Capital Weather Gang 2010-11 winter outlook
Volatile winter with about average snow and temps overall
Lead author: Matt Ross
After last year winter's historic snows, there is both excitement and dread as the new winter approaches, depending on one's perspective. Snow lovers would like nothing more than a repeat of Snowpocalypse, Snowmageddon, and Snoverkill. And there are certainly those (the silent majority?) who would like as little of the white stuff as possible. As long as both groups keep their expectations reasonable, we think this winter will have something for everyone.
First things first: we almost certainly aren't getting as much snow as last winter. Probably not even close. That was in a once-in-a-generation kind of winter. Furthermore, we're in a La Nina and that does not usually favor big snows around here (though there have been exceptions, like 1995-1996). But we do think this winter will be snowier than our typical La Nina winter (see more on La Nina below), and very likely to be snowier than the back-to-back La Nina winters of 2007-2008 and 2008-2009 when just 5 and 8 inches of snow fell.
It is also likely to be colder than our typical La Nina winter. But that doesn't mean those who despise winter weather are in for endless suffering. In many ways this winter should behave like a typical La Nina featuring long stretches without winter storminess and, after cold air outbreaks, welcome thaws.
If your benchmark for a successful winter is either multiple crippling blizzards or persistent sunny and mild conditions, then you probably won't be a happy camper. But a likely changeable winter of 2010-2011 promises to put a smile on your face at least occasionally...
While advances have been made in seasonal forecasting, there is still a great deal of uncertainty and limited skill in developing these outlooks. This is a low-confidence forecast, especially the overall snowfall estimate, where one big storm (or the lackthereof) could make or break the projections provided below...
Monthly and Seasonal Temperatures (based on 1971-2000 averages)
Overall Temperatures for December through February:
Slightly warmer than average (by up to one degree)
Monthly temperatures relative to average
December: one to two degrees colder than average
January: one to two degrees warmer than average
February: two to three degrees warmer than average
Note that monthly temperature predictions are less reliable than overall seasonal temperatures. A cold or warm pattern lingering a week too long or ending a week short can greatly alter a monthly average.
We are projecting five or six accumulating snowfall events. It is highly likely that several storms will not produce all snow for everyone as they track to our west, a track which favors mixed precipitation events. We believe our biggest event will be in the general 6"-10" range with a 3-6" event and a few 1-3" events rounding out the season. This does not include freezing rain events of which 1 or 2 are certainly possible.
Our snowfall projection covers November through April (current 30-year running averages in parentheses):
Overall: Around average
Reagan National Airport (DCA): 14" (15.4")
Dulles Airport (IAD): 19" (21.9")
BWI: 21" (20.1")
Fairfax/Loudoun/Montgomery counties: 15-22"
Arlington/Alexandria/PG/DC counties: 12-18"
Below are the factors that we have deemed most important in determining conditions this upcoming winter. No single factor tells the whole story, nor are the correlations between current/past conditions and future conditions always strong. But we have chosen factors that in the past have proven to at least have some predictive value. And when considered collectively, they help paint a picture of what we believe is most likely to happen this winter.
No two winters are alike, but we expect this winter to share some similarities with the La Nina winters of 1942-43, 1955-56, and 1964-65. These analogs helped to form the basis of our temperature and snow predictions because the weather in those years most closely matched the factors below.
Equatorial Pacific Ocean
A La Nina event that should persist throughout winter is underway. La Nina is indicated by anomalously cold sea surface temperatures in the Equatorial Pacific Ocean. In our region, La Ninas, particularly moderate to strong events, are often associated with dry, warm winters without much snow. This is usually because of two primary factors:
1) The frequent presence of a southeast ridge. It is a persistent area of high pressure near Bermuda that both keeps us in warmer air masses and pushes the storm track to our north and west.
2) A dominant northern jet stream and lack of a subtropical jet. Prevalent storm tracks along the northern branches of the jet stream typically cut to our west and/or redevelop as coastal storms to our north and we are left either warm and rainy, or dry.
La Nina's also tend to produce highly variable temperatures characterized by lots of warm thaws followed by Arctic plunges and back to warmth again.
