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Posted at 10:45 AM ET, 09/28/2010

The rain that wasn't

By Jason Samenow

* Drying out today, but stormy Thursday?: Full Forecast | NatCast *

National Weather Service doppler radar estimated precipitation totals vs. a rainfall projection from NOAA's Hydrometeorological Prediction Center yesterday morning. The "X" on the dopper estimate indicates Washington, D.C.'s location. In many areas less than 0.3" fell compared to 2-3+" forecast.

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"
Dulles: 0.52"
BWI: 1.05"

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. 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.

By Jason Samenow  | September 28, 2010; 10:45 AM ET
Categories:  Floods, Weather Checker  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Showers to wind down, afternoon sun
Next: Tropical depression forms, eyes Florida, East Coast


To paraphrase Walter, if we're going to get our knickers in a knot, save it for the snow.

Having said that, the NWS, especially, has been off the mark a good bit lately with respect to high temps and now precip.

Posted by: JerryFloyd1 | September 28, 2010 11:05 AM | Report abuse

Considering the rain bust yesterday, how is Thursday looking? Sue Palka last night was saying that the Western suburbs would get the bulk of the rain on Thursday, but I'm also hearing that it could stay mostly east like yesterday's rain event. Who's right at this point?

Posted by: FH59312 | September 28, 2010 11:10 AM | Report abuse

At 4:45am the rain was coming down so hard in Gainesville/Haymarket, VA there was about an inch on the road. I guess it didn't last that long but it had to have dropped an inch or so while it was pouring.

Posted by: dancermommd1 | September 28, 2010 11:20 AM | Report abuse


Working on a post that will address this question. But it's premature to be saying who will get the most rain from the Thursday storm at this point.

Posted by: Jason-CapitalWeatherGang | September 28, 2010 11:22 AM | Report abuse

I'm very interested in Thursday's rain, as well, because I'll be in NYC that day. If it's to the east, NYC will get it.

Posted by: JerryFloyd1 | September 28, 2010 11:29 AM | Report abuse

I saw these various model for Thursday's rain event, aka "Tropical Depression 16":

Posted by: FH59312 | September 28, 2010 11:51 AM | Report abuse

Thanks, FH59312. Looks like the folks at NOAA are talking to the NHC, since their models most closely align.

Posted by: JerryFloyd1 | September 28, 2010 12:32 PM | Report abuse

Arcola, near Dulles, we got close to an inch since Monday by my home-made gauge, a bit more than the 0.52 IAD reported. Waken by heavy rainfall at 4-5AM. It doubles the total from previous 5 weeks! Not a total bust.

Posted by: LoudounGeek | September 28, 2010 12:33 PM | Report abuse

My analysis last weekend of the "Cyclone Phase Evaluation" web site indicated a passage WEST of us and up the St. Lawrence region...hence I was not too surprised.

Nor am I confident over this "3 to 6 inch deluge" being predicted for Thursday! How can such an event happen when the Weather Channel's "tropical update" features spaghetti plots showing "Nicole" tracking well offshore to the east on most model runs??? True, we will get SOME rain on Thursday--we should be in the cool sector, and could get a 1.5" soaker due to tropical moisture overrunning the front to our east. However, I suspect that the huge deluge currently being forecast will fall well to our east, perhaps offshore Ocean City UNLESS the cyclone prediction charts show activity well to the west of the spaghetti plots.

Most likely we'll see soaking rain rather than convective showers Thursday, and I'd be surprised if two inches fall...we could get considerably less.

Posted by: Bombo47jea | September 28, 2010 12:36 PM | Report abuse

I wasn't surpised at all by the low rain totals. Just by looking a radar trends from Sun. - Mon. it was obvious, at least 2 me, that most of the rain would b East.
As usual, Spotsy falls in low range of the prec., been the trend since I moved here in 84.
Right now I'll go with low rain totals 4 most of the area on Thurs., & dry weather thru. 1st 2 weeks of Oct.

Posted by: VaTechBob | September 28, 2010 2:19 PM | Report abuse

Yes, collectively we may get more upset about a missed snow forecast; individually I get more upset about the rain one. I needlessly wore a rain coat yesterday and today on a warm, no jacket needed, day. In the winter I'm dressed for the temperature so precipitation mis-casts don't matter nearly as much. Of course, I'll be equally cautious on Thursday and equally peeved if I'm waiting for the bus wearing jacket under sunny, 80 degree sky.

Posted by: busgirl1 | September 28, 2010 2:29 PM | Report abuse

oh mah gosh...i've been quoted by the gang!

and paraphrased by jerry! i'm famous.

so which season's storms are the hardest to call in terms of "accumulation"? must be winter with the darned rain/snow line...

Posted by: walter-in-fallschurch | September 28, 2010 3:22 PM | Report abuse

What a bust of a forecast!

Yesterday's pathetic precipitation totals were as fully disappointing as the Northeast Corridor's rain/snow line! Appreciative, but still annoyed, that we received only .41" in Michigan Park since Sunday.

Record-setting, dreadful heat, stressed vegetation, well-below normal rainfall, and mounting drought combined to make this one awful summer. I am so grateful it's over!

Let's hope this storm doesn't disappoint. RAIN, please!!!!!

Posted by: TominMichiganParkDC | September 28, 2010 4:32 PM | Report abuse

Stop relying on "models" and do some old-fashioned forecasting. I can recall as a kid during the 70's and as a young adult during the mid-late 1980's weather forecasts were right on the money. And they only seemed to rely on radar. Since the advent of "computer model" forecasts and TV weather "personalities" w/o any meterological expertise (especially during the 1990's), have the weather forecasts been horrible and totally off the mark. This latest rain soaking "non-event" is a prime example of it.

Posted by: rkayblock1 | September 28, 2010 6:51 PM | Report abuse

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