An energetic, summer air mass streaming overhead – combined with an approaching cold front – may generate a line of strong to severe thunderstorms across the western suburbs and D.C. region late this afternoon and evening.
The most likely timing for storms is between 5 and 10 p.m. tonight. Heavy rain and frequent lightning are likely; damaging wind gusts are possible.
The large-scale setup
You can blame today’s severe weather threat on a conspiracy between the subtropical (Bermuda) high and an approaching disturbance in the jet stream. Figure 1 shows the surface-level elements at play (forecast chart for 8 p.m. this evening): (1) southerly winds importing large amounts of humid, unstable air; (2) a surface pressure trough (dotted orange line); and (3) a cold front pushing through the Ohio Valley. Green shadings indicate potential regions of strong convective showers and storms; red shadings suggest locations where storms may become severe.
Figure 2 shows the upper level forecast chart, for the same time (8 p.m.). The blue ribbon is the polar jet stream. The “U”-shaped bend in the jet is a trough (the proverbial “upper air disturbance”), crossing the Ohio Valley. The pocket of dinner-mint coloration is a jet streak, with winds up to 110 mph. The approaching trough and jet streak will generate uplift of air later today over the Mid-Atlantic, along with the approaching front at the surface. As unstable air is “scooped up” and lifted, a line of storms is expected to develop.
What does the Storm Prediction Center have to say?
The National Weather Service (NWS) Storm Prediction Center (SPC) is concerned enough to issue a 30 percent probability of damaging winds along a broad arc extending from the Tennessee Valley, through the Mid-Atlantic, into New England (Figure 3). Within western Pa. and southern NY state, the probabilities of damaging winds reach a maximum of 45 percent. The hail threat is 15%. There is a 5% chance of tornadoes across northwestern PA into central NY. From this diagram, the severe threat is MORE LIKELY to our north and west, but up to 15 percent close to DC, and especially the western suburbs.
The concern I have is for a line of storms developing along and ahead of the front, initially over the mountains –then moving east off the mountains, into the D.C. region. This is most likely after 5 p.m. today, continuing through the evening. The likelihood of severe weather should increase the further north and west you are from D.C. Western Md. and the W. Va. panhandle, and Mason-Dixon region of north central MD, are the most likely zones. A severe thunderstorm watch has already been issued until 8 p.m. for these areas.
What do the high resolution models predict?
There is a consensus among all of these models for a convective line of storms. I show three of these below – first the NAM (Figure 4), then the RAP (Figure 5), and finally the WRF-ARW (Figure 6). All of these forecast snapshots are valid between 6-8 p.m. this evening.
The dominant convective mode for today is linear, with possibly two strong lines crossing central PA. SPC has even mentioned the possibility of a derecho in this region north of the Mason Dixon line. The air mass is expected to become moderately unstable across the entire Mid-Atlantic; however, the wind shear – which organizes ordinary thunderstorms into strong/severe systems – gets stronger north of the Mason Dixon Line. This is where I expect the most widespread, severe activity today. In fact, both SPC and the high resolution models suggest the possibility of bowing line segments and supercells across PA-NY. Bowing segments are most likely to produce isolated downbursts, and supercells may generate a couple tornadoes and large hail.
The bottom line
Today is a heightened awareness day for our region. A severe thunderstorm watch is likely for regions north and west of DC, but may be extended to cover portions of the metro. Expect a line of strong thunderstorms to move off the Appalachians, crossing the metro region between 5-10 p.m. There may be isolated severe storms in this line. The greater risk is over far north and western suburbs, mainly for pockets of damaging wind and large hail. Damaging winds and hail become more widespread across central PA and NY, with the possibly of a few tornadoes. The convective line will weaken as it crosses the metro, as instability lessens toward sundown. The lowest threat for severe weather today and tonight will be in our far southern and eastern suburbs.