Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 16, 2017
Valid: June 17-21, 2017 (Saturday-Wednesday)

Summary:

The recent theme of daily afternoon convection will continue for much of the medium range period, with the exception of the western Mid-Atlantic on Saturday and the I-95 Corridor on Tuesday. Saturday will be warm and humid as the Mid-Atlantic moves into the warm sector of the next storm system. Clearing skies by lunchtime will allow for afternoon/evening thunderstorms in much of the region. Mostly clear skies and recirculating back trajectories in western PA will allow ozone to build, however, with the possibility of an isolated exceedance. The next cold front will slowly approach the region from the west on Sunday. Prefrontal thunderstorms will cover the Mid-Atlantic west of I-95, moving west to east starting around 15Z, and waning quickly into the evening hours. Clear skies and dry conditions are expected along and east of the I-95 Corridor, but lower Sunday emissions and strong onshore southerly flow will limit rising ozone. The cold front will slowly push through the Mid-Atlantic on Monday, triggering widespread, heavy precipitation, moving from west to east, which will suppress ozone formation. The front will be moving into western portions of the Mid-Atlantic by 12Z and slowly progress eastward to the vicinity of I-81/I-95 by 00Z Tuesday. The front will stall somewhere across the SMA on Tuesday, leading to clearing skies across the NMA and CMA. There is little in the way of an air mass change expected with the front, allowing warm and humid (but not as humid as the weekend) weather to persist. Although the I-95 Corridor will be post-frontal, sunny skies, light surface winds, and westerly back trajectories may allow for some ozone development. On Wednesday, potent shortwaves aloft interacting with a secondary cold front will bring another round of convective precipitation to the NMA.
Weather Model Discussion:

The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be strong consensus between the weather models with the key large scale atmospheric features in the medium range period. On Saturday, potent shortwaves moving in the northern stream flow, from west to east along the U.S./Canadian border, will reinvigorate the weak upper level trough that is currently over southern-central Canada. The trough will quickly deepen as it moves eastward, with its axis over the upper Midwest and its center of circulation over ON by 12Z Sunday. Strong shortwaves spinning around the trough will interact with an existing frontal boundary, developing a mid-level low over MI/ON and a corresponding surface low in roughly the same place by 12Z Sunday. The trough will continue to spin in place over ON, as it deepens over the upper Midwest and into the Mid-Atlantic by 12Z Monday. By 12Z Monday, the surface low will pull northeast into western QC, with its associated trailing cold front pushing into the Mid-Atlantic, eventually stalling along the Atlantic coast sometime on Tuesday. The GFS and EC have come into much closer consensus today regarding the upper level trough on Tuesday. Now, both models keep the trough extending into the Mid-Atlantic on Tuesday, reaching southward to the SMA (~NC). The trough will have a slight negative tilt, with its axis over the Great Lakes. On Wednesday, both models drop a potent shortwave around the axis of the trough, moving over the NMA. There are some slight timing and magnitude differences between the GFS and EC with the shortwaves, but both models have the same trend. The trough axis will move over the Mid-Atlantic between 18Z Wednesday and 06Z Thursday. The main difference for the end of the period in today’s models is that there will be a secondary cold front that will move into the NMA on Wednesday. WPC keeps this front farther to the north in this morning’s analysis, but the 06Z GFS/00Z EC model guidance brings it southward into the NMA. Then the potent shortwave interacts with this boundary on Wednesday, triggering an area of widespread convection across the NMA; the EC has stronger and more widespread precipitation.
The Dailies:

