Medium Range Air Quality Outlook
Issued: August 1, 2017
Valid: August 2-6, 2017 (Wednesday-Sunday)
Stagnating flow caused by a mid-level ridge will keep the risk of exceedances High on Wednesday, falling to Appreciable on Thursday, before a cold front arriving on Friday reduces the risk to Slight over the weekend. Wednesday is the day to watch for ozone exceedances. It will be the second day of strong subsidence and stagnation under the mid-level ridge, with bay/sea breezes likely. A weak shortwave moving over the NMA will bring the possibility for afternoon/evening thunderstorms, but they appear very isolated in nature. Localized back trajectories and a presumably modifying air mass suggest a High risk of exceedances along the I-95 and around the Chesapeake Bay. Thursday will be similar to Wednesday with a slightly increased chance for precipitation across the NMA and CMA and a shift to southerly flow aloft, which may be sufficient to take the edge off ozone along I-95. Risk of an exceedance will drop to Appreciable with the focus shifting to locations west of I-95 and inland areas of the SMA. Unsettled conditions will impact the Mid-Atlantic on Friday as the cold front of the next storm system approaches the western Mid-Atlantic. Scattered thunderstorms are possible ahead of the front in the afternoon hours while precipitation associated with the cold front will impact the western portions of the region throughout the afternoon and evening hours. Southerly flow will combine with periods of sunshine and clouds, temperatures and breezy surface winds to limit ozone accumulation and formation, lowering the risk of an exceedance to Marginal. Saturday will be seasonably cool and less humid across the Mid-Atlantic as a cold front pushes through the region to the SMA Atlantic coast and stalls. The arrival of a presumably clean air mass, along with unsettled conditions in the SMA, will drop the risk of an exceedance to Slight for the weekend.
Weather Model Discussion:
The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. The weather models have come into consensus with the synoptic setup through the end of the medium range period. A broad and weak upper level trough will be over most of the eastern U.S. by 12Z Wednesday as a strong upper level closed low moves over Hudson Bay. A series of shortwaves that develop across the Gulf Coast states will move through the base of the upper level trough over the Southeast U.S. By 12Z Thursday, concentrated shortwave energy riding in the northern stream flow will down across AB/SK and into the upper Midwest. This shortwave energy will strengthen quickly over MN/WI by 00Z Friday, reinforcing the existing broad through over the eastern U.S. and forming a mid-level low over NN/IA. Both the GFS and the EC develop a closed center of circulation over WI between 12Z and 18Z Friday before the shortwave energy begins to phase with the closed parent circulation over the Great Lakes/southern ON between 00Z and 12Z Saturday. As the upper level features phase, this will quickly pull the mid-level low and associated surface low pressure system through the Great Lakes region and into southeastern ON, while pulling a cold front into the Mid-Atlantic by 06Z Saturday. By 12Z Saturday, the closed low will be lifting northward, into ON/QC, and weakening. By 06Z Sunday, a broad and weak trough will remain over most of the eastern U.S. as shortwaves drop across the Midwest, forming another, weaker lobe of low pressure by 00Z Monday. This feature and other shortwaves across the eastern U.S. will keep troughing over the Mid-Atlantic throughout Sunday.
Wednesday is a day to watch for potentially quickly rising ozone due to high pressure over the region resulting in stagnation with very localized back trajectories. On Wednesday, weak mid-level high pressure moving over the Mid-Atlantic will keep conditions mostly dry across the region. A weak shortwave moving over the NMA brings the possibility for afternoon/evening thunderstorms, however. Weak subsidence will bring mostly sunny skies for most of the region in the morning hours but will give way to increasing probability of precipitation and clouds into the afternoon hours. The 03Z SREF shows that precipitation becomes likely in locations to the west of I-95 between 18Z Wednesday and 00Z Thursday. Hi-res weather models are suggesting that showers and thunderstorms will be more scattered to isolated in nature with periods of sunshine in between. Temperatures across the Mid-Atlantic will reach the mid-80s/low90s °F under mostly sunny skies in the SMA and partly sunny skies in the NMA and CMA. Near average temperatures and mostly sunny sky conditions will combine with stagnating flow aloft and at the surface to create an ozone friendly environment. This appears in the 06Z air quality models, with all three models developing isolated USG ozone in a few areas. The NOAA and BAMS models show USG forming in eastern PA (e.g., PHL, Lehigh Valley, Susquehanna Valley, metro BAL), while the NC model highlights Washington D.C. Moderate ozone is shown along I-95, PIT, and central NC. Given that Wednesday will be the second consecutive day of very warm and stagnant conditions, with bay/sea breezes likely, the risk of an exceedance will rise to High with a focus on I-95 and areas around the Chesapeake Bay.
