Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 11, 2018
Valid: July 12-16 (Thursday-Sunday)

Summary

A persistent but fluctuating mid-level ridge will keep conditions favorable for ozone formation throughout the medium range period. The center of this ridge is not very well-defined, however, which adds uncertainty to the forecast. The threat of smoke transport from upwind in ON seems diminished today, but we won’t know for sure until we see how ozone evolves this afternoon. So Thursday remains a day of interest due to the potential lingering effects of smoke, nearby presence of high pressure, and a surface trough along I-95 that will promote converging winds. The primary forecast questions for Thursday will be the magnitude of the effects of any lingering or transported smoke, the impacts of the surface trough along the I-95 Corridor, including the potential development of afternoon cloud cover and precipitation. The risk for an ozone exceedance will be Appreciable with a focus on the entire I-95 Corridor. An increased risk of an ozone exceedance will continue into Friday as high pressure builds over the Mid-Atlantic. A weak onshore component to the mid-level flow for the eastern portion of the region – associated with a weak lingering low out in the Atlantic – could have a positive impact on air quality for coastal regions but as a result could concentrate pollutants further inland. The primary forecast question for Friday will be the effects of the onshore flow/converging winds along the I-95 Corridor, as well as any lingering smoke. As a result, the risk of an ozone exceedance will increase to High with a focus on the I-95 Corridor and locations along I-76 throughout the NMA. Unsettled conditions are possible across most of the Mid-Atlantic on Saturday as a weak cold front approaches the region. Surface high pressure sinking into the SMA will promote south/southeasterly transport for the western Mid-Atlantic and another day of weak onshore flow in the eastern Mid-Atlantic. The primary forecast questions for Saturday will the coverage of precipitation and cloud cover associated with the frontal boundary that will likely dissipate throughout the day. If cloud cover is sparse across the NMA, regional ozone could quickly rise due to weak surface winds and weak flow aloft. The risk for an ozone exceedance will drop to Appreciable due to uncertainty in the forecast, with exceedances possible along the I-95 Corridor from DC to NYC and in western PA. There is a great deal of uncertainty in the specifics for Sunday’s forecast but it appears that a surface trough will develop over the east coast, enhancing the possibility for unsettled conditions. Recirculating back trajectories along the I-95 Corridor and westerly transport across the rest of the NMA will be favorable for ozone formation, while weak onshore flow and partly cloudy skies keep ozone in check across the SMA. Given the uncertainty in the forecast and lower Sunday emissions, the risk of an ozone exceedance will lower to Marginal with a focus on the I-95 Corridor and western PA. On Monday, dry conditions across the eastern half of the region will be favorable for ozone formation but the effects of southerly surface winds will be a primary forecast question, given the divergence in the GFS and ECMWF guidance. As a result, the risk of an ozone exceedance will remain Marginal.

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features until late in the medium range period. A reinforced upper level trough will move over the Northeast U.S. on Thursday, with the trough axis reaching the Atlantic coast by 00Z Friday. A shortwave lobe from this trough will spin off over the Atlantic Ocean around 18Z Friday and remain roughly southeast of Cape Cod through midday Sunday. As the upper level trough continues to move eastward overnight, an upper level ridge centered over the Mississippi River Valley will begin to build eastward, edging into the Mid-Atlantic by 12Z Friday. The axis of the upper level ridge will be tilted and elongated along the St. Lawrence River Valley on Friday as an upper level trough centered over Hudson Bay will limit the northward extent of the ridge. The southward push of this Canadian trough will gradually depress the northern edge of the ridge throughout the day Friday, pinching the ridge axis over the Mid-Atlantic and Northeast by 00Z Saturday. As this occurs, a shortwave trough developing across the Midwest will slowly crest the ridge, pulling an associated cold front towards the NMA by 12Z Saturday. While this shortwave trough passes over the NMA, additional shortwave energy will drop down over New England by 12Z Sunday. By 00Z Monday, the upper level troughing will push to the Atlantic coast, picking up the lingering spinning vorticity over the Atlantic, and continue moving eastward. The departure of this trough will again allow upper level ridging to build eastward over the Mid-Atlantic. Similar to Friday, the ridge axis will be pinched and tiled along the St. Lawrence River Valley by 18Z Monday, with the Mid-Atlantic remaining under the influence of high pressure through 00Z Tuesday. At mid-levels, a ridge associated with the Mississippi River Valley upper level ridge will build over the Mid-Atlantic by 12Z Thursday and persist over much of the eastern CONUS throughout the medium range period. The center of this ridge is not very well-defined, which adds uncertainty to the forecast. As a result, day to day fluctuations in the strength and center of this mid-level ridge may have substantial impacts on the air quality forecast.

