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Forecasting Made Easy, for Kids and Grownups Alike

Dan Stillman @ 11:10 PM

The easy part of the forecast comes first. Simply put, today and tomorrow will be delightful. The hard part comes for Friday and the weekend, as retreating high pressure allows rain chances back into the forecast ...

The Easy Part

Today: Sunny. High near 75. Tonight: Clear and cool. Lows in the upper 40s downtown, low-to-mid 40s in the burbs. Tomorrow: Sunny. Highs in the upper 70s to near 80.

The Hard Part

Friday: We'll have to watch the progress of a storm system approaching from the southwest. As of now, I think we'll be able to escape at least a good part of the day with partly cloudy skies and a high in the mid-to-upper 70s. Increasing clouds later in the day with rain moving in during the evening. The exact timing of the precipitation is still in question, and thus so is its impact on outdoor evening activities.

Friday Night: Rainy with a low in the mid-to-upper 50s. Saturday: Steadier rain early in the day should diminish into scattered showers by afternoon. The clouds and rain will keep temps from climbing much past the mid 60s. Saturday Night and Sunday: Looks like we'll continue to see showers in the area (I wouldn't call it a washout, though) with a Saturday night low around 50 and a Sunday high in the mid-to-upper 60s.

Kids vs. Sue Palka

As if meteorologists didn't have enough competition from amongst themselves, now kids have a chance to get in on the action. Today's KidsPost section in the Washington Post features a weather forecasting contest for children in grades 3 through 8 (which happens to be organized by me).

Kids are challenged to predict by 5 p.m. tomorrow the high temperatures in Washington for this coming Friday, Saturday and Sunday. The most accurate forecasters could get to appear on air with Channel 5 meteorologist Sue Palka or receive a school visit from her.

Teachers, parents and kids can check out all the details in the hard copy of today's KidsPost, which includes tips for how kids can go about making their temperature predictions.

Pictured: Sue Palka, courtesy WTTG.

Forecasting Lesson for Grownups

Jealous that the kids get to forecast the weather and you don't? Well, I can't offer you a contest, but today does provide a perfect opportunity to share one of the ways meteorologists go about predicting the high temperature. The technique involves looking at one of the many forecast maps produced by "the models" that we so often talk about.

The models predict what conditions will be like at ground level and at various pressure levels in the atmosphere -- pressure decreases with height. While the models do forecast ground-level temperatures, these forecasts are often not as accurate as those for higher up in the atmosphere. One reason for this is that the models have a hard time simulating the impact of friction from the Earth's surface on the flow of air.

At the 850-millibar pressure level, which corresponds to about 5,000 feet (1.5 kilometers) up, the effects of friction are much weaker. Thus, model predictions at this level are more reliable. In particular, a rule of thumb known as the "850-rule" says that on a clear day with light winds, the high temperature at the surface will be equal to the 850-millibar temperature plus 15 degrees Celsius. The addition of 15 degrees accounts for the typical increase in temperature -- approximately 10 degrees Celsius per kilometer -- as you move down in the atmosphere.

Let's try applying the rule to today's forecast. At right is the 850-millibar forecast of temperature (colored contours) in degrees Celsius for this morning at 12Z, which is 8 a.m. for those who aren't familiar with Z-time. The temperature at this height doesn't experience much in the way of diurnal change, so there's no problem with using a morning forecast map even though we're trying to predict the afternoon high.

Using the legend below the map, the temperature for D.C. looks to be about 7C. Add 15C to that and you get 22C, which is equal to about 72F. As it turns out, the high temperature in the Washington area this time of year often works out to be a few degrees Fahrenheit higher than the value given by the 850-rule. With that in mind, I wouldn't be surprised if we top out around 75F today.

I should highlight once again that the 850-rule works best with clear skies and light winds. In fact, it can be a very poor indicator of high temperature when the weather is cloudy and unsettled. It is also just one of many ways that meteorologists go about predicting temperature. Each forecaster has his or her own favorite tricks, which may or may not include the 850-rule. This is what makes weather forecasting as much an art as it is a science. has more for those who want to delve further into the technical side of forecasting temperature and plenty of other variables.

Forecast map courtesy Unisys.

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