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If it's going to be windy during the forecast day, you have to worry about the possibility that the air mass is changing. To put it simply, you must figure out where tomorrow's air is today.
Here's a simple example: suppose you're forecasting for Rapid City. The hypothetical weather in Rapid City was clear, with a high of 64. The wind was out of the south at 15 knots. Meanwhile, the weather in Goodland, KS was clear with a high of 77. Suppose you know that the weather pattern is supposed to be identical tomorrow: same skies, same winds, etc. What is your temperature forecast for Rapid City? MAP
It'd better not be 64. After a day of 15 kt S winds, the air in Goodland will easily make it to Rapid City. A better forecast would be something like 77. Even though the winds and clouds won't change, the temperature on the second day will be warmer than the first because of the warm advection.
FORECASTING TECHNIQUE: Air Transport
Within an air mass, while temperature advection is rarely constant, the wind does tend to blow fairly steadily for one or two days at a time. If you are expecting such a situation, with nearly steady winds for twelve or twenty-four hours, the following method is perhaps the best way of forecasting temperature:
- Estimate how far the air will travel in 24 hours. The distance is 4 degrees of latitude for every 10 kt of wind speed. Use the peak wind speed at the surface or an average of surface and 850 mb wind speeds. Go that distance upstream from the forecast city. That is your "source region".
- To forecast the min, use the 12Z temperatures in the source region as your reference point. To forecast the max, use the afternoon temperatures in the source region as your reference point.
- Take into account differences in conditions (cloudy vs. clear, etc.) to determine your final forecast.
This method is usually accurate to within about five degrees. Numerical forecasts are usually better than that, but if the forecast you get from this method deviates significantly from the numerical forecast, you should doubt the numerical forecast.
The Kansas Rule
"Help, my weather map doesn't have a distance scale!". Yes it does: use the Kansas rule. Kansas is 3 degrees (or 180 nautical miles) from its northern border to its southern border. You can use the state of Kansas as a distance scale for estimating distances anywhere in the continental United States. If you dislike Kansas for some reason, you can use Nebraska, South Dakota, or North Dakota: they're all 3 degrees tall. Or try Colorado, Wyoming, or Montana: they're 4 degrees.
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