Tracking the thaw: How warm will it get?
How do meteorologists and forecast models calculate temperature?
Another pattern change is headed for Minnesota.
Minnesota meteorologists know Minnesota is the “land of 10,000 weather extremes.” Wild swings in weather here can be a form of job security for Minnesota weather forecasters. They can also be a nightmare for Minnesota commuters.
This week is a great example.
Temperatures bottomed out at negative 38 in Cotton and Brimson in northern Minnesota Thursday. This weekend, temperatures likely hit 40 degrees in parts of Minnesota. So we’ll see a statewide temperature swing of 75 to 80 degrees in Minnesota in just 48 to 72 hours.
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Diurnal variation
One temperature concept meteorologists deal with is the diurnal temperature variation. Simply put, diurnal variation is the difference between the daily high and low temperatures.
For example, Feb. 20’s average high and low temperatures at the Minneapolis-St. Paul International Airport are 30 degrees and 14 degrees. So Feb. 20’s average diurnal temperature range is 16 degrees. All things being equal, it would be about 16 degrees warmer on the afternoon of Feb. 20 than it would in the early morning.
But in the weather, all things are rarely equal.
Tracking extreme temperature swings
Forecast models do an excellent job overall of tracking big weather changes. But tracking temperatures is often more complicated than it may seem.
There are many variables that impact our daily temperature range.
Temperature correction factors
In my first weather job at Chicago’s Weather Command, we learned how to do temperature computations. They begin with assuming the diurnal variation, then apply correction factors based on what’s happening in the atmosphere that day.
Correction factors include snow cover, wind, cloud cover or sunshine, and advection of warmer of colder air masses. Any of these elements could add or subtract several degrees form the eventual high or low temperature that day.
Forecast models essentially do that same thing. But it’s amazing to me that this process hasn’t become more refined or accurate than it is today. I still trust my “old school” daily temperature computations over most model output these days. The meteorologists at Weather Command had a great way to predict temperatures.
It’s also interesting to note that forecast models today still fail to accurately gauge the extent of extreme temperature changes. Forecast models often underpredict warm spells in advance by a few degrees.
My computation numbers show we’re likely to exceed 40 degrees in the Twin Cities and much of southern and western Minnesota between Friday and Monday. The models have been reluctant to crank out those numbers. If we don’t exceed 40 degrees, I may have some minor egg on my forecast face next week.
Stay tuned.