Why Were Recent Vermont Storm Forecasts So Bad?

How it began: Forecast from the National Weather Service
issued Sunday afternoon for the storm that hit
Sunday night and continued into Monday. 

 We've had three pretty substantial winter storms here in Vermont since November 27. One of the storms, on December 4, was pretty well forecast. 

The other two storms, one on November 27 the other Sunday and yesterday, were not so well predicted.

The storm on November 27 was surprisingly ferocious. What was expected to be mostly a rain storm turned out to be a wet, sloppy snowstorm that cut power to as many as 34,000 Vermonters.

The most recent storm was supposed to be something of a monster. We were variously warned to expect Flooding, widespread heavy, wet snow, travel trouble, widespread power outages and strong winds.

Even just 12 hours before the storm hit, it was supposed to be bad. As we all know, it was just.... meh.

Are local meteorologists suddenly falling down on the job? Have all the computer forecasting models gone haywire?

Nope and nope.

Winter storms in New England are notoriously hard to predict. I don't think I've ever seen a Vermont snowstorm that played out precisely as forecast. More often than not, the storm predictions are quite accurate, but you're going to get a few surprises somewhere no matter what. 

However, two of the recent storm forecasts fell short. That was mostly due to very slight forecast inaccuracies that had big real-life effects

WHAT WENT WRONG?

In all three recent storms, temperatures hovered very close to the freezing mark. It's normally no big deal if you predict a temperature of say, 34 degrees and it ends up being 32 degrees. Close enough. When there's a lot of precipitation around, that ever so slight error in temperature can mean surprise snow, or an unforeseen wet washout. 

Both relatively poorly forecast storms were mostly a case of just slight errors in precipitation intensity. Those very minor mistakes had super noticeable effects.

In the November 27 case, the storm was stronger than forecast, which increased the intensity of the precipitation.  Heavier precipitation tends to cool the atmosphere more than lighter sprinkles and inconsequential snow.

Since temperatures were just a little above freezing at the start of the November 27 storm the extra oomph of moisture dropped temperatures just a degree or two below what had been expected. The result was that very unpleasant surprise snowstorm and power outages.

The storm Sunday and Monday had the opposite problem. The storm - and its associated precipitation - turned out to be less intense than expected.

It was still a very wet storm, especially for this time of year. Almost all of Vermont had at least an inch of rain and melted snow, and some places had two inches.

Still, it was less precipitation than expected, especially at the stage of the storm when rain was to change to snow.

There was lots of moisture available to the storm, so it did look like we were in for a lot of snow. What was to be an intense storm moving northward through eastern New England turned out be only of moderate strength. v

How it ended: Map of actual snow totals from 
storm that ending Monday afternoon. As you can
see, it was noticeably less than expected.

That means there was less lift in the air than anticipated, so the precipitation once temperatures fell to near the freezing mark wasn't so intense. 

 In the valleys, the temperature stayed up by a degree or so because of the less intense precipitation. So what was to have been a wet, heavy snow was just a yucky slush bomb.

For instance, in Burlington, the temperature stayed at 33 or 34 degrees for the entire snow event, so the National Weather Service measured just 1.9 inches of new snow. 

Even where it was a bit colder, at slightly higher elevations, an expected band of intense snow never materialized.  So most everybody got just four to seven inches of snow, with just a couple spots seeing eight or nine inches.  That was well short of the 12 to 18 inches that was expected.

Forecasters do get bonus points for accurately figuring out where the deepest snow would accumulate. It just didn't pile up as much as planned.

At least in this second wrong forecast, the effects of the storm weren't as bad as feared, so any frustration with an off prediction was tempered by relief things turned out OK.

The National Weather Service office in South Burlington also has a range of forecasts with each storm.  They offer what amounts to a 10 percent chance of storm over-performing, and a 10 percent chance of it under-producing.

This latest storm was just slightly larger than the 10 percent chance of a whiff - meaning lighter snow.

Of course, there is one success story in here. The December 4 storm played out pretty much as meteorologists expected.  Sure, there was a little more rain instead of snow in the valleys, but the forecast was fairly close. 

For the latest storm, there were still nearly 2,500 homes and businesses in Vermont without power as of 8 a.m. today. Even though the storm was less intense than forecast, it still had some real impacts,

As far as this week goes, the only weather trouble I see is the potential for briefly heavy snow showers this evening. That could make some slick spots on the roads, but accumulations should be under an inch for almost everybody, except maybe some spots in the northern Green Mountains.

Those of you who are frustrated by some winter storm forecasts that don't turn out as planned: Remember this: Forecasts as little as six or 12 hours before the storm will give you a general idea of what will happen. 

The storm will almost always turn out to be a little more intense, or a little less dramatic than the forecast indicates. If meteorologist say you will see four to eight inches of new snow, that's probably what will happen, but be prepared for a little more or a little less, just in case.