What Our Latest Nor'easter Reminds Us About Weather Forecasts
Our most recent nor'easter was a good illustration of some things we should remember when the next one rolls around.
Yesterday's storm occurred 130 years and one day after the Blizzard of 1888 was hammering New England and eastern New York. The Blizzard of 1888 was one of the worst storms in recorded history and the blizzard all other New England blizzards are compared to. It reportedly produced 50" of snow in Middletown, Connecticut and 58" in Saratoga Springs, New York, with drifts as high as 50 feet in places! It completely crippled the region and hundreds of people perished. And.....no one had any warning that is was coming. Usable weather forecasts did not exist at that time. If I could go back in time and explain to someone who had been unable to leave their house or get back home for a week that we would acquire the ability to know in advance what the weather would do and would eventually become so proficient at forecasting weather conditions before they happened that people would take to sending out snarky "tweets" if the exact amount of snow that was going to fall in their neighborhood wasn't provided to them ahead of time, they would have thought I was daft. But the fact that they do is a testament to how accurate weather forecasting actually has become - people now expect it to be right.
This wasn't even the case during recent history, when the #2 blizzard that all other New England blizzards are compared to occurred - the Blizzard of 1978. One of the reasons that that storm had such a large impact on the populace was that no one believed the forecast of heavy snow and blizzard conditions enough to cancel anything in advance. So once everyone got to school and to work that day, and the blizzard occurred as forecast, many people were trapped on the roads or where they were at the time. Another thing that the Blizzards of 1888 and 1978 had in common was that there were no cell phones around during either storm. If you were stuck in a remote location, it might be a while before anyone knew about it.
Fast forward to yesterday's storm, which produced blizzard conditions in parts of eastern New England and between 2.5" and 23.0" of snow in Connecticut. Here was the forecast I issued prior to the storm juxtaposed with the NWS map of snowfall reports from the storm (click to enlarge)
Considering that the NWS totals are through 1:00 A.M. Wednesday and my forecast totals are only through 7:00 P.M. Tuesday, I was relatively happy with the general result. But the Devil, as always, is in the details. The largest disparity was in the heavily populated Connecticut River Valley/Pioneer Valley of Massachusetts and Hartford Basin region in Connecticut, where many snow amounts came up short. In addition to that, the combination of wet ground before the accumulating snow began, the lowering water content of the snow during the day (i.e. it became fluffier), and the amount of solar radiation that gets through the clouds this time of year, in a number of these spots half the snow that did fall was gone by evening.
The eastern areas of Connecticut were under a heavy band of precipitation related to the coastal storm, while the higher totals produced in western Connecticut and the Berkshires were the result of a phenomena known as "frontogentic forcing", aided by upsloping along the east-facing higher terrain. This band decreased in coverage and intensity as it drifted into central Connecticut. The CT Valley and Hartford Basin ended up in no-man's land, with heavy snow to the east & west and the result was some unimpressive totals.
A lot of mesoscale phenomena, including frontogenetic forcing is frequently not depicted accurately enough in the forecast modeling ahead of time. It can be located on radar after a storm is in progress, but it still isn't easy to determine how long it will last in a given area or when/whether it will move to another area. So what's the bottom line from all of this? I have never seen or issued a snowfall map forecast that turned out to be 100% accurate. I don't expect them to be and you shouldn't either. The snowfall map forecast is a guideline. It is a general forecast. It tends to over-homogenize snowfall zones and doesn't account sufficiently for small-scale effects that will occur. Also, snowfall amount probabilities change before a storm begins and while it is occurring. The forecast you saw before you went to bed is no longer valid when you get up and the one you saw in the morning is no longer valid half way through the storm. As the atmosphere constantly changes, the probability of what you are most likely to experience changes with it.