A few storms still possible in D.C. area this evening

July 27, 2014

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11 p.m Update: A heavier storm (with heavy rain and frequent lightning) in eastern Frederick County could clip far northern Montgomery County and northern Howard County during the next 45 minutes or so as it races to the east. Otherwise, it appears to be just moderate showers moving west to east across the immediate metro area over the next approximately 1.5 hours. The strong to severe storms that once looked likely did not materialize, and amazingly our nice weekend weather streak is basically still intact.

9:55 p.m. Update: Some strong to severe storms due west of D.C. are now moving east out of West Virginia. Hard to say whether or not they will fizzle on approach, but if they hold together they could move west to east through the metro area around 10:45 p.m. to midnight.

7:30 p.m. Update: Storms have yet to develop for the D.C. area and our winning weekend weather streak isn’t over yet. We could still see a few storms this evening, most likely during the 9 p.m. to midnight window. If they develop they could be strong with isolated damaging winds, though best chance of strong storms may stay north of the immediate area (where a severe thunderstorm watch is in effect until 2 a.m. for Frederick and Carroll counties and points north and west).

4:30 p.m. Update: A broken batch of mainly light to moderate showers should move across the D.C. area during the next couple hours. So far these are fast-moving and not too consequential, but we’ll keep an eye in case they strengthen. Otherwise, we’re still watching a couple areas of showers and storms well to the west that still have the potential to bring more widespread showers and storms, a few possibly strong to severe, after approximately 6-7 p.m., though it should be noted that the Storm Prediction Center recently decreased our chance of damaging winds from 30% to 15%.

From 11:48 a.m. … Sunday evening is a time for severe storm vigilance, as a strong weather system approaches.  However, less than optimal timing of the cold front’s arrival, combined with displaced location of the strongest winds aloft, should spare the D.C. region from a widespread, significant severe day.

In a nutshell, here is what we expect:

Timing: 5 p.m. – 11 p.m.
Region: Entire D.C. region
Storm Coverage: 50%-60% coverage
Impacts: Showers and thunderstorms, some with locally severe wind gusts; golfball-size hail; one or more isolated, weak tornadoes

Let’s discuss the details…

Technical Discussion

The large-scale (synoptic) pattern

Figure 1 shows the forecast surface chart for 8 p.m. this evening, during the window of most significant storm activity.   The weather-maker is a strong low pressure system crossing the Great Lakes, with an associated cold front.  Ahead of the front, we are in the system’s warm sector – comprised of warm, humid southerly flow.   But note the location of the cold front at this time – still over Ohio.   Accordingly, the most widespread, severe storms (red shaded region) remain well to our west, across the Appalachians.


Figure 1. Forecast surface analysis for 8 p.m. Sunday. (NOAA)

In Figure 2, we show the jet stream-level airflow forecast for 8 p.m..   The ribbon of pale blue and light green is the jet stream, kinked to the south by a shortwave trough crossing the Lakes.    The trough causes air to rise ahead of it, helping to destabilize the atmosphere.  The pocket of winds exceeding 90 knots (turquoise) is a jet streak moving through the trough.  Note that the jet noses to the southwest of our region, focusing the highest winds over the southern Ohio and Tennessee Valleys.


Figure 2. Forecast jet stream from the NAM model at 8 p.m. Sunday. (Unisys)

Figure 3 shows the variation of wind speed with altitude, called wind shear, at 8 p.m..   Even this morning, the Dulles balloon sounding reveals significant wind shear across our region (42 knots, quite strong for July).   However, the really significant shear – 50 to 60 knots – should remain to the south and west of the D.C. region.

Caption
Figure 3. Wind shear. (twisterdata.com)

This morning, a thick layer of clouds has streamed in from the west, where sub-severe convective storms have been ongoing, and enhanced by a weak “precursor” shortwave crossing through.    The lack of sun has limited the atmospheric destabilization, somewhat.    However, I expect at least partly sunny conditions through the afternoon, as the lead shortwave exits.    In terms of convective available potential energy (CAPE – a measure of buoyant energy feeding thunderstorms) – model guidance suggests 1500-2000 Joules/kilogram will materialize.  This is sufficient to energize strong to severe thunderstorms.

Today’s severe risk areas

How this all comes together is portrayed in Figures 4 and 5, the outlook maps from the NOAA Storm Prediction Center (SPC).


Figure 4. Tornado outlook for Sunday. (SPC)

Figure 5. Severe wind outlook for Sunday. (SPC)

The general severe risk (not shown) remains in the Slight category for the immediate D.C. region, but a small zone of Moderate Risk encompasses Appalachia.    This is the zone most proximal to the cold front, coincident with time of maximum heating (destabilization), and very large wind shear (50-60 knots).    This region could very well experience a “mini-severe weather outbreak” today – including one or more strong tornadoes (10% probability) and widespread wind damage from supercells and bowing line segments.

The tornado and wind damage threat is significantly lower across our region, but still worth a heightened state of vigilance.   The combination of potentially moderate instability and strong shear is enough to generate supercells and bowing line segments – albeit less organized and less widespread than over Appalachia.  Accordingly, we are at a 30% chance for damaging wind and only 5% for tornadoes.   Our large hail probability is 15%.

We need to monitor the pros at SPC throughout the day, for any adjustments to the risk categories – both in terms of location and in magnitude.

What do the models show?

I typically examine three of the mesoscale (high resolution) models, including the WRF-ARW, HRRR and 4-km NAM.   Snapshots of simulated (not actual) radar coverage are shown below, in Figures 6, 7 and 8, respectively, all valid for 7-8 p.m. this evening.


Figure 6. Forecast radar from the high resolution NAM model. (stormvistawxmodels.com)

Figure 7. Forecast radar from the HRRR model. (NOAA)

Figure 8. Forecast radar from the WRF-ARW model. (NOAA)

There is some lack of agreement among the models regarding coverage and intensity of storms.  The HRRR does not appear to have the best handle on the situation, as it portrays no convection at all across Maryland and NOVA during the prime time.

The WRF-ARW and NAM, I feel, offer more legitimate solutions, given the expected destabilization and strong wind shear.  Both of these models generate a short squall line sweeping through the region.   This line or band may contain bowing segments, generating localized strong to severe winds, and an isolated, weak tornado.   One or more supercell storms may initiate within or ahead of this line, with associated hail and an isolated tornado.

I feel the threat may continue into the evening, then a continued chance of widespread showers and sub-severe thunderstorms overnight into the early morning.   The front should clear the region by Noon Monday, shifting Monday afternoon’s severe thunderstorm potential to the Tidewater region.

Dan Stillman is a meteorologist and editor for the Capital Weather Gang. He earned an M.S. in Meteorology from Texas A&M University, and a B.S. in Atmospheric, Oceanic and Space Sciences from the University of Michigan.
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