Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 22, 2016
Valid: July 23-27, 2016 (Saturday-Wednesday)

Summary:

It will be a weekend full of uncertainty in terms of the air quality forecast for the Mid-Atlantic, due to questions about the placement and impact of a cold front that will arrive across the northern Mid-Atlantic on Saturday, before stalling near the Mason-Dixon Line through at least Sunday morning. The forecast region will be under oppressive heat through Monday, due to presence of upper and mid-level ridging patterns. On Saturday, there is a growing chance for the development of clouds and scattered precipitation along the front, but this should be isolated enough to have a limited effect on ozone formation. Intense heat and the presence of a modified air mass south of the front will support rising ozone across the southern and parts of the central Mid-Atlantic. As a result, the chance for an ozone exceedance will be Appreciable on Saturday. The upper and mid-level ridges will be directly over the Mid-Atlantic on Sunday, subduing convection along the stalled front and causing winds to subside. Additionally, the stalled front is expected to lift northward as a warm front during the afternoon, allowing the modified air mass to surge northward. Therefore, the chance for an ozone exceedance will remain Appreciable on Sunday. There are questions on Monday regarding the speed and extent of clouds and precipitation ahead of a second approaching cold front and its associated pre-frontal trough. At this point in time, it appears there will be sufficient cloud cover and scattered precipitation during the afternoon to limit rising ozone. Therefore, the chance for an ozone exceedance will drop to Marginal on Monday. On Tuesday, the cold front will move into the region and push southward towards the central Mid-Atlantic. Widespread clouds and precipitation along the front, as well as the arrival of a cleaner air mass across the northern Mid-Atlantic, will keep the chance of an ozone exceedance Marginal for Tuesday, mainly for locations south of the front. The front will progress further southward on Wednesday, encouraging another round of clouds and precipitation. The chance for an ozone exceedance will remain Marginal for the southern Mid-Atlantic, in the event the front slows down.

Discussion:

The weather forecast models are in relatively close agreement with the main features of the synoptic pattern during the medium range, with the exception of the NAM regarding the strength of its upper level features during the weekend. For this reason, the NAM is considered an outlier for both Saturday and Sunday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A relatively shallow upper level trough will shift eastward across southern QC and into the Northeast U.S. on Saturday, amplifying somewhat, and reaching ME by 00Z Sunday. This feature will push a cold front to the Mason-Dixon Line by Saturday morning. Behind this trough, a mid-level ridge centered across the southeastern U.S. will build back northward into the Ohio River Valley. On Sunday, the upper level trough will lift northeastward toward the Canadian Maritimes, while a second trough will move eastward across MB and into southern ON. A slightly amplified upper level ridging pattern will set up over the Mid-Atlantic on Sunday, in between this pair of troughs, with the mid-level ridge also shifting eastward across the forecast region as well. The second trough will suppress the mid-level ridge and pull a cold front into the Ohio River Valley beginning 12Z Monday. On Tuesday, this trough will lift northeastward, pushing the cold front into the northern and central Mid-Atlantic. After the trough departs on Wednesday, the Mid-Atlantic will return to mostly zonal flow aloft.

There is more consensus in today’s guidance regarding the placement of the approaching cold front on Saturday, but the exact impact of the front itself on air quality remains uncertain. It appears that the front will push southward across PA and stall in the vicinity of the Mason-Dixon Line by 12Z. There is slightly more support in both the deterministic models and Hi-Res guidance this morning for the development of clouds and scattered precipitation along this front. Both the GFS and EC show light precipitation across southern PA and portion of the central Mid-Atlantic beginning 18Z, likely fueled by diurnal heating. There are hints this as well in the 00Z NMM/ARW, 03Z SREF, 06Z 4 km NAM, and 06Z 13 km GFS. Even though this convection and precipitation would likely be isolated, it could act to limit ozone formation along the front. If this precipitation does not form, however, convergence along and near the front could cause locally enhanced ozone formation. A presumably clean air mass will arrive behind the front (based on current upwind conditions this morning), with 06Z back trajectories for the I-95 Corridor shifting northwesterly, though not particularly fast. The 06Z air quality models all show a band of Moderate ozone with pockets of USG across southeastern PA, DC, eastern MD, and central and southern DE, which lends some confidence to the ability of near-frontal convergence to promote ozone production. The NCDENR model shows a similar pattern, but at a lesser magnitude, with no USG ozone modeled, and only low Moderate ozone across the aforementioned locations. However, given the intense heat across the region expected over the weekend, and the modified air mass that will remain in place ahead of the front, there will be enough factors in place to support rising ozone, mainly at locations south of the eventual location of the front. As a result, the chance for an ozone exceedance will be Appreciable for locations along and south of the cold front.

