Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 25, 2015
Valid: May 26-30, 2015 (Tuesday-Saturday)

Summary:

With a warm and humid summer-like air mass persisting over the Mid-Atlantic region, there is an appreciable chance for Code Orange/Red air quality through the end of the medium range period. The main factors which limit higher chances for Code Orange ozone are the impact of maritime-influenced transport aloft and the chances for clouds and convection essentially every afternoon. The chances for afternoon clouds/precipitation are greatest on Thursday due to the expected arrival of a cool front from the west, which will progress to roughly the central part of the region before dissipating. The next substantial cold front will arrive in the Saturday to Sunday period, but it is uncertain how quickly it will progress into the Mid-Atlantic region.

Discussion:

The weather forecast models are in overall agreement on the synoptic features of the medium range period, but there are some slight differences, especially later in the period. The 00Z ECMWF, 00 CMC, 12Z NAM, and 06Z GFS were consulted for this analysis. The overall picture has not changed for the Mid-Atlantic. An upper level ridge will remain in place over the region through the end of the week, with surface high pressure anchored offshore. As a result, an unseasonably warm and humid summer-like air mass will persist over the Mid-Atlantic through the medium range period. The next strong, air mass-changing cold front will not impact the region until Saturday/Sunday at the earliest. The models are in close agreement on shifting the upper level ridge axis eastward on Tuesday, which will place the Mid-Atlantic on the far western edge of the ridge. A series of shortwaves will move though the flow essentially every day; the models disagree slightly on the timing and placement of these small shortwaves. The bottom line for air quality is the increased chance for clouds and convection every afternoon. The models all move a strong area of shortwave energy across the Great Lakes and into southern ON/QC on Wednesday and Thursday, although there are small differences with the timing and location. This feature will slowly move a cool front into the northern Mid-Atlantic (NMA) from the west on Wednesday before the front dissipates roughly over central PA on Thursday. The main effect of this feature is expected to be greater chances for organized convection on Thursday afternoon. On Saturday, a large shortwave/vort max drops down from north-central Canada and amplifies the semi-permanent upper level trough over eastern Canada. This feature will bring the next significant cold front into the Mid-Atlantic, sometime in the late Saturday to early Sunday time frame. The global models show the usual discrepancies regarding frontal passages, with the GFS the fastest with the vort max and surface front, followed by the EC, and then the CMC. The main effect for air quality on Saturday will be how fast an organized line of convection reaches the eastern Mid-Atlantic, as discussed below.

Given the eastward location of the upper level ridge, back trajectories continue to show an onshore component over the NC/SC coastlines for essentially the entire period, with the strongest maritime transport influence on Tuesday and Wednesday. As has been the case since last week, the air quality models (e.g., NOAA/EPA, BAMS, NCDENR) continue to keep air quality essentially in the Good range across the region on Tuesday and Wednesday. Presumably this is in response to the maritime-influenced transport pattern, although we did see that the models were biased roughly 10-15 ppb low for ozone over the weekend. The air quality models do show scattered Moderate ozone and PM2.5 conditions on Thursday and Friday across the NMA, mostly in the vicinity of the I-95 Corridor, likely in response to lighter winds later in the week.

Tuesday will be warm and increasingly humid, with mostly sunny skies and breezy south/southwesterly winds in the afternoon, mainly along the eastern part of the region. The main forecast question will be the impact of maritime-influenced upper level transport. We have seen this transport pattern suppress ozone formation in the past, even under full sun and high temperatures ≥ 85 °F. The degree to which ozone rises this afternoon across the region will give us an idea of the possible impact of the maritime-influenced transport. Particle concentrations may be suppressed as well, particularly at coastal locations, where concentrations are in the Good range this morning (i.e., Delaware) and are only very slowly rising despite the increasing humidity.

Wednesday, Thursday, and Friday have increased chances for afternoon clouds and convection, especially on Thursday due to the arrival of the cool front. The convection looks to be more likely across the western and central parts of the region, with most models keeping it to the west of the I-95 Corridor on Wednesday and Thursday. Winds will continue out of the south/southwest, with breezy conditions along the coast on Wednesday and then diminishing wind speeds on Thursday and Friday.

Saturday’s forecast is very uncertain given the possible approach of the next significant cold front. The GFS is the fastest with the front, and places widespread precipitation across most of the region by the afternoon, including the I-95 Corridor. The CMC is the slowest of the global models and keeps most of the region dry on Saturday.

-Huff