Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 13, 2017
Valid: July 14-18, 2017 (Friday-Tuesday)

Summary:

Passage of a slow-moving cold front late Friday into Saturday lead to high confidence in lower risk of an exceedance through Sunday, but uncertainty about a weak upper level tough and associated cold front reduce confidence late in the period, making Monday and Tuesday days to watch. A cold front approaching the region from the northwest will bring unsettled conditions to the NMA and parts of the CMA for much of the day on Friday. Scattered afternoon/evening thunderstorms will assist in limiting ozone accumulation. Temperatures will fall to near or below average for most of the Mid-Atlantic on Saturday as Friday’s cold front pushes into the central SMA. The arrival of a presumably clean air mass in the wake of the front coupled with unsettled conditions along the stalled boundary in the SMA will limit ozone production on Saturday. High pressure settling over the Ohio River Valley on Sunday will promote mostly sunny skies and light/calm surface winds. These conditions are favorable for ozone accumulation, but fast northerly back trajectories from southern ON and lower Sunday emissions should counter ozone production. There is uncertainty, however, as the BAMS air quality models show a quick surge in ozone, bringing ozone up to the mid-to-upper Moderate range with a few isolated/scattered areas of USG ozone in PIT, MD, and PHL. Uncertainty remain in the forecast for Monday and Tuesday regarding the fate of the stalled front across the SMA and the approach of the next weak cold front from the northwest. The stalled southern front is especially hard to see in the weather model guidance, but WPC shows a wave of low pressure developing along it on Monday over eastern VA. This feature will move northward slightly on Tuesday, possibly phasing with the weak low along the northwesterly cold front. The 06Z air quality models are suggesting that the front and associated precipitation may get hung up across the NMA and CMA. The uncertainty in the precipitation forecast associated with the weak frontal boundaries and lows will impact Tuesday, as both the GFS and EC show the potential for lingering precipitation in the NMA. As a result, only a Minimal chance for an exceedance is expected currently, but Monday and Tuesday are days to watch for potential ozone formation.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. Strong consensus between the weather models continues until 12Z Monday, when the GFS and EC diverge regarding the shape and strength of an upper level trough and the placement of associated shortwaves. At 12Z Friday, shortwaves will continue to dig out an upper level trough over the Great Lakes. At the same time, this upper level trough will interact with a frontal boundary, developing a surface low over the Great Lakes. As the upper level trough pulls eastward, with the axis passing over the Mid-Atlantic around 06Z Saturday and moving into New England by 12Z Saturday, the associated surface low will pull a cold front through most of the Mid-Atlantic, stalling in the SMA by 00Z Sunday. The shortwave will quickly rotate northeastward along the trough, as another series of shortwaves begin to develop over southwestern ON/Great Lakes by 06Z Sunday. Despite the shortwave energy moving out of the Mid-Atlantic, the longwave trough will remain over the region, extending into the Southeast US. By 12Z Sunday, the shortwave to the northwest will be moving over south/central ON and will begin to reinforce the longwave trough. By 06Z Monday, the shortwave, along with the upper level trough, will be over the central Great Lakes. Following the same trend as in yesterday’s runs, the GFS and EC continue to vary on the strength of this shortwave, but the track and outcome of its presence are similar. The EC still has a stronger, more organized shortwave at the center of the weak trough, while the GFS persists in spreading out the shortwave energy. At 12Z Monday, the EC briefly develops a closed low over southeastern ON with the trough axis oriented north-south, whereas the GFS has a weaker open, positivity tilted trough over the same location. The difference between the two models will impact the strength and progression of a cold front/area of low pressure that will move into the Mid-Atlantic on Monday. The WPC seems to be putting more weight in the GFS as the orientation and progression of the cold front symbolizes the GFS outcome more closely. Despite the small uncertainty in the models, the upper level trough axis will move over the Mid-Atlantic by 12Z Tuesday, with shortwaves moving over most of the region. By 18Z Tuesday, the GFS elongates the weak trough along the Atlantic Coast with shortwaves sluggishly moving across the Mid-Atlantic and Southeast through 00Z Wednesday. The EC on the other hand, keeps a much tighter trough over the NMA and CMA on Tuesday, with the axis and shortwaves moving offshore by 00Z Wednesday. Both models quickly weaken the trough later on Tuesday and move it northeastward, out of the Mid-Atlantic, ahead of strong mid- and upper level ridging spreading across the eastern US on Wednesday.
The Dailies:

A cold front approaching the region from the northeast on Friday will bring unsettled conditions to the NMA and parts of the CMA for much of the day. The front will be slow-moving, not reaching the I-95 Corridor until 03-06Z Saturday. Near average temperatures, dew points in the low 70s °F, and overhead shortwaves will drive afternoon/evening thunderstorms, which will be the primary source of precipitation across the region. As shown in the 03Z SREF, precipitation is expected to move into the western Mid-Atlantic around 15Z, moving from northwest to the southeast, reaching to the Atlantic coast by 00Z Saturday. The latest hi-res models are showing thunderstorms initiating across central/eastern PA and MD around 21Z Friday and lasting through 00Z Saturday; this precipitation looks more scattered compared to what will be occurring today. With the expected scattered nature of the thunderstorms, partly cloudy skies in the morning and afternoon hours will combine with light converging winds along the I-95 Corridor, creating favorable conditions for some ozone formation, assuming sufficient afternoon sun. The SMA will remain mostly dry on Friday as weak high pressure remains centered over western NC. Mostly sunny skies and temperatures in the mid-to-upper 90s °F will allow for some ozone formation in the SMA as well. The 06Z air quality models are not in strong agreement with ozone across the region on Friday. The NC and NOAA models are buying into the precipitation across the eastern NMA as they bring ozone well into the Good range. The BAMS-CMAQ is closer to the NC and NOAA model but keeps low Moderate in the PIT and DE area. The BAMS-MAQSIP is the most aggressive of the air quality models with mid-to-upper Moderate ozone east of I-95 and in the PIT metro area, similar to yesterday’s runs. The air quality models do agree on a strip of mid-Moderate ozone across central NC, likely in repose to temperatures, clear skies and southwesterly flow. Given the likelihood of afternoon thunderstorms and partly to mostly cloudy skies across much of the region, risk of an exceedance on Friday will be Marginal.