Thursday will be similar to Wednesday with a slightly increased chance for precipitation across the NMA and CMA. The main difference will be a shift to southerly flow aloft and at the surface in response to high pressure aloft started to shift eastward. Temperatures will remain in the upper80s/low 90s °F under mostly sunny skies in the morning and early afternoon hours. Skies in the NMA and CMA will become partly sunny by 18Z Thursday as the chance for showers increases. The 03Z SREF shows that showers and thunderstorms are most likely between 21Z Thursday and 00Z Friday in locations to the west of I-95 in the NMA and CMA. These showers and thunderstorms will be driven by instability as the Mid-Atlantic will be in the warm sector of a storm system moving across the Midwest. Surface winds will increase as thunderstorms will be in the area, but breezy and gusty winds will be localized, with essentially calm winds in the morning and light southerly in the afternoon for most locations. The rest of the region is expected to remain dry with mostly sunny skies as surface high pressure remains centered in western NC. A shift to southerly flow aloft will increase humidity but also result in a weak onshore flow for the NMA, but the nearby surface high pressure will result in localized flow aloft in the SMA. The BAMS air quality models keep the eastern NMA in the Moderate range with a swath of USG ozone along I-95 and central MD, while the NC model buys into thunderstorms and stronger onshore flow, dropping the eastern NMA into the Good range. The NC model also picks up on the subsidence in the SMA, keeping Moderate ozone across NC. The BAMS models also highlight the SMA but slightly higher than the NC model as they develop a few areas of USG ozone in NC (CLT). Risk of an exceedance will drop to Appreciable, due to higher chances for thunderstorms and a shift to southerly flow aloft, with the focus on locations west of I-95 in the NMA/CMA and inland areas of the SMA.
Unsettled conditions will impact the Mid-Atlantic on Friday as the cold front of the next storm system approaches the western Mid-Atlantic. Mostly sunny skies are expected throughout the region in the morning hours before giving way to partly sunny skies in the afternoon. Clouds will increase as afternoon convection begins to take place, resulting in scattered thunderstorms for the third consecutive day. Precipitation associated with the cold front will impact the western portions of the region throughout the afternoon and evening hours. Temperatures will be limited to the mid-to-upper 80s °F due to periods of clouds and precipitation across the region. Stronger southerly flow aloft will combine with periods of sunshine and clouds, temperatures and breezy southerly surface winds to limit ozone accumulation and formation. Although the BAMS air quality models respond the conditions in the SMA appropriately by developing widespread Good ozone, they keep upper Moderate/low USG ozone across the Susquehanna River Valley and BAL metro. This is either due to the BAMS model over-predicting or the models not buying into the possibly scattered thunderstorms. Despite USG ozone in the models in the NMA, risk of an exceedance will lower to Marginal due to stronger southerly flow aloft and breezy surface winds.
Saturday will be seasonably cool and much less humid across the Mid-Atlantic as the cold front pushes through the region to the SMA Atlantic coast. A few showers and thunderstorms are possible in the early morning hours as the front pushes into the region, but most of the showers during the daylight hours are expected to remain in the SMA, where the front will stall along the NC coast. A cool Canadian air mass filtering in behind the front will drop temperatures into the mid-to-upper 70s °F in the NMA and into the 80s °F in the CMA and SMA. A combination of a fresh air mass, breezy northwesterly surface winds, below average temperatures and a shift to northerly flow aloft will bring regional ozone into the Good range. This appears in the 06Z BAMS air quality models as they bring ozone across the NMA and CMA into the Good range with a band of Moderate ozone along the frontal boundary in the eastern SMA. Risk of an exceedance will be Slight on Saturday.
Sunday will be another pleasant day in the NMA and CMA while the stalled frontal boundary will result in unsettled conditions in the SMA. Mostly sunny skies in the NMA and CMA will allow temperatures to rebound back into the 80s °F. Continued northerly flow aloft will prevent ozone accumulation from becoming excessive, despite slackening surface winds as high pressure moves directly overhead and offshore. In the SMA, showers and thunderstorms are expected across the southern half of NC throughout the day. A combination of mostly cloudy skies, precipitation and the cool Canadian air mass finally pushing into the region, temperatures will fall into the low-to-mid 80s °F. A combination of a recent change in air mass in the NMA and unsettled conditions in the SMA will keep the risk of an exceedance Slight on Sunday.