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Posted at 2:30 PM ET, 10/ 1/2010

Rain review: Took some time to get it right

By Jason Samenow

* Cool weekend: Full Forecast | Record rains for DC, Baltimore *

Nailed the first half. Took several tries on the second half.

That basically sums up our forecast performance for Thursday's record rainfall.

We correctly warned ahead of time of the likelihood for heavy rain and the potential for flooding. And, for the first wave, we got the timing of heaviest rain right (late Wed. night into midday Thurs.) and essentially the amount. But we did not initially expect the rainfall double whammy in the metro region.

Our initial thinking on Wednesday was that the rain would come in two pieces - but timed close together and over different areas. We envisioned one inland area of rain (fed by tropical moisture streaming out ahead of tropical storm Nicole), closely followed by one along the coast (fed by the remnants of Nicole). Thus, our rainfall map issued Wednesday showed two rainfall maximums: one west of D.C. and one east of D.C.

In fact, there were two areas of rain, but they were 6-12 hours apart, and, rather than tracking to the east and west, both came right up the gut of the metro region. The outcome was a double dose of heavy rainfall.

Although we initially whiffed Wednesday on the second half of the forecast, we correctly identified the potential for a second wave of rain by early Thursday morning. As CWG meteorologist David Streit wrote: "There is a decent chance (50-60%) that a second surge of tropical rains comes through overnight with the best chance east of the Bay (70% chance). . . . The big question is whether we get more than 2.76 inches before midnight and set a 24 hour rain record at Reagan National."

The primary reason it took until early Thursday to correct the second half of the forecast related to the uncertainty around tropical storm Nicole. Because the location of the center of Nicole (which died within about six hours of being named) was so elusive and poorly defined, the track of Nicole's remnants and the timing of their arrival were extremely difficult to pin down.

This was clearly a complicated forecast -- with the challenge of trying to predict how a rapidly evolving and complicated tropical system would interact with the more temperate weather patterns of the mid-Atlantic. It took some iterating but ultimately, we think we provided helpful information - even if we didn't provide as much lead time as we would've hoped.

By Jason Samenow  | October 1, 2010; 2:30 PM ET
Categories:  Weather Checker  
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Next: PM Update: Quiet and cool Friday night

Comments

Please credit Nicole's remnants for that second rain surge last evening.

Continued "tropical" humidity yesterday afternoon indicated that more rain was likely due overnight.

Posted by: Bombo47jea | October 1, 2010 2:47 PM | Report abuse

Jason and CWG, I just want say that doing these quick response post mortems of the forecast is fantastic. Keep up the good work.

Posted by: imback | October 1, 2010 5:10 PM | Report abuse

@imback

Thanks.

Posted by: Jason-CapitalWeatherGang | October 1, 2010 5:22 PM | Report abuse

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