Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 6, 2015
Valid: July 7 – 11, 2015 (Tuesday – Saturday)

Summary:

Warm, humid, and partly sunny conditions will keep chances for USG conditions Marginal on Tuesday, mainly along the I-95 Corridor. Chances for USG ozone will drop to slight for the rest of the period. Tuesday will be pre-frontal, with southwesterly flow, afternoon clearing, and development of an Appalachian lee trough. Ozone will likely reach the Moderate range along the I-95 Corridor, with the highest concentrations expected along and east of the I-95 Corridor north of DC. Southerly flow today has kept another plume of wildfire smoke mostly to the north and west of the region, and this trend is expected to continue tomorrow under southwesterly flow. A cold front will weaken as it moves into the region from the northwest on Wednesday, stalling in the vicinity of the Mason-Dixon Line. Substantial clouds and convection triggered by the front will limit ozone to the generally Good range on Wednesday. A dilute plume of Canadian wildfire smoke may surge back into the region ahead of the cold front overnight Tuesday into Wednesday, making low-to-mid Moderate particle concentrations likely on Wednesday. Starting on Wednesday, the Mid-Atlantic frontal pattern we have seen often this summer will dominate, with the front oscillating north- and southward in the vicinity of central MD throughout the next several days. A series of surface waves moving eastward through the Mid-Atlantic will keep clouds and showers somewhere in the region through the end of the period. There will be a pattern change towards the end of the work week as an upper level ridge builds over the central and southern US, with the axis setting up over the Great Lakes by Saturday. This would normally point toward chances for deteriorating air quality, but the Mid-Atlantic will be downwind of the “ring of fire” of shortwaves cresting the ridge. In particular, an upper level short wave is expected to bring a surface wave through the Mid-Atlantic on Saturday, with the associated clouds and showers limiting ozone production despite the prime synoptic set-up.

Discussion:

The weather models are in very close consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a prominent but weakening upper level shortwave is centered over WV and moving northeastward with the flow. As the shortwave weakens, the associated surface warm front will weaken as well, making for a very diluted frontal boundary moving northward through the northern Mid-Atlantic (NMA) today. Throughout the day Tuesday, a large shortwave moving eastward through ON will pull a cold front across the northern Plains and towards the Mid-Atlantic, which will weaken as it crosses the Great Lakes overnight. By 12Z Wednesday, the front will be slowly moving southeastward through PA, bringing early morning convection to northwestern PA. Small, relatively weak shortwaves moving eastward along the frontal boundary will determine the exact location of the heaviest periods of rain. Additional widespread convection on Wednesday afternoon ahead of the front will extend southward to the vicinity of the Mason-Dixon Line (MDL). The front is expected to stall somewhere in the vicinity of the MDL around 12Z Thursday, although model discrepancies suggest it may end up settling farther south in northern VA. Throughout the day Thursday, there is agreement that the upper level flow will become zonal, preventing further movement of the front through the central Mid-Atlantic (CMA). The advancement of the front will also be arrested by a mid-level ridge over the Southeastern US. Small short waves moving with the flow just north of PA will promote clouds and showers along and north of the frontal boundary, with an area of surface high pressure building south of the front in the southern Mid-Atlantic (SMA). By Friday morning, there will be less shortwave action in the vicinity of the NMA, keeping only scattered areas of cloud cover and rain showers along the frontal boundary somewhere in the CMA –the EC consistently shows the boundary farther south than the GFS does, and a blend of the two solutions provides the best confidence at this time. Throughout the day Friday, a wide upper level ridge over the central and eastern US will become prominent and continue to build. Saturday will see more strengthening of this large ridge, with some discrepancies arising as to the exact placement of the ridge axis. The EC solution shows a more defined ridge peak over the southern Hudson Bay, with the axis extending through the Great Lakes and towards the Mississippi River Valley. Shortwaves cresting the ridge axis and moving along the “ring of fire” into the Mid-Atlantic on Saturday will continue chances for clouds, rain, and convection, despite the strengthening ridge.

