Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 12, 2018
Valid: July 13-17 (Friday-Tuesday)

Summary

The trend in today’s weather models is for stronger upper level ridging on Saturday and Sunday, as well for a stronger cold front pushing into the region on Tuesday. As a result, nearby high pressure will keep an elevated risk for an ozone exceedance through the weekend, before the risk drops early next week. Friday remains a day of concern as the center of high pressure extending from the surface into the mid-levels moves directly overhead. The persistent smoke continues to be a concern as well, although it is still not clear yet how much of an effect it is having on air quality in the Mid-Atlantic. Visible satellite imagery this morning clearly shows a thicker plume still over ON, flowing southward, and thinner plumes over the NMA. For the northern half of the region, localized back trajectories, mostly sunny skies and near average temperatures will be favorable for ozone formation. The primary forecast question will be the effect of light southeasterly surface winds across the Mid-Atlantic that could clean out coastal locations but concentrate pollutants further inland. These conditions will result in a High risk of an ozone exceedance with a focus along the I-95 Corridor, western PA (PIT), and the western half of NC. High pressure will continue to influence conditions on Saturday, as the surface high pressure sinks southward to VA. In a change from yesterday, the weather models have weakened and slowed the progression of the weak cold front/surface trough that was expected to impact the NMA on Saturday. This feature is now expected to move into the NMA overnight/early Sunday morning as a weak surface trough, making it less likely as a convection-initiator and more likely to act as a line of converging winds. Mostly sunny skies, near/above average temperatures, and light surface winds will be favorable for ozone formation across most of the NMA and CMA, especially along the I-95 Corridor. The primary forecast question for Friday will be the strength of surface winds along the I-95 corridor as they shift to a more southerly component in the afternoon. The risk of an ozone exceedance will remain High with a focus on the I-95 Corridor and western PA. Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA on Sunday, surface high pressure lingering just east of the Chesapeake Bay will keep ozone conducive conditions throughout the Mid-Atlantic. The southward push of the surface trough will be the primary forecast question for Sunday as the spatial coverage of precipitation and cloud cover are in question, as well as converging surface winds in the trough axis. The risk of an ozone exceedance will drop to Appreciable only due to uncertainty in the precipitation forecast. With the consensus in today’s NWP guidance for longwave trough development beginning on Monday, the risk for exceedances running into next week has lessened. Unsettled conditions may a return to the Mid-Atlantic on Monday as a mid-level trough moves over the Midwest and towards the Mid-Atlantic. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The primary forecast questions for Monday will be the coverage of afternoon precipitation and associated cloud cover, primarily across the western and central NMA, and the impact of breezy surface winds along the I-95 Corridor. As a result of these conditions, the risk of an ozone exceedance will drop to Marginal with exceedances possible along the I-95 Corridor. Widespread unsettled conditions will impact the Mid-Atlantic on Tuesday as a strong cold front pushes into the region from the northwest. Although the eastward push of showers and thunderstorms are uncertain, cloud cover and precipitation, in addition to persistent southerly flow aloft and gusty surface winds, should be enough to keep ozone formation in check across the region, keeping a Marginal risk for an exceedance on Tuesday.

NWP Model Discussion

The weather models have come back into remarkably close agreement today on the synoptic features of the medium range period, including the arrival of the next major cold front on Tuesday. By 12Z Friday, the upper level trough passing over the Northeast U.S. today will continue to gradually push eastward, allowing an upper level ridge centered over the Mississippi River Valley to build eastward. One change in the guidance today is a stronger ridge over the Mid-Atlantic on Friday and Saturday. The mid-level ridge associated with the upper level ridge is already in place over the Mid-Atlantic where it will remain, fluctuating in strength across the eastern CONUS until the arrival of a mid-level trough on Monday that will push the mid-level ridge eastward and out to sea. The axis of the upper level ridge will be tilted to the northeast along the St. Lawrence River Valley by 18Z Friday, before an upper level low in the northern stream flow, moving over the Hudson Bay and into OC, weakens and depresses the eastern part of the ridge over the Mid-Atlantic by 12Z Sunday. At the same time, a shortwave trough wrapping around the crest of the depressed ridge over the Upper Midwest, will move into the NMA. This shortwave trough will be picked up by the southern extent of the QC upper level trough, as the axis of this longwave trough stretches southward into the Northeast U.S., paralleling the Atlantic coast by 00Z Monday. Upper level ridging will attempt to build in the wake of the Canadian trough through the second half of Sunday, but lingering shortwave perturbations across the Ohio River Valley and NMA will keep weak troughiness over the northern half of the Mid-Atlantic until an upper level closed low begins to affect the region on Monday. This feature, centered over southern Hudson Bay/northern ON between 18Z Monday and 00Z Tuesday, will begin to develop a longwave trough over the eastern US. By 12Z Tuesday, the trough axis will pivot across eastern ON, giving a more neutral tilt to the trough as it strengthens and moves over the Mid-Atlantic between 18Z Tuesday and 06Z Wednesday.

