Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 20, 2015
Valid: July 21 – 25, 2015 (Tuesday – Saturday)

Summary:

Tomorrow’s cold front will bring drier and cleaner air to the mid-Atlantic. Air quality will slowly deteriorate throughout the remainder of the work week as an upper level ridge builds in from the west. Cleaner, drier air behind the front will drop ozone and PM2.5 into the Good range on Wednesday. Persistent high pressure over the Northeast will allow clear skies and calm winds to prevail over the Mid-Atlantic throughout the upcoming week, promoting the steady formation and buildup of ozone and particles. Favorable weather conditions for deteriorating air quality will peak on Saturday, making it the most likely day for a USG forecast.

Discussion:

The weather models are in very good consensus throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. A large trough currently over ON will continue eastward over the next couple of days, digging southward as it does so. This feature will be reflected at the surface via a cold front that will sweep west to east through the Mid-Atlantic tomorrow, finally bringing a drier air mass into the region. The front will continue southeastward through Saturday, slowing as it penetrates the Southeast. A ridge currently aligned with the western edge of the Rockies will continue to strengthen and build in behind this trough, with its axis reaching through the Great Plains by 00Z Thursday. In the Mid-Atlantic, zonal flow aloft will slowly shift southeasterly as the ridge slowly builds in from the west. Mostly sunny skies will prevail over the Mid-Atlantic following the passage of tomorrow’s cold front, with little chance for precipitation until Sunday.

Despite the passage of a cold front currently over eastern PA, Tuesday will be another hot and humid day across the Mid-Atlantic. Westerly surface winds and back trajectories originating from the Ohio River Valley (ORV) will set the stage for an atmosphere rich in ozone precursors. A dissipating stationary front along the Mason-Dixon Line (MDL) will act as a focus for convergence for the buildup of ozone and PM2.5. A cold front will push west to east through the Mid-Atlantic tomorrow, reaching the coast by 00Z Wednesday. Impacts will include building clouds ahead of and along the front, as well as cooler, drier air filtering in behind it. Convection is likely west of the I-95 Corridor in the NMA and will be widespread in the SMA. The resulting cloud cover will limit ozone formation, with Moderate ozone in areas that see periods of afternoon sun ahead of the front, particularly along and east of the I-95 Corridor. Stagnant morning air will help particles to peak in the lower Moderate range at scattered locations along and east of the interstate as well.

Frontal convection overnight Tuesday will help to clean the atmosphere of pollutant precursors by Wednesday morning. Seasonally average temperatures, dew points in the 50s F, and Canadian back trajectories will combine to limit both ozone and particles to the Good range despite mostly sunny skies.

Thursday’s weather will be very similar to Wednesday’s, but with diminishing surface winds as the weather pattern in the Mid-Atlantic settles. With the cold front continuing to drop into the Carolinas, a sprawling area of high pressure will spread into the Northeast from the west. This will promote lighter winds and continue to maintain clear skies, allowing the air mass over the region to begin to modify and allow ozone to rebound to the Good/Moderate threshold downwind of the I-95 Corridor. Particles will build just into the Moderate range in the western Mid-Atlantic, where the center of surface high pressure is expected to settle and result in the calmest winds.

Friday will be another day of deteriorating air quality as high temperatures persist in the mid-80s F and dew points linger around 60 F. Back trajectories will shorten compared to the past few days, with the air aloft originating from MI. Full sun and light winds will allow ozone to form, unhindered, throughout the day. Northwesterly surface winds will contain the areas of highest ozone to locations downwind of the I-95 Corridor in the eastern Mid-Atlantic. The highest particle concentrations will spread eastward from the center of surface high pressure in the ORV, with Moderate PM2.5 likely across the entire Mid-Atlantic south of central PA.

A small ridge will be over the mid-Atlantic on Saturday. This will be the day of poorest air quality in the period. A resulting area of surface high pressure will engulf the entire Mid-Atlantic, promoting calm surface winds and abundantly sunny skies. However, northerly back trajectories and the lack of weekday commuter emissions will keep ozone and particles from increasing much compared to Friday with Moderate air quality expected.

– Eherts/Ryan