There are a lot of competing factors on Sunday, but the synoptic set up is more favorable for high ozone than on Saturday. A ridging pattern aloft and the presence of a mid-level ridge/Bermuda High on Sunday will keep temperatures well above average across the Mid-Atlantic. Subsidence from these features should be sufficient to limit clouds and convection along the stalled front, which will remain across the MDL through 12Z, and slowly lift northward as a warm front sometime during the afternoon. 06Z back trajectories for the I-95 Corridor remain northwesterly but slow appreciably. That said, oppressive heat is expected across the forecast region, with subsiding winds and ample sunshine pushing temperatures into the upper 90s °F. As a result, the BAMS models shows Moderate ozone across much of the northern and central Mid-Atlantic, with USG ozone over western PA, DC, BAL, eastern MD, most of DE, and central and southern NJ (along and east of I-95). Though this seems overdone, the northward advancement of the presumably modified air mass, combined with light winds and intense heat, may be sufficient for ozone to rise rapidly. One caveat to this is that it will be Sunday, when anthropogenic emissions are statistically lower, which may decrease the likelihood of such rapid ozone formation. To that end, NCDENR is much more reserved in its solution, keeping most of the northern mid-Atlantic in the upper Good and low Moderate range, with only a few patches of USG ozone over DC, southeastern VA, and western NC. Though this solution is certainly possible, depending on how far south the clean air mass behind the front can penetrate, the presence of the upper and mid-level ridges makes the likelihood of rising ozone as high or higher than Saturday. Therefore, the chance for an ozone exceedance on Sunday will remain Appreciable.

There are still questions on Monday regarding the speed and extent of clouds and precipitation expected to develop ahead of an approaching pre-frontal trough and trailing cold front, which will be nearing the Ohio River Valley by 12Z. The GFS remains slightly slower with the progression of this precipitation, with the heaviest rain reaching the I-95 Corridor by 00Z Tuesday. On the other hand, the EC shows a slightly faster solution, with widespread precipitation reaching I-95 by 18Z Monday. The 03Z SREF keeps the highest probabilities of precipitation just to the north and west of the I-95 Corridor through 18Z Monday. Even if this precipitation doesn’t arrive until the late afternoon or early evening, clouds ahead of the rain should be sufficient to limit ozone formation. The 06Z 15-panel NAM shows substantial cloud cover developing over the I-95 Corridor as early as 15Z Monday. The BAMS models are struggling to resolve the impacts (and likely the timing) of this front, with the MAQSIP-RT model keeping the most of the northern and central Mid-Atlantic under Moderate ozone, with USG ozone modeled across northern NJ. The CMAQ model shows this USG ozone stretching down into southeastern PA and northern MD as well. 06Z GFS back trajectories for the I-95 Corridor are localized, and recirculate at 500m AGL, which could lend some confidence to the CMAQ solution. Due to the likelihood of afternoon clouds and precipitation, however, the chance for an ozone exceedance will drop slightly to Marginal on Monday.

On Tuesday, the cold front will move into the northern Mid-Atlantic and push southward towards the central Mid-Atlantic by 12Z. Widespread clouds and scattered precipitation are expected along this front, which will limit ozone formation. Furthermore, a shift to northwesterly transport behind the front should usher cleaner air into the forecast region. The BAMS models reflect this clearly, bringing most of the northern Mid-Atlantic down into the Good range for ozone on Tuesday. The focus for rising ozone will therefore shift southward across the central and southern Mid-Atlantic, where the modified air mass from previous days will remain in place. That said, the BAMS models do not resolve any USG in these locations, likely due to ample convection and precipitation. The chance for an ozone exceedance on Tuesday will drop to Slight as a result.

The cold front will push even further south into the central and southern Mid-Atlantic on Wednesday, triggering another round of widespread clouds and precipitation. The building in of a cleaner air mass across the northern and central Mid-Atlantic will limit rising ozone, despite ample sunshine and continued heat. As a result, the chance for an ozone exceedance will remain Slight on Wednesday.

-Brown/Huff