Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Friday, August 5, 2016
Valid: August 6-10, 2016 (Saturday-Wednesday)
Summary:
Saturday remains a day of interest for the Mid-Atlantic, but with lower confidence due to the persistently clean regional conditions over the past few days. This morning’s deterministic models have all slowed the track of an approaching cold front by about 3 hours, with the front now reaching the I-95 Corridor by 00Z Sunday. Depending on the extent of clouds and precipitation ahead of the front (which remain questionable), above average temperatures, periods of intense sunshine, and lightly converging winds will support rising ozone ahead of the front. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable. Despite its slowing trend on Saturday, the cold front will move briskly on Sunday after it clears the I-95 Corridor, reaching the VA/NC border by 12Z. The arrival of a much drier and presumably clean air mass behind the front, as well as a shift to northwesterly transport at the surface and aloft, will drop the chance for an ozone exceedance down to Slight on Sunday. This clean transport pattern will remain in place on Monday, but surface high pressure will drift eastward over the western NY/PA border by 12Z. The close proximity of the high will promote sunny skies and diminishing winds, allowing for some rising ozone along the I-95 Corridor. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday. On Tuesday, high pressure will move directly over the I-95 Corridor in the morning, before shifting offshore by the afternoon. Stagnating morning winds, recirculating back trajectories, and clear skies will support ozone formation along I-95, but a shift to onshore flow as the high moves eastward may temper rising ozone. As a result, the chance for an ozone exceedance will remain Marginal on Tuesday. There is uncertainty in Wednesday’s forecast due to model differences with the overall synoptic pattern, specifically regarding the possibility of afternoon convection and precipitation moving into the northern Mid-Atlantic. The GFS keeps precipitation to the west of the I-95 Corridor, while the EC brings it to I-95 beginning at 18Z. Given this uncertainty, and the presence of weak onshore transport, the chance for an ozone exceedance on Wednesday will remain Marginal.
Discussion:
The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, but the GFS and EC diverge beginning on Tuesday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad upper level trough currently over central ON will progress eastward into QC on Saturday, pulling its associated cold front into northwestern PA by 12Z. There were hints of this front slowing down in the yesterday’s guidance, and that trend is more evident in today’s models runs. The GFS and EC have adjusted to the NAM solution, with the front only reaching the I-95 Corridor by 00Z Sunday; this is about 3 hours slower than yesterday’s guidance. On Sunday, the upper level trough will pivot northeastward throughout the day towards the Canadian Maritimes, with its cold front pushing southward toward the VA/NC border by 12Z. A broad and weak upper level ridge will build eastward across the northern Plains on Monday, with its leading edge remaining over the Mid-Atlantic for the majority of the day. Concurrently, the semi-permanent mid-level ridge centered across the southeastern U.S. will creep back northward across the northern and central Mid-Atlantic. The presence of these ridging patterns aloft will allow a broad area of surface high pressure to drift eastward across the Great Lakes, Ohio River Valley, and northern Mid-Atlantic, with its center over the western NY/PA border by 12Z Monday. On Tuesday, the upper level ridge will shift eastward and fully cover the Mid-Atlantic, though it will remain relatively weak and may even flatten out across southern ON/QC by 00Z Wednesday. Surface high pressure will pass directly over the I-95 Corridor around 12Z Tuesday, before slowly moving offshore during the afternoon. There are differences in the GFS and EC solutions regarding the upper level setup on Wednesday, though these difference are reflected most clearly in the precipitation forecast. The GFS keeps the upper level ridge firmly over the Mid-Atlantic, but brings lobes of vorticity shearing off of a disturbance over the southern US towards the northern Mid-Atlantic by 00Z Thursday. The EC, on the other hand, does not show these vorticity lobes, but instead has the upper level ridge sagging slightly, allowing the southern periphery of a broad trough over southeastern Canada to brush the northern Atlantic beginning 12Z. The other main difference is that the EC develops a relatively strong tropical system over FL late Sunday and retrogrades it slowly westward along the Gulf Coast through Wednesday. The GFS has hints of organized vorticity on Tuesday, but it never forms into a tropical system. The NHC gives this disturbance a 20% chance of forming into a tropical system in the next 5 days.
Saturday is still a day of interest for portions of the northern and central Mid-Atlantic. As discussed above, the deterministic models have slowed down the progression of the approaching cold front, only bringing it to the I-95 Corridor by 00Z Sunday. This suggests that the majority of I-95 and the entire central Mid-Atlantic will remain ahead of the front through the afternoon. 06Z GFS back trajectories for the I-95 Corridor are southwesterly from eastern TN/western NC. Though conditions upwind are relatively clean this morning, southwesterly transport will certainly be less clean than the persistent onshore transport that has remained in place the past few days. Additionally, a shift to southwesterly flow will funnel heat and moisture into the forecast region, allowing temperatures to jump above average ahead of the front. The development of a pre-frontal trough could provide convergence, causing locally enhanced ozone formation ahead of the front. This will depend, however, on if the convergence provided by this pre-frontal trough will be strong enough to trigger the formation of clouds and precipitation. Though the GFS and EC have slowed to match the NAM’s slower track of the cold front, they still develop an organized (albeit weak) line of clouds and precipitation reaching the I-95 Corridor between 18Z and 21Z. The NAM continues to show more scattered precipitation, the heaviest of which doesn’t develop until 00Z Sunday. This precipitation pattern is supported by the 06Z 4 km and 12 km NAM models and the 00Z NMM/ARW, with the ARW model going so far as to keep the I-95 Corridor essentially dry through Saturday. Furthermore, the 06Z 15-panel NAM only shows scattered clouds across the northern and central Mid-Atlantic during Saturday afternoon. If a NAM solution verifies, then ample sunshine and lightly converging winds ahead of a sluggishly progressing front will support rising ozone along the I-95 Corridor. The 06Z air quality models seem to at least partially favor this type of solution, with all of the models bringing the I-95 Corridor up into the Moderate range for ozone. The NOAA model also develops an area of USG ozone across west central NJ, just to the east of PHL/TTN. One caveat that must be mentioned, however, is that several days of onshore transport and moderate cloud cover have kept ozone levels quite low across the region. Ozone concentrations yesterday across the northern and central Mid-Atlantic were well below what was expected, even with the presence of widespread NOx titration in the morning. If the regional air mass remains sufficiently clean through this afternoon, it may be slow to modify on Saturday, especially if afternoon convection is generated ahead of the front. That said, there are enough factors in place on Saturday which favor ozone formation, which will keep the chance for an ozone exceedance Appreciable for another day.
