Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 8, 2016
Valid: July 9-13, 2016 (Saturday-Wednesday)

Summary:

The primary period of interest will be the latter half of the medium range, when upper and mid-level ridges are expected to develop across the Mid-Atlantic. The only complicating will be speed and track of a shortwave disturbance aloft which the deterministic models are struggling to resolve. On Saturday, a cold front will push eastward into the Mid-Atlantic, reaching the I-95 Corridor by 00Z Sunday, triggering clouds and precipitation across much of the forecast sunshine. The air mass in place will be slow to respond to this passage, however, which combined with intense afternoon sunshine ahead of the front will allow ozone levels to rise. As a result, the chance for an ozone exceedance will be Marginal on Sunday for locations along and east of the I-95 Corridor. On Sunday, the cold front will pass to the south and east of the Mid-Atlantic, allowing most of the forecast region to feel the effects of an air mass change behind the front. Persistent northwesterly winds at the surface and aloft will usher cleaner and slightly cooler air into the northern and central Mid-Atlantic. The chance for an ozone exceedance will drop to Slight for NC, which will remain close enough to convergence along the cold front to experience locally enhanced ozone formation. On Monday, an approaching upper level ridge and a developing mid-level ridge will promote clear skies and subsiding winds across the Mid-Atlantic. A clean transport pattern will remain in place aloft, but intense afternoon sunshine should overwhelm its effects and promote ozone formation near the I-95 Corridor. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday for locations along and east of the I-95 Corridor. There is uncertainty in the forecasts for both Tuesday and Wednesday, due to the discrepancies discussed above regarding an approaching shortwave disturbance. At this point in time, it appears that this shortwave and its associated precipitation will remain to the west of the I-95 Corridor on Tuesday. The mid-level ridge will hook up with the Bermuda High, with another day of intense heat and light winds allowing for ample ozone production. The chance for an ozone exceedance will rise to Appreciable on Tuesday for the I-95 Corridor as a result. On Wednesday, the combine effects of the shortwave aloft and a warm front approaching from the west will trigger clouds and thunderstorms across the northern and central Mid-Atlantic. Therefore, the chance for an ozone exceedance on Wednesday will drop to Marginal.

Discussion:

The weather forecast models are in fair agreement through most of the medium range, but diverge during the latter half of the period with the speed and track of a developing upper level shortwave. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough currently over the Northern Plains will progress eastward on Saturday across the Great Lakes region, pulling a cold front across the Mid-Atlantic. The axis of the trough will move directly over the northern Mid-Atlantic by 12Z Sunday, but will slowly lift northeastward toward the Canadian Maritimes during the afternoon ahead of an upper level ridge amplifying over the Northern Plains. The eastern edge of this ridge will reach the Mid-Atlantic by 06Z Monday, and slowly build eastward throughout the day, with a mid-level ridge concurrently reforming across the eastern U.S. The GFS and EC begin to diverge by 12Z Monday regarding a small, yet potent shortwave disturbance developing beneath the ridge over the Tennessee River Valley. The GFS shows the shortwave slowly circulating northeastward over IL by 00Z Tuesday, whereas the EC progresses it quickly eastward into the southern Mid-Atlantic and strengthening. On Tuesday, the upper level ridge will fully envelop the Mid-Atlantic. The GFS continues to bring the shortwave northeastward into the Ohio River Valley by 12Z Tuesday, while the EC shows it tracking directly over the central and southern Mid-Atlantic. For the purposes of this discussion, we lean toward the GFS solution for Tuesday and Wednesday. Additionally, the mid-level ridge will hook up with the Bermuda High around 18Z Tuesday. On Wednesday, an approaching upper level trough across southern MB/QC will encroach upon the upper level ridge, but the exact placement of this trough remains a question. The GFS keeps the trough near the northern US/Canada border throughout the day, but the EC lifts it northeastward beginning 12Z Wednesday.

