Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: June 26, 2017
Valid: June 27-July 1, 2017 (Tuesday-Saturday)
Summary:
After unseasonably cool conditions to start the week, high pressure and rebounding temperatures will arrive late week leading a period of Appreciable risk in the NMA. Tuesday there will be a few showers in the eastern NMA but mostly dry conditions are expected elsewhere. Despite mostly sunny skies across the region, below average temperatures will prevent excessive ozone formation keeping risk for an exceedance Slight. A warming trend will start on Wednesday as high pressure begins to build into the Mid-Atlantic. Fast northwesterly flow across the NMA will keep Good air quality but slower flow in the SMA will allow for some ozone to accumulate. Despite mostly sunny skies, temperatures will remain below average, resulting in one last day of Slight exceedance risk. The Bermuda High will be in place on Thursday and this pattern remains solidly in place into the weekend. Clear skies, recirculating back trajectories and average temperatures along the I-95 Corridor will bring risk for an exceedance to Appreciable on Thursday. Friday and Saturday will also feature an Appreciable risk as the Bermuda High remains in place. The key forecast issues will be the magnitude of transported ozone and the degree of horizontal ventilation (strong southwest winds).
Weather Model Discussion:
There is strong consensus between the weather models with timing, placement, and strength of atmospheric features through the end of the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. An upper level trough that is currently over much of the eastern U.S., will progress eastward, with its axis moving over the western Mid-Atlantic by 12Z Tuesday. The trough axis will move directly over the Mid-Atlantic by 00Z Wednesday as a strong shortwave develops over the Appalachian Mountains, following the axis offshore by 12Z Wednesday. The trough will not be as strong at mid-levels due to the westward extension of the Bermuda High (BHWE). By 06Z Wednesday, the BHWE builds around the trough axis and into the SMA and Mississippi River Valley; into the rest of the Mid-Atlantic by 18Z Wednesday. Upper level ridging will try to build over the Ohio River Valley and into the Mid-Atlantic by 00Z Wednesday, but shortwaves moving across the Great Lakes will result in some lingering troughiness/zonal flow over the Mid-Atlantic through 06Z Friday. At mid-levels however, BHWE will continue to build into the Mid-Atlantic with a surface high developing over the SMA on Wednesday. The mid-level ridging will persist over the CMA and SMA through 12Z Saturday. In the NMA, shortwaves moving over the Great Lakes around 12Z Thursday will develop a low pressure system over the Upper Midwest/Great Lakes. As this system moves eastward, it will depress the mid-level ridging on Friday. By 12Z Saturday, another series of shortwaves will interact with a frontal boundary over the upper Midwest, developing another low pressure system over the Great Lakes. The center of this system will pass to the north, but the associated cold front will not reach the mid-Atlantic due to the strong upper level ridge.
The Dailies:
Unseasonably cool conditions across the Mid-Atlantic will continue on Tuesday as the upper level trough moves overhead. Mostly sunny skies are expected for most of the day, but a few clouds and isolated rain showers can be expected between 15Z and 18Z Tuesday in both the SMA and NMA. The 03Z SREF shows a slight chance of precipitation across the NMA between 15Z and 18Z with the greatest chance to the west of I-81. Strong westerly flow is expected across the NMA as the trough moves overhead, however in the SMA, high pressure will begin to build over the region, resulting in slow northwesterly flow at the surface but almost stagnant flow aloft. The 06Z NC air quality model is picking up on both the possible precipitation in the NMA and weaker flow across the SMA as upper Good ozone develops across the SMA and eastern NMA/CMA. The 06Z NOAA and BAMS models show a few isolated areas of upper Good/low Moderate ozone to the south of the SMA but more widespread low Good ozone across the Mid-Atlantic. The air quality models respond to the below average temperatures keeping ozone mostly in the Good range. As a result, there is a Sight risk for an exceedance on Tuesday.
A warming trend will begin on Wednesday as high pressure builds over the entire region. Clear skies will allow temperatures to rise about 5 °F warmer than Tuesday and precipitation is unlikely to occur anywhere in the Mid-Atlantic. Continued fast northwesterly flow in the NMA will keep ozone in the Good range, but slowing northwesterly flow in the SMA will allow for ozone accumulation. This rise in ozone is showing in the 06Z air quality models as more widespread upper Good and isolated low Moderate ozone begins to appear. Below average temperatures should limit ozone formation Wednesday keeping the risk for an exceedance Slight.
As high pressure resides over the Mid-Atlantic on Thursday, clear skies and a shift to slow southwesterly flow will bring an increase in temperatures and dew points. Temperatures are expected to rise another 5 °F across the region, while the slow southwesterly flow will bring dew points in the mid/upper 60’s °F into the Mid-Atlantic. As mentioned before, a low pressure system will develop over the Great Lakes by 12Z Thursday. The associated warm front will develop in the NMA, placing the Mid-Atlantic in the warm sector as it lifts northward. The front will form north of I-80, bringing a few scattered showers to the NMA between 15Z Thursday and 06Z Friday. In addition to precipitation associated with the warm front, clear skies and increased humidity will bring the possibility for some afternoon/evening thunderstorms across the NMA. Generally clear skies, rising temperatures and unfavorable back trajectories are causing the 06Z air quality models to rise ozone into the Moderate range across much of the Mid-Atlantic. At this time the main area of concern is the I-95 Corridor as the NC model develops an isolated area or two of USG ozone and the BAMS models develop upper Moderate. Back trajectories for locations along I-95 in the NMA, are recirculating over the SMA before moving northeast along the interstate. Risk of an exceedance will rise to Appreciable.
Appreciable risk of an ozone exceedance will continue on Friday. Temperature and dew points will continue to rise. Mostly sunny skies in the morning/afternoon will give way to scattered clouds and afternoon/evening thunderstorms. Precipitation in the NMA is most likely to remain to the west of I-81, keeping the area of concern along the I-95 Corridor for Friday. A combination of the formation of a lee trough in the eastern NMA, warm temperatures, and slow southwesterly flow will promote ozone formation and accumulation along the interstate. Ozone is expected to return to the Good range across the SMA on Friday. The key question for the NMA will be the magnitude of transported regional ozone. Overall conditions are favorable for ozone formation locally, however, so that the risk continues to be Appreciable in the NMA.
The large scale pattern remains the same on Saturday and the risk for an exceedance will remain Appreciable with a focus on the I-95 Corridor. Clouds and precipitation will impact the western NMA in the morning hours, clearing as it reaches central PA by 18Z. Mostly clear skies in the eastern NMA will once again lead to the possibility of afternoon/evening thunderstorms. If thunderstorms are isolated or do not develop, southwesterly back trajectories and mostly sunny/partly cloudy skies will allow for ozone formation. In the SMA, Saturday will be similar to Friday but with more widespread afternoon/evening thunderstorms. Continued onshore flow will limit any ozone formation to the Good/low Moderate range.
-Enlow/Ryan