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Posted at 11:00 AM ET, 03/ 4/2008

In Focus: This Evening's Storm Threat

By Jason Samenow

The storm passing through tonight means business. A Flood Watch is in effect for the risk of torrential rains and the Storm Prediction Center in Norman, Okla., indicates there is a slight risk of severe thunderstorms.

precip-mar4-5_08.gif
Simulated rainfall from tonight's showers and thunderstorms from the Global Forecasting System (GFS) model between 7 p.m. and 1 a.m. The light blue shades represent 1-1.5" of rain, the dark blue between 0.5-1", and the dark green shade indicates 0.25-0.5". Credit: NOAA

So what are the possible impacts, from most likely to least likely?

  • Bank on it (90%): Moderate showers, at least 0.25".
  • Pretty safe bet (70%): Heavy rain, at least 0.50"
  • Decent chance (50%): Very heavy rain exceeding 1" as well as thunder and gusty winds
  • A possibility (33%): Damaging winds, exceeding 50 mph
  • Can't rule it out somewhere (3%): A tornado

When will the rain mostly occur?

  • West of town: starting between 5 and 8 pm, ending between 8 p.m. and 11 p.m.
  • Inside the beltway: starting between 6 p.m. and 9 p.m., ending between 9 p.m. and midnight
  • East of town: starting between 7 p.m. and 10 p.m., ending between 10 p.m. and 1 a.m.

Why is it happening?

llj.gif
Simulated wind speeds about one mile above the surface (850mb) at 1 a.m. tonight. The ribbon of light red in the middle of the image (from south to north) is the low level jet, where winds are from the south at almost 80 knots or 90 mph. Credit: College of DuPage Next Generation Weather Lab

Low pressure developed in the deep South yesterday and is intensifying as it moves northeastward up the spine of the Appalachian mountains. As the low is passing just to our west, we 're in the warm sector of the storm characterized by moist southerly flow from the counterclockwise circulation around the low. This southerly flow is enhanced by an area of high pressure to our east whose clockwise flow is steering our wind in the same southerly direction. The funneling of the wind from the low to the west and the high to the east is forming what's known as a low level jet (pictured), an area of strong winds about a mile up that will transport tremendous amounts of moisture into the region.

When the strong cold front trailing the low pressure clashes with the warm, moisture-laden air, it may trigger a heavy line of showers and thunderstorms as it passes through the region this evening. Some thunderstorms may become intense enough to transfer some of the strong winds from the low level jet a mile up, projected to be near 90 mph, down to the ground.

We'll be closely tracking the development of showers and thunderstorms through the evening and will post any watches and warnings for severe weather.

By Jason Samenow  | March 4, 2008; 11:00 AM ET
Categories:  Thunderstorms  
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