Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, May 24, 2015
Valid: May 25-29, 2015 (Monday-Friday)

Summary:

High chances for Code Orange/Red air quality in the Mid-Atlantic region continue on Monday and Tuesday, with appreciable risk for the rest of the period, but uncertainty persists. An upper level ridge will remain over the Mid-Atlantic through the end of the medium range period, which will keep a very warm and humid air mass over the region. The ridge axis will be centered over the Mid-Atlantic on Monday and slide eastward on Tuesday, placing the Mid-Atlantic on the western edge of the ridge for the remainder of the period. The main limiting factors for Monday and Tuesday will be the speed with which regional ozone rises and the impact of onshore transport over the NC/SC coastline. Increasing chances for afternoon convection are the main sources of uncertainty for Wednesday through Friday.

Discussion:

The weather forecast models are in general agreement on the synoptic features for the medium range period, but there are some slight differences. The 00Z ECMWF, 00 CMC, 12Z NAM, and 06Z GFS were consulted for this analysis. The main issue for air quality continues to be the upper level ridge that will remain in place over the Mid-Atlantic through the end of the medium range period, which will keep a very warm and humid air mass over the region. The ridge axis will be centered over the Mid-Atlantic on Monday and slide eastward on Tuesday, placing the Mid-Atlantic on the western edge of the ridge. The NAM and GFS are slightly faster to push the ridge eastward compared to the EC and CMC. The ridge will weaken Wednesday through Friday as a series of shortwaves move through the flow. In particular, the global models bring a strong shortwave across the Great Lakes on Wednesday and into southern ON/QC on Thursday, which will possibly pull a cool front into the northern Mid-Atlantic. The main impact of the shortwaves will be increasing chances for clouds and convection, especially on Wednesday and Thursday. Due to the eastward placement of the ridge, back trajectories continue to be a major factor limiting possible rising regional ozone, with onshore transport over the NC/SC coastline beginning on Monday at low levels and continuing at all levels on Tuesday and Wednesday.

Memorial Day will be warm and increasingly humid. The location of the upper level ridge will keep any shortwaves moving through the flow to the north of the Mid-Atlantic. As a result, Monday will be mostly sunny across the region. Light south/southwesterly winds will pick up in the afternoon, especially along the coast. Back trajectories at 1500 and 1000 m AGL are slow and westerly from the Ohio River Valley (ORV), while low level trajectories at 500 m AGL are onshore from over the NC coastline; all of the trajectories show strong subsidence. The forecast questions for tomorrow will focus on the degree to which low level transport aloft of maritime air may suppress rising ozone levels, particularly along the I-95 Corridor. Memorial Day holiday emissions are another factor, as will be the rapidity with which ozone rises today across the region and upwind in the ORV.

Tuesday will be slightly warmer and more humid than Monday, with increased chances for afternoon clouds and thunderstorms. The 09Z SREF paints a high probability of precipitation across most of the western Mid-Atlantic in the afternoon, but the 12Z 4 km nested NAM only shows isolated thunderstorms. The EC and CMC keep the region essentially dry on Tuesday. Back trajectories all have an onshore component, however, hooking to the southeast over the NC/SC coastline. As has been the case for the past several days, the air quality models have a clean out for Tuesday and Wednesday, with only the NCDENR model indicating isolated Moderate ozone and PM2.5 in eastern PA. Despite the uncertainty in possible clouds/precipitation and onshore transport, chances for at least isolated USG ozone remain high given the very warm conditions, normal weekday emissions, and a possibly modifying air mass.

Substantial uncertainty remains in the forecast for Wednesday through Friday. The global models all have increasing chances for clouds and precipitation, given the warm and humid conditions and the series of shortwaves moving through aloft. If the weak frontal boundary predicted by the global models verifies, it will act as a focus for clouds and convection on Wednesday and especially Thursday. Back trajectories shift westward, away from the Atlantic, on Thursday and Friday, however, with slow localized transport on Friday. In response, the air quality models have scattered Moderate ozone along the northern I-95 Corridor on Thursday. Thus, despite increased chances for afternoon convection later in the period, chances for USG ozone remain, given the persistent warm and humid conditions, increasingly modified air mass, and more westerly back trajectories. The next true air mass-changing cold front will not impact the region until next Sunday at the earliest.

-Huff