When you went to bed Friday night, the forecast called for 5-8" of snow for Boston. When you woke up, the forecast was for 1" or less. What happened?!?
First, let me tell you how the "sausage" is made so to speak. Forecasts beyond a few days are largely based on computer model predictions. Around 7pm EDT weather balloons are launched all over the country. The data from those balloons gets fed into super computers. The computers use that weather balloon data to complete specific calculations of atmospheric variables that give us a forecast. It takes even the fastest computers in the world several hours to complete this process. So, new computer model forecasts start coming in between about 11pm and 2am EDT. That’s why the forecast often changes between the evening newscast and the morning newscast.
Those calculations are based on assumptions and estimations that are incomplete, limited by computational resources, or inadequately representative of the actual atmosphere. Errors tend to become exaggerated over longer time scales. In other words, as I’ve said before, "models make mistakes." Like making sausage, its a messy process! Over the last few days, there were two camps. The majority of models had the storm coming up from the south and hugging the coast as it moved into New England. But, there were a few models predictions that showed the storm moving quickly to the east and passing well south of us on its way out to sea.
The energy for this storm was over the middle of the country yesterday evening, which means it was well sampled by weather balloons. When that data came in overnight, the models have flipped. Now the vast majority show the storm passing well south of us. The current forecast track reminds me a bit of the storm that brought 1/2" of snow to Boston on Friday, March 4.
There is still one outlier among the models that brings a few inches of snow to the Boston area. That’s why the National Weather Service hasn’t yet canceled the Winter Storm Watch for Boston, even though they did cancel the watch for Essex, Middlesex and Worcester. A Winter Storm Watch means 6" or more of snow is possible. A Winter Storm Warning means its going to happen. No warnings have been issued. So, in other words, because there is still a chance that this one model is correct, 6" of snow can’t entirely be ruled out yet.
The entire process is repeated with another weather balloon launch around 7am EDT. Computer model forecasts based on that balloon data comes in between about 11am and 2pm EDT. So the forecast will probably change again by the evening newscast. If the trend continues, the snow forecast will drop even more and the Winter Storm Watch will be canceled.
