Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 29, 2017
Valid: June 30-July 4, 2017 (Friday-Monday)

Summary:

With the exception of Saturday, high pressure across the Mid-Atlantic will dominate the medium range forecast. Despite scattered unsettled conditions on Friday, clear skies and above average temperatures in the NMA will allow for ozone to creep into the upper Moderate range. Although the air quality models are not indicating a widespread USG ozone event, an isolated exceedance cannot be ruled out, resulting in High risk for an exceedance on Friday particularly in the NMA. Saturday will be the lone day of Slight risk, as a cold front approaches from the west. Widespread showers/thunderstorms will washout any ozone accumulation from Friday. The risk will rise back to High on Sunday as high pressure builds into the Mid-Atlantic once again. The westward extension of the Bermuda high will build into the SMA, causing Saturday’s approaching cold front to stall across central PA. Average temperatures, southwesterly flow, clear skies and a lee trough forming along the I-95 corridor will be the determining factors in Sunday’s ozone levels. Clear skies and warm conditions will continue on Monday and Tuesday keeping an elevated threat for an exceedance into the Holiday.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be strong consensus between the EC and the GFS through the medium range period. The NAM has a similar overall setup through 18Z Saturday, but is more aggressive with respect to short wave disturbances moving across the eastern U.S. General upper level zonal flow will continue over much of the Mid-Atlantic by 12Z Friday, with a shortwave moving into the SMA. At the same time, an upper level trough will continue to develop over the upper Midwest, with a closed circulation over MB by 12Z. Shortwaves spinning through the trough will deepen it through 12Z Saturday, as the ridge axis moves over the upper Mississippi River Valley. At this time the shortwaves will begin to pull northeastward, slightly weakening the trough, and allowing the westward extension of the Bermuda high to build into the Southeast and lower Mississippi River Valley at mid-levels by 18Z Saturday. As the shortwaves pull to the northeast, an associated surface low pressure system will closely follow moving to the ON, QC border by 12Z Saturday. This low pressure system will slowly bring a cold front into the western NMA by 12Z Sunday. By this time, the upper level trough has moved over the Mid-Atlantic, but is battling with a building ridge at mid-levels. The ridge will be compressed over the NMA by 12Z Monday, as mid-level ridging build into much of the Mid-Atlantic and into the Midwest. As the ridge and trough compete over the Mid-Atlantic, Sunday’s cold front will stall across the NMA by 18Z Sunday, and linger in place through the end of the medium range period. Returning to upper levels, the trough will have pulled north of the NMA by 18Z Monday, allowing zonal flow to return over the Mid-Atlantic. Another round of strong shortwaves will spin across the Mississippi River Valley on Tuesday, but the most recent model runs disagree on the eastward progression of these features.

The Dailies:

Areas of unsettled weather will move into the Mid-Atlantic on Friday. A low pressure system developing in the Great Lakes region will place the Mid-Atlantic in the warm sector of this system by 15Z Friday. High pressure over the Atlantic will result in onshore southeasterly flow in the SMA, and southwesterly flow for the NMA. This southwesterly flow will funnel in humid air front the Gulf Coast into the NMA, bringing dew points into the upper 60’s. This increase in dew points, combined with above average temperatures will lead to increasing cloud cover and scattered afternoon/evening thunderstorms across the Mid-Atlantic. The 03Z SREF shows that precipitation will be most likely west of I-95 between 15Z Friday and 00Z Saturday. Breezy winds in the morning hours will increase with daytime heating in the afternoon hours. Onshore flow in the SMA will keep ozone in the Good range, but southwesterly flow in the eastern NMA will be favorable for ozone accumulation. As noted above, precipitation and cloud cover is expected to remain to the west of I-95, raising the concern for a high ozone event in this area. The air quality models are trending away from USG level ozone in the Mid-Atlantic, as the most recent model runs show widespread Moderate with an isolated USG appearing in northern NJ on the CMAQ version of the BAMS. Aside from the lone USG feature, the air quality models are in consensus that mid-upper Moderate ozone can be expected along the I-95 Corridor, with upper Moderate/USG most likely upwind (to the northeast) as winds “train” over the Interstate.
Increased convection is possibly the cause for the lower trend in the air quality models, as todays weather model runs are developing more widespread clouds and precipitation compared to yesterday’s runs. There is, however, an added concern as the latest NRL-NAAPS model in showing some very dilute smoke moving into the NMA after 12Z Friday. As a result of these factors, risk for an exceedance will remain High for Friday with a focus on the I-95 Corridor in MD/DE/PA/NJ.
Unsettled conditions will impact the entire Mid-Atlantic on Saturday as the cold front approaches from the west. Mostly cloudy skies in the morning hours will briefly clear around lunchtime, allowing for temperatures to rise to around average. A mixture of this mid-day clearing and dew points crawling into the low 70s F, will produce widespread afternoon/evening thunderstorms. The 06Z air quality models are recognizing the precipitation and clouds, as ozone returns to the Good range for most of the Mid-Atlantic. Risk for an exceedance will lower to Slight on Saturday.

High pressure will begin to build into the Mid-Atlantic on Sunday as the upper level trough pulls northeastward into QC. The majority of the region will be sunny, but a few more clouds will develop in the CMA as a frontal boundary lingers. Average temperatures across the region combined with mostly sunny skies and southwesterly flow will be favorable for ozone production. There is disagreement between the BAMS air quality models and the NC model in regard to the magnitude of the ozone increase on Sunday. The BAMS models show significant ozone formation along the CMA and NMA Atlantic coast with widespread Moderate, scattered USG and on the CMAQ, a few isolated areas of high ozone in the NMA. The NC model does increase ozone concentrations but only develops scattered Moderate with an isolated USG along the coast. The weather models are possibly developing this area of elevated ozone due to a lee trough developing along the I-95 Corridor, serving as an area of convergence. As a result, risk for an exceedance will increase to High for Sunday.

The threat of an ozone exceedance continues on Monday. A frontal boundary draped across PA will enhance the probability for scattered showers in central and western portions of the state. The rest of the region should expect mostly clear skies as high pressure continues to build over the SMA. Southwesterly flow will shift to northwesterly by 12Z Monday. Although temperatures will remain slightly below normal, mostly sunny skies will promote ozone formation. One area of uncertainty is the amount of vehicle emission on July 3rd. Typically, local emissions are lower near holidays but with July 4th falling on a Tuesday, the reduction is uncertain. Ozone in the Good range with scattered Moderate can be expected for most of the region, but the Atlantic coast in the CMA and NMA will experience widespread Moderate, with scattered USG and isolated High ozone levels. High risk for an exceedance will continue on Monday with a focus on the Atlantic Coast.

Risk for an ozone exceedance will lower to Moderate on Tuesday. Mostly sunny skies will continue as high pressure persists over the Mid-Atlantic. Northwesterly flow may help reduce ozone levels, but 48 hr back trajectories are relatively slow for locations in the NMA, and in the SMA, back trajectories recirculate for a few hours before shifting northwesterly. As a result of these factors, poor air quality is expected to continue in the NMA and extend into the SMA as surface flow begins to stagnate.

-Enlow/Ryan