... Weak low pressure will pass to the south tonight as a second, stronger area of low pressure moves from the upper Great Lakes into the Northeast. The second low will drag a cold front west to east across the Mid-Atlantic Thursday, then re-develop off the New England coast as high pressure nudges in from the Ohio River Valley Friday into Saturday. A broad area of low pressure will likely traverse the East Coast during the second half of the weekend, then another area of low pressure may approach around Tuesday.
NEAR TERM /THROUGH THURSDAY/...
Showers continue to move through our region this evening. The threat for showers should taper off overnight from the northwest to southeast. The heaviest precipitation is currently moving through areas south of I-66 with lighter precipitation to the north. The threat for any thunderstorms has ended for this evening.
Light rain/showers are ongoing this afternoon across most of the forecast area beneath a region of upper level diffluence. The showers are helping to keep temperatures steady in the 50s. Conditions are mostly dry across central Virginia, and locations there may start to break out into some sunshine shortly. A developing surface boundary is evident from central Virginia to the Northern Neck, with clouds, showers, northerly winds, and cooler temperatures to the north, and breaks of sun and southerly winds to the south. As heating continues, limited instability will build to the south of the boundary, with a few hundred J/kg of CAPE expected by later this afternoon. Hi-res guidance hints that a few storms may try to pop up in the vicinity of a weak area of low pressure which is starting to form along the aforementioned front. As a result, a passing thunderstorm can't be ruled out across Nelson or Albemarle Counties later this afternoon into this evening. With around 60 knots of 0-6 km shear present, any deep updraft would have the potential to become supercellular, but instability will be a limiting factor. If any stronger storms were to form, they could be capable of producing damaging winds or hail. Further north, just showers are expected within the more stable airmass
Shower activity is expected to wind down from west to east tonight. Low clouds will hang around even after steady precipitation moves out. Some patchy fog may be possible as well, given a saturated lower atmosphere and weak winds at low levels. Lows tonight will range from around 40 in the mountains, to the mid 50s in the cities and along the bay.
SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... The large upper low currently over the Great Lakes will drop southward toward our area by Thursday. At the surface, low pressure will track off the coast, placing us in northwesterly flow in its wake. The day will start out cloudy, but gradually clearing skies are expected over the course of the day as downslope flow kicks in and conditions start to dry out at low- levels. Despite the northwesterly winds, there won't be much in the way of cold advection, so mild temperatures are expected, with highs mainly in the 60s. As the upper low progresses to the east, a push of cold advection will surge in behind it during the late afternoon and evening hours. As temperatures cool, any upslope activity along the Allegheny Front will change over to snow. Hi-res guidance hints that some heavier snow showers may descend in northwesterly flow tomorrow evening and impact the Allegheny Front. Some models even show the snow squall parameter climbing above one tomorrow evening as limited surface based instability develops. Temperatures will be a limiting factor for snowfall accumulation, with most locations even in the higher terrain staying just above freezing. For now, we're forecasting around a coating to an inch of snow along the Allegheny Front tomorrow night, but locally higher totals appear plausible if heavier squalls were to form.
The upper low will linger to our north on Friday. Mostly dry conditions are expected, but a stray shower can't be ruled out as a weak disturbance rotates around the upper low. Highs on Friday will range from the 40s in the mountains, to the lower 60s across southeastern portions of the forecast area.
LONG TERM /SATURDAY THROUGH WEDNESDAY/... Quasi-zonal flow is progged over the Mid-Atlantic between two upper shortwave troughs/lows embedded within a larger cyclonic gyre/long wave trough over eastern North America to start the weekend. Subtle shortwave ridging between the two lows should lead to dry weather and near or slightly below seasonable temperatures Saturday.
Warm advection ahead of and an impulse of upper-level energy attendant to the western of the two aforementioned upper lows/troughs (approaching from the Midwest) heightens the chances of shower activity Saturday night into Sunday. A trailing jet streak/vort max may cause shower potential to linger Sunday, though there may be a relative lull in between. Overall, precipitation looks lighter and more intermittent with a complete washout unlikely.
Uncertainty is still moderate to high heading into the first half of next week. Another trough is slated to approach from the west sometime between Monday night and Wednesday, but its precise timing and position vary from model to model and run to run.