After the winter we have had I doubt many of you are upset that we were in the 40s this week. In fact the 40s are nice, but if it wasn't for that snowpack, we could have been warmer, like the 50s and maybe even as warm as 60°. I know I should be happy for the 40s and not say "Coulda, Woulda, Shoulda", but I wouldn't be doing my job if I did not tell you the whole story.
Forecasting high temperatures is all about observing the present and predicting the future. The present is the current 850 MB temperature. This is the actual temperature measured at about 5000 feet above the surface. Pressure is measured in millibars (MB). Sea Level is 1000 MB and an altitude of 5000 feet is 850 mb. The pressure continues to get less as you go up. The jet stream is at about 30,000 ft., which is 300 mb, and so on. Not only does the pressure decrease with height, so does the temperature. So, to make a long story short, maybe its too late, but temperatures increase when we bring that air to the surface. When we know the 850 mb temperature, the temperature at 5000 feet, we need to bring that down to the surface pressure to get what we call an equivalent potential temperature.
Lets take Tuesday into consideration. The 850 mb temperature was around 9° Celsius, 48° Fahrenheit. You can see the actual sounding (observation from the weather balloon).
If you bring that temperature down to the surface and calculate the increased pressure, you would get the upper 50s to low 60s. So our high, in a perfect world would have been about 60°F. It was not a perfect world yesterday as the deep snow pack was doing its job to knock down the potential high temperature. This work is actually done for us by looking at the maximum temperature, MAX T, highlighted on the upper right. That 12.4° C translates to 54°F, but it was warmer later in the day. Here on the surface the snow actually cooled the air and wind, plus the evaporation process of the melting snow cooled the air above it reducing our high from about 60° to 44°.
Now you know and your next party you can say, "Did you know that the temperature last week at the 850 mb level was equivalent to 60° and by bringing it down the dry adiabatic on a SKEW T chart and adjusting for evaporation cooling from our snowpack, you get a high of only 44°". Believe me this will impress your friends.
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