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Posted at 11:30 AM ET, 03/ 7/2008

In Focus: Timeline and More for Two-Day Storm

By Dan Stillman

*Flood Watch in effect through this evening*

For the second time this week a powerful March storm is set to barrel through the D.C. metro area. Like the Tuesday night storm, this one is expected to produce copious amounts of rain, with one inch or more a possibility. But this time the rain will come in two rounds over the course of a little more than 24 hours, rather than the single shot of precipitation we received earlier in the week in closer to nine hours.

Flooding is less of a concern with this storm since the rain will be spread out over a longer time period. Yet the threat of a period of heavy rain later this afternoon and early evening has prompted the National Weather Service to issue the Flood Watch linked to above. The main threat will be thunderstorms (slight chance today, better chance tomorrow morning) and strong winds (tomorrow afternoon and night).

Here's our best estimate on the storm timeline...

Round 1

  • 11 a.m. to 2 p.m. Fri: Rain spreads over area from southwest to northeast. Mid to upper 40s.
  • 2 p.m. to 8 p.m. Fri: Rain, heavy at times. Slight chance of thunderstorms. Mid 40s to near 50.
  • 8 p.m. to 11 p.m. Fri: Rain tapers from southwest to northeast. Mid to upper 40s.
  • 11 p.m. Fri to 5 a.m. Sat: Lull in precipitation. Misty, foggy with some lighter showers. Mid to upper 40s.

Keep reading for the Round 2 timeline and more storm details. See Camden's post for the full forecast through the weekend and beyond.

Round 2

  • 5 a.m. to 8 a.m. Sat: Rain redevelops from southwest to northeast. Temperatures steady or rising.
  • 8 a.m. to Noon Sat: Rain, heavy at times. Maybe thunderstorms, some with isolated damaging winds. Temperatures spike into the mid 50s, possibly upper 50s to near 60 south and east.
  • Noon to 3 p.m.: Rain tapers from southwest to northeast. Increasingly breezy. Temperatures steady or falling through the 50s.
  • 3 p.m. to 7 p.m.: Winds sustained at 15-20 mph, gusting to about 25 mph. Chance of lingering rain showers, or snow showers mainly north and west. Temperatures dropping through the 40s.
  • 7 p.m. to 3 a.m.: Winds sustained at 20-25 mph, gusting to about 35 mph. Temperatures dropping into and through the 30s.

Will there be any snow?

Most of the precipitation will have probably departed the area to the northeast by the time air cold enough to make snow in the upper levels of the atmosphere arrives from the west. That said, some non-accumulating snowflakes aren't out of the question later in the afternoon or early evening, with the best chance being well north and west of the metro area.

Model prediction of temperatures at the 850 millibar pressure level (about 5,000 feet up in the atmosphere) show a 12-degree Celsius (22 degrees Fahrenheit) drop from 1 p.m. Saturday to 7 p.m. Saturday. Colder air often takes longer to filter in at the surface than higher up in the atmosphere, so here on the ground temperatures probably won't fall quite as fast. Credit: College of Dupage

When is the greatest chance for thunderstorms, and could they be severe?

While there's a slight chance of thunderstorms this afternoon and evening, the action really picks up as the cold front associated with the storm approaches and passes through tomorrow. The morning hours of Saturday hold the greatest potential for thunderstorms (especially south and east of town) that could produce heavy downpours and isolated high wind gusts, fueled by a surge of warm air from the south just ahead of the approaching front. Then, as and after the front passes through, the afternoon and evening are likely to bring a gradual end to the precipitation, but with the likelihood of gusty winds and falling temperatures.

The concern for severe storms is not as high as with the last storm because the atmosphere is expected to be more stable. Remember, before the storms hit on Tuesday night, daytime highs had reached the low 70s, the warmth providing enough energy to fuel severe weather that night. Still, there is the potential for some thunderstorms to produce isolated damaging winds.

Model forecast of rainfall between 7 a.m. Saturday and 1 p.m. Saturday, as well as surface pressure (black lines) and temperature (blue and orange lines) at 850 millibars (about 5,000 feet up in the atmosphere) at 1 p.m. Saturday. Note the predicted position of the center of low pressure (marked by the "L") almost directly over the metro area. Credit: NOAA

Why such strong winds late tomorrow afternoon and night?

