Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 24, 2016
Valid: May 25-29, 2016 (Wednesday-Sunday)

Summary:

There is a High chance for an ozone exceedance in the Mid-Atlantic on Wednesday and Thursday, given a stronger and farther northward extending upper level ridge compared to yesterday’s guidance. The weather forecast models now place the Mid-Atlantic completely under the ridge both days, which limits the chances for clouds and precipitation. Upwind ozone has risen quickly, with multiple exceedances on Monday from MI all the way to southern LA/AL. Back trajectory forecasts for Wednesday and Thursday bring this dirty air directly into the Mid-Atlantic. This pattern, combined with hot and sunny weather, makes exceedances likely for the next two days. Chances for an ozone exceedance drop to Marginal on Friday, as back trajectories shift south/southwestward and speed up, which will presumably transport cleaner air into the region. The weekend looks like a possible clean out, as the models bring a weak tropical system to the coast of the Carolinas, which will shift back trajectories onshore.

Discussion:

The weather forecast models continue to be in close agreement for most of the medium range period regarding the main synoptic features that will impact air quality conditions. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. There are some substantive changes to the synoptic pattern in today’s model runs. The first part of the period is still impacted by a broad upper level ridge that will build eastward today and move into the Mid-Atlantic tomorrow, as the pesky closed low exits to the northeast over the Canadian Maritimes. Similar to yesterday’s model runs, the deterministic models keep the ridge broad, without a sharp axis, on Wednesday. There are still several small shortwaves embedded under the ridge and at least three larger shortwaves in a broad upper level trough over the western U.S. The strength and placement of the eastern U.S. mid-level ridge mirrors that of the upper level ridge; at the surface, the Bermuda High begins to extend westward. On Thursday, the ridge continues to shift eastward, with the axis moving over the Mid-Atlantic. Today’s model runs build the ridge more northward compared to yesterday’s analysis. The main impact on air quality is that clouds and showers triggered by shortwaves moving through the flow on the edge of the ridge will be diverted northward on Wednesday and Thursday. The Mid-Atlantic remains completely under the ridge on Friday, which will shelter most locations from the impact of shortwaves moving through the flow aloft. There is a new wrinkle in the forecast for the weekend. Both the GFS and EC deterministic models and the MREF ensemble members bring a weak tropical system into the southeastern U.S. coastal waters on Saturday, and move it to the coast of the Carolinas on Sunday. The EC and GFS diverge somewhat, with the GFS bringing the weak system inland on Sunday, while the EC keeps it along the southern Mid-Atlantic coast. The impact of this system will be generate strong onshore flow aloft for the weekend. The other difference in today’s model analyses is that the weak back door cold front that will be located along the northern edge of the upper level ridge stays farther north on Wednesday through Friday – well into southern Canada. However, it begins to move southward on Saturday, and may travel as far south as the PHL-ILG metro region by Sunday morning.

Wednesday and Thursday continue to be the days of most interest for the period. As the ridge builds over the region, temperatures will quickly climb into the upper 80s to low 90s °F. It will be sunny across most of the region both days, with slow west/northwesterly back trajectories. Ozone air quality deteriorated quickly across the Great Lakes and Mississippi River Valley yesterday, with scattered exceedances from MI to southern LA/AL. Code Orange ozone forecasts are posted for most of those same locations today. Therefore, upwind ozone is expected to be high on Wednesday and Thursday. The air quality models are still not predicting any areas of USG ozone for either Wednesday or Thursday for the Mid-Atlantic. They are keeping ozone in the mid-to-upper Moderate range at locations across most of PA, NJ, MD, and DE, with scattered locations in VA and NC. The models could be responding to chances for diurnal afternoon thunderstorm activity, particularly on Thursday. The 03Z SREF still has a high probability of precipitation west of I-81 for Wednesday, and the 06Z 4 km NAM has a few scattered storms in western PA on Thursday afternoon. Overall, the chances for precipitation on Wednesday are much reduced compared to yesterday’s guidance, and there is still some uncertainty for Thursday, but with the ridge shifted farther north, it seems likely that the deterministic models’ precipitation forecast is overdone. As a result, there is a High probability for an ozone exceedance somewhere in the Mid-Atlantic on Wednesday-Thursday, most likely along the I-95 Corridor.

PM2.5 is also rising quickly to our west, with concentrations this morning well into the Moderate range, including areas of western PA. The air quality models bring PM2.5 into the Moderate range across the entire Mid-Atlantic region beginning tomorrow and continuing through Friday. This seems reasonable given rising humidity and increasing upwind concentrations.

On Friday, back trajectories shift south/southwestward and increase in speed. It will still be hot and sunny, with continued chances for diurnal afternoon convection. The air quality models begin back off on both ozone and PM2.5 levels on Friday – the locations of highest ozone and PM2.5 remain the same (e.g., I-95 Corridor), but the intensity decreases, possibly in response to the shift in upper air transport. The main forecast question for Friday will be how high ozone reaches on Thursday, particularly at upwind locations. Given the uncertainty in the transport pattern and ozone levels upwind, the chances for an exceedance drop to Marginal on Friday.

In contrast to yesterday’s analysis, Saturday and Sunday now look clean – back trajectories shift south/southeast with a strong onshore component in response to the weak tropical system that will be moving into the coastal southern Mid-Atlantic. If this onshore transport pattern verifies, it will clean out the region, with the possible exception of locations west of I-95, where Moderate ozone conditions may persist. The other wild card is the strength and progression of the back door cold front, particularly on Sunday. If the front reaches as far south as the Mason-Dixon Line, or even into central NJ, it will help to clean out the northern Mid-Atlantic. Given the shift to onshore flow and questions about the back door cold front, chances for an ozone exceedance drop to Slight for the weekend. PM2.5 also will likely see a downward trend for the weekend, which is supported by the air quality models.

-Huff/Brown