Late day thunderstorms could be severe

>> Update, 2:24: p.m.: Follow this blog post for the latest updates: Tornado watch for entire area until 10 p.m. (LIVE UPDATES)

Update, 2:03 p.m.: A tornado watch has been issued for the entire region until 10 p.m. 

Update, 1:21 p.m.: The National Weather Service Storm Prediction Center says there is an 80 percent chance a severe thunderstorm or tornado watch will be issued for the region this afternoon.

(National Weather Service)
(National Weather Service)

From 11:34 p.m.: An energetic and very juicy weather system scooting through the region today could set off severe thunderstorms late this afternoon and evening (3-9 p.m. most likely time for any severe weather).  A few storms could produce damaging winds and perhaps an isolated tornado.


The high-res NAM model’s simulated radar suggests strong storms in the region this evening. (WeatherBell.com)

Torrential rains are likely to be the most widespread hazard.  Recall, a flash flood watch is in effect for the entire region. Downpours could quickly drop 1-2 inches of rain in localized areas.

storm threat

The National Weather Service’s Storm Prediction Center places the D.C. area in a slight risk zone for severe thunderstorms and assigns the following probabilities for the various storm hazards:

* Tornadoes: 5 percent chance within 25 miles of a point
* Damaging straight line winds: 15 percent chance within 25 miles of a point
* Large hail: Not expected


Summary graphic of today’s severe weather potential (National Weather Service)

Severe weather is certainly not a given, but if the sun manages to pop out this afternoon, that would increase the chances, by supplying the atmosphere with some of the needed fuel, or Convective Available Potential Energy (CAPE).

The trigger for the thunderstorms is an approaching surface cold front and upper level trough (low pressure center at high altitudes).


Graphic shows upper level trough, an area of low pressure at about 18,000 feet, approaching the region this evening. Yellow and orange shaded areas represent vorticity – or spin in the atmosphere, which can feed into developing storms downwind. (StormVistaWxModels.com)

A roaring push of warm, humid air from the south at an altitude of about 5,000 feet  – known as a low level jet – will feed into developing storms, supplying them with rich, tropical moisture.  Some of the strong winds from this jet could be thrust down to the surface in storm downdrafts.


Simulation of the low level jet from the NAM model, the area of strong winds (35 to 40 knots, or about 40-45 mph) – shaded in yellow – at around 5,000 feet.

As winds at even higher altitudes (18,000 feet) will be coming from more of a westerly direction, that may provide enough turning – or wind shear – for a tornado or two.  The so-called “significant tornado parameter” is greater than 1 over the D.C. area this afternoon, indicating instability and spin in the atmosphere may be sufficient for tornado development.


Significant tornado parameter this afternoon is over 1 in the D.C. area, suggesting the atmosphere hypothetically could support tornado activity. (National Weather Service)

If tornadic storms form, we think they would be most likely along and east/southeast of I-95 (but cannot rule them out farther west).

We will keep you posted on storm development and any watches and warnings.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Jason Samenow · June 10, 2013