MICHELE NORRIS, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

MELISSA BLOCK, host:

And I'm Melissa Block.

Another heavy snowfall is predicted for the mid-Atlantic region tonight and tomorrow. The storm moving in from the Midwest, it could bring 10 to 20 inches of snow from Virginia up to New York. Operative word there: could.

We wondered what goes into a snow forecast, and we've called Doug Hill to find out. He's chief meteorologist at ABC 7 News, Washington's ABC affiliate.

Welcome to the program.

Mr. DOUG HILL (Chief Meteorologist, ABC 7 News, Washington): Thank you very much. Very happy to be here.

BLOCK: And, Doug, how do you make these calculations of projected snowfall? What goes into them?

Mr. HILL: It's a long process but at the very beginning, at the very foundation of what we do, we take a look at computer models. You've heard meteorologists talk about that, I'm sure, on radio and TV many times. These are mathematical equations derived from observations, and actual raw weather data is put into the model - to the beginning of each computer model run. And the programs are able to quite accurately predict what is going to happen at different levels and layers in the atmosphere over two, three, four, five, six, seven days or longer.

BLOCK: So when you, as a meteorologist, are taking that information that the computer spits out and you're applying your human intelligence to that, what else are you bringing to bear?

Mr. HILL: Well, actually I'd like to explain it sometimes as weather forecasting is an art form that is based on science. In the real chaotic world we live in, our atmosphere, the way it is and the way patterns change, you have to have that ability that is beyond pure science. You have to understand the area in which you're forecasting, understand how storms normally affect your area and compare one to the other to give you that intuitive advantage that you add on top of the scientific knowledge and data. And that's how you go about making the forecast.

BLOCK: You know, I'm thinking back to this snowstorm this past weekend, the really big one, and meteorologists were predicting three inches of snowfall each hour at the height. How do you figure that out?

Mr. HILL: Well, by the rate of fall, you can tell by intensity on Doppler radar. Once again, thanks to the wonders of computers, we have algorithms that can detect for us the rate of fall of the precipitation based on its intensity - how fast and how strong does the radar return - come back to the radar transmitter so we can do a quick calculation and convert that to inches.

BLOCK: And how much of this is affected by the amount of moisture in the snow, in terms of how much snowfall ultimately piles up?

Mr. HILL: That is really important, and that's called the snow ratio. If, just an example, you take an inch of liquid - can't call it rain 'cause we're not turning rain to snow - but just say equivalent of an inch of liquid of water. At normal ratios, that would produce 10 to 12 inches of snow. But as the air is colder and the dendritic formation - the actual snowflakes as they begin to form - if it forms in a much colder, dryer environment aloft, what can happen is that ratio can climb to 20 to one, 30 to one, maybe sometimes 40 to one under ideal circumstances.

So given the same amount of liquid, what we have to do is forecast what that ratio will be. If we have the ratio all wrong, then we're going to be completely wrong on the forecasted snow amount.

BLOCK: The wetter snow would be denser and more compact?

Mr. HILL: Right, more water in it. And the dryer snow would be fluffier and would pile up more. Here's a good example, the big weekend storm. That was 30 inches of wet snow with high water content in the snowflakes. When that accumulated, it started compacting. And because of the amount of water per snowflake, the weight of that 30 inches of snow was much heavier than 30 inches of light, fluffy dry snow.

BLOCK: Yeah. And having been out there shoveling it, I know.

(Soundbite of laughter)

Mr. HILL: Right.

BLOCK: I can tell how heavy that was. Do you think, Doug, that this has all gotten more accurate over time? I mean, the snowstorm this past weekend, it just seemed like it started exactly when people said it would start. It ended just about exactly when people said it would end. And the accumulation was just about spot-on.

Mr. HILL: All of us who make our living forecasting weather are very happy with the couple of the computer models these days. And I think any meteorologist that's being honest with you will tell you that it's the excellent performance of two particular computer models. There are quite a few we look at, but two in particular that have been excellent for the past couple of months.

BLOCK: So fist bumps around the weather room after that storm?

Mr. HILL: Absolutely. Or keyboard bumps, yes, same idea.

(Soundbite of laughter)

BLOCK: And what about this one?

Mr. HILL: Well, this time around, the computer models are not as much in agreement as they were on Saturday but pretty darn close to it.

BLOCK: Doug Hill, thanks so much.

Mr. HILL: My pleasure.

BLOCK: Doug Hill is chief meteorologist and vice president of weather at ABC 7 News here in Washington, D.C.

Well, one day, will you be president of weather, Doug?

Mr. HILL: You know, I'm happy with what I have right now. I don't want the extra headaches.

(Soundbite of laughter)

BLOCK: Okay. Transcript provided by NPR, Copyright NPR.

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