Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 27, 2015
Valid: July 28 – August 1, 2015 (Tuesday – Saturday)

Summary:

Clear skies and light winds associated with a strengthening upper level ridge will increase the chances for USG ozone to Appreciable on Tuesday and High Wednesday, although substantial uncertainty remains. Temperatures will be in the low to mid 90s °F on both days, allowing for favorable conditions for ozone formation. The air quality models are in closer agreement today by showing upper Moderate to low USG level ozone along the I-95 Corridor on Tuesday. The main forecast questions on Tuesday pertain to persistence, air mass transport, and the possible impact of a weakening frontal boundary that may trigger afternoon clouds and convection west and south of Washington, DC. Chances for USG ozone increase on Wednesday, although there is still uncertainty regarding southerly low level transport, which may be sufficient to limit rising ozone. The air quality models show another day of USG ozone on Wednesday, but they differ as to the location (either north or south of the Mason Dixon Line). Clouds and convection on Thursday afternoon ahead of a cold front will end the threat of USG ozone. Slightly cooler and noticeably drier air will filter into the region on Friday behind the frontal passage, returning ozone to the Good range for most locations. Ozone may rebound into the Moderate range on Saturday under sunny skies and warmer conditions, but increasing concentrations will be limited by a relatively clean air mass.

Discussion:

The weather models are in close consensus on the synoptic features throughout the medium range period. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. An upper level ridge extending over the southern US will strengthen on Tuesday, with its axis over the eastern periphery of the Mississippi River. The ridge axis will progress relatively quickly eastward in the flow, reaching the Ohio River Valley on Tuesday and moving over the Mid-Atlantic on Wednesday. At this time, a closed upper level low will move northeastward from the northern Plains to Hudson Bay. This closed low will bring a cold front into the Mid-Atlantic on Thursday and begin to dig out a broad upper trough over the northeastern US. The cold front will move from northwestern PA at 12Z Thursday to the I-95 Corridor by about 00Z Friday, and will reach eastern NC by 12Z Friday. Although all of the mesoscale and global forecast models are in quite close consensus on the timing of the frontal passage, this front may slow down as it runs into a very hot air mass over the region, with temperatures in the upper 90s °F. The trough associated with the closed low over Hudson Bay will broaden and remain in place over the Northeast through Saturday.

Tuesday and Wednesday continue to be the days of most interest during the medium range period, due to the approach of the upper level ridge. Surface high pressure centered over the Great Lakes on Tuesday will promote clear skies and light winds throughout most of the region, with temperatures reaching the low 90s °F. The air quality models are in closer agreement this morning on showing upper Moderate to pockets of USG ozone along the I-95 Corridor, particularly BAL/DC/northern VA. Ozone in the residual layer will be the main factor in whether or not the USG threshold will be exceeded. Regionally, widespread clouds and morning convection will keep ozone in the Good range today. There is uncertainty regarding the air mass transport forecast for tomorrow. The weather models are having a tough time pinning down the back trajectory forecast. Today’s 06Z GFS back trajectory analyses ending at PHL indicate southeasterly/southerly onshore flow at 500 AGL on both Tuesday and Wednesday. If this southerly flow verifies, it will limit the chances for USG ozone to locations north and west of I-95. Another limiting factor will be the fate of the weak cold front currently draped across southern PA. This dissipating frontal boundary will move southward today and may act as a focus for afternoon thunderstorms across northern and central VA. The 06Z 4km NAM and 03Z SREF show agreement on producing clouds and convection to the south and west of the DC metropolitan area. However, the clouds and precipitation associated with this convection does not look substantial enough to limit rising ozone over the northern part of I-95 Corridor, from ILG to PHL/TTN to NYC.

Currently, PM2.5 concentrations are in the Good range along and east of I-95, due to southerly flow aloft and at the surface. Inland, west of I-95, particles are into the Moderate range, with the highest concentrations in southwestern PA. The forecast for tomorrow will depend on air mass characteristics and flow aloft. The potential for onshore flow along the Atlantic coast may limit rising PM2.5 concentrations east of I-95, while more stagnant conditions inland will likely allow particles to rise higher into the Moderate range.

Wednesday will be another hot and humid day as surface high pressure continues to dominate the weather. Skies will be clear and surface winds will be light, allowing for another day of rising ozone. Based on the current analysis, the chances for USG ozone are higher on Wednesday, since the air mass will mostly likely be more modified compared to Tuesday. In addition, the remnants of the frontal boundary that may trigger scattered convection on Tuesday will have dissipated by Wednesday, allowing for more widespread clear skies. The main forecast question for Wednesday continues to be the air mass transport pattern. The 06Z GFS back trajectory analyses ending at PHL for Wednesday is similar to Tuesday, with low level (500m AGL) transport from the south, along the east coast and originating near HAT. The air quality models continue the trend of USG ozone for Wednesday, but they are split on the location of the USG conditions. The BAMS models place upper Moderate to USG ozone at locations north of the Mason-Dixon Line, particularly PHL/TTN and NYC. This is likely in response to the light southerly surface winds pushing I-95 emissions northward. In contrast, the NCDENR model is showing USG to Unhealthy ozone levels in and around the DC metropolitan area and southern NJ. Some of this is likely an artifact of the Chesapeake Bay/Atlantic Ocean and may be overdone. The air quality models seem to be responding to different aspects of the transport pattern, with the BAMS siding with more southerly (lower level) flow and the NC model responding to more northerly (mid-level) flow. This adds considerable uncertainty to the forecast. Given that Wednesday will be the second day in a row with temperatures in the 90s °F, light surface winds, and mostly sunny skies – at least along northern portions of the I-95 Corridor – the risk for at least isolated USG ozone seems High. Persistent humid conditions along with the light surface winds will keep PM2.5 in the Moderate range on Wednesday for much of the region, with the possible exception of locations along the Atlantic coast.

A cold front will arrive early enough in the day on Thursday to end the threat of USG ozone. The mesoscale and global models are consistent in developing a pre-frontal line of clouds and convection that reach the I-95 Corridor in the late afternoon. Although the 03Z SREF, 06Z 12km NAM, and 06Z 13km GFS do not show a strong line of storms, they do have widespread cloud cover, which should limit ozone production. Very humid conditions will persist throughout most of the day on Thursday. As a result, PM2.5 concentrations will linger in the Moderate range for another day for most of the region. Locations in northwestern PA will see particle concentrations decrease as the front passes through, however, for a possible return to the Good range.

The cold front will reach southeastern NC by 12Z Friday and stall along the NC/SC border, bringing slightly cooler and noticeably drier conditions to the region. Skies will be clear and winds will die down behind the front, but the new, cleaner air mass should limit ozone and PM2.5 concentrations to the Good range. There may be a few locations in NC along the stalled front that may see low Moderate PM2.5 concentrations due to light converging winds and light rain showers. Saturday may see ozone rebound into the Moderate range as temperatures rise back into the low 90s °F.

-DeBoe/Huff