Local forecasters get a new crystal ball

Correction: An earlier version of this article quoted WJLA meteorologist Bob Ryan but omitted his first name and identifying information. This version has been corrected.

February 22, 2012

The Washington area doesn’t typically get a lot of heavy snowfalls; the usual winter torment here is a slippery combination of rain, sleet, freezing rain and snow.

So we’re lucky to be getting “dual-pol.”

That’s the dual-polarization weather radar upgrade taking place at the National Weather Service’s Baltimore-Washington Forecast Office in Sterling. Work began Friday and is expected to take a week.

Forecaster Matthew Kramar said the biggest benefit here should be “the ability to detect the intensity of precipitation” in winter storms and to distinguish exactly what type of precipitation is going to fall where. In the summer, meteorologists say, dual-pol will do a better job of measuring heavy rain, helping the Sterling office make better flash flood forecasts. NWS has been testing dual-pol since 2009 and began installing it in select offices last April.

Paul Schlatter, who’s in charge of training meteorologists to use the technology, said the snowstorm that hit Seattle on Jan. 19 demonstrated dual-pol’s value. Forecasters could locate the transitions between snow, sleet and freezing rain along with how the winds were pushing the storm, allowing them to predict when changes would arrive in different parts of the Seattle area. The knowledge enabled road crews to treat affected areas before the freezing-rain arrived. “This will be perfect for the D.C. area,” said Schlatter.

How is dual-pol different from older technology?

Doppler radars, including the one operating at Sterling since 1992, send out microwave pulses, some of which reflect back from things they hit: raindrops, snowflakes, hailstones and even insects or birds. The amount of energy coming back to the radar dish shows how much rain, hail or snow is in the air and where it is but not whether it’s frozen.

It also doesn’t indicate the shapes of objects in the air — and that’s important. Raindrops smaller than approximately .08 of an inch are round, as are most hailstones. As a falling raindrop grows in size, air pressure on the bottom pushes it into a hamburger-bun shape, but hail remains round.

Microwave pulses from older weather radars are horizontally polarized — think of a wavy line parallel to the ground. This polarization does a good job of detecting big, hamburger-shaped raindrops that could cause flooding. But it can’t distinguish between these and round hailstones.

Adding a vertical polarization (that’s the “dual” part of dual-pol) enables radars to detect vertical dimensions of objects. That means the radars can give forecasters information about the shapes of objects in the air.

Dual-pol radar also can show at what altitude precipitation is melting or freezing in the air — another help in predicting what form it will take on the ground.

The NWS expects to have all 160 of its Doppler radars upgraded to dual-pol by May 2013 at a cost of $50 million. NOAA, the NWS’s parent agency, estimates economic benefits at $690 million annually. Says WJLA meteorologist Bob Ryan: “It’s certainly a cost-efficient upgrade.”

— Jack Williams, Capital Weather Gang

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