Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Monday, August 1, 2016
Valid: August 2-6, 2016 (Tuesday-Saturday)
Summary:
The first half of the medium range will be characterized by the competition between clean onshore regional transport and possible local ozone production under mostly sunny skies. A wave of low pressure will depart offshore and high pressure will be centered over QC/NS on Tuesday. On Wednesday, the high will settle over New England, and then will drift slightly southward on Thursday into the Gulf of Maine. These synoptic features will promote onshore flow aloft and at the surface, as well as seasonable temperatures, across the Mid-Atlantic all three days. Although there may be some local isolated spikes in hourly ozone under sunny skies, the predominant onshore flow should limit rising ozone, for a Marginal chance of an ozone exceedance Tuesday-Thursday. An approaching cold front and the development of high pressure across western NC on Friday will shift surface winds southwesterly, funneling heat and moisture into the forecast region. Rising temperatures and return flow around the high will support rising ozone along the I-95 Corridor, which will remain under sunny skies through the early evening. Therefore, the chance for an ozone exceedance will rise to Appreciable on Friday. There is uncertainty in Saturday’s forecast regarding the extent of clouds and precipitation developing ahead of the cold front, as well as its track into the northern Mid-Atlantic. The GFS does not develop clouds and precipitation ahead of this front, while the EC shows more organized precipitation. The front will likely reach to somewhere around southern PA/NJ or the Mason-Dixon Line by Saturday evening. Locations along and ahead of the front will likely be in a modified air mass under sunny skies, which would promote rapid ozone formation. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable for locations south of the front.
Discussion:
The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences on Saturday regarding the arrival of a cold front and its associated precipitation. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough will depart the Mid-Atlantic Tuesday morning ahead of a weak upper level ridge, which will move eastward and fully envelop the forecast region by 00Z Wednesday. At the surface, high pressure will gradually move eastward, from southeastern QC/NS on Tuesday, to New England on Wednesday, to the Gulf of Maine on Thursday. The position of this high will promote onshore flow into the Mid-Atlantic during this period. On Wednesday, the upper level ridge will flatten out across PA/NY, returning most of the Mid-Atlantic to zonal flow aloft. This will allow some weak shortwave energy on the underside of the ridge to stream across portions of the northern and central Mid-Atlantic. Concurrently, a mid-level subtropical ridge, which is currently centered over southeastern U.S., will build northward across the entire eastern U.S. An upper level ridge will re-amplify over the Mid-Atlantic and move eastward throughout Friday, ahead of an upper level trough churning across ON, which will pull a cold front into the Great Lakes beginning 12Z Friday. On Saturday, this trough will progress eastward into QC, with the northern Mid-Atlantic remaining under its southern periphery throughout the day. Its associated cold front is expected to move into the Mid-Atlantic from the northwest on Saturday, but the EC and GFS differ slightly on the exact timing of its arrival. The EC is slightly faster, bringing the cold front to the Mason-Dixon Line by 00Z Sunday, whereas the GFS only brings the front to roughly central PA/NJ.
The cold front that has been quasi-stationary across the central Mid-Atlantic since last week will push southward toward the VA/NC border by 12Z Tuesday. Though there will be clearing behind this front, showers circulating onshore around the departing low may bring some morning clouds and precipitation to the northern and central Mid-Atlantic, especially coastal locations. The NAM in particular develops heavy precipitation along the I-95 Corridor through 12Z. In the afternoon, diurnal convection and thunderstorms will bring a second round of precipitation to much of the Mid-Atlantic, especially across VA and NC in the vicinity of the cold front. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL around the low positioned offshore, but short and westerly at both 1000m and 1500m. The combination of this onshore transport at lower levels, clouds and scattered precipitation, and northeasterly surface winds will act to temper ozone formation along I-95. The main question will be if there is sufficient afternoon sunshine for rising ozone to counteract these limiting factors. The 06Z air quality models suggest that there will enough clearing and sunshine, with all of the models developing an area of USG ozone in the vicinity of DC/northern VA. The NOAA model also shows USG ozone across PH and northern DE, but the BAMS and NCDENR models only resolve upper Good and low Moderate ozone in these locations. The air quality models have been consistently over-forecasting for most locations since last week. Therefore, these solutions may be slightly overdone, especially given the number of factors in place that should suppress ozone formation. Given the possibility of clearing skies and possible air mass modification, the chance for an ozone exceedance on Tuesday will be Marginal.