A complex weather pattern featuring two frontal boundaries will trigger widespread clouds and precipitation across the northern Mid-Atlantic on Saturday. A cold front will reach the western Mid-Atlantic by 06Z Saturday, and progress eastward throughout the day, reaching the I-95 Corridor by 00Z Sunday. At the same time, a back door cold front will drop southward across NY into eastern PA and northern NJ by 12Z. The deterministic models show scattered precipitation across the northern and central Mid-Atlantic throughout the day, with the heaviest precipitation focused right along the back door? front itself. This precipitation pattern is further supported by the 03Z SREF and 06Z 4km nested NAM. 06Z GFS back trajectories for the I-95 Corridor are short and recirculating, but this should not pose a problem for high ozone given the likelihood of rain and persistent cloud cover. The NOAA and BAMS air quality models seem to favor sunnier conditions, with the NOAA model developing upper Moderate ozone along the I-95 Corridor and a patch of USG ozone over PHL, and the BAMS model showing USG ozone across central NJ, PHL, DE, and eastern MD. These solutions seem unlikely, however, given that these models have been over-predicting ozone concentrations during the current weather pattern. The NCDENR model, which has been the most accurate during these days, shows only low Moderate ozone along and east of the I-95 Corridor, with the rest of the Mid-Atlantic in the upper Good range for ozone. Temperatures will remain above average, which combined with breaks of afternoon sunshine could allow ozone levels to rise. However, there are enough limiting factors that the chance for an ozone exceedance will drop to Marginal for locations along and east of the I-95 Corridor.

On Sunday, the cold front will push to the east and south of the Mid-Atlantic, allowing most of the Mid-Atlantic to experience an air mass change in its wake. Mid-level cooling across the northern and central Mid-Atlantic will knock temperatures back down to more seasonable values. Back trajectories for the I-95 Corridor shift northwesterly, originating from southern ON, which will usher cleaner air into the forecast region. The air quality models clearly reflect this atmospheric ventilation, with both the BAMS and NCDENR models showing the majority of the Mid-Atlantic well into the Good range for ozone. The only location which might experience slightly more ozone production is southern NC, where the convergence along the nearby cold front could cause ozone to rise into the Moderate range. As a result, the chance for an ozone exceedance will drop to Slight on Sunday for NC.

The presence of a mid-level ridge across the eastern U.S. and surface high pressure centered over western PA will promote clear skies and subsiding winds across the northern and central Mid-Atlantic on Monday. Back trajectories remain northwesterly for the I-95 Corridor, which should slightly limit ozone production, but intense afternoon sunshine and light surface winds will likely overwhelm its moderating effects. The BAMS models are currently showing low to mid Moderate ozone along and east of the I-95 Corridor on Monday, as well as across western NC. There in uncertainty regarding the track of an upper level shortwave over the Tennessee River Valley and its associated precipitation which could affect ozone production across the central and southern Mid-Atlantic, with the EC developing heavy precipitation across southern VA and the entirety of NC beginning 18Z Tuesday. However, the northern half of the forecast region should remain clear, allowing the chance for an ozone exceedance on Monday to rise to Marginal for the I-95 Corridor.

Questions regarding the track of the upper level shortwave continue into Tuesday. The GFS has the shortwave pulling northeastward, keeping the majority of its precipitation west of the I-95 Corridor through 00Z Wednesday. The EC, however, shows the shortwave tracking due easterly, bringing heavy precipitation into the central Mid-Atlantic throughout the day. It is worth noting that this track would still keep the northern extent of the I-95 Corridor under mostly clear skies. The exact placement of the precipitation will be the determining factor regarding which locations will experience rapid ozone formation. Surface high pressure is expected to remain across the Mid-Atlantic, with the mid-level ridge also connecting with the Bermuda High. These features will promote rising temperatures and light surface winds, which means that the locations which remain unscathed by the approaching precipitation will likely see rapidly rising ozone concentrations, especially along the I-95 Corridor. Back trajectories for the I-95 are short and recirculating on Tuesday, which is a typical “red flag” for ample ozone production. The BAMS models appear to favor the GFS solution at this point in time, bringing the northern Mid-Atlantic and much of the I-95 Corridor up into the Moderate range for ozone, with the MAQSIP-RT developing an area of USG ozone across south central PA and western MD. Given the uncertainty in the precipitation forecast and the potential for substantial ozone formation, the chance for an ozone exceedance will rise to Appreciable on Tuesday.

On Wednesday, a weak front stemming from a low pressure system centered over southern MB is expected to push eastward into the western edge of the forecast region. This will promote another day of intense heat, with the potential of diurnal showers and thunderstorms across the western Mid-Atlantic. The main forecast question will continue to be the track of the shortwave and its precipitation. Following the GFS solution as we did for Tuesday, the northern and central Mid-Atlantic will experience sufficient clouds and precipitation to limit rising ozone along the I-95 Corridor. The chance for an ozone exceedance will fall to Marginal on Wednesday as a result.

-Brown/Ryan