Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 27, 2018
Valid: July 28-August 1 (Saturday-Wednesday)

Summary

A Marginal risk of an ozone exceedance will persist through Tuesday as each day, there will be sufficient sunshine and light winds to potentially promote isolated rising ozone. Saturday will be pleasant throughout most of the Mid-Atlantic as weak high pressure begins to build in behind Friday’s cold front. Aside from a few lingering showers in the morning along the I-95 Corridor in the NMA and CMA and a few isolated afternoon showers across the NMA, the majority of precipitation will be confined along Friday’s cold front that will be hung up across the SMA. Mostly sunny skies, light westerly flow across the NMA and CMA, sea/bay breezes developing along the Atlantic coast and Chesapeake Bay, and the possible transport of remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry. Upwind PM2.5 concentrations behind the cold front are quite low this morning, however, suggesting that the smoke (seen aloft in satellite imagery) is not mixing to the surface. The possibility for smoke transport and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor, especially areas affected by bay/sea breezes. The arrival of a drier air mass will bring comfortable conditions to most of the NMA and parts of the CMA on Sunday, while a developing low pressure system in the Midwest will promote unsettled conditions throughout the southern half of the region. The trend in yesterday’s and today’s weather models is to slow the northward progression of Friday’s lingering frontal boundary across the SMA as a warm front. As a result, it now appears the stalled front will remain across the SMA on Sunday. Light westerly flow and weak surface winds, in conjunction with the continued risk for smoke transport, could be favorable for some ozone formation in the NMA and CMA, but a mix of sun and clouds and lower Sunday emissions should be enough to prevent substantial ozone formation across the northern half of the region. Nevertheless, nearby surface high pressure, very light surface winds, and a mix of sun and clouds will keep the risk of an ozone exceedance Marginal on Sunday, with a focus on the I-95 Corridor, especially areas affected by bay/sea breezes. Unsettled conditions will impact the southern half of the region on Monday as a developing low pressure system over the Mississippi River Valley starts to slowly pull Friday’s cold front northward as a warm front, pushing into VA by Monday evening. The primary forecast question on Monday will be the coverage of clouds throughout the NMA and CMA as light westerly flow aloft, another day of weak surface winds, and near average temperatures could be favorable for ozone formation. One possible limiting factor along I-95 will be flow aloft turning weakly southerly, which may be enough to limit any rising ozone, especially along the southern and central portions of the Corridor (e.g., DC to PHL). Flow aloft will still be light westerly at locations farther inland, such as PIT, putting them and the northern branch of the Corridor at higher risk. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ and locations along the I-95 Corridor from BAL to NYC, given questions about the effects of weak southerly flow aloft and the northern extent of approaching precipitation. A low pressure system will move into the Ohio River Valley on Tuesday, promoting widespread showers and thunderstorms throughout the Mid-Atlantic. Widespread, locally heavy precipitation, cloudy skies, and sustained surface winds will limit ozone formation throughout the region. The primary forecast question on Tuesday will be how quickly unsettled conditions impact locations throughout the northeastern NMA. Given the uncertainty in the precipitation forecast and the recent trend for a slower northward migration of the warm front and associated precipitation, the risk of an ozone exceedance will remain Marginal with a focus on locations throughout the northeast NMA, especially along the I-95 Corridor. Precipitation will become more widespread on Wednesday as the Mid-Atlantic lies in the warm sector of the low pressure system centered in the Ohio River Valley. Strong southerly flow will continue to push a saturated air mass northward into the Mid-Atlantic promoting widespread precipitation and cloudy skies throughout the day, dropping the risk of an exceedance to Slight.

 

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features throughout the medium range period, although there are some slight differences that develop on Tuesday and Wednesday. By 12Z Saturday, an upper level low will be pushing eastward very slowly over the ON/QC border, reaching as far east as eastern QC by 00Z Sunday. Additional shortwave energy stretching across the northern half of the CONUS will keep broad upper level troughing across the CONUS into the work week. By 12Z Monday, another surge of shortwave energy will drop southward across the northern Great Plains, digging out a defined upper level longwave trough over the Central Plains by 12Z Tuesday. This feature will strengthen quickly on Tuesday, as the models briefly develop a closed low over the Mississippi River Valley by 00Z Wednesday. By 12Z Wednesday, the Mississippi River Valley trough will be influenced by a strong shortwave passing through southern Canada, which will pull the center of the Mississippi River Valley trough northward, resulting in a broad upper level trough centered over the Great Lakes region and Mississippi River Valley by 00Z Thursday. With an amplified longwave trough centered just to the east of the Mid-Atlantic and the western edge of the Bermuda High just off the east coast, the pattern through the first half of next week will be reminiscent of the pattern throughout this past weekend and the first half of this week, although somewhat weaker. Southerly flow will set up as early as 18Z Monday, gradually strengthening on Tuesday and Wednesday, promoting widespread unsettled conditions throughout the region early next week.

