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Wednesday March 28, 2018

Assignment: Predict the 24 hr minimum temperature (deg F) for Alturus, CA (KAAT)

Predicting the minimum temperature can be challenging, because subtle differences in cloud cover and wind speed can lead to large differences in nighttime radiative cooling, and therefore large differences in the temperature. A good example is the Wednesday minimum temperature forecast for Alturas, California. The GFS and NAM MOS forecasts are far apart (34 or 20 F, a big difference!), probably because of uncertainty in the cloud and wind speed forecasts. Let's predict the Tmin for Alturas and see what happens. Alturas is a small town in the northeast corner of California, close to the Oregon and Nevada borders, at an elevation of more than 4000 feet. The town's motto is "Where the West Still Lives".

Question: If the wind speed decreases to near zero and the skies clear during the night, would you expect the minimum temperature to be lower or higher than for a windier, cloudier night?

What Happened?

Early Wednesday morning turned out to be a good, but not perfect, night for radiative cooling at Alturas. Looking at the observations, the period from midnight until 6 am was mostly clear with mostly calm winds. So the low temperature dropped to 25 F, far below some of the MOS forecasts (in the 30's). But there were brief periods with light winds and fog or mist, so it didn't get as cold as some other MOS forecasts predicted (20 F). So to answer the question, clear skies and light winds usually allow temperatures to drop more than with cloudier, windier conditions, sometimes by a lot.

Forecasts for Alturus, CA (KAAT)
Forecaster Tmin, deg F (error) Comment Day Score