... High pressure will build across the central Appalachian Mountains today before moving offshore of southeastern Virginia tonight. Low pressure will develop over the Tennessee River Valley Friday, and then strengthen as it tracks northeastward near or just south of our area Friday night into Saturday. Weak high pressure will return for Sunday, but a clipper-like system may affect the region early next week.
NEAR TERM /THROUGH TONIGHT/... 12z IAD sounding shows a fairly substantial subsidence inversion around 850 hPa, along with ample dry air in the low-mid levels. As mixing commences, temperatures should quickly climb into the 40s, before topping out in the low to mid 50s later this afternoon. With dry air in place in the low-mid levels, and high pressure settling to our south, conditions will remain precipitation free. Although skies are currently mostly sunny, high clouds are evident on satellite imagery to our west, and will start to stream in on westerly flow aloft in advance of a system over the center of the country. These high clouds will push into western portions of the forecast area over the next couple hours, before ultimately reaching locations further east later in the afternoon. Previous discussion follows...
Broad upper- level troughing is evident in moisture channel imagery early this morning across southeastern Canada into the northeastern United States. A cutoff low is spiraling over the south-central Plains headed for the middle Mississippi River Valley. At the surface, high pressure is building across the central Appalachian Mountains.
The high will shift eastward just south of the local area through the day. Abundant high clouds ahead of the cutoff low over the middle of the country will move quickly east in westerly flow in the upper levels, so we anticipate a broken deck of cloud cover by nightfall.
As the high shifts by to our south, a light southerly wind of 5 to 10 mph should push temperatures into the lower to middle 50s this afternoon east of the higher terrain. The light southerly wind will likely linger overnight, and this coupled with high clouds should prevent a sharp drop in temperatures for most areas. Low temperatures are forecast to stay above freezing east of the Appalachian Mountains, though a few sheltered/valley locations could drop quickly this evening.
SHORT TERM /FRIDAY THROUGH SUNDAY/... Guidance continues to come into better agreement for the area of low pressure that is expected to affect the CWA at the end of the week. A track generally from the middle Mississippi River Valley across the Tennessee River Valley to across southern Virginia Friday into Friday night looks most likely, before the low moves offshore and up the coast toward southeastern New England during the first part of the weekend.
Without a strong blocking high pressure area and source of cold air to the north, precipitation should start as rain across the entire area (though a brief light mix can't be completely ruled out near and west of the Blue Ridge Mountains first thing Friday morning). A feed of moisture from the Gulf of Mexico should lead to another soaking rain near and just north of the surface low track. Most ensemble means favor 1 to 1.5 inches, with isolated totals of up to 2.5 inches possible mainly from central Virginia to southern Maryland and up I-95. Given the recent rainfall earlier this week, this may very well cause isolated/minor flooding issues, though rainfall rates probably won't be quite as high as the event earlier in the week. Rain should peak in intensity Friday night into early Saturday morning.
As the low pulls off the coast and strengthens rapidly, cold air will wrap around the system. Depending on how quickly the low strengthens, lingering moisture and cold air overlapping near the higher terrain may result in a quick period of snow. Any accumulating snow would be most likely over the Allegheny Highlands, though a coating of snow can't be ruled out over the Blue Ridge or Catoctin Mountains. A few wet snowflakes could even mix in over the lower elevations north and west of the major cities/I-95 corridor Saturday morning, if enough moisture is left as colder air moves in.
A brisk northerly wind is forecast Saturday into Saturday night as the strengthening low pulls away and temperatures tumble. Wind gusts could exceed 30 mph, and when combined with air temperatures falling through the 40s into the 30s, widespread wind chill values in the 20s are likely by Saturday evening.
Most precipitation should come to an end by Saturday evening, with perhaps a few lingering snow showers near the Allegheny Front into the first part of Sunday morning. Continued brisk conditions are forecast on Sunday as high pressure tries to nose into the region.
LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... An upper-level low associated with the northern stream of the jet will approach Sunday night before passing through our area Monday through Monday night. There will be southern stream energy around at this time, but it appears that it will not phase with the northern stream system until it's well offshore. If this is the case, then this system will have very little moisture to work with. Therefore, most areas may end up dry and chilly with just a few snow showers near the Allegheny and Potomac Highlands (with perhaps a flurry or sprinkle farther to the east). If this system does phase with the southern stream energy, even partially, then more widespread rain and snow is possible. We will continue to monitor.
High pressure will likely return for Tuesday and Wednesday, bringing the likelihood for dry and seasonable conditions, though some ensemble members have additional shortwave trough energy pivoting overhead.