Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 23, 2018
Valid: May 24-28 (Thursday-Monday)

Summary

Friday remains the primary day of concern in the period, with an Appreciable risk of an ozone exceedance, focused along the I-95 Corridor in the CMA and NMA. An amplifying ridge of high pressure located over the Mississippi River Valley on Thursday will progress eastward, reaching the Ohio River Valley early Friday before weakening over the Mid-Atlantic on Saturday. Surface high pressure moving over the Mid-Atlantic Thursday-Friday morning will promote clear skies with light surface winds and low atmospheric humidity. Ample vertical mixing of air originating upwind from western ON was expected to help limit rising ozone, however, wildfires in the southern Canadian Prairies are producing moderate to dense smoke that is moving eastward into ON. The NAAPS model keeps the smoke plumes west and north of the Mid-Atlantic through the end of the medium range period, and surface PM2.5 concentrations are still in the Good range upwind, but given the advancing ridge, smoke transport is a wild card for Thursday and Friday’s ozone forecast. On Thursday, there is a Marginal risk for an exceedance, primarily at locations impacted by bay/sea breezes. Friday will be sunny, warm, and dry across the NMA and CMA, which will promote ozone formation, with the highest levels expected along the I-95 Corridor. For the Memorial Day weekend, the Mid-Atlantic will be caught between an advancing shortwave along the US/Canadian border and a tropical low in the Gulf of Mexico. These features will promote widespread showers and thunderstorms in the CMA and SMA on Saturday, and across the entire region on Sunday and Monday. There is still some question about how clear it will remain in the NMA on Saturday, but today’s models are trending wetter. Given the uncertainty, the risk of an exceedance will drop but only to Marginal on Saturday, since it will be very warm, with light southwesterly surface winds, so any locations along I-95 that remain clear will experience another day conducive for ozone formation.

 

 

NWP Model Discussion

There is continued agreement between the weather models for the start of the medium range. Conditions across the Mid-Atlantic will be heavily influenced by high pressure on Thursday. An amplifying ridge at upper levels, centered over the Mississippi River Valley around 12Z Thursday, will slowly progress over the Great Lakes/Upper Midwest by 00Z Friday. This ridge will push the center of surface high pressure out of the Ohio River Valley and through the CMA, to the eastern Atlantic coast by 00Z Friday. The placement of the surface high pressure just off the NC/VA coast and a weak influence of the western edge of the Bermuda high will set up a return flow across the southeastern US on Saturday. This return flow will bring a large plume of tropical moisture into the Mid-Atlantic over the weekend. By 12Z Friday, shortwave energy cresting the upper level ridge over southern MB/upper Midwest will begin to flatten the upper and mid-level ridges as it propagates into the western Great Lakes by 00Z Saturday. This wave will sweep across the Great Lakes, moving over southeastern ON by 00Z Monday. The nearby presence of this wave will enhance the northward flow of moisture that begins to move into the Mid-Atlantic on Friday. At the same time, a cold front that has been lingering in central ON/southwestern QC will be pushed southward by the shortwave on Saturday, before it stalls across central NY on Sunday. In addition, a tropical disturbance will be moving into the Gulf of Mexico early Saturday; there is still some uncertainty about the fate of this system. As of 12Z Wednesday, the NHC has a 60% chance of cyclone development over the next 5 days, noting that environmental conditions could improve over the next few days. Even if this system does not organize as it moves through the Gulf, its track will have large impacts on the SMA and parts of the CMA starting on Saturday. The ECMWF brings the disturbance into the central Gulf on Saturday and the Mississippi River Delta around 12Z Sunday. The GFS has a weaker and more eastward track to this disturbance, bringing it along the east coast of FL on Sunday and reaching the GA/SC coastline by 06Z Monday. The GFs solution appears to bring more localized heavy rain to the eastern half of NC on Monday, where the ECMWF solution appears to bring less intense and more widespread heavy rain across the entire SMA. The WPC is favoring the more westward, ECMWF solution. The GFS appears to be an outlier for now regarding the strength and track of the tropical system, as well as the northern shortwave. Despite this uncertainty in the specifics of the weather features, the presence of a wave passing to the north and this tropical disturbance surging from the south will bring widespread unsettled conditions across the Mid-Atlantic on Sunday and Monday.

There is also substantial uncertainty in the magnitude of ozone predicted across the Mid-Atlantic by the air quality models. The models have been extremely unreliable so far this season, swinging from substantial under-prediction the first week of May, to substantial over-prediction last week (e.g., predicting USG ozone when Good conditions were observed). Several of the models develop upper Moderate to USG ozone on Thursday and Friday. At this point, we recommend interpreting these predictions with extreme caution.

