In Focus: The Spin on Yesterday's Bad Forecast
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.
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.
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