The Dailies

Day 1 (Saturday): Saturday will be pleasant throughout most of the Mid-Atlantic as weak high pressure begins to build in behind Friday’s cold front. Aside from a few lingering showers in the morning along the I-95 Corridor in the NMA and CMA, and a few isolated afternoon showers across the NMA, the majority of precipitation will be confined along Friday’s cold front that will be hung up across the SMA. Mostly sunny skies and light westerly flow across the NMA and CMA, in addition to possible remnant ON wildfire smoke transport from the west, could be favorable for ozone formation across locations that remain dry. Upwind PM2.5 concentrations behind the cold front are quite low this morning, however, suggesting that the smoke (seen aloft in satellite imagery) is not mixing to the surface. Weak westerly flow under mostly sunny skies across the NMA and CMA will allow for the development of sea/bay breezes that could enhance ozone formation along the Atlantic and Chesapeake Bay coasts. The air quality models are generally in agreement with widespread upper Good range ozone across most of the Mid-Atlantic, with a strip of Moderate ozone along the I-95 Corridor and around the Bay. This Moderate ozone is likely in response to weak westerly flow and the development of sea/bay breezes concentrating pollutants. The possibility for smoke transport and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor, especially areas affected by bay/sea breezes.

Day 2 (Sunday): The arrival of a drier air mass will bring comfortable conditions to most of the NMA and parts of the CMA on Sunday, while a developing low pressure system in the Midwest will promote unsettled conditions throughout the southern half of the region. Although the majority of the NMA and CMA will remain dry as surface high pressure moves over PA, a few afternoon showers cannot be ruled out as shortwave perturbations pass overhead. The trend in yesterday’s and today’s weather models is to slow the northward progression of Friday’s lingering frontal boundary across the SMA as a warm front. As a result, it now appears the stalled front will remain across the SMA on Sunday. Light westerly flow and weak surface winds, in conjunction with the continued risk for smoke transport, could be favorable for some ozone formation in NMA and CMA, but a mix of sun and clouds, along with lower Sunday emissions, should be enough to prevent substantial ozone formation across the northern half of the region. Throughout the SMA, Friday’s lingering cold front will promote cloudy skies and widespread showers and thunderstorms. These conditions, combined with southerly onshore flow, will keep ozone formation minimal across the SMA on Sunday. The BAMS air quality models are in agreement with scattered Moderate ozone east/south of I-95 in the NMA/CMA and a strip of Moderate ozone along the warm front across the SMA, with Good ozone elsewhere. The BAMS models possibly develop the Moderate ozone along the front in the SMA in response to converging surface winds but unsettled conditions will be widespread enough to keep ozone formation in check. Nearby surface high pressure, very light surface winds, and a mix of sun and clouds will keep the risk of an ozone exceedance Marginal, with a focus on the I-95 Corridor, especially areas affected by bay/sea breezes.

Day 3 (Monday): Unsettled conditions will impact the southern half of the region on Monday as a developing low pressure system over the Mississippi River Valley begins to slowly pull Friday’s cold front northward as a warm front, pushing into VA by Monday evening. Although a few scattered showers could develop in the NMA or CMA, the majority of precipitation will be confined to the SMA along and head of the lifting warm front. Southerly flow pushing a plume of moisture northward over the front will promote widespread showers and thunderstorms across the SMA. The primary forecast question on Monday will be the coverage of clouds throughout the NMA and CMA as another day of weak surface winds and near average temperatures could be favorable for ozone formation. One possible limiting factor along I-95 will be flow aloft turning weakly southerly, which may be enough to limit any rising ozone, especially along the southern and central portions of the Corridor (e.g., DC to PHL). Flow aloft will still be light westerly at locations farther inland, such as PIT, putting them and the northern branch of the Corridor at higher risk. The BAMS air quality models develop widespread Moderate ozone throughout the NMA and CMA with the highest ozone concentrated along the Mason-Dixon Line and stretching northeastward through NJ and into NY. Converging surface winds along the I-95 Corridor could aid ozone formation. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ and locations along the I-95 Corridor from BAL to NYC, given questions about the effects of weak southerly flow aloft and the northern extent of approaching precipitation.

Day 4-5 (Tuesday-Wednesday): A low pressure system will move into the Ohio River Valley on Tuesday, which will continue to pull the warm front northward, promoting widespread showers and thunderstorms throughout the Mid-Atlantic. The presence of this system to the west and the Bermuda High to the east will result in strong southerly flow that will push a moist air mass northward into the Mid-Atlantic. Widespread, locally heavy precipitation, cloudy skies, and sustained surface winds will limit ozone formation throughout the region. The primary forecast question on Tuesday will be how quickly unsettled conditions impact locations throughout the northeastern NMA. The air quality models suggest that precipitation could be slow to push into the northeastern NMA, as they develop Moderate and USG ozone throughout PA, NJ, and NY, north of the Mason-Dixon Line. Unsettled conditions throughout the rest of the Mid-Atlantic result in Good ozone in the air quality models. Given the uncertainty in the precipitation forecast and the recent trend for a slower northward migration of the warm front and associated precipitation, the risk of an ozone exceedance will remain Marginal with a focus on locations throughout the northeast NMA, especially along the I-95 Corridor.

Precipitation will become more widespread on Wednesday as the Mid-Atlantic lies in the warm sector of the low pressure system centered in the Ohio River Valley. Strong southerly flow will continue to push a saturated air mass northward into the Mid-Atlantic promoting locally heavy precipitation and cloudy skies throughout the day. Precipitation and cloud cover, combined with strong southerly flow sourcing from the Gulf of Mexico, will keep ozone formation minimal throughout the region on Wednesday, dropping the risk of an ozone exceedance back to Slight.

-Enlow/Huff