The rain that wasn't
It doesn't happen that often, but - once in a great while - we miss a forecast. Badly.
CWG's predicted rain totals of 1-3", with isolated higher amounts didn't come close to verifying in many locations. The National Weather Service's forecast of 2-4" with isolated amounts over 5" fared even worse.
Here are approximate totals from the three local airports (since Monday):
Reagan National: 0.21"
In fairness to ourselves and other forecasters, some locations did receive upwards of 2" of rain in eastern Montgomery county and into western Howard county. Also, parts of the Eastern shore and far western suburbs saw totals over 1" as reported by the National Weather Service. But for most of the metro area, totals were a very underwhelming 0.25-0.5"! The "D.C. split" - whether you think it's real or imagined - was in full effect.
Frankly, from the surprising Sunday morning showers to the hours of "zippo" when heavy rain was forecast Monday afternoon and night, the entire "event" was forecast less than satisfactorily. Had this been a snow forecast, no doubt many folks would've been up in arms. Fortunately, D.C. weather consumers are more forgiving for missed rain forecasts - especially if it rains less rather than more.
As one of our readers, Walter-in-Falls-Church - a snow fanatic - commented:
wow...funny how little (i.e., not at all) i'm disturbed by an "underperforming" storm this time of year.
december-february...now THAT's a different story. let's avoid these underperforming, dry-slotting, dc-splitting events this winter, shall we?
So what happened this time?
In short, the models we rely on for forecasting precipitation totals had a very poor handle on the complicated upper level low pressure system impacting the region. I mentioned this yesterday in my post discussing rainfall potential, quoting NOAA's Hydrometeorological Prediction Center: "Putting it simply..model [precipitation forecasts] have not been doing very well handling the rainfall with this system to this point in time." Nonetheless, I felt that the deep plume of moisture extending into the Gulf of Mexico would manifest itself in bands of heavy rain moving over the area, energized by the the upper level low pressure to the west.
But the upper level low's location west of the region was precisely the reason rainfall was so unpredictable and so variable. Whenever our region is to the east of a storm, that means we're on the "warm" side of the storm, reliant on more showery, narrow rain bands rather than large areas of steady rain. In these scenarios, precipitation totals can be wide-ranging due to the hit-or-miss nature of small-scale rain features. And that's precisely what we got or didn't get yesterday.
If there's a lesson CWG can learn from this kind of event, we need to manage expectations by highlighting the uncertainty in the timing, distribution and amount of rain that will fall in this weather set up. It's among the most difficult to forecast.
| September 28, 2010; 10:45 AM ET
Categories: Floods, Weather Checker
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