Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 11, 2016
Valid: August 12-16, 2016 (Friday-Tuesday)

Summary:

Above average heat and stifling humidity will continue across much of the Mid-Atlantic through the weekend, but persistent southerly flow will keep clean maritime air flowing into the region. Ample regional heat and moisture and the approach of a cold front will support the development of diurnal convection and precipitation essentially every day during the medium range, but the exact extent of this precipitation remains a question. On Friday, this afternoon precipitation is looking increasingly scattered, which will allow for periods of intense sunshine at some locations. Despite this sun, the persistent southerly transport pattern should limit any areas of ozone formation to the Moderate range at most, keeping the chance for an ozone exceedance Slight. Similar conditions will be present on Saturday, with the possibility of periods of afternoon sunshine allowing for some ozone formation, but the clean transport pattern will keep the chances for an ozone exceedance Slight once again. More widespread clouds and precipitation are expected on Sunday as the cold front enters the Ohio River Valley, further enhanced by the development of pre-frontal surface trough. As a result, the chance for an ozone exceedance will remain Slight on Sunday. The cold front will pass through the northern Mid-Atlantic on Monday morning, and stall near the Mason-Dixon Line. Another round of clouds and precipitation along the front will limit ozone formation across the forecast region. Therefore, the chance for an ozone exceedance on Monday will remain Slight. On Tuesday, the cold front will remain stalled across the central Mid-Atlantic, acting as a focus for clouds and precipitation. Although surface high pressure moving across the northern Mid-Atlantic will support the chance for ozone formation along the northern extent of the I-95 Corridor, the presumably clean post-frontal air mass in place should keep the chance for an ozone exceedance Slight on Tuesday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with differences at the end of the period regarding the placement of a slow-moving cold front. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad upper level trough extending southward across the Plains will progress eastward on Friday, with its center traversing MB/southern ON. The leading edge of this trough will be positioned just to the north of the Mid-Atlantic, across NY/New England, for the entirety of Friday. Concurrently, a semi-permanent mid-level ridge centered over the Bermuda High out on the Atlantic will remain in place on Friday persist through the medium range. The axis of the upper level trough will shift slightly eastward on Saturday towards the Great Lakes, pulling its associated cold front into the Ohio River Valley during Saturday afternoon. The NAM shows a shortwave disturbance across the Gulf Coast, which caused record flooding to much of the southern U.S., getting absorbed into the periphery of the upper level trough beginning 12Z Saturday. The GFS and EC have a similar thing happening, but not until roughly 12Z Sunday. On Sunday, the upper level trough will pivot across the Great Lakes, reorienting its cold front east-west, and bringing it right to the northern edge of the Mid-Atlantic by 00Z Monday. The upper level trough will flatten and contract northward on Monday, with its axis shifting northeastward towards the Canadian Maritimes. Its associated cold front will move southward towards the central Mid-Atlantic, reaching the vicinity of the Mason-Dixon Line by 00Z Tuesday. On Tuesday, the Mid-Atlantic will remain under mostly zonal flow aloft, allowing a line of potent vorticity, streaming off from the decaying Gulf Coast low underneath the flattened upper level trough, to approach the Mid-Atlantic. The GFS keeps this vorticity mostly to the north of the forecast region through Tuesday, while the EC brings it to the MDL by 00Z Wednesday. At the surface, the cold front will stall across the central Mid-Atlantic, in the vicinity of the MDL.

Friday will be another scorching day for much of the Mid-Atlantic, with above average temperatures, dew points in the 70°F, and heat indices climbing above 100 °F. The NWS has placed portions of the I-95 Corridor, including northern DE, PHL, and western NJ under an Excessive Heat Warning through Saturday evening. Ample regional heat and moisture will support the formation of afternoon convection and thunderstorms. The GFS and EC both continue to develop widespread precipitation across the northern and central Mid-Atlantic beginning 18Z, with this rain reaching the I-95 Corridor by 00Z Saturday. The NAM, however, shows more scattered precipitation, which is also supported by the 06Z 4 km and 12 km NAM, as well as the 00Z NMM/ARW. The 03Z SREF sides with the GFS/EC solution, with high probabilities of precipitation across the northern and central Mid-Atlantic by the afternoon. 06Z GFS back trajectories for the I-95 Corridor remain relatively fast and southerly, bringing a persistent flow of clean maritime air into the forecast region. Surface winds will be southwesterly and relatively light, which will keep highway emissions along I-95. If a NAM-like solution verifies, periods of intense afternoon sunshine may allow for some ozone formation along the I-95 Corridor. The 06Z air quality models reflect this possibility, with all of the models developing a strip of Moderate ozone along I-95, more so than they did in yesterday’s model runs. The main forecast question will be how quickly this relatively clean air mass can modify, but the continued presence of a clean regional transport pattern suggests that any ozone that forms will not reach beyond the Moderate range. As a result, the chance for an ozone exceedance will remain Slight on Friday.

