Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Thursday, July 17, 2014
Valid: July 18- 22, 2014 (Friday-Tuesday)
Summary:
A center of high pressure will station itself over the northern Mid-Atlantic (NMA) on Friday. Sunny skies, light to calm winds, and an afternoon sea breeze could develop isolated upper Moderate/USG ozone along I-95, but consistent cloud cover predictions from the weather models add uncertainty to this forecast. PM2.5 will reach the Good/Moderate threshold due to low atmospheric moisture and light winds. As the center of high pressure moves northeastward on Saturday, it will extend its ridge down the Mid-Atlantic, keeping most of the region dry. As a series of shortwaves pass through the region, mostly cloudy skies will prevail, limiting ozone to the lower Moderate range for most locations. PM2.5 will reach the low Moderate due to increasing dew points. Sunday looks to be the cleanest day of the period due to sustained onshore flow. Monday and Tuesday will see a gradual rise in temperatures and dew points but the air quality forecast is still uncertain; Good to Moderate air quality is expected.
Discussion:
The weather forecast models are in close agreement for the medium range period. Friday will start out with a weak upper level trough over the Mississippi River Valley, which was created by the development of shortwaves moving in from the southern Plains. The trough will move into the Mid-Atlantic, where it will stay until Monday afternoon, when it will begin to flatten into zonal flow for the rest of the period. The 00Z ECMWF and 06Z GFS diverge on Tuesday, with the GFS developing weak ridging over the eastern U.S., while the ECMWF continues the zonal flow aloft.
High pressure overhead on Friday will result in mostly sunny skies and very light to calm winds, with a switch in the predominant surface flow from northwesterly to northeasterly/easterly. The 06Z 4 km and 12Z 12 km NAM this morning also indicate a line of convergence along central NJ, associated with a sea breeze front. These conditions have been conducive for isolated upper Moderate/USG ozone several times so far this summer along the I-95 Corridor in the northern Mid-Atlantic (NMA). However, for the past few days, including today, the weather forecast models have predicted cloudiness on Friday, although this morning’s GFS run is somewhat slower with the eastward movement of the clouds. The models may be responding to rising motion from the upper level trough to the west of the region. Given that the center of surface high pressure will be directly over the NMA, actual conditions will most likely be sunnier than predicted by the models. The NOAA and BAMS air quality models are consistent in predicting Good ozone for Friday, most likely due to the cloud cover forecast. Nevertheless, with light to calm winds, high pressure overhead, an afternoon sea breeze, and short back trajectories, there is a chance that ozone will reach the upper Moderate/USG range along the I-95 Corridor in the NMA. There will be a chance of late afternoon rain showers on the very southwestern side of the Mid-Atlantic as a series of shortwaves approach the region aloft, which will lead to Good ozone in this part of the region. PM2.5 will be limited to the Good range due to low atmospheric moisture. Remnant smoke from wildfires burning in western and central Canada has reached the Great Lakes region this morning, which is upwind of the Mid-Atlantic for Friday. The smoke appears to be aloft at this point, and not impacting the surface significantly, and the NAAPS smoke model keeps the impact of the smoke north of the Mid-Atlantic on Friday. However, given the high pressure center over the region, the impact of the smoke plumes will need to be monitored.
The shortwaves aloft will travel through the region on Saturday, triggering clouds and precipitation along the western Mid-Atlantic. The center of high pressure will move northeastward and will extend its ridge down through the Mid-Atlantic, keeping the rest of the region dry. PM2.5 will reach the Good/Moderate threshold due to rising dew points and morning stagnation. The passing of the shortwaves will leave the skies mostly cloudy. Ozone will reach the Moderate range in locations with the most sun, yet most of the region will see ozone in the Good range. Given the forecasted surface winds, the chance for mid-to-upper Moderate ozone along I-95 will need to be monitored, but at this point, it looks like there will be sufficient clouds to limit ozone to the lower Moderate range.
The surface ridge will remain over the Mid-Atlantic on Sunday as a stationary front will move northward as a warm front along the Southeast coast. The precipitation forecast is still somewhat uncertain due to weather forecast model discrepancies. The weather models, particularly the GFS, want to develop a wave of low pressure along the stalled frontal boundary, but it’s not clear when this will occur. The GFS develops the wave off of the Delmarva coast as early as Sunday morning, which brings clouds and precipitation along the NMA coastline. The ECMWF keeps the chance for rain showers mainly along the southwestern part of the Mid-Atlantic and the Southeast coast. Regardless of the exact precipitation forecast, air quality will be reduced to the Good range for most locations due to sustained easterly winds, with the exception of the chance for Moderate ozone in inland areas.
On Monday, the center of high pressure will move farther northeastward over the Atlantic before returning back overhead on Tuesday. Temperatures and humidity will gradually rise. Substantial uncertainty exits for the air quality forecast due to questions about when/where a wave of low pressure will form along the stationary front off of the East Coast, as well as the differences in flow aloft, as discussed above. For now, we expect Good to Moderate air quality, with the highest chance for mid-to-upper Moderate ozone in the NMA along I-95, particularly if the GFS forecast verifies.
-Catena/Huff