the deterministic models in their development of an upper level shortwave trough over the Plains, which translates to differences in the precipitation forecast. The GFS shows a broader and faster shortwave, with heavy precipitation reaching the I-95 Corridor by 00Z Saturday. The EC slows this shortwave down and amplifies it across the Midwest, keeping the heaviest of its precipitation to the west of the I-95 Corridor through 00Z Saturday, with only light showers reaching I-95 by this time. 06Z GFS back trajectories for the I-95 Corridor are still recirculating, but have more of an onshore component due to a weak surface low which is expected to be over the Delmarva area by 12Z Friday. Depending on how far east the precipitation reaches by the afternoon, the I-95 Corridor could experience another day of ample sunshine, intense heat, and convergence along the stalled front which is expected to remain in the vicinity of the MDL. These factors would support rising ozone along the I-95 Corridor. However, a GFS solution would bring substantial clouds and periods of heavy rain into the forecast area soon enough to limit ozone formation along the I-95 Corridor. The BAMS models appear to favor the GFS, bringing most of the northern and central Mid-Atlantic down into either the Good or Moderate range, with only the MAQSIP-RT model showing a small area of USG ozone across northeastern PA and northern NJ. Due to the likelihood of at least scattered afternoon clouds and precipitation associated with approaching shortwave aloft, the chance for an ozone exceedance will remain Marginal on Friday.

Saturday’s forecast is also uncertain, due to continued differences between the deterministic models regarding their upper level features. The EC brings an organized upper level trough across the Great Lakes, with moderate precipitation across the northern Mid-Atlantic through 00Z Sunday. The GFS keeps weak shortwave energy across the northern and central Mid-Atlantic aloft, allowing for scattered diurnal showers, mostly near and ahead of the stalled front (still roughly over the MDL), which may lift even further northward into PA by 12Z. 06Z GFS back trajectories for the I-95 Corridor are turn clockwise and onshore at 500m and 1000m AGL, and originate from northern PA at 1500m. Though it is unclear which solution will verify, it appears that there will enough clouds and scattered precipitation once again to limit ozone formation across most of the Mid-Atlantic. Therefore, the chance for an ozone exceedance on Saturday will remain Marginal.

-Brown/Huff