The Dailies

Day 1 (Thursday): The threat of smoke transport from upwind in ON seems diminished today. The smoke plume appeared to become diluted yesterday afternoon, based on satellite imagery. In addition, PM2.5 concentrations are not elevated this morning at locations in NY and northern New England, which would have been the first to feel the impacts of the smoke. There should be substantial vertical mixing (>2km) this afternoon in the NMA, so that may mix down some smoky air. We will have to monitor the potential impacts of the smoke closely through Friday.

Nevertheless, Thursday remains a day of interest due to the potential lingering effects of smoke and the nearby presence of high pressure. The center of surface and mid-level high pressure will be located to the northwest, over the Great Lakes region. This will set up a flow around the ridge of high pressure, resulting in another day of north/northwesterly flow aloft across the northern half of the region. A plume of smoke can still be seen this morning in visible and true color satellite imagery stretching across interior Canada and southeastward across MB and into southern ON. Given northwesterly flow at mid and lower levels across MB and ON throughout the day today, and back 36 hour back trajectories from the western part of the NMA sourcing from just north of Lake Huron, transport of smoke will remain a factor in the air quality forecast for Thursday. It will be important to monitor today’s air quality conditions to gauge the effects that the smoke is having on the regional air mass to get a better feel to how the air mass will modify throughout the rest of the work week. Aside from the potential for smoke, influence from high pressure will promote light northeasterly surface winds across most of the NMA and CMA under mostly sunny skies. Although most of the region will remain dry, a few of the convection allowing models are hinting that a few afternoon showers and thunderstorms are possible along the I-95 Corridor where a surface trough will promote converging winds. A weak cold front will linger in the SMA on Thursday, combining with a plume of moisture to provide the opportunity for scattered showers and thunderstorms throughout the afternoon hours. Despite the nearby frontal boundary, the air quality models keep regional ozone in the Moderate range across the SMA in response to periods of mid-July sunshine, localized back trajectories and light surface winds. Across the NMA and CMA, the air quality models continue to respond to northerly transport aloft by dropping regional ozone mostly into the Good range. The BAMS and NC models are in agreement that ozone will remain in the Moderate range for locations east of I-81 with a few isolated pockets of USG along the I-95 Corridor. This strip of elevated ozone is likely in response to the surface trough and corresponding converging surface winds along the eastern Mid-Atlantic. The primary forecast questions for Thursday will be the magnitude of the effects of any lingering or transported smoke and the impacts of the surface trough along the I-95 Corridor, including the potential development of afternoon cloud cover and precipitation. Given questions about the smoke and the surface trough, the risk for an ozone exceedance will be Appreciable with a focus on the entire I-95 Corridor.

Day 2 (Friday): An increased risk of an ozone exceedance will continue into Friday as high pressure builds over the Mid-Atlantic. Surface high pressure over southwestern PA and mid-level high pressure directly overhead will promote localized back trajectories across the entire Mid-Atlantic. A weak onshore component to the mid-level flow for the eastern portion of the region – associated with the spinning Atlantic Ocean low – could have a positive impact on air quality for coastal regions but also could concentrate pollutants further inland. Mostly sunny skies, seasonable temperatures, and light surface winds will blanket the Mid-Atlantic. The air quality models are responding to the continued influence from high pressure by developing widespread Moderate ozone across the region and a few pockets of USG ozone in the NMA and CMA, mainly for locations along and west of I-95. The BAMS models also pick up on the weak onshore component of the mid-level flow for the east coast as they drop ozone into the Good/low Moderate range for locations along and east of the I-95 Corridor. The NC air quality models are not as strong with this feature as they develop a few pockets of USG ozone along the I-95 Corridor. The primary forecast questions for Friday will be the effects of the onshore flow/converging winds along the I-95 Corridor, as well as any lingering smoke. Given the ozone-conducive features in place, the risk of an ozone exceedance will increase to High with a focus on locations along/west of the I-95 Corridor and along I-76 throughout the NMA.