A weakening warm front moving northward through the NMA today will continue to spin up convection and cloud cover across the region, which will taper off throughout the first half of the day on Tuesday. This will leave a short window for clear skies tomorrow afternoon before the impacts of the cold front progressing across the Midwest are felt. Temperatures in the high 80s ͦF, dewpoints approaching 70 ͦF, and afternoon periods of sun will push ozone concentrations into the Moderate range at scattered locations. The formation of an Appalachian lee trough just west of I-95 will aid in ozone formation as well. With sustained southwest surface winds, areas along and east of the I-95 Corridor north of DC are most likely to see concentrations reach into the upper Moderate range. The models are responding to this accordingly, with the BAMS and NCDENR air quality models resolving a corridor of Moderate ozone concentrations from southwestern CT to the DC Metropolitan area; the NOAA/EPA model did not run this morning.

Yesterday, another plume of wildfire smoke swirled southward out of the flow from Canada into the Great Lakes. Today’s warm front and associated surface wave are pulling clean and moist air northward into the Mid-Atlantic this morning, which is keeping the smoke out of the region, with the exception of parts of PA and NJ. Surface PM2.5 concentrations this morning are well into the Moderate range at locations surrounding the region, across the Great Lakes and New England. For tomorrow, southwesterly flow should continue to push the smoky air north and west of the Mid-Atlantic. Moderate PM2.5 may continue in parts of PA, and low Moderate conditions are possible along I-95 due to rising humidity.

Wednesday will see the arrival of the cold front from the northwest, with the impacts becoming less organized as the front slows down through the NMA. While northwestern PA and WV are expected to see organized convection overnight Tuesday into Wednesday morning, most of MD along with DE, NJ, and VA are expected to see clouds and scattered thunderstorms on Wednesday afternoon and evening. The warm, humid air mass ahead of the front, along with a zonally-shifting upper level flow and developing Southeastern mid-level ridge, will slow and stall the front somewhere in the vicinity of the MDL late on Wednesday. Where the heaviest rain sets up will depend on the exact path of some small but strong shortwaves moving eastward aloft. Although it will be hot and humid, widespread optically thick cloud cover will inhibit ozone formation throughout much the Mid-Atlantic. The humid conditions, along with the possibility of pre-frontal advancement of smoky air from the west and encroaching Saharan dust moving up from the Gulf Coast, will keep Moderate levels of PM2.5 likely on Wednesday, especially to locations south of the front.

By Thursday, the front will take on the characteristics of most Mid-Atlantic fronts so far this summer. Surface waves will ride along the boundary as it oscillates north- and southward around central MD, keeping varying chances for clouds and showers in the Mid-Atlantic for multiple days. Shortwaves aloft are expected to move eastward with the flow along the NY/PA border on Thursday, promoting clouds and showers along and north of the frontal boundary. The mesoscale models show that the clouds will lift northward later in the day, with clear skies taking over much of the region by nightfall. The expanse of cloud cover throughout the day will limit ozone to the Good range, but particle concentrations may linger in the Moderate range in areas where rainfall has not yet cleared out any Saharan dust, mainly in the SMA.

Friday will see continued chances for clouds and scattered showers along the frontal boundary in MD, but a lack of shortwave energy aloft will limit the extent of any precipitation and cloud cover. Light westerly winds and a weak focus for convergence in MD and DE may help to promote ozone production in areas where clouds do not linger, specifically to the east of the I-95 Corridor. However, long back trajectories from the Mississippi River Valley will prevent ozone from reaching substantially into the Moderate range. Moderate particle concentrations will take a break on Friday as smoke and dust both retreat away from the Mid-Atlantic.

On Friday, an upper level ridge will start to build in the central and southern US, with an axis establishing itself through the Great Lakes region by Saturday morning. A ridge with its axis west of the Mid-Atlantic is a classic set-up for deteriorating air quality, but in this case, shortwaves cresting the ridge along the ring of fire will be moving into the Mid-Atlantic, which will limit any chances for rising ozone. In particular, a strong upper level short wave moving over the OH/WV border will bring a rainy system right through the Mid-Atlantic on Saturday. Despite the mesoscale nature of this event, very close agreement between the global models regarding this featre increases confidence in its likelihood. Despite the western ridge, warm conditions, and light surface winds, this surface wave and precipitation will limit ozone to the Good range for most locations on Saturday. Areas where the rain does not reach until Sunday, namely eastern Mid-Atlantic locations such as PHL, BAL, and DOV, may see concentrations of both ozone and PM2.5 reach into the Moderate range. In the event that this surface wave forecast changes in the coming days, the ridge to the west could pose an air quality threat for the entire region this coming weekend.

– Eherts/Huff