Warm and humid conditions will continue across the Mid-Atlantic on Saturday. Light winds in the morning hours will pick up into the afternoon, resulting in a strong southerly flow across the SMA, with weaker flow in the NMA. It will be warm and very humid across the region, with above average temperatures and dew points in the low 70s °F. Mostly cloudy skies in the morning will clear west to east across the region, giving way to mostly clear skies by the afternoon, with the most clearing expected in western portions of the Mid-Atlantic. Given above average temperatures, clearing skies and the humid air mass, scattered thunderstorms will develop across much of the Mid-Atlantic between 18Z Saturday and 00Z Sunday. The 03Z SREF and 06Z GEFS show probability for precipitation is highest across all of the CMA, SMA and the eastern NMA, roughly east of I-99. Hi-res, convection allowing models (e.g. NMM and ARW) are in consensus with the SREF and GEFS, as thunderstorms and showers are focused across the SMA and CMA, with mostly dry conditions in the western NMA. This clearing trend in the west is also showing up in the 06Z NOAA and BAMS air quality models, with scattered Moderate/isolated USG ozone is appearing in western PA. We have seen the past two days that the BAMS models have been aggressive in developing isolated USG ozone at locations along the stalled frontal boundary. Our instinct was to discount this guidance, as it seemed overdone, but we have observed isolated USG ozone in the past two days, most notably in Hagerstown, MD yesterday and metro DC region on Wednesday. So the lesson seems to be that the air mass in place behind the stalled front is still modified enough that any clearing can result in rapid ozone formation. As a result, locations in western PA are at most at risk, where clear skies, recirculating back trajectories, and little precipitation is expected. All air quality models are in consensus that the rest of the Mid-Atlantic will mostly be in the Good range on Saturday with scattered Moderate ozone in heavily populated areas. There is some concern for parts of the SMA for the possibility of dilute smoke being transported into the region. The most recent NOAA HMS fire and smoke analysis and the HRRR-Smoke model are showing a number of wildfires occurring in southern GA/FL, where back trajectories for locations in NC begin, as well as a fire in eastern NC. The most recent NRL-NAAPS model is not indicating any smoke transporting into the SMA at this time. There does not appear to be abundant smoke from the fires currently, but it is something to monitor. Although thunderstorms are expected across much of the Mid-Atlantic on Saturday, the risk for an exceedance will increase to Appreciable, with an emphasis on the western NMA due to a recirculating air mass with mostly clear skies under the strong June sun.

Unsettled conditions are expected for the Mid-Atlantic on Sunday as a cold front slowly approaches from the west. Even though the region will be pre-frontal on Sunday, the position of the synoptic features is such at the flow aloft will be southerly rather than southwesterly. Back trajectories for the I-95 Corridor, for example, will be fast and due south, from the GA/FL coastal waters, which should be a very clean transport pattern. Another round of afternoon/evening thunderstorms will occur as above average temperatures continue and dew points crawl into the mid 70s °F. Convective thunderstorms are expected to trigger around 18Z across the region, moving west to east, continuing into the evening as the front approaches. It the heaviest precipitation is expected in the western Mid-Atlantic, where more organized pre-frontal convection will occur. The 03 SREF and 06 GEFS show precipitation most likely between 15Z Sunday and 00Z Monday across the central portions of the Mid-Atlantic. The 06Z air quality models are indicating that heavy, widespread precipitation will occur as they show ozone in locations behind the precipitation falling into the Good range. Locations west of the Appalachian Mountains have scattered low to mid Moderate ozone as storms ahead of the front die down into the evening. The focus for air quality will be on the I-95 corridor for Sunday as precipitation may not push that far eastward, resulting in mostly clear skies allowing some ozone formation to take place, however, lower Sunday emissions and back trajectories that are strong and southerly will limit substantial ozone formation. As a result, risk for an exceedance will lower to Slight for Sunday.

The cold front will slowly push through the Mid-Atlantic on Monday, triggering widespread, heavy precipitation. The front will be moving into western portions of the Mid-Atlantic by 12Z and slowly progress eastward to the vicinity of I-81/I-95 by 00Z Tuesday. Mostly cloudy skies and widespread periods of heavy precipitation will suppress ozone formation across the Mid-Atlantic, keeping risk for an exceedance Slight.

The slow moving cold front will continue to impact the SMA and locations east of I-95 in the NMA and CMA on Tuesday. By 12Z Tuesday, the front is expected to stall somewhere across the central SMA, extending northeastward along the Atlantic coast. Given the nearby frontal boundary, conditions across the SMA are expected to be unsettled, with widespread precipitation throughout the day. The GFS is more aggressive with precipitation, while the EC has clearing across the SMA by 00Z Wednesday. In the NMA and CMA, skies will clear behind the front. Unlike with most strong fronts, there will be little in the way of an air mass change. Seasonable temperatures, light surface winds, and westerly back trajectories suggest there could be some ozone formation along the I-95 Corridor on Tuesday, even though it will be post-frontal. The forecast questions will focus on upwind ozone transport from the Ohio River Valley. Given this uncertainty, the risk for an exceedance will rise to Marginal on Tuesday.

Wednesday will be a typical summer day for most of the Mid-Atlantic. Clear skies will allow temperatures to remain near average across the SMA. In the NMA, the potential approach of a secondary cold front adds uncertainty to the forecast. As mentioned in the model discussion, potent shortwaves moving overhead will interact with this frontal boundary, bringing a round of widespread showers and possibly thunderstorms to the NMA. As a result, the risk for an exceedance will return to Slight.

-Enlow/Huff