Day 3 (Saturday): Unsettled conditions are possible across most of the Mid-Atlantic on Saturday as a weak cold front approaches the region from the northwest. Although high pressure over the Mid-Atlantic will likely cause the front to dissipate as it pushes into the NMA, shortwave perturbations aloft will continue to round the ridge and may be strong enough to promote scattered showers throughout the northern half of the region in an increasingly humid air mass. Model guidance suggests that most of the precipitation will be limited to locations west of I-81 throughout the NMA and CMA. Although a moist air mass will also be in place over the SMA, the upper level perturbations will likely remain too far to the north to promote unsettled conditions on Saturday. Surface high pressure sinking into the SMA will promote south/southeasterly transport for the western Mid-Atlantic and, with the Atlantic low still out there southeast of Cape Cod, another day of weak onshore flow in the eastern Mid-Atlantic. Light surface winds and seasonable temperatures under periods of mid-July sunshine are evident in the air quality models as they keep the NMA and CMA under widespread Moderate ozone. The models are not in agreement with the development of USG ozone across the NMA and CMA as the NC-GFS2 and the BAMS-MAQSIP develop scattered USG ozone across the western half of PA and in a few locations along the I-95 Corridor. In comparison, the BAMS-CMAQ actually keeps most of the NMA under low Moderate ozone with upper Moderate in the PIT metro area and in the DC metro area. The primary forecast questions for Saturday will the coverage of precipitation and cloud cover associated with the frontal boundary that will likely dissipate throughout the day. If cloud cover is sparse across the NMA, regional ozone could quickly rise due to weak surface winds and weak flow aloft. The risk for an ozone exceedance will drop to Appreciable due to uncertainty in the forecast. Exceedances will be possible along the I-95 Corridor from DC to NYC and in western PA.

Day 4-5 (Sunday-Monday): Persistent humid conditions and increased warmth on Sunday will extend the chances for scattered showers across the region throughout the day. There is a great deal of uncertainty in the specifics for Sunday’s forecast but it appears that a surface trough will develop over the east coast and will enhance the possibility for unsettled conditions. If conditions remain dry across the NMA, recirculating back trajectories along I-95 and converging surface winds in the surface trough could be favorable for ozone formation. Across the rest of the NMA, a shift to westerly transport aloft, mostly sunny skies and above average temperatures will also be favorable for ozone formation. Despite the center of weak surface high pressure nearby and above average temperatures, partly cloudy skies and weak onshore flow will likely keep ozone in check across the SMA. The air quality models respond to these conditions with a mix of Good and low Moderate ozone across the SMA and widespread mid-to-upper Moderate ozone across the NMA with a few isolated areas of USG. Given the uncertainty in the forecast and lower Sunday emissions, the risk of an ozone exceedance will lower to Marginal with a focus on the I-95 Corridor and western PA.

The EC and GFS diverge significantly on Monday regarding the low pressure system moving through southern Canada and an associated cold front. The GFS has a stronger upper level low, which pushes farther south, which results in more southerly flow across the Mid-Atlantic on Monday. The EC is weaker and farther north with the upper level low, which allows more westerly flow across the Mid-Atlantic on Monday. At this time, WPC is siding with the dirtier EC solution, but it’s hard for us to take a side this far out. Monday should be warmer and more humid, with above average temperatures. A plume of moisture will push eastward into the region ahead of the cold front moving through the Midwest. Daytime heating and increasing moisture will promote the opportunity for unsettled conditions across the western half of the region. Dry conditions across the eastern half of the region will be favorable for ozone formation, but if the GFS solution verifies, the effects of southerly surface flow will be a primary forecast question. Given the uncertainty, the risk of an ozone exceedance will remain Marginal.

 

-Enlow/Huff