Usually in moderate-to-strong events. the warm outbreaks are more common than the cold outbreaks and the cold outbreaks are typically dry. Not all La Nina's are the same and there are other factors that drive our weather. However, we expect a moderate to strong La Nina event this winter that starts to wane in January/February. Thus we expect it to be a major player in the type of weather we experience.
North Pacific Ocean
The Pacific Decadal Oscillation (PDO) is a measurement of the intensity and location of sea surface temperature anomalies in the North Pacific. When it is strongly positive it often correlates with a cold and stormy pattern for the mid-Atlantic. When it is sharply negative, it is often, but not always, warm and dry. Measured monthly, it also oscillates in predominant cycles that can last 2-3 decades. From the late 1940s to mid-1970s we were in a predominantly negative PDO cycle, and from the mid-1970s to late 1990s a predominantly positive PDO cycle. Since then, it is somewhat unclear what phase of the PDO we are in, though there is evidence we may be in a predominantly negative phase.
During La Niña events the PDO tends negative and during El Niño events it tends positive. We expect the PDO to average negative this winter. We are probably in a long term negative phase and during a La Nina event it is highly unlikely that the PDO will act counter to the current decadal phase. It has been sharply negative this fall and we expect it to be somewhat to strongly negative throughout winter. Sea surface temperatures in the Gulf of Alaska are running anomalously cold which is consistent with a negative PDO index and typically supports a persistent area of low pressure in the Gulf. This is usually not a good sign for snow lovers as an area of low pressure farther west over the Aleutian Islands is the preferred setup for wintry weather in our region.
North Atlantic Oscillation (NAO)
The NAO is technically a measurement of the differences in air pressure over the North Atlantic Ocean. However, it is often indicated by either an upper level low pressure area (positive phase) or upper level high pressure area (negative phase) over or near Greenland. A negative NAO in the winter months strongly correlates with a cold and stormy pattern in our region. This was the major factor in our historic winter less than a year ago. Blocking over or near Greenland helps push the storm track further south and east, often creating storm tracks that are cold and snowy for our region.
It is likely that we are currently in the first few years of a long term negative phase of the NAO. It has averaged negative for 12 consecutive months running and 25 out of the last 29 months. This could be the saving grace for snow lovers. Good periods of high latitude blocking can sometimes negate or offset the negative signals in the Pacific. If we don't "cash in" during these periods, winter is looking pretty bleak for snow lovers. However, we expect that we will have enough extended periods of blocking for at least one or two decent snowstorms.
The late spring pattern of a trough in the western U.S. and a ridge in the east persisted throughout the summer and led to our record setting heat. This fall has also been quite warm in the region, but the pattern has changed a bit with the heat spreading further west. October finished above normal here, but was tied with August as our least warm month with respect to normal since February, and for the first time in 7 months, the warmest temperature anomalies were centered west of the Mississippi.
What are other forecast outlets predicting for the coming winter? Like ours, outlooks such as those issued by the National Weather Service (NWS) and AccuWeather suggest this will not be a repeat of last winter. They also suggest close to average temperatures and snowfall.
What is the Farmer's Almanac predicting? Is it a legitimate forecast? As we mentioned in an earlier article, the Farmer's Almanac is calling for above average snowfall in the mid-Atlantic region with generally below average temperatures. We do not believe the Farmer's Almanac outlooks are skillful or credible.
Last winter in review
Winter of 2009-2010 snowfall map
The winter of 2009-2010: Could it have been colder?
Why was the winter of 2009-2010 so snowy? Part I and Part II
The varying predictability of snowstorms
Past winter outlooks
2009-2010 winter outlook
2009-2010 winter outlook recap
2008-2009 winter outlook
2008-2009 outlook live chat
2008-2009 winter outlook recap
2007-2008 winter outlook
2007-2008 winter outlook recap
2007-2008 winter outlook
2006-2007 winter outlook
2005-2006 winter outlook
(In addition to CWG's Matt Ross, the lead author, CWG's Wes Junker, Jason Samenow, Matt Rogers, Don Lipman and Dan Stillman also contributed to this outlook.)
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