This week, I’d like to begin by briefly going over the outcome of my summer forecast for Aroostook County. I forecast sufficient rainfall for a good harvest and, according to Tim Hobbs on our Potato Picker’s Special, that worked out well.
With regard to my prediction of how many days there would be with a high temperature of 80 degrees or warmer, with the average being 26 such days, I forecast 30, and we ended up with 41, which tied for the third most in 77 years of record keeping at Caribou.
So I forecast a warmer than average summer, I just didn’t go warm enough! I think an overall summer forecast grade of B- seems fair (always easy to grade yourself, right?!) It would be a B, but I am deducting for the fact that parts of southern Aroostook, mainly south of Houlton were quite dry.
Now, we’ve turned the page to Autumn, and soon enough we’ll be tracking winter storms! Sometime, you might hear a forecast for accumulating snow, yet notice that the temperature is several degrees above freezing, causing you to think “no big deal, it’ll just melt.” However, through a process called evaporative cooling, the entire column of atmosphere, from your backyard up to where the precipitation-producing clouds are, can cool right down to freezing, if the air the precipitation is falling into is quite dry.
Why is this? Because the precipitation will fall through that dry air and evaporate, and evaporation is a cooling process (it requires heat for water to go through a phase change from liquid/frozen water to water vapor).
So don’t be lulled into a sense of security if you hear a forecast for evening accumulating snow and the thermometer at your house reads something like 37. Evaporative cooling can easily cool surface temperatures by 5 degrees, making your 37, suddenly the not-so-magic 32, where wet roads freeze up.
This same process, wherein the precipitation initially evaporates, will sometimes lead people to think something is wrong with the radar, because it shows precipitation falling at their location, yet they look outside and see nary a flake. Well, the falling snow is picked up just fine by radar, but again, the reason they see nothing is that in the early stages of a storm, the precipitation that is being detected aloft, can be all gone, “eaten up” by evaporation, by the time it makes it to the ground! Eventually, if the low-levels become sufficiently saturated, and the snow can make it to the ground.
Handy clue: For significant snow to start making it down to the ground as a winter storm arrives, the relative humidity typically has to make it up to around 80-85 percent. If lower, evaporation will keep gobbling it up! Once it gets to that 80-85 percent range, accumulation will be “free” to occur, and slippery roads will soon follow!
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
