This past weekend, I received a question from a reader who wondered why meteorologists seem to have a hard time deciding on a forecast, particularly in the winter. Last Friday’s non-event is a good example of what he meant. First, the predictions were for freezing rain, then maybe rain and sleet, then maybe just rain, then some wintry mix… The combinations were slightly different depending on the meteorologist and the forecast you were reading or watching and what time you saw them.
There are a couple of good reasons that Friday morning was such a difficult forecast. First, when just a degree or two makes the difference between frozen precipitation and liquid precipitation, it’s hard to be confident in a forecast. I challenge anyone to forecast an overnight low in a developing situation down to the exact degree. It can be done, but it’s harder than you think. Thirty-two degrees or below means the precipitation would be frozen. Thirty-three degrees or above means the precipitation would be liquid. Add to that the need to forecast with the same preciseness the temperature in the layers above the surface, and things get more complicated.
The second reason Friday was difficult actually ties in with the first. The computer models were wavering with each update. If all the models agree, it’s easier for a human forecaster to be confident in his or her predictions since they are based on the models. If the models disagree, or the outputs change within the same model with each run, then the meteorologist has a hard time being confident. Those who have a good feel for what has happened historically in an area will base their decisions on their own experience. It’s good to have someone who knows the nuances of the region in those situations, but even the local experts get it wrong sometimes.
I understand that what frustrated so many of you about Friday’s forecast was the way the school districts decided to delay the start of the school day when little to no freezing on the roads occurred in the Triangle. Given what a close call the forecast was (just one degree cooler for an hour could have made a huge difference on the roads), most would agree that it’s better to be safe than sorry when the safety of children is concerned.