Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 14, 2014
Valid: July 15- 19, 2014 (Tuesday-Saturday)

Summary:

Tuesday will see potentially severe thunderstorms with heavy rain, mostly cloudy skies, and southerly surface flow for Good air quality. Wednesday-Thursday will have mostly sunny skies with light northwesterly surface flow as surface high pressure moves in from the west. Both days will have Good PM2.5 with scattered Moderate ozone. The center of surface high pressure will move overhead for Friday and Saturday, creating conditions for increasing ozone concentrations, possibly into the upper Moderate range. A warm front moving into the region from the south will result in showers primarily on Saturday and possibly on Friday, however, which will limit rising ozone and keep the chance for USG ozone low. PM2.5 will increase into the Moderate range in most locations Friday-Saturday, but may fall into the Good range depending on the precipitation forecast.

Discussion:

The weather forecast models are in excellent agreement for the medium range forecast. The period begins with a strong upper level closed low centered over the lower peninsula of MI, which is anchoring a longwave trough. The closed low will slowly move northeastward through southern ON and QC and exit to eastern Canada by Saturday, pulling the longwave trough over the eastern U.S. A secondary mid-level trough, created by a shortwave, will develop over the southern Great Lakes and move into the Mid-Atlantic on Saturday, which will trigger showers across the region on Saturday.

The passage of a slow-moving cold front along with the development of a surface trough over the Mid-Atlantic will produce heavy rain and thunderstorms on Tuesday. There is a potential for severe thunderstorms mainly in NJ, DE, and MD, especially in the afternoon. The day will be overcast with southerly surface winds. Tuesday will see Good air quality as the rainstorms wash everything out.

The cold front will move off of the East Coast on Wednesday leaving most of the region dry. With the front just off the coast, there is a chance for morning precipitation for locations closest to the shore. Northwesterly flow will bring cleaner and drier air into the area. The day will be partly sunny with low humidity keeping PM2.5 in the Good range. With light surface winds and afternoon sun, scattered locations may see Moderate ozone.

Surface high pressure, centered over the Midwest, will fully extend over the Mid-Atlantic on Thursday. The day will be mostly sunny with continued light northwesterly surface flow and low humidity. PM2.5 will be limited to the Good range while ozone will reach the Moderate range at scattered locations in response to the light winds and sunny skies.

The center of high pressure will move overhead for Friday and Saturday, resulting in light northerly/northeasterly surface flow and mostly sunny skies. Back trajectories are localized both days, with recirculation on Saturday. This pattern has resulted in high Moderate to USG ozone along the I-95 Corridor several times already this summer. However, the remnants of Tuesday’s cold front seem likely to reorganize and move back north as some kind of warm front, which, in conjunction with Great Lakes shortwave, will trigger showers primarily on Saturday and possibly on Friday. The chance for clouds and precipitation keep the chances for USG ozone low at the end of the medium range period, but we will have to see how conditions evolve. PM2.5 seems likely to reach the Moderate range in most locations, but may fall into the Good range depending on the precipitation forecast.

-Catena/Huff