NASHVILLE (AP) — The winter weather outlook in Tennessee is for slightly warmer than normal temperatures and precipitation that is a bit above average.
Translation: keep the ice scraper and the rock salt handy.
The major factor is the La Nina surface conditions in the Pacific Ocean, which means the sea temperature is cooler than usual.
However, a phenomenon that’s predictable only a week or so before it happens could prove a spoiler, said Bobby Boyd, a forecaster for the National Weather Service in Nashville.
Last winter was a La Nina winter, too, cautions Boyd, but Nashville had 10 snows of at least a half-inch, tying 1978 as a winter with several snow events.
“I was really surprised at the statistic of 10 half-inch snows,” Boyd said.
Perhaps that was because none were that deep. A 2.6 inch snowfall on Dec. 12 and 2.5 inches that fell on Jan. 10 were the deepest snowfalls recorded in Nashville during the winter of 2010-11. It added up, though, to a total depth of 12.5 inches for the year — the most snow since 2002-03. The winter even included a white Christmas, the first in the city since 1993.
Back to that hard-to-predict phenomenon that could put the winter outlook into the recycle bin statewide: it’s called Arctic oscillation.
What it does is open the door for frigid Canadian, Arctic and sometimes even Siberian air to flow down into Tennessee. How it does it is a bit complicated, but here is the short answer: An increase in the air pressure in the polar latitudes, coupled with lower pressure in the mid-latitudes sends cold air plunging southward. When the pressure differences are reversed, the prevailing winds tend to keep the coldest air up north. Predicting whether Artic oscillation is going to be in the positive phase (blocking the flow of cold air southward) or negative (spilling the chilled air into the southern U.S.) can only be accurately accomplished within two weeks.
An area from Reelfoot Lake eastward across the Cumberland Plateau is expected to get more rain and snow than normal this winter. The predicted wetter spot is generally north of Interstate 40. The Delta region, most of the Tennessee River Valley and the mountains are expected to receive near-normal precipitation.
The greater Memphis area is expected to see warmer than normal winter temperatures and the outlook holds true, to a lesser extent, for the rest of the state, Boyd said.
That combination of not-as-cold temperatures coupled with wetter-than-normal conditions could bring on one of winter’s less enjoyable attributes.
“We can get into borderline situation of snow vs. ice,” Boyd said. “We may be looking at a pattern where Tennessee might be prone to icy conditions as opposed to snow.”
Boyd recalled the ice storm of 1994, during which there were widespread electricity outages, some lasting for as long as a week in rural areas.
The meteorological autumn is winding down with a precipitation deficit. NWS forecasters don’t expect the rest of November and most of December to improve that situation much, but Boyd said the precipitation — in whatever forms it takes — is lurking out there, beginning near year’s end.
Published in The Messenger 11.18.11