The National Weather Service released its winter outlook yesterday which of course receives a lot of press. The outlook from this agency is a probabilistic one. They use a probability system much the same as a chance of showers. On a day where the chance of rain is say 40 percent, there is enough of a risk to mention this in the forecast, but there is still a large chance, 60 percent, you won?t see them.
The winter outlook from NOAA should be thought of in much the same way. Those numbers which lean warmer,colder, wetter, or drier only means the forecast is leaning in that direction.
When you look at the winter forecast from NOAA?s Climate Prediction Center you find southern New England is just slightly tilted to a higher probability of a warmer-than-average winter and on the northern edge of a wetter-than-average winter.
Remember, this outlook is for December 1, 2015 through February 29, 2016. It doesn?t include a possible snowy and cold March or an early spring after a potentially colder winter.
What Type Of Storm Track?
The pattern assumes a mean storm track into the west coast, with hopefully drought-relieving rain which continues across the southern tier before turning up the east coast.
The conundrum for New England will be whether these storms continue northeast along the coastline or head out to sea. The pattern also assumes the core of the coldest air stays locked up in Canada with fewer arctic outbreaks than last year.
Winters in southern New England have tremendous variability. In Boston and surrounding areas, we have seen years where not one snowstorm reached the six-inch mark and others when multiple storms over 12 inches fell. This clustering of storms is often a result of patterns locking in for six-week periods. Sometimes the patterns repeat themselves again for another 6 weeks, and thus you have three intense months of winter.
Most of the forecasts being issued since August take into account the status of the ocean waters around the globe. While there are other factors in play, the vast expanse of water and its subsequent relationship to the upper level wind patterns makes looking at the state of the oceans in fall necessary to help predict the upcoming winter.
The two maps below show the state of the ocean waters last February followed by where we are as of this week. Notice the enormous changes that have occurred.
Doesn’t El Nino Make Predicting The Winter Easier?
While El Nino exists this year, each El Nino cycle is different. The placement of the warm water, the depth of it, and the extent in all directions each play an important role in driving the atmosphere. While the waters off South America might be warmer than average, a cold pool of water in the North Atlantic can shift upper level winds enough to have storms hit or miss.
In October and November of 2014, no one foresaw the snowiest winter on record in Boston. While I expected a very cold February, along with many others, I didn?t imagine or forecast the depth of snow that would pile up in six weeks.
Last year?s severe winter snow was quite localized, areas west of Worcester, south of Hartford, and much of the western half of Northern New England saw typical or even below-average snowfall. It was the cold which was so pervasive across the entire northeast corner of the United States.
What Is Typical?
On average there are 11 snow events with an inch or more per year in Boston, the number increases to 17 in Worcester. Last year there were 19 of these in Boston and 18 in Worcester. The timing of any snow is critical to its impact. Four inches of snow from 8 p.m. until 1 a.m. has a very different effect than if it occurs 12 hours earlier.
The long term models have El Nino weakening late in the winter and towards spring. Were this to occur, other factors would likely come into play beyond El Nino.
Forecasters often use analog winters as a way to see how the atmosphere will perform in a given year. This year meteorologists are using years like 1957-58, 1986-87, 1987-88, 2009-10, and others. The problem with the analogs is this year?s situation isn?t the same as the past, but they can give us an idea for the future.
Last Year?s Predictions
At this time last year NOAA also made a winter prediction. You can see it below. The forecast was good in some parts of the country, but abysmal in the northeast, especially New England.
At this time last year there were some signs of a possible El Nino, but this never materialized and was likely a strong piece as to why the forecast didn?t verify.
Predicting the entire winter is difficult. Even with the latest models and fastest computers, unforeseen or less understood factors can change these predictions. Each month in the winter, I try to outline my own best forecast for the upcoming 4 weeks. The shorter period of time to forecast has a higher accuracy rate.
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