Temperatures will fall to near or below average for most of the Mid-Atlantic on Saturday as Friday’s cold front pushes into the central SMA. Locations in the SMA ahead of the front can expect above average temperatures and uncomfortably humid conditions to continue into the weekend, with unsettled conditions. Precipitation is expected to occur along and head of the front, starting around 18Z Saturday and lasting through 00Z Sunday as shown in the 03Z SREF. In the NMA and CMA, weak high pressure building in the Ohio River Valley will inhibit precipitation, with skies clearing by the afternoon. The arrival of a presumably clean air mass on northwesterly flow aloft should limit ozone formation in the NMA and CMA on Saturday, while widespread precipitation and cloudy cover will limit ozone in the SMA. The 06Z air quality models are responding to these factors, as they show ozone levels dropping to widespread Good with isolated Moderate. The majority of the Moderate ozone is shown in the CMA, in locations between the precipitation in the SMA and the cloud cover in the NMA. As a result, risk for an exceedance will drop to Slight on Saturday.

Uncertainty enters the forecast on Sunday as cloud cover comes into question. We currently expected skies to be mostly sunny across the NMA and CMA on Sunday as weak high pressure remains over the Ohio River Valley. Weak overhead shortwaves may increase cloud cover in the NMA during the peak heating hours of the day. Despite the uncertainty, near normal temperatures are expected across the NMA as above average temperatures continue in the SMA. The frontal boundary that has been in place in the SMA will remain and once again trigger unsettled conditions; mostly cloudy skies will give way to periods of sunshine and a few isolated showers throughout the day across NC. With the center of high pressure nearby, mostly sunny skies and light/calm surface winds will be favorable for ozone accumulation. This is appearing in the 06Z air quality models as areas of Moderate ozone increase. The NC model is the most conservative as it develops upper Good range ozone across the NMA with scattered Moderate across the SMA. The BAMS models are more bullish as widespread mid-to-upper Moderate ozone develops with a few isolated/scattered areas of USG ozone in PIT, MD, and PHL, where surface winds are expected to be light/calm and converging. On the negative side for ozone formation, transport aloft will be northerly from southern ON, which presumably will be clean, although there is a lot of dilute smoke across Canada currently from raging fires in AK and western Canada. In addition, lower Sunday emissions should help limit ozone formation. The main forecast question will be how quickly the air mass modifies under stagnating surface winds. Given lack of consensus in the air quality models, risk of an exceedance will remain Marginal, with a focus on PIT, and the I-95 Corridor in the CMA and NMA on Sunday.

Uncertainty remains in the forecast for Monday as a weak cold front, associated with the weak trough aloft, approaches the Mid-Atlantic. In addition to this cold front, the WPC indicates that shortwaves moving over the SMA on Monday will interact with the stalled frontal boundary and develop a wave of low pressure in the vicinity of RIC. This feature is hard to see in the weather models witch is causing some difficulty in Monday’s forecast. Despite this uncertainty, much like Sunday, average temperatures are expected in the NMA while above average Temperatures will persist in the SMA. Both the operational GFS and EC show precipitation moving through the NMA with the approach of the weak cold front, starting between 12Z Monday and 18Z Monday, lasting through 00Z Tuesday. The 06Z air quality models are suggesting that the front and associated precipitation may get hung up across the NMA and CMA, as both BAMS develop a large swath of USG ozone in MD, southeast PA, and NJ, with an isolated spot of USG in VA and NC. In addition to the possibly slowing front, light/calm surface winds and localized back trajectories favor ozone accumulation. Looking at 850 mb, the GFS and EC shift flow aloft more S/SE beginning around 18Z, which may, along with SE surface winds, keep enough clean onshore flow along I-95 to limit ozone production. The NC model is much more conservative as it shows widespread Good ozone with areas of Moderate ozone in the eastern NMA. Risk of an exceedance will remain Marginal on Monday due to the uncertainty in the forecast.

The uncertainty in Monday’s forecast will carry over into Tuesday. Both the GFS and EC show scattered precipitation in the NMA, but not in the same places. The GFS has the most organized precipitation, keeps it along the I-95 Corridor through 00Z Wednesday. At 850 mb, the EC brings the weak low and cold front farther east into the Mid-Atlantic compared to the GFS. WPC brings low pressure over the Delmarva at 12Z, which could favor unsettled weather along I-95. Temperatures will be similar to Monday’s across the Mid-Atlantic. The WPC shows that high pressure will begin to build in the Ohio River Valley which will promote mostly sunny skies in the western portions of the NMA and CMA. Given all of this uncertainty with the weak synoptic features on Tuesday, the risk for an exceedance will remain Marginal on Tuesday.

-Enlow/Huff