Capital Weather Gang 2009-10 Winter Outlook
Odds favor cold, snowy winter
After a string of lackluster winters, this one could deliver for cold/snow lovers. We are forecasting the coldest winter since 2003-04 and the snowiest winter since the epic winter of 2002-03 when 40-60 inches buried the region, unless El Niño (an event characterized by warmer-than-normal sea-surface temperatures over the equatorial Pacific that can lead to shifts in weather worldwide) has some tricks up its sleeves.
Already, this El Niño event has reached moderate levels. The biggest threat to our forecast is the possible development of a strong El Niño. Often, when an El Niño gets too strong, we get flooded with warm air resulting in mild air and rain rather than snow. We're giving this a 30% chance. On the other hand, if the El Niño weakens rather than strengthens, it would likely result in similar (or even deeper) cold to what we're expecting, but it would probably give us a bit less snow. There is a 10% chance of this possibility.
Overall, we find chances for a large snowstorm of 8"-12"+ are much higher than normal this coming winter. No two winters are alike, but we expect this winter to share some similarities with the winters of 1957-58 (Reagan National Airport snow: 40.4"), 1965-66 (28.4"), 1986-87 (31.1"), 1994-95 (10.1"), 2002-03 (40.4"), and 2006-07 (9.5").
While significant advances have been made in seasonal forecasting, there is still a great deal of uncertainty 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 forecast.
Monthly and Seasonal Temperatures
Overall Temperatures for December through February:
1 to 2 degrees colder than average
Monthly temperatures relative to average
January: 1 degree colder than average
February: 3 degrees colder 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 mean.
Much of our seasonal snowfall could come in several large events. There will be plenty of periods with little or no snow as happens in even our snowiest winters.
Our snowfall projection covers November through April (current 30-year running averages in parentheses):
Overall: 150% to 175% of average
Reagan National Airport (DCA): 24" (14.2")
Dulles Airport (IAD): 31" (20.4")
BWI: 30" (18.0")
Fairfax/Loudoun/Montgomery counties: 26"-36"
Arlington/Alexandria/PG/DC counties: 22"-30"
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.
Equatorial Pacific Ocean
After a 2 year La Niña event, we are currently in the midst of a rapidly strengthening El Niño. El Niño is an oceanic phenomenon in which sea surface temperatures along the equator from the coast of South America to just beyond the international dateline run unusually warm. Based on modeling, past El Niño events, and current evolution, we are forecasting the persistence of a moderate El Niño event through December that starts to weaken during January and into February. Since the Niño regions (where El Niño establishes itself) are thousands of miles long, where the most anomalously warm water sets up is important. We believe this will be a center/west based event.
Typically when the core anomalies set up in the center and western flanks of the Niño regions, the potential strength of the El Niño is capped in the moderate range. Some past winters with a similar strength and configuration were cold and snowy such as 2002-03 and 1986-87, while others had more mixed results such as 1994-95 and 2006-07.
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 unclear what phase of the PDO we are in, though there is some 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. After 2 years of sharply negative readings, the PDO flipped to its positive phase in August, September and through October. We expect the PDO to lean positive this winter (though not decidedly so) and perhaps act stronger than the index implies.
We are starting to see sea surface temperatures in the North Pacific take on a more favorable signature for a ridge in the West and trough (dip in the jet stream) in the East, conducive to cold air in our region. While not as impressive looking as 2002, which had a huge warm pool in the Gulf of Alaska, we should still expect a more stormy pattern than the last 2 winters, with an active subtropical jet stream.
The evolution of the PDO in 1957-58 serves a possible analog to what we may experience this year. It was somewhat positive overall, occurred after a La Nina period of negative readings, and ran counter to the predominant phase. It also acted stronger than the index indicated. We received 40" of snow that winter.
Just as this past winter was our first below normal temperature winter since 2003-04, this summer was our first below normal summer since 2004, even if a hot August tried to throw a wrench in that. The predominant theme this summer was a huge trough in the center of the country leading to much cooler than normal temperatures in the Great Plains and Upper Midwest. After a September that was warm in the Northern Plains and cool in the East, we have seen the predominant summer pattern repeat itself in October with the coldest anomalies setting up in the same places it did this summer. Good analogs for the overall summer pattern are 1965 and 1994. As we have gone through Fall, we have seen a pattern quite reminiscent of 2002 set up. It should also be noted that this October featured our 5th colder than average month in the last six, something we haven't seen since 2003.
The Quasi-Biennial Oscillation (QBO) is a measurement of stratospheric winds over the equator. It is currently in a strengthening negative/easterly phase. In the past this has been very favorable for high latitude blocking or upper level high pressure at northern latitudes. In particular, it correlates well with a Greenland Block or negative North Atlantic Oscillation (-NAO) which is typically favorable for a cold and stormy pattern in the mid-Atlantic.
Other factors that were considered, but carried less weight, include 2009 hurricane season, local and nationwide precipitation patterns of the summer and fall, significant events, and persistence (the idea that predominant patterns/phases can persist for months/years until something significant changes them).
Stay tuned throughout winter for updates and insights, as well as detailed coverage of every event.
(In addition to CWG's Matt Ross, the lead author, CWG's Jason Samenow, Matt Rogers, Dan Stillman, Andrew Freedman, Steve Tracton, Greg Postel, Camden Walker, and Josh Larson also contributed to this outlook.)
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