Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 3, 2015
Valid: June 4-8, 2015 (Thursday – Monday)

Summary:

There is still only a slight chance for USG ozone across the Mid-Atlantic through the medium range period. On Thursday, the persistent upper level disturbance over the Southeastern US and a stalled frontal boundary along the coast of the Carolinas will continue to promote widespread clouds and scattered precipitation, concentrated south of the Mason-Dixon Line. On Friday afternoon, the next cold front will advance into northwestern PA. Clearing skies to the south and east, under the influence of surface high pressure centered over northeastern PA, as well as light and converging surface winds will allow ozone and particles to rise. Particles in particular may reach the Moderate range at scattered inland locations. The arrival of slightly cooler and drier air behind the front will help to return air quality to the generally Good range throughout the northern Mid-Atlantic on Saturday, while afternoon clouds and rain showers across the central and southern Mid-Atlantic will limit ozone formation. Particles may linger in the Moderate range, depending on the track and intensity of pre-frontal precipitation. On Sunday and Monday, Saturday’s cold front will move back north as a warm front, and on Monday, the next cold front will approach from the west. There should be sufficient clouds to limit ozone formation at the end of the period, but high atmospheric humidity and light winds will enhance PM2.5 formation, making it the leading pollutant.

Discussion:

The forecast models are in general agreement in the beginning of the medium range period, but they begin to diverge on Saturday. The 06Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. Currently, there is a short wave over the southeastern United States (US) in TN and a weak upper level ridge over the northeastern US. A stationary front continues to sit along the NC/VA coastline, but the front will slowly weaken and dissipate by 12Z Friday.

The models continue to handle the movement of the southern shortwave very well, inching it southeast from TN, reaching SC by 18Z Thursday and coastal NC by 12Z Friday. The slow eastward progression of the southern shortwave, along with the stationary front draped along the east coast of the southern Mid-Atlantic (SMA), will enable a consistent onshore flow across the SMA. This onshore flow will keep persistent clouds and showers across most of the region through Friday, particularly south of the Mason-Dixon Line.

By 12Z Friday, a strong shortwave will drop down in the northern stream, moving over the ON/QC border. This feature will rotate around the base of the upper level trough axis over eastern Canada and will pull a cold front into the northern Mid-Atlantic (NMA) from the northwest Friday afternoon into Saturday. At this point in time, discrepancies in the models become evident. The CMC diverges from the other models, making it an outlier for the remainder of the period. There are some minor, typical differences between the models regarding the timing of the front, but there is general agreement that the cold front will move quickly and clear the SMA by 12Z Sunday as a weak ridge of high pressure builds behind the Canadian short wave.

The GFS and EC develop a short wave over the northern Plains by 12Z Sunday. Shortwave shearing from a closed low over the western Rocky Mountains will help reinforce the northern Plains shortwave as it digs out an upper level trough over the northeastern Plains. The EC has a much stronger and more amplified trough than the GFS. By 06Z Monday, the models have this upper level trough moving eastward into the Great Lakes region. The EC has the southern edge of this trough reaching into southern IL, while the GFS has it reaching into southern MI. This feature will pull a cold front just west of the Appalachians, bringing widespread rain showers to the NMA on Monday. The stronger EC solution creates more convection and precipitation ahead of the cold front, but both models show widespread precipitation across the Mid-Atlantic on Monday, mainly west of the I-95 Corridor.

Onshore winds at the surface and aloft will promote scattered showers for the entire region tomorrow, particularly south of the Mason-Dixon Line. The oscillating stationary front along the Carolina coast will provide a focus for convection in the southern part of the region, while surface high pressure centered over New England (NE) will push the rain showers and the thickest cloud cover south of Pennsylvania. This widespread cloud cover and rain should keep particle and ozone concentrations low across the region on Thursday.

Friday’s weather will vary little from Thursday’s at first, as light onshore flow and a similar synoptic set-up will continue to drop rain across the southern and central Mid-Atlantic (SMA, CMA), but high pressure to the north will keep PA showers mostly at bay. An afternoon cold front approaching from the northwest will be preceded by clearing skies in PA; the front will reach Lake Erie by 18Z Friday. It will sweep from NW to SE through the state during the evening, and it will generate a line of showers and possible thunderstorms across NW PA on Friday. Lighter, converging surface winds and the chance for sun to the east and south will allow for rising PM2.5 and O3 concentrations, mainly along the I-95 Corridor. PM2.5 in particular may reach the Moderate range at scattered locations inland, given increasing atmospheric humidity and light and converging surface winds.

Northerly winds will follow behind the front, bringing lower dew points along with them to the NMA on Saturday. As skies clear and the sun comes out, Saturday will see the first stark diurnal temperature differences of the week in the NMA. There does not appear to be a significant change in air mass behind the front. Temperatures and dew points will drop slightly, but not dramatically. Nevertheless, the arrival of a new air mass in the NMA should keep air quality in the generally Good range. The front will generate clouds and scattered showers across the CMA on Saturday afternoon, which will gradually push southward into the SMA in the evening, following the path of the frontal boundary as it drops through the rest of the region. Afternoon clouds and showers south of the Mason-Dixon Line will limit ozone formation, while particles may linger in the Moderate range at scattered locations, depending on the strength and location of precipitation. The BAMS and NCDENR air quality models are showing widespread Moderate ozone and PM2.5 across the region on Saturday, centered along the I-95 Corridor and the CMA. The magnitude of the model predictions is probably overdone, but the location and general increasing trend in particles toward the Moderate range seems accurate.

The upper level shortwave over the northern plains will begin to impact Mid-Atlantic weather on Sunday. A surface low will develop in MN and move eastward across the Great Lakes. As the low moves eastward, Saturday’s cold front will move back northward into the Mid-Atlantic as a warm front on Monday. The most prominent impact of this frontal passage will be to increase dew points; however, the EC and GFS are in general agreement on bringing clouds and rain showers into the Mid-Atlantic from the west throughout the evening ahead of an approaching cold front. Broken cloud cover and the chance of widespread precipitation will again keep ozone low, with particles remaining in the Moderate range at scattered locations, dependent on the intensity of precipitation.

-Huff/Eherts/DeBoe