Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 11, 2015
Valid: August 7-11, 2015 (Wednesday – Sunday)

Summary:

Back trajectories from the Ohio River Valley coupled with clear skies and light winds associated with surface high pressure centered over WV will increase the chances of USG ozone to Marginal for the end of the medium range period, but there is considerable uncertainty in the forecast. A seasonably cool and much drier air mass filtering into the region behind today’s cold front will keep air quality mostly in the Good range for Wednesday and Thursday. A mid-level ridge will build over the region through the end of the period, allowing a return to above average temperatures. Weak troughing aloft at 500 mb will limit rising temperatures to the upper 80s °F, however. Friday looks like the day with the best chance for widespread Moderate ozone, particularly along the I-95 Corridor, due to westerly transport aloft and light southwesterly surface winds. A key forecast question for the weekend will the fate of a weak cold front moving through the northern Mid-Atlantic on Saturday. The global models are showing clouds and precipitation associated with the front, which would limit significant ozone production. However, this front may weaken faster than model guidance is showing as it moves into a warm air mass. This could result in the cold front acting as a line of convergence, possibly pushing ozone into the upper Moderate range, particularly on Sunday. The other main forecast question will be the flow aloft; 500 m AGL back trajectories suggest that the flow will shift southerly for Saturday and Sunday, with an onshore component. This could limit any rising ozone to the Good to low Moderate range.

Discussion:

The weather models are in generally close consensus, but they begin to diverge on Friday, which adds considerable uncertainty to the air quality forecast for the end of the medium range period. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, the dominating upper level features are a western US ridge centered over the Rockies and a longwave trough over the eastern US. This longwave trough is bringing a cold front through the Mid-Atlantic today. The 09Z WPC surface analysis shows the cold front currently in northwestern PA, and it is draped southward along the western periphery of the Appalachian Mountains. This front will clear most of the region by Wednesday morning but will linger along eastern NC before moving completely offshore by Thursday morning. Seasonably cool and much drier air will filter into the Mid-Atlantic behind the front for Wednesday and Thursday. Beginning on Thursday afternoon, the longwave trough will retreat northeastward and allow the center of surface high pressure to settle over WV/western VA on Friday. As the upper level trough retreats, it will leave a weak trough behind over the Southeastern US, while the flow over the northern and central Mid-Atlantic (NMA, CMA) becomes more zonal. A mid-level (850 mb) ridge will build over the Mid-Atlantic on Friday, bringing a return of warmer air to the region through the end of the period. Also on Friday, a shortwave will crest the upper level ridge in the northern stream flow and drop down into the Great Lakes region, digging out a shallow trough, which will pull a weak cold front into the NMA on Saturday. The GFS is stronger and faster than the EC in regard to this shallow trough over the Great Lakes, and as a result, the GFS has a stronger cold front than the EC. The front has rotated slightly compared to previous days’ analyses, and is now looking less like a back door cold front and more like a traditional cold front. The differences in the model guidance translate into uncertainty regarding the impacts of this front on air quality in the NMA and CMA on Saturday and possibly Sunday, as discussed below. On Sunday, the western US ridge will build northeastward into the the Great Lakes. A broad area of shortwave energy stretching from the Gulf of Mexico to New England will attempt to coalesce into a longwave trough over the eastern US on Sunday. The EC and GFS both have this feature, but to varying degrees. This weak trough will prevent the western ridge from building over the Mid-Atlantic on Sunday.

Today’s cold front will continue moving through the Mid-Atlantic today and clear the NMA and CMA by Wednesday morning. The front will remain in eastern NC for much of Wednesday, reaching the east coast of NC by 00Z Thursday. 850 mb model analyses definitively show Canadian air briefly filtering into the Mid-Atlantic on Wednesday and Thursday. The air mass will be seasonably cool, with about average temperatures for this time of year. The air will be much less humid, however, with dew points in low 50s °F by Thursday. Surface winds on Wednesday will be sustained northwesterly, shifting more westerly and diminishing on Thursday. There is a chance for scattered clouds, rain showers, and thunderstorms in western and central parts of the NMA on Wednesday as the upper level trough sits overhead, but most of the region should see mostly sunny skies, improving to full sun on Thursday. Back trajectories are north/northwesterly both days, with the strongest flow on Thursday (from interior ON). Given the arrival of the new air mass, which is expected to be low in ozone and PM2.5 precursors, mostly Good air quality is expected region-wide. There is a chance for isolated pockets of Moderate ozone on Thursday, given the lighter surface winds. The air quality models keep ozone in the Good range across the board through Thursday.

The air quality forecast is uncertain for Friday-Sunday. A mid-level (850 mb) ridge will build over the region, allowing temperatures to rebound to roughly 3-5 °F above average (i.e., upper 80s to around 90 °F). The center of surface high pressure will move eastward on Friday and sit over WV/western VA through Sunday, leading to mostly clear skies and light southerly surface winds. These conditions should promote rising ozone, but there are some mitigating factors that lend considerable uncertainty to the forecast. The main question mark is the weak frontal boundary that will move into the NMA on Saturday. The global models are showing clouds and precipitation associated with this front, but the strength and coverage are still uncertain. The faster and stronger GFS brings clouds and rain down to MD/DE on Saturday, while the EC keeps them north and west of I-81. The fact that both models have some rain suggests that the front will be strong enough to generate cloud cover. It will weaken and dissipate on Sunday, however, somewhere in the vicinity of southeastern PA. WPC has the front reaching PHL by 12Z Sunday. However, the cold front will be moving into a warm air mass. This may slow down the front and dissipate it more quickly, impacting the chances for precipitation. There is also the possibility that the dissipating front will act as a line of convergence, which could allow for explosive afternoon ozone formation, similar to what occurred in the PHL metro area on July 28-29. Another question is the direction of air mass transport aloft. Back trajectories ending at PHL on Saturday and Sunday morning indicate southerly onshore flow at 500 m AGL. This may be sufficient to keep a flow of clean maritime air moving into the region. There is also a question as to the strength and direction of surface winds, particularly east of I-95. By the weekend, more of a south/southeasterly surface flow may be established, which would also help to inhibit ozone formation.

Given the information we have today, Friday seems like the day with the highest chance for more widespread Moderate ozone, particularly at locations north and east of the I-95 Corridor, due southwesterly surface winds, Ohio River Valley (ORV) back trajectories, rising temperatures, and sunny skies. The BAMS-CMAQ and NCDENR air quality models are showing Moderate ozone along I-95, including DC/BAL, ILG/PHL/TTN, and NYC metro. There is also a hint of a coastal sea breeze, which would enhance ozone production. Continued Moderate ozone certainly seems possible into the weekend, with the best chance at locations farther to the south and east in the NMA and CMA, which will be less impacted by the possible clouds and precipitation associated with the cold front on Saturday. The BAMS and NCDENCR air quality models are somewhat split for Saturday, with only the BAMS-CMAQ showing mid-to-upper Moderate ozone along the I-95 Corridor. Sunday could be anywhere from Good to upper Moderate, depending on air mass transport (clean) or a lingering weak line of convergence associated with Saturday’s cold front (dirty). PM2.5 concentrations will likely stay in the Good range for most of the region through Friday due to the continued low humidity. Locations in the southern Mid-Atlantic may PM2.5 rise into the Moderate range in response to the light surface winds and higher humidity.

-DeBoe/Huff