4:00 p.m. update: The persistent, widespread overcast across central Maryland has greatly limited destabilization of the atmosphere north of a line from Front Royal, Va. to Salisbury, Md. A special weather balloon release from NWS Sterling this afternoon revealed very low instability across northern VA. In spite of favorable wind shear and a stationary front through the D.C. region, widespread strong to severe tstorms north of this line (from Front Royal to Salisbury) are very unlikely.
In contrast, the air mass south of this line has warmed considerably, and a moderately unstable air mass is in place. Cities including Fredericksburg, Charlottesville, and Harrisonburg are more likely to experience strong to severe storms. The Storm Prediction Center may still issue a severe thunderstorm watch for central and southeastern VA this afternoon.
2:34 p.m. update: The National Weather Service says it’s likely to issue a Severe Thunderstorm Watch (80 percent chance) for late this afternoon and evening. It writes:
SCATTERED THUNDERSTORMS ARE EXPECTED TO FORM OVER THE NEXT FEW HOURS…WITH SEVERE HAIL AND WIND POSSIBLE.
Stay tuned to CWG for the latest…
From 12:34 p.m.: The National Weather Service Storm Prediction Center (SPC) in Oklahoma has outlined a 30 percent chance of severe thunderstorms across the greater D.C .region later today – the most likely timing 6-9 p.m. But there’s a big “IF” attached to this. Read on to discover why this may not be the ideal setup for severe storms across our region.
Quick overview of today’s setup
Irrespective of whether storms are “severe”, showers and storms are likely to roll through the region with thunder, lightning, and some strong winds gusts. The thorny question is whether storms reach “severe” criteria – with wind gusts exceeding 58 mph, hail over half an inch in diameter and tornado activity.
Figure 1 (below) shows that our region is in a high-end “slight risk” area (see this graphic for more on what a slight risk means) for severe weather, which includes a 2% probability of tornadoes, 30% chance of damaging winds, and 15% likelihood of large hail.
It’s been an active weather morning in parts of the Mid-Atlantic. Not too far to our north, a corridor of intense convective storms has set up across central Pa. (Figure 2 below). The region is under flash flood watches and warnings for training storm cells – that is, heavy rain-producing cells repeatedly passing over the same location.
The culprit is a stalled weather front draped from northwest to southeast across PA (Figure 3) with moist flow aloft moving parallel to this boundary.
Wind shear, the change in wind speed with altitude, is significant over much of the Mid-Atlantic. The strong flow is from the NW, and embedded in this flow are a couple of weak upper-air disturbances approaching from the Ohio Valley.
SPC clearly sees this front as a continued focus for production of severe weather through the day. Our region remains on the south side of this boundary – the potentially warm and humid side. The big question is: Will our air mass become unstable enough to energize severe thunderstorms?
A more detailed assessment
The widespread convective activity to our north has created a dense overcast, and this is limiting solar heating and destabilization of the air mass across D.C.-Baltimore. The false-color infrared satellite image (Figure 4, below) shows multiple layers of cloud streaming across our region on northwest winds aloft.
The 10:30 a.m. surface observations reveal aggressive warming and moistening over south-central Va., where temps have risen into the mid-70s, and dewpoints in the mid-upper 60s. The cloud cover over the Mason-Dixon has created quite a thermal gradient, as our temps remain the low-mid 60s with dewpoints in the upper 50s-low 60s. At 11 a.m., it was 61 F in Frostburg and 81 F in Charlottesville.
The convective potential (i.e. thunderstorm development potential) in this morning’s Dulles sounding is not that great. This is based on an assumed degree of surface warming late in the day, and the most unstable layers located above the surface. Based on these analysis methods, the predicted CAPE (convective available potential energy) is only a few 100 J/kg. This is a weakly unstable atmosphere.
Convective potential can also be assessed using the numerical forecast models. This morning’s run of the high resolution NAM suggests that CAPE will rise to about 1300 J/kg by 8 p.m. this evening. The morning SREFs (Short Range Ensemble Forecast) implies that our region will warm into the upper-70s, with a forecast afternoon CAPE in the 1200-1700 J/kg range.
Basically, the models are somewhat bullish on the realized degree of heating and predict modest CAPE…whereas morning satellite trends suggest that the potential for significant destabilization may be blunted by continued or intermittent cloud cover.
If the cloud cover does break this afternoon, even a couple hours of heating could produce enough destabilization to trigger strong to severe thunderstorms across our region – given that other key ingredients are in place, including (1) a nearby frontal zone to focus convergence; (2) approaching shortwaves to trigger a round of convection; and (3) decent wind shear.
To help resolve the conundrum of destabilization, the Washington NWS office plans to launch a special radiosonde (weather balloon) at 2 p.m. this afternoon.
What type Of severe weather are we talking about?
Given the elements in place (described above), any severe storms would be more widespread and organized, rather than spotty. The primary mode is likely to be short line segments or clusters of multicell storms. These storms may include small bowing segments capable of producing damaging wind. Large hail (golfball size) is also possible.
Tornadic potential is there, but low. Anytime you have bowing segments, you run the risk of small spin-ups embedded in bow echoes. Also, with a frontal boundary nearby, sometimes enough cyclonic wind shear will concentrate along that boundary to produce an isolated, low-top supercell (mini-supercell).
Localized flash flooding is also possible. While the precipitable water is nowhere near off-the-charts values of our recent floods, there is 1.2”-1.3” concentrated over our region, along the front. And with upper-level flow parallel to the frontal boundary, there is the potential for echo training. As mentioned early, this process has already lead to serious problems about 200 miles to our north.
Below are the morning mesoscale model predictions (HRRR in Figure 5, WRF-ARW in Figure 6) for 7-8 p.m. tonight. The HRRR is somewhat spotty, with scattered multicell storms. The WRF-ARW is more aggressive, showing intense multicells/short bowing segments. In both cases, a clustering of these cells moves through our region from 6-9 p.m. this evening, pushing off the Appalachians from NW to SE.
We’re keeping an eye on the sky this afternoon
To sum up, today’s severe thunderstorm forecast is a bit dicey. Atmospheric destabilization may favor more of a severe focus to our southeast. We will have to see if the overcast breaks in the next few hours, in order to realize the full potential for widespread severe weather.
Accordingly, recognizing this limited potential, CWG will keep the Storm Threat Level at Condition 1. Check back with us later in the afternoon for updates to this forecast.