Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Sunday, July 27, 2014
Valid: July 28 – August 1, 2014 (Monday-Friday)
Summary:
Air quality will be in the Good range across the region at the start of the period as a cold front moves through on Monday and ushers in an unseasonably cool and dry air mass. An omega blocking pattern will keep a longwave trough aloft over the region through the end of the period, which will promote persistent Good air quality for most locations. However, PM2.5 and ozone will rise into the Moderate range Wednesday to Friday at isolated to scattered locations, primarily along the I-95 Corridor, as surface high pressure moves overhead.
Discussion:
The weather forecast models continue to be in excellent agreement on the large scale features of the medium range period. An intense shortwave will dig out an upper level longwave trough over the eastern U.S. on Monday and pull a cold front through the Mid-Atlantic region. The movement of the cold front has slowed slightly compared to yesterday’s analysis; it is now expected to clear the region by 12Z Tuesday morning. The potent shortwave will round the eastern side of the trough and merge with an existing closed low/shortwave over eastern ON/western QC early Tuesday. This will reinforce the deep longwave trough over the eastern U.S. and set up an omega block over North America that will persist through the medium range period. The main closed low/shortwave energy will begin to retrograde northward on Friday, but the longwave trough will persist over the eastern U.S. A series of small shortwaves will rotate through the trough over the Mid-Atlantic during the medium range period, keeping the chance for daily isolated showers in the forecast.
With the amplified upper level trough locked in over the eastern U.S. and a cool pool aloft over the Mid-Atlantic, Good air quality will be the rule for most locations, with isolated to scattered Moderate conditions later in the period.
The cold front will move through the region on Monday, bringing an unseasonably cool and dry air mass in its wake. Showers and thunderstorms overnight into the morning will clean out the atmosphere, and gusty winds will provide ample ventilation. Good air quality is expected across the region.
Tuesday-Friday will be dominated by the upper level trough and high pressure at the surface, moving down from Canada and gradually progressing eastward to the Mid-Atlantic by Thursday-Friday. Temperatures will be below average through the period, and dew points will remain low for the end of July (e.g., mid-upper 50s °F in the northern Mid-Atlantic). Forecast guidance suggests a gradual trend toward warmer and more humid conditions late in the period, as surface high pressure moves overhead, but it will still be relatively cool and dry (e.g., high temperatures reaching the low-mid 80s °F and dew points in the upper 50s/low 60s °F by Friday in the northern Mid-Atlantic).
With the arrival of the Canadian high pressure system on Tuesday, air quality will be in the Good range across the region. As surface high pressure moves eastward on Wednesday and surface winds begin to stagnate, PM2.5 may creep up into the Moderate range at scattered locations, continuing through Friday. It will depend on how quickly the air mass modifies and if any of the smoke from the persistent wildfires in the western U.S. and Canada is pulled down to the surface over the Mid-Atlantic by subsiding air associated with the high pressure system. For the second day in a row, the NAAPS model shows smoke moving into the region from the west beginning on Wednesday and continuing into Thursday. The possible impact of the smoke will need to be monitored to determine its effect on Mid-Atlantic PM2.5 air quality later in the medium range period.
There is also the chance for ozone to climb into the Moderate range at isolated to scattered locations beginning on Wednesday and continuing to Friday, mainly along the I-95 Corridor. The 12Z 4 km NAM shows a sea breeze on Wednesday afternoon, which seems likely to develop, considering the light synoptic surface flow. Sea/bay breezes also seem a good bet for Thursday and Friday. In addition, the upper level transport becomes slow and localized on Thursday and Friday, which will help to promote rising ozone mixing ratios, despite the relatively cool temperatures. As with PM2.5, it will depend on how quickly the air mass modifies and how much vertical mixing is promoted by the cool air aloft. The air quality models are not in agreement regarding the later part of the period. The BAMS ozone models keep conditions in the Good range through Thursday, while the NC ozone models (CMAQ C and D) develop Moderate ozone along I-95 on Wednesday (isolated) and Thursday (widespread). With full sun and light to calm surface winds, possibly aided by sea/bay breezes, Moderate ozone seems likely somewhere along I-95 late in the period.
-Huff