Despite its slowing trend on Saturday, the cold front will move briskly on Sunday after it clears the I-95 Corridor, reaching the VA/NC border by 12Z. The northern and central Mid-Atlantic will be post-frontal, with a much drier, slightly cooler, and presumably clean air mass arriving behind the front on northwesterly surface winds. 06Z GFS back trajectories for the I-95 Corridor shift northwesterly and are quite fast. These factors, combined with a drop to more seasonable temperatures, should limit ozone formation along the I-95 Corridor in spite of post-frontal clearing. The 06Z air quality models reflect this cleaning, with the BAMS and NCDENR models bringing most of the Mid-Atlantic down into the Good range for ozone, with only a few isolated areas of Moderate ozone across northern VA. One concern with this new air mass, however, is the presence of diffuse smoke across parts of Canada. If this smoke is still intact enough by the time it reaches the Mid-Atlantic, it could have an influence on air quality. The current HMS analysis identifies plenty of remnant smoke out across Canada and the Great Lakes, some from as far away as Russia. But AOD is not elevated, and surface PM2.5 concentrations behind the cold front are relatively low, which suggests that the incoming air mass is indeed relatively clean. Given the shift to likely clean transport pattern both at the surface and aloft, the chances for an ozone exceedance will drop to Slight on Sunday.
On Monday, an area of surface high pressure will drift eastward, with its center over western NY/northwestern PA by 12Z. The close proximity of this high will promote clear skies and diminishing winds across the northern and central Mid-Atlantic, which will favor rising ozone along the I-95 Corridor, despite the clean air mass in place. 06Z GFS back trajectories for the I-95 Corridor remain fast and northwesterly, suggesting that clean transport will continue through at least Monday morning. The main forecast question will be how clean the regional air mass will be in light of Sunday’s frontal passage. Thus far this summer, we have seen that weak frontal passages like the one experienced on Saturday have only a minimal cleaning impact on the Mid-Atlantic, with much of the forecast returning to Moderate ozone conditions soon after the frontal passage. If this prediction holds true for this front, then local ozone production may be sufficient to counterbalance clean regional transport. The BAMS air quality models keep most of the Mid-Atlantic in the Good range for ozone, but brings the central Mid-Atlantic up into the Moderate range. Given the support for at least some rising ozone, the chances for an ozone exceedance will rise to Marginal on Monday.
High pressure will move directly over the I-95 Corridor around 12Z Tuesday, before moving offshore during the afternoon. This will cause a period of stagnation Tuesday morning, followed by another day of abundant afternoon sunshine. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating, reflecting the position of the high in the morning. As this high pressure shifts eastward, however, the transport pattern will shift onshore both at the surface and aloft in the late morning, which could counteract the impacts of the high pressure that otherwise favor rising ozone. That said, if surface winds are sufficiently light during the afternoon, they may push highway emissions just to the north and west of the I-95 Corridor. The BAMS models reflect this possibility, bringing the entire I-95 Corridor up into Moderate ozone, with the highest concentrations located just to the west of the highway. Additionally, the BAMS CMAQ model shows hints of USG in far northern NJ. Though the impact of onshore transport in the afternoon may be sufficient to temper rising ozone, the nearby position of the high, with recirculating morning back trajectories, will keep the chances for an ozone exceedance Marginal on Tuesday.
There is uncertainty in Wednesday’s forecast, due to the differences in the overall synoptic pattern modeled by the GFS and EC, specifically regarding the possibility of afternoon convection and precipitation. A warm front may approach the western Mid-Atlantic by 12Z, with a weak back door cold front approaching from the north in the afternoon. The GFS keeps most of this precipitation to the west of the I-95 Corridor through 00Z Thursday, while the EC shows scattered precipitation across I-95 by 18Z Wednesday. Interestingly enough, the 06Z 13 km GFS sides with the EC, with scattered thunderstorms across the I-95 Corridor by 18Z. If these clouds and precipitation do not form, the northern and central Mid-Atlantic will experience another sunny day, with southwesterly winds funneling heat and moisture into the forecast region. That said, 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m and 1000m, which may limit rising ozone along I-95. Given these questions, the chance for an ozone exceedance on Wednesday will remain Marginal.
-Brown/Huff