 

The Dailies

Day 1 (Thursday): Thursday will be a pleasant day for almost the entire Mid-Atlantic as the presence of a ridge building to the west becomes evident. Noticeably drier air will filter into the NMA and CMA by 12Z. Ample vertical mixing and cool air advection aloft at 850mb will limit daytime surface temperatures to around normal values. Surface winds will be light and northwesterly to start the day, diminishing in the afternoon as the surface high moves overhead. In addition, a strong sea breeze will develop along coastal regions in the afternoon. In the SMA, today’s cold front will linger just to the south of NC, promoting a few areas of clouds and isolated precipitation in the southern half of the state. As expected across inland portions of the Mid-Atlantic, the air quality models develop widespread low to upper Good ozone due to vertical mixing, average temperatures, and fast northerly flow aloft. Although back trajectories are fast and from interior ON, which is typically a clean transport pattern, there is moderate to dense smoke in the vicinity, due to wildfires burning in the southern Canadian Prairies. This morning, PM2.5 concentrations across most of the upper Midwest and ON are still in the Good range, suggesting that is smoke is not mixing to the surface. The NAAPS model keeps the smoke to the north and west of the Mid-Atlantic through the end of the period. But with the advancing ridge aloft, smoke may be advected into our region, making smoke transport a wild card for the air quality forecast. The NC models seem to be slightly more aggressive as they develop more widespread Moderate ozone across most of the region, excluding central PA. Impacts from sea/bay breeze are also evident in the air quality models as each develops a strip of elevated (Moderate) ozone along coastal/bay regions. The NC models are the most aggressive with widespread mid-Moderate ozone with isolated USG ozone around the DC area. BAMS models bring ozone into the upper Moderate and the NOAA model into the upper Good range. There is also a swath of elevated (Moderate) ozone across central NC that appears in all of the air quality models. Again this could be due to weaker northerly flow possibly tapping into the transport of pollutants from the I-95 Corridor or Ohio River Valley. Given these conditions, and the questions about smoke transport, the risk of an ozone exceedance increases to Marginal for Thursday.

Day 2 (Friday): Friday is the primary day of interest as the risk of an ozone exceedance increases to Appreciable. It will be warm with light southwesterly surface winds. A second day of relatively low atmospheric humidity will favor rising ozone. Strong influence from an upper level ridge centered over the Ohio River Valley and the presence of a surface high pressure passing through the CMA and SMA will bring another day of sunny skies to the region. As mentioned above, the passing surface high pressure will set up return flow across the region, increasing mid-level moisture and allowing temperatures to reach above average values. This increase in moisture could result in a few isolated showers in the SMA and CMA in the afternoon hours. Southwesterly surface winds, transport aloft shifting westerly, and above average temperatures are causing elevated ozone concentrations in the air quality models across the NMA and CMA. Yet again the NC model is the most aggressive with widespread upper Moderate ozone across the NMA with USG ozone along the I-95 Corridor and western/central PA. Both versions of the BAMS are similar to the NC with USG ozone developing along the I-95 Corridor, however, they back off throughout the rest of the Mid-Atlantic with low/mid-Moderate ozone. Smoke transport from the northwest will once again be a consideration for the forecast. The I-95 Corridor from Washington, DC northward is the primary location of concern.

 

Day 3 (Saturday): Unsettled conditions will return to the Mid-Atlantic on Saturday as a noticeably more humid air mass continues to lift into the region, and the effects of the tropical low in the Gulf of Mexico begin to affect the SMA and CMA. Increased moisture, slightly above average temperatures and mostly sunny skies in the morning and afternoon hours will provide the opportunity for scattered thunderstorms to develop across the entire Mid-Atlantic in the afternoon and evening hours. Onshore flow in the SMA combined with unsettled conditions across most of the area should limit ozone formation, however, southwest flow, and timing of clouds and precipitation in the NMA and CMA could be problematic for the I-95 Corridor. In today’s model runs, however, the trend seems to be toward bringing more showers and storms further east, reaching into the NMA. This is a change from previous days, which kept the NMA relatively clear. This uncertainty in the timing of unsettled conditions is apparent in the air quality models as both BAMS versions and the NC-GFS keep an area of Moderate ozone along the I-95 Corridor. The air quality models also appear to respond to relatively slow, possibly polluted, southwest transport aloft for the western NMA as ozone concentrations reach into the mid-Moderate range. Onshore flow in the SMA will be enough to limit ozone to the Good range despite above average temperatures. The return of unsettled conditions will lower the risk of an exceedance but uncertainty in timing and ozone favorable transport will only bring the risk of an exceedance to Marginal on Saturday.

Day 4-5 (Sunday-Monday): Uncertainty enters the precipitation forecast on Sunday, as a wave of low pressure passes to the north and a tropical disturbance to the southwest brings tropical moisture into the region. As of now, we are confident that Sunday and Monday will be unsettled across the Mid-Atlantic, however, the uncertainty lies in the specifics. Continued northward transport of tropical moisture into the Mid-Atlantic will promote widespread showers and thunderstorms on Sunday and Monday. Unsettled conditions combined with clean southerly back trajectories should be sufficient to limit ozone formation across the Mid-Atlantic, resulting in a Slight risk for an exceedance.

-Enlow/Huff