Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 27, 2016
Valid: May 28 – June 1, 2016 (Saturday-Wednesday)

Summary:

An Appreciable chance for an ozone exceedance remains for Saturday across the northern Mid-Atlantic, mainly north and west of I-95. The main forecast questions for Saturday will be the influence of a shift to southerly onshore flow around the circulation of a weak tropical/sub-tropical low developing off of the Southeastern U.S. coast and the eastern extent of diurnal afternoon convection. The flow aloft turns completely onshore for Sunday and Monday, which should clean out the region from south to north and will drop the chances for an ozone exceedance to Slight. The highest ozone (Moderate) will be on Sunday, in the northern Mid-Atlantic, which will be the sunnier and drier of the two days. There is higher than usual uncertainty in the air quality forecast for Tuesday and Wednesday. A weak cold front will move into the northern Mid-Atlantic on Monday and the central Mid-Atlantic on Tuesday, before finally stalling along the Atlantic coast on Wednesday. The fate of the tropical/sub-tropical low is also uncertain; the low may possibly interact with the stalled front on Wednesday. At this point, Tuesday appears to have slightly higher chances (Marginal) for an ozone exceedance at locations ahead of the cold front. Chances for an ozone exceedance drop to Slight on Wednesday due to slightly lower temperatures and a somewhat higher chance of clouds and precipitation. PM2.5 will follow a gradual downward trend in concentrations, with mid-Moderate conditions continuing on Saturday along I-95.

Discussion:

The weather models are in close agreement on the main features of the medium range through Monday, but they diverge at the end of the period. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. The big story remains the tropical or sub-tropical low developing off the Southeastern U.S. coast on Saturday. The National Hurricane Center gives this system a 90% chance of developing over the next 48 hours. To that end, all of the deterministic models bring the low to the SC coast by the end of the day Saturday; the NAM is still slightly slower than the GFS and EC. The upper level ridge remains over the Mid-Atlantic on Saturday, but it will begin to be pinched on its southern edge by the tropical system. On Sunday, the tropical/sub-tropical low will spin over SC. There are some subtle differences in the models regarding the strength and exact track of the low. At that same time, a strong vort max over the Plains will move northeastward, into ON, and will carve out a shallow upper level trough, which will gradually move eastward across southern ON into southern QC on Monday and push the upper level ridge eastward to the Canadian Maritimes. The GFS/NAM and EC continue to differ slightly on the fate of the tropical low; the GFS and NAM keep it over the Carolinas through Tuesday, while the EC brings it northeastward along the NC coast. The Canadian upper level trough will slowly push a weak cold front into the northern Mid-Atlantic on Monday and Tuesday. Both models bring the front roughly parallel to the Eastern Seaboard on Wednesday, where it interacts with the tropical low to varying degrees. The EC is much drier on Tuesday and Wednesday, while the GFS is wetter. Both models build a relatively weak upper level ridge back over the Mid-Atlantic on Wednesday, but the EC is faster and farther north with the ridge than the GFS.

Saturday is the day of most interest during the period. With the ridge still over the region, it will be hot and humid. The main questions are afternoon cloud cover/convection and the impact of the shift to onshore flow aloft, which will begin tomorrow at lower levels (500 m AGL). Yesterday, there was a general eastward shift in the locations of highest ozone, and this trend is expected to continue today. The highest ozone seems to be following the eastward progression of the air mass, which very likely contains dilute smoke from the western Canadian wildfires. Tomorrow, 06Z GFS back trajectories for the I-95 Corridor swing due south at 500 m AGL, and surface winds will also be southerly, albeit light inland. Historically, onshore flow aloft cleans the region out quickly from south to north. The coarse resolution models and the 03Z SREF continue to show plenty of afternoon convection, triggered by topography, west of I-95. The 06Z 4 km NAM shows more isolated thunderstorms, however. The air quality models may finally be catching up observed conditions. The 06Z NOAA model shows isolated USG ozone in BAL, western PA, and northern NJ, with upper Moderate along the rest of the I-95 Corridor and western NC. The 06Z BAMS models show a similar pattern without the pockets of USG ozone. How high ozone rises today, especially in the southern parts of the region, should give us a better idea of how quickly the air mass is cleaning out (if at all). Certainly, there is much less NOx titration this morning along I-95, and PM2.5 is lower as well across the western and southern parts of the region, which suggests the air mass is not quite as dirty as the past two days. However, an Appreciable chance for an ozone exceedance continues for Saturday, mainly at locations along and just to the north and west of I-95, where there will presumably be sufficient afternoon sun, and where emissions will be pushed by southerly flow. PM2.5 seems to be slightly lower along I-95 this morning, at least in PHL, but it remains well inside the mid-Moderate range. Moderate conditions should continue across most of the central and northern Mid-Atlantic on Saturday, as PM2.5 will be slower to respond to the southerly flow compared to ozone.

On Sunday and Monday, 06Z GFS back trajectories aloft for the I-95 Corridor shift completely onshore, from the southeast. This should drop ozone down to the Good to Moderate range, with the highest ozone remaining in the northern part of the region. The tropical low does not look like it will be strong enough to generate enough subsidence along its periphery to cause a spike in ozone. The circulation will be coming from the southeast, where there will be presumably clean tropical/maritime air. There is a risk that the polluted air mass that was over the Mid-Atlantic on Wednesday and yesterday could be recirculated back into our region, but given the location of the tropical low, this risk seems small. Sunday will still be very warm and mostly sunny across the northern Mid-Atlantic, with the usual chances for diurnal convection. Monday looks much cloudier with greater chances for more widespread precipitation due to a cold front approaching from the northwest and an influx of humidity and possibly tropical moisture. It looks like a washout both days for the southern Mid-Atlantic. As a result, the chance for an ozone exceedance will drop to Slight both days, with the highest ozone expected on Sunday (Moderate) across the northern part of the region. The air quality models support this trend, with Good ozone in locations impacted by the tropical low. PM2.5 will follow a similar trend, with a gradual decrease in concentrations through the holiday weekend.

There is a great deal of uncertainty in the air quality forecast for Tuesday and Wednesday. The track and strength of the tropical low remains a question mark. A weak cold front will be moving into the central Mid-Atlantic on Tuesday and possibly settling along the Atlantic coast on Wednesday and interacting with what is left of the tropical low. In addition, a weak ridge may be building over the region by Wednesday. At this point, Tuesday looks more favorable for rebounding ozone, mainly at locations east and south of I-95, which will be ahead of the cold front. Wednesday may be a bit cooler with northwesterly (although slowish) transport behind the cold front. So for now, we assign a Marginal chance of an ozone exceedance for Tuesday, dropping to Slight for Wednesday in response to lower temperatures and possibly a greater chance for clouds and precipitation.

-Huff/Brown