On Wednesday, high pressure will settle over New England around 12Z. Though this will promote more substantially sunny and clear skies, the position of the high will funnel clean, maritime air into the northern and central Mid-Atlantic, both at the surface and aloft. This once again should limit ozone formation. There may still be a slight chance for some weak afternoon convection and showers, but there is much less support in the deterministic models and Hi-Res guidance in comparison to Tuesday. As on Tuesday, the key to the air quality forecast will be determining which of the two battling factors – clean regional transport and local ozone production under mostly sunny skies – will win out. The 06Z air quality models side with the clean transport pattern in place, with the BAMS and NCDENR models resolving only Moderate ozone along and west of I-95. That said, given widespread afternoon sunshine and questions about how quickly the air mass in place will modify on Tuesday, the chance for an ozone exceedance will remain Marginal on Wednesday.
Surface high pressure will drift slightly southward on Thursday, centered off of the MA coast in the Gulf of Maine. The placement of the high will continue to support onshore flow at the surface and aloft, which should temper ozone formation, especially along I-95, for a final day. If onshore surface winds are lighter than expected, they may serve to push I-95 emissions north and west of the highway, promoting ozone formation in those locations. The 06Z air quality models are split regarding the magnitude of ozone levels. The BAMS models are more conservative, showing a solution very similar to that of Wednesday, with Moderate ozone developing to the west of the I-95 Corridor, suggesting that the clean transport pattern and onshore surface winds will be sufficient to temper rising ozone. The NCDENR model, however, develops a large area of USG ozone across PHL and the entire northern half of NJ, which could mean the surface winds will be sufficiently light for ozone to rise rapidly directly along I-95. The NC model solution may be overdone, but it hints at a regional setup that could potentially be conducive to local ozone production. As a result, the chance for an ozone exceedance on Thursday will remain Marginal.
Friday and Saturday are the days of most interest in the medium range period. An approaching cold front will bring clouds and thunderstorms into the western Mid-Atlantic by 00Z Saturday, but these will remain well west of the I-95 Corridor, which will see mostly sunny skies. The pre-frontal set-up will shift winds to the south/southwest, aided by the presence of high pressure over western NC, with its associated return flow pushing warm and moist air into the forecast area. 06Z GFS back trajectories ending at 12Z along the I-95 Corridor are still southeasterly and still onshore, but they are slow, especially at 1000m and 1500m, hinting at a shift to continental transport. The main forecast question is how modified the air mass in place will be after three days of sustained but relatively light onshore flow. There will also be a pre-frontal or possibly weak lee trough in place ahead of the front, which may promote afternoon convection, since there will not be a strong upper level ridge overhead. Interestingly, the BAMS and NCDENR models do not develop any USG ozone, keeping the entire I-95 Corridor in the Moderate range for ozone. The chance for an ozone exceedance will rise to Appreciable, given the favorable synoptic conditions for rising ozone.
On Saturday, the cold front will move into northwestern PA, with clouds and precipitation ahead of the front impacting the northern and central Mid-Atlantic. Though the deterministic models show relatively the same placement and track of this precipitation, they disagree regarding its intensity. The EC develops much more substantial and organized precipitation ahead of the front, while the GFS has only scattered patches of precipitation. Furthermore, the 06Z 13km GFS does not show much cloud cover developing ahead of the front. Additionally, 06Z GFS back trajectories for the I-95 Corridor turn westerly on Saturday. The main forecast question is how far south the front progresses. Locations along and ahead of the front will likely be in a modified air mass under sunny skies, which would promote rapid ozone formation, similar to what occurred in southern Maryland and along the western Chesapeake Bay coastline the first part of last week. As a result, the chance for an ozone exceedance will remain Appreciable on Saturday, mainly for locations which remain ahead of the approaching front.
-Brown/Huff