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Cloudy Forecast, Confused Models

Dan Stillman @ 11:20 PM

A persistent flow off the Atlantic Ocean will continue to sock the D.C. area in with overcast skies and scattered light showers for at least today and tomorrow. Expect high temps in the low-to-mid 60s and lows in the mid-to-upper 50s.

There's a good chance the cloudy and showery conditions will stay with us through Friday. We finally start to clear out over the weekend, with the sun returning and temps on the rise.

Model Misbehavior

It doesn't take a meteorologist to understand the difficulty of this forecasting challenge: Less than 12 hours before precipitation was expected to spread over the Washington area on Monday night, the two primary models that forecasters use to predict weather in the United States had very different opinions on how much rain would fall and where.

This lack of consensus is illustrated in the above image. Both maps show predicted precipitation amounts for a 24-hour period beginning at 8 p.m. Monday and ending at 8 p.m. Tuesday. The left graphic is the forecast from the Global Forecast Model, known as the GFS, while the right graphic is the forecast from the North American Model, known as NAM. The GFS scenario gave D.C. nearly 0.5 inches of rain, with higher amounts just to the west. But according to NAM, only about 0.1 inches were expected to fall.

These kinds of disagreements are typically more of a concern during the winter, when accurate snowfall forecasts are of the utmost importance. But following the record-setting rains of this past weekend, there was worry that another shot of significant rainfall could cause additional flooding.

The National Weather Service erred on the side of caution, issuing a flood watch Monday afternoon and calling for the possibility of 1-2 inches of rain (an earlier GFS run had suggested even more precipitation than that indicated by the graphic above). It soon became clear, however, that heavy precipitation was not in the offing for the immediate metro area. NWS dropped the flood watch early Tuesday morning, and in the end it was NAM that was most on the mark, as Reagan National came in with a 24-hour total of only .07 inches of rain.

For all you weather wonks out there, this isn't the first time GFS has busted badly compared to NAM.

Images courtesy the National Centers for Environmental Prediction.

A Scathing Review

The Miami Herald is in the midst of publishing an in-depth series of articles chronicling mishaps and mistakes at the National Weather Service and National Hurricane Center during the past few decades. This article from today's edition includes links to other articles in the series and an "about the series" summary.

I haven't had a chance to read through all the stories yet, but on first glance it is clear that the Herald left no stone unturned. It'll be interesting to see what kind of fall out, if any, is triggered by these stories.

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