The Dailies

Day 1 (Friday): Friday remains a day of concern as the center of high pressure extending from the surface into the mid-levels moves directly overhead. The persistent smoke continues to be a concern as well, although it is still not clear yet how much of an effect it is having on air quality in the Mid-Atlantic. Visible satellite imagery this morning clearly shows a thicker plume still over ON, flowing southward, and thinner plumes over the NMA. For the northern half of the region, localized back trajectories, mostly sunny skies and near average temperatures will be favorable for ozone formation. Although back trajectories for locations along I-95 will be weakly onshore, they are more localized in nature, especially at mid-levels. The primary forecast questions will be the effect of light southeasterly surface winds across the NMA and CMA that could clean out coastal locations but also concentrate pollutants along the I-95 Corridor, as well as air mass characteristics (including any effects from smoke). For the SMA, the location of high pressure to the north will promote onshore flow, near average temperatures, and possibly a few afternoon clouds. The air quality models respond to the presence of high pressure with widespread Moderate and isolated USG ozone across the NMA and CMA. The areas highlighted with USG ozone are southeastern PA, north/west of the I-95 Corridor from DC to NYC, and western PA. The models have the patches of USG ozone slightly shift to the northwest of these locations in response to the southeasterly surface winds pushing pollutants inland. Across the SMA, the air quality models respond to the onshore flow as they are mostly in agreement by developing upper Good/low Moderate ozone across eastern VA and NC, while locations further inland, particularly the NC Piedmont could see ozone levels rise into the mid-to-upper Moderate range. These conditions will result in a High risk of an ozone exceedance with a focus along the I-95 Corridor, western PA (PIT), and the western half of NC.

Day 2 (Saturday): Saturday has become more interesting due to the trend for a stronger upper level ridge over the region. High pressure will continue to influence conditions on Saturday, as the surface high pressure sinks southward to VA. Mostly sunny skies in the morning hours will allow temperatures to rise slightly above average values, but increasing moisture ahead of an approaching surface trough will promote partly cloudy skies in the afternoon hours. In a change from yesterday, the models have slowed the progression of the weak cold front/surface trough that was expected to impact the NMA on Saturday. This trough is now expected to move into the NMA overnight/early Sunday morning as a weak surface trough – the main trend being for a weaker feature. Back trajectories along the I-95 Corridor will be localized due to the close proximity of surface and mid-level high pressure, while flow across the rest of the NMA and CMA veers southwesterly. Mostly sunny skies, near/above average temperatures, and light surface winds will be favorable for ozone formation across most of the NMA and CMA, especially along the I-95 Corridor. Another day on onshore flow in the SMA will promote slightly below average temperatures despite mostly sunny skies. This persistent onshore flow will likely be able to limit ozone formation across most of the SMA. As expected, the air quality models continue to clean out the SMA due to the persistent onshore flow. The air quality models generally continue to increase regional ozone across the NMA and eastern CMA due to the lingering center of high pressure near the I-95 Corridor. A few of the NC model versions are outliers with Good ozone creeping into the eastern NMA and CMA. Widespread Moderate across the NMA and CMA, with scattered USG ozone along the I-95 Corridor and across western/central PA is the air quality model consensus. The primary forecast question for Friday will be the strength of surface winds along the I-95 corridor as they shift to a more southerly component in the afternoon. The risk of an ozone exceedance will remain High with a focus on the I-95 Corridor and western PA.

