Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 7, 2014
Valid: June 8 – June 10, 2014 (Sunday-Thursday)

Summary:

An approaching low pressure system will end the sunny weekend with clouds and a slight chance for rain Sunday night. However, due to humid and hot conditions all day, ozone and PM2.5 will reach the Moderate range. The low front will bring scattered rain and thunderstorms Monday, dropping the ozone to the Good range, but not enough to lower to PM2.5, keeping it in the Moderate range. There is still major uncertainty in Tuesday’s forecast due to weather forecast model discrepancies. Good to Moderate ozone and PM2.5 depending on timing and location of precipitation and cloud cover. Wednesday and Thursday will also bring on scattered rain and thunderstorms, limiting ozone to the Good range. PM2.5, however, should stay in low Moderate to high Good.

Discussion:

The weather forecast models are in good agreement on the large scale features. There are some differences regarding the amplification of the upper level trough in the southern Great Plains Monday and Tuesday with the ECMWF being the slowest (most amplified) and the NAM the fastest (less amplified). In any event, they all end up in concert at the end of the period with a progressive trough near IL 1200 UTC Wednesday.

Because the strongest synoptic scale forcing remains well to our west through the forecast period, most of the sensible weather (e.g., rain) will depend on smaller scale features associated with weak frontal boundaries and mesoscale features. As a result, we have low confidence in the timing and extent of cloud cover and precipitation after Monday and this leads to low confidence in the ozone forecast. With high humidly and light to moderate winds, PM2.5 should be relatively stable.

Sunday will be the warmest day and most conducive to ozone formation. However, the latest model runs suggest substantial high cloudiness building in late in the afternoon. This will take the edge off peak concentrations, particularly in the western mid-Atlantic. Rising humidity and light winds will allow PM2.5 to increase. Moderate concentrations for both.

Monday continues to look like cloudy and rainy with Good ozone but Moderate PM2.5. Note that there are major differences between the forecast models on timing of rain Monday and location (north to south) but all models have significant cloud cover.

Large differences on Tuesday with respect to rain although the GFS has deep PBL mixing which might trigger convection late in the day. This is the most uncertain day with ozone and PM2.5 Good to Moderate depending on location of rain and extent of cloud cover.

SE winds dominate Wed and Thursday, ECMWF still much wetter than GFS. Maritime tropical air mass will keep PM2.5 in the low Moderate to high Good (cleanest eastern mid-Atlantic), ozone looks to straddle the Good-Moderate line depending on rain chances.

-Ryan/Catena