Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Sunday, July 20, 2014
Valid: July 21- 25, 2014 (Monday-Friday)
Summary:
The chances for upper Moderate/USG ozone early in the period continue to trend downward due to the faster arrival of the next cold front. Although temperatures will reach the low 90s °F by Wednesday, clouds and precipitation and a cleaner transport pattern aloft are expected to limit ozone to the low to mid Moderate range through Wednesday, with Moderate PM2.5 as well. Widespread rain and the arrival of a clean and dry air mass will return air quality to the Good range for Thursday and Friday.
Discussion:
The weather forecast models are in good agreement this morning regarding the synoptic pattern for the medium range period. The 00Z ECMWF, the 06Z GFS, and the 12Z NAM were used for this analysis. The upper level flow over the Mid-Atlantic will turn zonal on Monday and transition to weak ridging on Tuesday. A series of shortwaves will crest the strong ridge over the western U.S. on Tuesday and begin to dig out a trough over the western Great Lakes. This trough will amplify and shift eastward on Wednesday, and then build over the Mid-Atlantic on Thursday and Friday, pulling a cold front through the northern Mid-Atlantic (NMA) in the late Wednesday to early Friday time frame. At the mid-levels, a ridge will try to build over the region Tuesday and Wednesday before being displaced by the upper level trough.
Monday-Wednesday are still the days of most interest in the period, but the chances for upper Moderate to USG ozone continue to trend downward today, due to the progressive development of the upper level trough and faster arrival of the next cold front. Although the mid-level ridge over the region will increase temperatures and dew points through Thursday, high temperatures are trending downward, with highs only reaching the low 90s °F in the NMA on Wednesday (the hottest day of the period), compared to the mid- 90s °F in yesterday’s analysis. In addition, the GFS and ECMWF agree that the next cold front will begin moving into the NMA on Wednesday afternoon/evening, which will trigger a line of showers and thunderstorms across the region, according to the 12Z versions of the12 km and 4 km NAM as well as the 09Z SREF. The BAMS air quality models have continued to back off on their previous predictions of USG ozone along the I-95 Corridor as well. This morning’s runs keep ozone in the Good range across the region on Tuesday and Wednesday.
Surface high pressure will move back over the region on Monday, resulting in a period of stagnation overnight and then light surface winds shifting from E/SE to S by the end of the day. Shortwaves moving over the region will result in a mixture of sun and clouds. Flow aloft will be onshore (E) at lower levels (e.g., 500 and 1000 m AGL) and veer to the SW at 1500 m AGL. The 06Z NOAA ozone model does indicate an isolated area of upper Moderate/USG ozone in northern NJ and northeastern PA, likely in response to the light onshore flow and closer proximity to the center of surface high pressure. Given the onshore winds and partly cloudy skies, Good to low Moderate ozone is expected elsewhere in the NMA. An area of shortwave energy interacting with the persistent stationary front along the Gulf Coast and Southeast will trigger clouds and rain that will keep air quality in the southern Mid-Atlantic (SMA) in the Good range. PM2.5 will be in the Moderate range at inland locations, roughly west of I-81. Smoke from the central and western Canadian wildfires is still located aloft over the northwestern parts of the Mid-Atlantic; the NAAPS model continues to keep the smoke north of the region through the end of the period, so it is not expected to impact surface air quality.
Winds become southerly on Tuesday as a return flow sets up around the center of surface high pressure, which will be located off of the NJ coast. It will be warmer and more humid, with high temperatures in the upper 80s °F in the NMA. Mostly sunny skies and light winds in the NMA will promote rising ozone levels, but the transport aloft will have an onshore (S/SE) component, which will likely limit ozone to the low-to-mid Moderate range. As noted above, the BAMS ozone models keep conditions in the Good range across the region. Afternoon clouds and rain, per the SREF, look to keep ozone in the Good to low Moderate range in the SMA. PM2.5 will likely reach the Moderate range across the region due to increasing humidity.
Although Wednesday will be hot and humid, the mesoscale models develop a line of rain and thunderstorms in the afternoon, triggered by the cold front approaching from the NW. Back trajectories are again more S/SE, with some onshore (maritime) component, which will continue to limit rising ozone mixing ratios. The model trend is toward a faster cold front passage, compared to previous days’ analyses, which is resulting in more clouds and precipitation on Wednesday. Thus, Moderate ozone is probably limited to along and east of I-95, with little chance of USG ozone. PM2.5 will fall into the Good range in the western part of the region as the advancing precipitation cleans out the atmosphere; Moderate PM2.5 will linger along the eastern part of the region until the cold front arrives late Wednesday into Thursday morning.
Thursday and Friday will feature Good air quality across the region as the cold front moves to the SMA by Friday morning and a cooler and dry Canadian high pressure system builds in behind the front.
-Huff