The explosive development of low pressure at the center of the storm will create a strong pressure gradient -- that is, a sharp change in pressure across a relatively short distance -- as indicated by the tightly packed black lines in the adjacent model forecast for 1 p.m. Saturday. Each line is called an isobar, or a line of constant pressure, and represents a four-millibar increase as you move outward from the center of the low. The sharper the pressure gradient, the stronger the wind.

By Dan Stillman  | March 7, 2008; 11:30 AM ET
Categories:  High Winds  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Rainy and Stormy Through Saturday
Next: CommuteCast: Rain to Cause Slow Travel


Well we do need rain.

Posted by: Period | March 7, 2008 12:11 PM | Report abuse

St. Charles, VA under a winter storm watch? Huh?

Posted by: Period | March 7, 2008 12:16 PM | Report abuse

Period...just did a NWS search and that's all the way down in the bottom tip of VA near KY and TN. The watch is mainly geared towards the higher elevations though.

Just starting to rain here in Lake Ridge...

Posted by: weatherdudeVA (Lake Ridge) | March 7, 2008 12:31 PM | Report abuse

Whoops. I was looking at St. Charles, MD, but I typed in St. Charles, VA instead.

Posted by: Period | March 7, 2008 12:54 PM | Report abuse

Very informative, nice job CWG.

Posted by: jeffc | March 7, 2008 12:55 PM | Report abuse

Rain began in Arl 2 minutes ago

Posted by: Period | March 7, 2008 1:10 PM | Report abuse

It is currently snowing in Shreveport LA but we get a rain event. How is this possible? We are snow cursed and I blame VaTechBob...LOL

Posted by: HEELS | March 7, 2008 1:12 PM | Report abuse

Will MoCo have school on Monday?? Just kidding.

Posted by: NeedaLaugh | March 7, 2008 1:28 PM | Report abuse

Here's a question that may look completely off-topic, but it does have some bearing on the operation of the site:
Do any of you use Comcast broadband west of I-270 and north of the Beltway in MoCo in an area where FIOS is not available?
If so, what is the reliability, in particular with respect to slowdowns and/or disconnects from congestion in late afternoon or evening?
If you'd rather not answer in public, you can use the "Ask the Gang" link at the tippy top of the page.

Posted by: Steve, Capital Weather Gang | March 7, 2008 2:01 PM | Report abuse

Sorry to JT for the outlash and thanks Capital weather team for answering my question.

Posted by: Big Goob | March 7, 2008 2:26 PM | Report abuse

Steve - I had similar slow/disconnect issues with COX cable in Fairfax. I was told that it was due to increased usage once kids were out of school. It was like clock work from about 3:00 - 7:00 PM Monday - Friday this would happen. Lucky for us FIOS became available.

Posted by: John - Burke | March 7, 2008 3:05 PM | Report abuse

Chances of air travel in and out of the time frame of 7am - 11am?

Posted by: bob | March 7, 2008 3:22 PM | Report abuse

After a grand total of 6 inches of snow this winter......what a proper requiem...39 and rain....

Posted by: Mike from the Blue Ridge | March 7, 2008 3:32 PM | Report abuse

bob -- considering the rain and potential for thunderstorms, I'd expect some delays at the airport, but not necessarily cancellations.

Posted by: Capital Weather Gang | March 7, 2008 3:49 PM | Report abuse

6"? I should be so lucky here in Manassas. We had about 2-3" accumulated on that morning in December and hardly a flurry since. It feels like I missed winter.

Posted by: missy | March 7, 2008 4:17 PM | Report abuse

We had 3" in December, 5" in January and 2 dustings in February. BOOOOOOOOOOO

But heck, I like gloomy weather - so I guess I'll just have to live with the fact that there is always next winter.

Bring on the thunderstorms!

Posted by: weatherdudeVA (Lake Ridge) | March 7, 2008 4:37 PM | Report abuse

The comments to this entry are closed.

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