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Posted at 10:45 AM ET, 10/28/2008

In Focus: The Spin on Yesterday's Bad Forecast

By Jason Samenow

Yesterday's forecast seemed so easy: a cold front would generate some clouds in the morning, move away and leave us with a partly sunny and windy afternoon with temperatures rising into the 50s. We all know that didn't happen. Instead, light rain developed and temperatures held in the 40s.

So what did I miss? I neglected the power of vorticity. If vorticity sounds like a geeky term, it is, but it's simply a measure of how fast the air is spinning. Vorticity is associated with rising air. Generally speaking, the more vorticity there is, the faster air rises. And when air rises, it can then cool, condense and form clouds and rain. Yesterday, there was a lot of vorticity, and I didn't pay enough attention.

Keep reading for more on vorticity and yesterday's forecast bust. For the weather through the weekend, see our full forecast.

Vorticity forecast yesterday evening simulated by the GFS model. Courtesy NOAA.

The image to the right is a model forecast of vorticity valid yesterday afternoon and evening. It shows a large area of vorticity -- shaded in yellow and orange -- from the Carolinas through Pennsylvania. All of the models simulated this feature, but predicted very little or no rain in conjunction with it. As it turns out, a solid line of light to moderate rain developed in association with this area of vorticity. In hindsight, even with the models forecasting hardly any rain, a better forecast would've been to assign at least a low chance of showers given the strong vorticity signature.

Missing the rain caused me to miss on temperatures. The rain knocked temperatures down locally from 50 at noon to 41 degrees at 3 p.m. when my forecast called for partial sunshine and mid-to-upper 50s. Ouch. At least, I was not alone in missing this forecast. The National Weather Service and media outlets forecast temperatures well into the 50s with no afternoon rain likewise.

Interestingly, in the District and points east, the rain lasted well into the evening and overnight -- well beyond the predictions of human forecasters or models. This had to do with the aforementioned vorticity feeding the development of an upper level low pressure area which was effective at tapping moisture from the south, prolonging the rain. The models often underestimate precipitation with these upper level lows, and this has led to some snowy surprises in the past during winter.

Another area of vorticity will approach the area late Tuesday night. So don't be surprised if we squeeze out some rain drops or snow flakes as mentioned in Matt's forecast. But a repeat of yesterday is not expected as there will be less moisture to work with and the strongest vorticity may pass to our north.

By Jason Samenow  | October 28, 2008; 10:45 AM ET
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Next: I Heard It Was Out There: Did You See It?


thanks for the explanation. I got caught in the rain yesterday on my scooter! Would have taken the metro if I knew rain was in the forecast.

Posted by: skoyles | October 28, 2008 10:50 AM | Report abuse

You need not be any more perfect than I am in my job. Besides, some of the "spice of life" is the unexpected things. So yesterday, during the rain, I spent some extra time playing with the munchkins instead of raking leaves. Hard to say I'm the worse for it.

Next time you're wrong I hope it's under-forecasting snow accumulation :-)

Posted by: ASColletti | October 28, 2008 11:58 AM | Report abuse

Neat. I am coming to see this blog as one of the "hidden gems" of the (online?) Post. For a long time I never clicked on any of the teaser links above the weather forecast for it, but am fast becoming a regular reader. Learning some interesting things. Thanks!

Posted by: B2O2 | October 28, 2008 12:17 PM | Report abuse

Its all good... forecasting seems like an art sometimes rather than a science. There is something in that saying, "You can't predict the weather." You guys do a great job with giving us the forecast.

Posted by: crucialruddy | October 28, 2008 12:29 PM | Report abuse

You guys do such a good job here at Capital Weather that I was convinced that I must have misread your forecast. I was sure that if it was raining, it must have said that somewhere on the blog. I actually went back to find the section I missed.

Posted by: Snowlover2 | October 28, 2008 1:46 PM | Report abuse

Thanks. Lets hope that pollsters take into account vorticity in their current forecasts. I'm hoping for an obama win.

Posted by: jojo2008 | October 28, 2008 1:55 PM | Report abuse

@B2O2 (and others)

Hopefully you will become a regular. We strive to provide not only the "forecast and the temps," but also a discussion on what is happening and why. We know you can get the temps anywhere on the web, however we hope that it's the why, how, and what (and accuracy and specifics ;) ) that keep you coming here instead.

As always, if anyone has any questions on how or why we do or forecast what we do, shoot us an email from the link at the top of the page. Thanks again for reading!

Posted by: JJones-CapitalWeatherGang | October 28, 2008 2:16 PM | Report abuse

I gotta say I truly appreciate the explanation of what happened yesterday as I made a comment last night about it and I didn't mean it to sound rude but I was more looking for an explanation of why things occurred. It's definitely a rarity that ANY weather site/newstation/channel actually apologizes for the wrong forecast and explains why it was wrong. Cheers to you all for that!

Posted by: paul-Sterling | October 28, 2008 4:06 PM | Report abuse

This is why I like coming to CWG. While other forecasters pretend they never missed a forecast or were led astray by the models, you acknowledge it and explain what happened. I suspect going through that process makes you better forecasters.

Posted by: SouthsideFFX | October 28, 2008 4:15 PM | Report abuse

Apparantly there were some very cold temperatures aloft with that system yesterday. The rain forecast was not only unforseen, but where I was, we also got a significant amount of sleet and ice pellets as well....the earliest I have seen this in the D.C. area since 1979.

Posted by: MMCarhelp | October 28, 2008 7:57 PM | Report abuse

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