Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 12, 2015
Valid: August 13-17, 2015 (Thursday – Monday)

Summary:

Steadily increasing temperatures and persistent sunny skies will promote rising ozone along and east of the I-95 Corridor, with concentrations peaking on Saturday and an increase to Appreciable chances for USG ozone. Clean mid-level air will limit concentrations mostly to the Good range on Thursday, but shortening back trajectories and rising temperatures will allow areas of Moderate ozone to form along and east of the I-95 Corridor on Friday. Despite model discrepancies regarding a back door cold front moving into the northern Mid-Atlantic, the skies over this usual area of concern will remain clear and allow ozone to continue to rise on Saturday. Recirculating back trajectories, a possible sea breeze, and surface high pressure centered over WV will aid in increasing concentrations. Despite persisting warm and sunny conditions, onshore, and looping back trajectories, Sunday’s chances for USG ozone will be limited to Marginal. The fate of the weak back door cold front on Sunday is still uncertain and will have to be monitored in the coming days. If the front continues to linger around PHL, the light converging winds may be enough to increase ozone into the upper Moderate range. Monday will see a continuation of the warm and relatively humid conditions and onshore looping back trajectories, again limiting ozone to the top of the Moderate range. PM2.5 will rise steadily throughout the period as humidity increases and winds remain light and west/southwesterly, plateauing in the mid-Moderate range over the weekend.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but show some discrepancies on Saturday and Sunday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. A large upper level trough – reaching as far south as GA – is still prominent over the East Coast, with an equally large ridge over the western US. At the surface, however, a large area of surface high pressure centered over IL will extend eastward tomorrow, allowing clear skies to dominate the weather in the Mid-Atlantic. This surface high will slowly move eastward, with its center reaching WV by 12Z Friday. This surface high pressure is expected to remain centered over the region through Monday, although its impacts in the northern Mid-Atlantic (NMA) may be compromised by a weak surface low over the Great Lakes Saturday into Sunday morning. As in previous analyses, a broad area of shortwave energy aloft stretching from the southern Plains to New England will prevent the western US ridge from building eastward over the Mid-Atlantic on Sunday and Monday.

Thursday will be a warm but dry day, influenced both by the cool mid-level air and increasing surface pressure. Despite very light westerly surface winds and clear skies, ozone will stay mostly in the Good range due to fast northwesterly back trajectories from interior ON and the clean air mass in place over the region. Some isolated areas of Moderate ozone are possible along and downwind – to the east – of the I-95 Corridor. Particles will stay in the Good range for most of the region as low humidity continues on Thursday. Locations in the southern Mid-Atlantic (SMA) where the humidity is the highest may see PM2.5 concentrations reach into the Moderate range.

The models have come into closer consensus for much of Friday, but diverge around 18Z. Focusing on the WPC’s decision to side with an EC/GFS consensus, the backdoor cold front moving into PA on Friday evening will slow and weaken as it approaches the state. The GFS analyzes a faster and stronger front, as it did in yesterday’s analysis, and shows clouds starting to build into northern PA around 18Z Friday. The EC shows the front dissipating sufficiently as to not promote notable cloud cover at all. Regardless of which solution verifies, since the cloud cover likely will not reach the I-95 Corridor, Moderate ozone concentrations are expected downwind to the east and northeast of the interstate. The light southwesterly surface winds and increased dewpoints will allow Moderate PM2.5 to spread northward and engulf most of the Mid-Atlantic on Friday.

Saturday is the main day of interest during this period as temperatures continue to rise, surface winds remain light and southwesterly, back trajectories become local and recirculating, and skies stay mostly clear over the I-95 Corridor. It is also another day of continuing model discrepancies, with the GFS keeping the cloud cover over much of the NMA from Friday afternoon through early Sunday morning. Regardless, temperatures are expected to persist just above average and light southwesterly surface winds will allow dewpoints to continue to rise. Shorter, recirculating back trajectories will allow the air mass to continue to modify and promote Moderate ozone formation along and downwind of the I-95 Corridor. The lack of strong synoptic forcing may also allow a sea/bay breeze to form, which would increase concentrations higher into the Moderate range. Particles will continue to rise higher into the Moderate range as the air mass continues to modify and humidity increases.

Conditions on Sunday will be very similar to those on Saturday. The remnants of the weak back door cold front are expected to reach PHL by 12Z Sunday. The front will have weakened considerably by the time it reaches PHL, decreasing the chances for clouds and precipitation along its boundary. Instead, the front would act as a line of convergence at the surface, which would push ozone concentrations into the upper Moderate range or low USG range, depending on how dirty the air mass will be. The BAMS air quality models are giving some credence this forecast by showing upper Moderate to USG ozone in parts of MD and NJ. Some of this is expected to be an artifact due to overrunning from the local waters, but the presence of the weak frontal boundary makes us hesitant to completely rule out high ozone concentrations. More localized onshore, looping back trajectories may limit ozone precursors in the usual areas of concern along the I-95 Corridor. This clean transport would keep ozone concentrations in the Moderate range. The 06Z 36km NCDENR air quality model is leaning towards this solution as it keeps ozone in the Good range in most of the Mid-Atlantic. The NCDENR is only showing an area of Moderate to low USG ozone in northeastern NJ. Historically, Sundays this summer have been unable to reach USG ozone concentrations, regardless of prime synoptic and mesoscale set-ups. With discrepancies in the air quality models, the chances for USG ozone will remain Marginal on Sunday. We will have to continue to monitor the situation, possibly increasing the chances for USG ozone on Sunday to Appreciable. PM2.5, on the other hand, will linger in the Moderate range as relatively high dew points and light winds continue.

Monday will be another day of continuing heat and humidity, allowing ozone and particles to continue to rise as well. The center of high pressure previously stationed over WV will move eastward out to sea, allowing short, onshore, looping back trajectories to persist. Although these trajectories will be a limiting factor to ozone production, concentrations will most likely persist in the mid- to upper Moderate range. Particles will follow the same pattern as humidity and light westerly winds persist over the Mid-Atlantic, with Moderate concentrations expected.

– Eherts/DeBoe/Ryan