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Wednesday, October 23, 2013
Over the years, I’ve become less enamored with the idea of winter seasonal forecasts.
No matter how skilled and insightful the forecaster, most of them are mostly wrong.
A season is not so much a single long atmospheric drama as a series of short one-act plays. Averages, season totals and individual perceptions of the whole often belie the parts, and conversely, particular periods of winter can distort the perception of the whole.
But, nevertheless, it’s time to throw out at least a few thoughts today, as we are headed into a few days of windy, cold weather that could yield at least few snowflakes in the mountains.
Often, a baseline for winter forecasts is the El Nino Southern Oscillation, the irregularly recurring patterns of sea surface temperatures in the equatorial Pacific along a narrow stripe from the coast of Peru thousands of miles westward.
El Nino — when those waters are warm, relative to normal — and La Nina — when the waters are cool — are not absolute controllers of our winter weather patterns, but can send strong signals that do correlate to various general expectations of the winter as a whole.
As it stands now, this oscillation is in its neutral phase, neither warm nor cool, and is generally expected to stay there through the winter.
These “La Nada” winters, as they’re sometimes called, with no strong El Nino or La Nina signal, have run the gamut for us, historically, in terms of snowfall. They include 1959-60, with a record high 62.7 inches of snow for Roanoke, and 1990-91, with a record low 1.2 inches.
There is a little bit of a lean toward healthy snow totals, however. Of the 19 winters since 1950 without an El Nino or La Nina, eight have produced at least 30 inches of snow for Roanoke, and six others have produced somewhere between 16 and 30 inches. About 18 inches is the historic average for Roanoke annual snowfall.
The most recent “La Nada” winter was last year, which produced 18.3 inches of snow, though about half of it was technically in what would be considered meteorological spring, after March 1. (About one-third of it was in what everyone would call spring, falling March 25-26 and April 4.)
Some forecasters put a lot of emphasis on Eurasian snow cover in October as a harbinger of potential cold air masses for winter. That has spread rapidly and appears to be running ahead of last year, which was above normal. The projected cold patterns did develop last year, but not until mid-January and beyond, holding stubbornly well into spring.
The state of the northern Pacific is sometimes considered as well. The sea surface temperatures there have been running low. That would imply the likelihood of low pressure troughs developing over that region, which often leads to strong jet stream wind blowing across the U.S. and high pressure ridges in the central and eastern U.S. that generally lead to milder weather.
These are only a very few factors out of multitudes forecasters consider.
The bottom line: Mixed signals. Almost every winter presents some level of mixed signals, but this one seems to present a particular puzzle.
My expectation: We are neither headed toward an exceptionally snowy or non snowy winter. I would lean to slightly higher than normal temperatures and slightly less snowy than normal, but with a couple of significant winter storms, owing to some questions I have about whether there will be sufficient high pressure blocking near Greenland to lock in cold air masses frequently this winter.
But I could be very wrong, and wouldn’t be at all surprised if I am.
Think about your expectations for winter, using whatever factors you want. I’ll be asking for entries on Roanoke and Blacksburg snowfall sometime in November.
Weather Journal runs on Wednesdays.
Weather JournalStorm track isn't very snowy for us