Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 29, 2015
Valid: June 30- July 4, 2015 (Tuesday – Saturday)

Summary:

A progressive pattern will continue in the Northeast as the upper level trough persists, bringing a series of low pressure systems across the Mid-Atlantic. A frontal boundary moving into the region tomorrow will persist to varying degrees through the end of the medium range period, oscillating between PA and NC and promoting clouds and showers in its vicinity. Despite the chances for clouds and rain on Wednesday throughout the region, encroaching Canadian wildfire smoke makes the middle of the week the most likely day for USG conditions. The next possible exception in an otherwise clean atmosphere exists on Saturday, as there is model disagreement regarding an area of rain moving across the east coast. Depending on how far south this disturbance tracks, the northern Mid-Atlantic may see a day of increased ozone and PM2.5 concentrations.
Discussion:

The weather models are in generally close agreement on the overall synoptic features for the majority of the medium range period. The 06Z NAM, 06Z GFS, 00Z CMC, and 00Z ECMWF were consulted for this analysis. There is agreement regarding the placement of the upper level trough currently dominating east coast weather, with all models placing the base of the trough well into the southeastern US. Currently, there is a shortwave aloft just north of ME and a developing vorticity maximum in the base of the trough over the Ohio River Valley (ORV).
The evolution of this large scale trough, its embedded vorticity maxima, and the surface reflections of these in the form of a variety of complex boundaries are the critical forecast uncertainties throughout the medium range period. It is encouraging that the numerical models have come to a closer agreement on these features, however, the extent of clouds and precipitation in this kind of synoptic situation are difficult for the numerical models to accurately resolve beyond the short range (1-2 days). The other important uncertainty during the medium range period is the possibility of wildfire smoke encroaching on the mid-Atlantic, perhaps as early as Tuesday. More details on the smoke forecast, which may affect PM2.5 forecasts, are found below.

Although the forecast models are generally in agreement, by 12Z Thursday the NAM is a clear outlier, as it is the only model showing a very strong short wave developing over the Great Lakes. The CMC and EC develop a weaker shortwave over the NY/Canada border, while the GFS develops a very strong shortwave along the northern border of ME. Despite these differences among model solutions, they all bring clouds and precipitation to the Mid-Atlantic on Thursday.
An embedded “mini” ridge is developed near 00Z Friday but the model solutions diverge regarding its speed of advance with the EC the fastest. The models continue to diverge throughout the day Friday, providing significantly different upper level solutions by Saturday.
This is then reflected in different precipitation forecasts with the EC being the strongest, and furthest north of the solutions.

In the daily forecasts, a surface low pressure system currently in the Tennessee River Valley (TRV) will move northeastward into the NMA on Tuesday, and pull an accompanying warm front northward. This low pressure system will be weaker than Saturday’s storm, limiting scattered clouds and precipitation to the central and northern parts of the region. The higher resolution models, including the 06Z 4km “nested” NAM and 03Z SREF, agree on keeping the clouds and precipitation north of central VA. The precipitation in PA is likely to be widespread, while the precipitation in MD and northern VA is more likely to be convection. The nested NAM shows a disorganized line of convection just west of the I-95 corridor at 21Z. Locations in southern VA and NC will likely see clear skies for most of the day on Tuesday. The high solar zenith angle in conjunction with clear skies is favorable for ozone formation. The BAMS air quality models respond to this by showing areas of Moderate ozone in NC. Good air quality is expected throughout the region with the exception of locations in southern VA and NC which will see more sun.

Dense smoke from wildfires in AK and western Canada is currently in place over the Dakotas and the central Plains, reaching south into MO. This air in MO is expected to get incorporated into the circulation of the low pressure system in the TRV and flow into the Mid-Atlantic.
It is difficult to determine at what height the smoke will be when it arrives overhead. To the extent that any smoke is mixed downward into the boundary layer, as occurred earlier this month, PM2.5 and ozone will increase markedly into the Moderate range or worse. We will continue to monitor the fate of this pulse of smoke.

Wednesday will the day of most interest in the medium range period as above average temperatures, clear skies, light winds, and possible smoke may enhance ozone and PM2.5 production in the Mid-Atlantic. Ahead of an approaching cold front, there will be clearing throughout most of the region. Convergence along a diffuse boundary, expected to set up in the vicinity of the I-95 corridor Wednesday afternoon, may increase pollutant concentrations. The air quality models agree with this by showing Moderate ozone along the I-95 corridor. Back trajectories are expected to be from the TRV, suggesting that the smoke from the Canadian wildfires may impact the Mid-Atlantic on Wednesday. Again, if the smoke is mixed downward, we could see ozone and PM2.5 rise well into the Moderate range.

On Thursday, a cold front will move through the NMA in the morning, promoting widespread clouds and precipitation. The heaviest precipitation will be along the frontal boundary. As this front drifts southward, it will weaken and stall over the central Mid-Atlantic. This increases the chances that clouds and precipitation will be widespread throughout the region on Thursday, keeping air quality in the generally Good range. The smoke from the Canadian wildfires will still have to be monitored for impacts to the region on Thursday.

Friday and Saturday will be reminiscent of the past couple weeks as a frontal boundary stalls in central VA, promoting clouds and showers in its vicinity. Throughout the day Friday and into Saturday, discrepancies arise in the weather models regarding the location of this oscillating boundary, with the EC showing the heaviest and most northerly area of precipitation. The GFS, however, keeps the bulk of the clouds and showers south of MD. The WPC analysis shows a blend of these two solutions, placing the frontal boundary across northern VA and DE at 12Z Saturday. Depending on the location of this boundary, the NMA may see a day of clear skies, making for a beautiful 4th of July although clear skies and firework smoke may lead to a possible USG air quality day. However, at this time, it is too early to determine the background state of the atmosphere or make a call as to the placement of the front.

-DeBoe/Eherts/Ryan