Another round of diurnal clouds and precipitation is expected on Saturday, ahead of the cold front arriving across the Ohio River Valley during the afternoon. The GFS and EC bring the leading edge of this precipitation to the I-95 Corridor by 00Z Sunday, while the NAM keeps this precipitation just to the north and west of I-95. 06Z GFS back trajectories for the I-95 Corridor remain fast and southerly, again promoting a persistent flow of clean maritime air. Surface winds will also remain southwesterly, but will slightly stronger and more persistent than on Friday. Even if heavy precipitation does not reach I-95 by Saturday afternoon, there will likely be enough nearby cloud cover to temper ozone, which can be seen in the 06Z 15-panel NAM. As on Friday, however, the potential for period of sunshine lends support to the possibility of some ozone formation along the I-95 Corridor. The air quality models are split, with the BAMS models bringing most of the Mid-Atlantic back down into Good range, while the NCDENR model continues to shows a line of Moderate ozone right along I-95. How high ozone rises on Friday will give a sense of how quickly local ozone production can counteract the clean regional air mass and transport pattern on Saturday. Given the continued strong support for a clean transport pattern aloft, the chance for an ozone exceedance will remain Slight.

More widespread clouds and precipitation are expected on Sunday as the front progresses to just north and west of the Mid-Atlantic. This will be further enhanced by the development of a pre-frontal surface trough expected to form, roughly aligned with the I-95 Corridor. The GFS and EC both develop precipitation beginning 18Z and reaching I-95 by 00Z Monday. 06Z GFS back trajectories for the I-95 Corridor turn more west and south, coming from the Gulf Coast, and they remain fast. The BAMS models keeps the entire Mid-Atlantic in the Good range for ozone, while the NCDENR model develops a few patches of Moderate ozone along the northern extent of the I-95 Corridor. One question will be how high ozone levels climbed both Friday and Saturday. If Moderate conditions were observed both of those days, then any periods of sunshine or converging winds along the surface trough before precipitation arrive could lead to spikes in ozone concentrations. However, given the support for more widespread precipitation, and continued clean transport aloft, local ozone formation should be even more limited than it was the two preceding days. As a result, the chance for an ozone exceedance will remain Slight on Sunday.

The cold front will move through the northern Mid-Atlantic during the morning and early afternoon on Monday, approaching the Mason-Dixon Line during the latter half of the day. The GFS and EC differ with their placement of precipitation along this front. The GFS shows precipitation further south, across the central and southern Mid-Atlantic by 18Z. On the other hand, the EC places this precipitation further northward across the northern and central Mid-Atlantic, indicating a slightly slower front. 06Z GFS back trajectories for the I-95 Corridor are westerly, which suggest that the front will not have cleared the I-95 Corridor by 12Z. The effects of the air mass change behind the front will not be felt immediately, meaning that locations behind the front will still experience sweltering heat and humidity. Post-frontal surface winds will be northwesterly, but relatively light. Therefore, locations just behind the cold front may experience enough sunshine and near-front convergence to support some ozone formation. The BAMS air quality models shows hints of this, with a line of Moderate ozone roughly aligned with the MDL in the vicinity of the front. All of this said, widespread clouds and precipitation on Saturday are expected to help to keep the regional air mass clean, which may make it difficult for the air mass to modify quickly on Sunday. Additionally, there should be ample cloud cover near the front to temper ozone formation, despite the factors discussed above. Therefore, the chance for an ozone exceedance will remain Slight on Monday.

On Tuesday, the cold front is expected to stall in the vicinity of the Mason-Dixon Line, providing a focus for another round of diurnal convection and precipitation. Again, the GFS and EC differ with their placement of this precipitation. The GFS has this precipitation focused across the central and southern Mid-Atlantic, while the EC develops it across the northern and central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are northwesterly from MI. Despite post-frontal conditions across the northern Mid-Atlantic, surface high pressure moving across the Great Lakes into PA and NJ. will promote clearing skies and diminishing winds at some locations, despite its close proximity to the cold front. This will allow for the chance of ozone formation along the northern extent of the I-95 Corridor. But with a presumably clean post-frontal air mass in place, any ozone that forms should remain inside the Moderate range. As a result, the chance for an ozone exceedance will remain Slight on Tuesday.

-Brown/Huff