Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: June 23, 2017
Valid: June 24-28, 2017 (Saturday-Wednesday)
Summary:
Scattered showers and widespread cloud cover on Saturday will give way to unseasonably cool conditions through the end of the medium range period keeping risk for an exceedance Slight. A cold front stalling along the eastern Mid-Atlantic on Friday will bring scattered rain showers with widespread cloud cover to most of the Mid-Atlantic. Some ozone formation will occur in locations west of the precipitation but strong northwest flow will keep ozone in the Good range. A cooling trend begins on Sunday as below average temperatures are funneled into the Mid-Atlantic from southern Canada. Converging winds and clear skies will allow for some ozone accumulation along the I-95 Corridor, but cool temperatures and northwesterly flow will keep ozone in the Good range across the rest of the Mid-Atlantic on Sunday. Monday Tuesday and Wednesday will be similar in the CMA and SMA as clear skies and cool temperatures persist with high pressure building in from the west. The NMA varies on Monday as shortwaves interact with wave of low pressure, bringing scattered cloud cover and precipitation to the northern NMA.
Weather Model Discussion:
The weather models remain in consensus regarding key atmospheric features during the medium range period. The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. An upper level trough will be in place over much of eastern Canada and U.S. by 12Z Saturday. As potent shortwaves spin around the trough and over the NMA, the trough will slowly progress eastward with its axis over the Great Lakes by 12Z Sunday. The trough will spin in place over the Mid-Atlantic on Sunday and into Monday. At the same time a ridge will build over the western U.S. at upper and mid-levels, pushing the axis slowly eastward over the Mid-Atlantic by 12Z Tuesday. The upper level ridge will pinch the trough over the Mid-Atlantic starting 18Z Tuesday, quickly forcing the axis offshore by 12Z Wednesday, as the ridge now builds into the western Mid-Atlantic. At mid-levels, the ridge will build over the SMA and Great Plains as soon as 18Z Monday, building eastward behind the trough through the end of the medium range period. The ridge will be over the entire Mid-Atlantic at mid-levels by 12Z Wednesday, as it merges with the westward extension of the Bermuda High and appears to persist into the weekend. This ridge will be the primary focus for the medium range period as it appears to be the only major threat for poor air quality in the Mid-Atlantic.
The Dailies:
A cold front will push through some of the Mid-Atlantic Friday night, making it to the I-95 Corridor, extending southwest through the SMA by 12Z Saturday. In the morning hours, a few lingering showers across the NMA and CMA will give way to mostly sunny skies as the front pushes offshore by 18Z Saturday. In the SMA, the cold front will stall, lingering into Sunday, resulting in widespread cloud cover and scattered precipitation ahead and along the front. The 03Z SREF, 06Z GEFS and 06Z 3km NAM all show a line of thunderstorms developing east of the Appalachian Mountains in the SMA, along the front, around 18Z pushing eastward through 00Z Sunday. Skies across the western Mid-Atlantic will clear as cloud cover moves eastward, allowing for some ozone formation to take place. This appears in the 06Z air quality models as a few isolated areas of low Moderate ozone develop in locations west of the precipitation. Despite the clearing skies, southwesterly onshore flow and cooler temperatures behind the front will result in a Slight risk for an exceedance on Saturday.
A cooler and much drier air mass will move into the Mid-Atlantic on Sunday. A surface high pressure over the Mississippi River Valley and a trough moving across the Great Lakes will funnel below average temperatures and dry air from Canada into the Mid-Atlantic. The high pressure building to the west will bring mostly sunny skies and dry conditions to the SMA. This clearing appears in the 06Z air quality models as isolated Moderate ozone continues in populated locations. In the NMA, the air quality models are showing widespread Moderate ozone developing along the eastern coast, likely in response to clearing skies and converging winds in the afternoon/evening. The hi-res weather models are all showing these converging winds along the Atlantic Coast between 18Z Sunday and lasting through 00Z Monday. This convergence combined with clear skies will allow ozone to accumulate along the I-95 Corridor but below average temperatures will prevent ozone from rising into the mid-Moderate range. As a result risk for an exceedance will remain Slight.
Cool conditions will continue for the Mid-Atlantic on Monday as northwest flow persists. Shortwaves moving over the NMA will interact with a wave of low pressure over the Great Lakes, advancing a weak cold front into PA by 18Z Monday. Mostly cloudy skies and scattered showers are expected between 18Z Monday and 00Z Tuesday across the northern NMA. Precipitation and clouds will not push southeast of I-81, but below average temperatures will keep ozone formation at a minimal along the I-95 Corridor. In the SMA, high pressure will keep skies mostly sunny but continued northwesterly flow and below average temperatures will keep ozone well within the Good range. Risk for an exceedance will remain Slight.
Tuesday will be a final day of the cooling trend as temperatures drop about 10 °F below average before warming up on Wednesday. High pressure building into the Mid-Atlantic will bring mostly clear skies and another day of fast northwesterly flow. A mixture of below average temperatures and northwesterly flow will overcome mostly sunny skies and keep ozone in the Good range on Tuesday. As high pressure moves overhead on Wednesday, flow both aloft and at the surface will switch to northerly. Another day of clear skies will bring temperatures up a few degrees, but will remain below average. Risk for an exceedance will be Slight for both Tuesday and Wednesday.
-Enlow