Day 3 (Sunday): Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA, surface high pressure lingering just east of the Chesapeake Bay will keep ozone conducive conditions throughout the Mid-Atlantic on Sunday. The southward push of the surface trough will be the primary forecast question for Sunday as the spatial coverage of precipitation and cloud cover are in question. The NAM and ECMWF keep the trough across the NMA, northwest to southeast, throughout the day on Sunday, with scattered showers and cloud cover moving southward towards I-80 in the afternoon hours. The GFS is slightly different than the NAM and ECMWF with the position and speed of the trough, as the GFS pushes the trough southward near the MDL by 00Z Monday, with more scattered showers across the western NMA and CMA and isolated showers to the east. The position of the trough, more specifically converging winds, will be important for air quality due to the modified air mass anticipated to already be in place, combining with a shift to westerly flow across the NMA and CMA. If the trough remains further north, similar to the NAM and ECMWF solutions, near average temperatures, light surface winds and westerly flow aloft will be favorable for ozone formation. The surface high pressure just east of the southern Delmarva will keep onshore flow across the SMA for another day. This transport of clean air will again limit ozone formation cross the SMA despite mostly sunny skies and temperatures reaching into the low 90s °F. The onshore flow across the SMA is apparent in most of the air quality models as they keep ozone in the Good/low Moderate range. The NC-GFS2 is the lone outlier that increases ozone into the Moderate range across the entire SMA with isolated USG in central/western NC. Converging winds along the surface trough and westerly transport across the NMA and CMA is clearly depicted in the air quality models as they develop a strip of USG/upper Moderate ozone along and head of the trough, roughly following I-76 across PA and along the I-95 Corridor from DC to NYC. The risk of an ozone exceedance will drop to Appreciable only due to uncertainty in the precipitation forecast. If the trough or associated precipitation and cloud cover are more widespread and impact locations along the I-95 Corridor and along I-76, then ozone formation could be limited. If the trough and associated precipitation and cloud cover are slower to push south or conditions remain dry along the trough, then ozone formation will be enhanced by the converging winds along the trough, despite lower Sunday emissions, and exceedances will be likely across much of the southern NMA and CMA.

Day 4-5 (Monday-Tuesday): With the consensus in today’s NWP guidance for longwave trough development beginning on Monday, the risk for exceedances running into next week has lessened. Unsettled conditions could make a return to the Mid-Atlantic on Monday as a mid-level trough moves over the Midwest and towards the Mid-Atlantic. A shift to southwesterly flow will pull a plume of moisture into the region, combining with slightly above average temperatures, resulting in uncomfortable conditions. This increase in moisture will combine with mostly sunny skies in the morning hours allowing scattered showers and thunderstorms to develop in the afternoon. The GFS and ECMWF disagree with precipitation across the region as the GFS develops precipitation across most of the NMA and the western half of the Mid-Atlantic, while the ECMWF focuses precipitation across the NMA while keeping most of the SMA dry. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The air quality models are not in agreement with ozone across the region on Monday as the BAMS models generally drop ozone into the Good/Moderate range while the NC-GFS2 and GFS3 keep a few pockets of USG ozone along the I-95 Corridor and western PA. The air quality models are in agreement that strong southerly flow in the SMA will be sufficient to limit ozone formation for another day. The primary forecast questions for Monday will be the coverage of afternoon precipitation and associated cloud cover, primarily across the western and central NMA, and the impact of breezy surface winds along the I-95 Corridor. As a result of these conditions, the risk of an ozone exceedance will drop to Marginal with exceedances still possible along the I-95 Corridor.

Widespread unsettled conditions will impact the Mid-Atlantic on Tuesday as a strong cold front pushes into the region from the northwest (finally). Showers and thunderstorms will push west to east ahead of the cold front, starting as early as lunchtime. Although the front itself will only advance as far as the NMA, cloud cover and precipitation, in addition to persistent southwesterly flow aloft and gusty surface winds, should be enough to keep ozone formation in check across the region. The primary forecast question will be the coverage/timing of pre-frontal precipitation and cloud cover across the eastern NMA/CMA and the entire SMA. This uncertainty in the forecast will keep a Marginal risk of an ozone exceedance on Tuesday with a focus on the I-95 Corridor and locations across the SMA.

-Enlow/Huff