Hold on tight. A rapidly intensifying low pressure system and powerful cold front charge through the region late Wednesday, providing two chances for strong wind gusts – Wednesday afternoon/evening along a possible line of showers and storms, and Wednesday night into Thursday morning behind the storms. As the winds crank, temperatures tank: from near 70 Wednesday afternoon to below freezing Thursday morning.
- Wednesday early-to-mid afternoon: Variable clouds, very breezy, warm, isolated showers. Temps 65-70
- Wednesday late afternoon through early evening (3-8 p.m.): Showers and storms likely, possible strong winds, gusting over 30 mph (isolated 40 mph+ in storms). Temps falling into 50s.
- Wednesday night through Thursday morning (8 p.m. to 8 a.m.): Partial clearing, very windy and sharply colder. Winds gusting to 40-50 mph. Temps falling into 20s by morning, wind chills 7-14.
Strong squall line (or narrow, intense line of showers/storms) possible late Wednesday
Computer models predict that a vigorous area of low pressure will pass just to our north on Wednesday, across northern Pennsylvania. This places our region in the storm’s warm sector, with a stiff southerly wind and temperatures surging to near 70 degrees. The surface system is being energized by a strong wave in the upper-level jet stream.
As the cold front approaches, a convective line of showers and possible thunderstorms – or squall line – arrives late Wednesday afternoon. The strong March sun and southerly flow of mild air will warm the low levels, while mid-levels will cool as the wave disturbance passes through. These processes will destabilize the atmosphere, a pre-requisite for showers and thunderstorms. A narrow band of convective cells is expected to develop just ahead of the cold front where uplift of air is most vigorous.
At the same time, wind shear in the low levels is expected to increase, as a low-level jet stream intensifies in the warm sector. The sharp increase in wind speed with altitude may enhance the vigor of some of the cells, pushing them to strong levels.
The figure below illustrates the simulated line of convection late Wednesday afternoon. The forecast chart is valid at 7 p.m. Wednesday evening. The convective line first develops along the Blue Ridge then sweeps eastward across the entire region. (Note: the timing of this feature is subject to change; strong storms could sweep through as early as the late afternoon hours).
Will storms be severe on Wednesday?
At this point, the setup does not appear conducive to widespread severe weather. The atmosphere is expected to be only mildly unstable. Cloudy skies may limit the amount of solar heating, although any breaks in the overcast may allow some pockets of stronger instability to develop.
There seems to be consensus among both NWS Sterling and the Storm Prediction Center (SPC) that any thunderstorms could approach strong levels, with only an isolated risk of severe (< 5% chance at any location) – see Figure 2 below. Wind gusts are the primary threat, up to 40-50 mph, along with heavy downpours and some sporadic lightning.
The convection may take the form of a dynamically-forced squall line, technically known as a “high shear low cape” (HSLC) type of system.
The CWG staff will monitor model trends with these storms throughout the next 48 hours and issue updates as necessary.
A second round of strong wind gusts likely Wednesday night
Act II of this storm sets up during the overnight on Wednesday, AFTER the cold front moves through. The area of low pressure is predicted to rapidly deepen as it swings through the Mid-Atlantic, with central pressure dropping close to 980 mb.
Combined with a strong arctic anticyclone (high pressure center) approaching from the west, this will create an intense pressure gradient.
Figure 3 (below) is a model prediction of the surface pressure field (solid lines are isobars) and 10-meter wind speed. The forecast is valid at 11 pm Wednesday evening. Note the narrow spacing between isobars over the Mid Atlantic. Also shown are 10-meter wind speed values. 25 mph sustained flow (over 20 knots) overlies much of the region.
Shown next (Figure 4) is a depiction of maximum expected wind gust, valid at 8 a.m. Thursday morning. Widespread 35-40 kt (40-45 mph) wind gusts are the rule just about everywhere, all night long. Winds relax by mid-morning Thursday.
The dark orange shadings to the west of our region denote the crest of the Blue Ridge. Here, peak gusts will likely hit the 45-50 mph range. There will be a core of strong winds about 5,000 feet above sea level behind the front, and the mountain tops project into this jet-like layer.
Here’s a simulation of winds Wednesday night from the RPM model…
— Howard Bernstein (@hbwx) March 11, 2014
NWS issues a Wind Advisory when gusts are expected to exceeding 45 mph. A Wind Advisory is likely Wednesday evening into Thursday morning, especially for the higher elevations. CWG will monitor the situation and issues appropriate updates.
Will there be power outages Wednesday night?
Several rounds of heavy snow and accumulating ice this winter have stressed vegetation. Additionally, the soil is moist from recent snowmelt, and heavy showers on Wednesday afternoon may saturate soils in some locations.
These aspects, combined with winds gusting to 40 mph or higher for several hours, could lead to some isolated to scattered power outages. At this time we are not anticipating major, long-lived power outages from this one-two, windy punch.
Bottom Line: Expect major, sudden shifts in the weather beginning Wednesday afternoon
This vigorous, transitional weather system promises to deliver extreme weather changes in the space of 12-18 hours. Starting with mild conditions on Wednesday, the region will experience gusty showers/thunderstorms, followed by a night of howling winds, then an arctic chill on Thursday with temps struggling to reach the mid-30s. Wind chills will dip into the single digits in our outlying areas to the north and west and teens in the immediate metro region.