Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 24. marraskuuta 2010 klo 17:38 (GMT)
Meteorological winter officially begins on December 1, but winter has begun a week early across much of North America, thanks to a significant cold blast that has broken dozens of daily low temperature records across much of western Canada and the Western U.S. Sheridan, Wyoming set a new record for the date this morning with -17°F, and Oakland California had its coldest November 24th with a reading of 34°F this morning. The cold blast is expected to be short-lived, though, with near-average conditions returning by the weekend. The long-range 1 - 2 week forecasts from the GFS and ECMWF models do not show the jet stream getting "stuck" in place for the beginning of meteorological winter next week, and it appears that the first two weeks of winter will be rather ordinary.
Figure 1. Forecast temperature and precipitation for the U.S. for the upcoming winter, as predicted by NOAA.
Latest winter forecast from NOAA
We currently have moderate La Niña conditions over the tropical Pacific ocean, which means that a large region of cooler than average waters exists along the Equator from the coast of South America to the Date Line. Cooler than average waters in this location tend to deflect the jet stream such that the Pacific Northwest experiences cooler and wetter winters than average, while the southern U.S. sees warmer and drier winter weather. NOAA's forecast for the upcoming winter issued on October 21 calls for a typical La Niña winter over the U.S.--warm and dry over the southern portion of the country, cool and wet over the Pacific Northwest, warmer and wetter than average over the Ohio Valley, and near average over the remainder of the country. According to NOAA's latest La Niña discussion, La Niña is expected to remain solidly entrenched throughout the coming winter and into spring.
Figure 2. Observed temperature and precipitation departures from average for the last three winters with a La Niña in the "moderate" or "strong" category. The current La Niña is right at the borderline between "moderate" and "strong." The anomaly patterns from the past three La Niña winters were dominated by the winter of 1999 - 2000, which was the warmest winter in U.S. history, and 1998 - 1999, which was the 2nd warmest in U.S. history. Image credit: NOAA/ESRL.
What happened during the last three La Niña winters?
The last three winters with moderate to strong La Niña conditions occurred in 2007 - 2008, 1999 - 2000, and 1998 - 1999. These winters were extremely variable. The most recent La Niña winter, in 2007 - 2008, was near average in temperature and precipitation; the other two winters were the two warmest winters in U.S. history. The winter of 1998 - 1999 set a world record for the greatest seasonal snowfall in history, when a seemingly endless parade of winter storms across the Pacific Northwest left an astonishing 1,140 inches (95 feet) of snow at Mt. Baker in northwestern Washington. It's worth noting that two of these three La Niña winters (2007 - 2008 and 1998 - 1999) saw record levels of tornado activity. Of the three winters, I believe that the winter of 2007 - 2008 may be the best historical analogue for the coming winter, since Arctic sea ice loss, which can significantly affect winter weather, was most similar to the conditions observed this year.
A look back at the winter of 2007 - 2008
The La Niña winter of 2007 - 2008 started slowly, but ended up piling up quite a bit of snow across much of the U.S. New York experienced its wettest winter on record, and Colorado, Connecticut, Pennsylvania, and Vermont had their second wettest winter on record. As is typical during a La Niña winter, Texas was drier than normal, but the rest of the south had near-average precipitation.
According to The Northern Tier Rules: The 2007-2008 Snow Report by David Robinson, Weatherwise, Mar-Apr 2009, eleven major cities reported more than 125 percent of average snowfall. This compares with only three in 2006-2007, and was the most since thirteen cities in 2003-2004.
Record high snow seasons occurred in Madison, Wisconsin (101.4 inches, previous record of 76.1 inches in 1978-1979); Youngstown, Ohio (102.8 inches, previous record of 90.2 inches in 2005-2006); and Caribou, Maine (197.8 inches, previous record of 181.1 inches in 1954-1955). Two stations came very close to establishing seasonal seasonal records; Spokane, Washington, 92.6", 0.9" below the 1949-1950 record, and Flint, Michigan, 82.8", just 0.1" below the record set in 1974-1975.
Figure 3. Snowfall totals for the winter of 2007 - 2008. Image credit: The Northern Tier Rules: The 2007-2008 Snow Report by David Robinson, Weatherwise, Mar-Apr 2009.
Wildcard number 1: What will the NAO do?
The North Atlantic Oscillation (NAO) is a climate pattern in the North Atlantic Ocean of fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores High. It is one of oldest known climate oscillations--seafaring Scandinavians described the pattern several centuries ago. Through east-west oscillation motions of the Icelandic Low and the Azores High,the NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. A large difference in the pressure between Iceland and the Azores (positive NAO) leads to increased westerly winds and mild and wet winters in Europe. Positive NAO conditions also cause the Icelandic Low to draw a stronger south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward. In contrast, if the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), westerly winds are suppressed, allowing Arctic air to spill southwards into eastern North America more readily. Negative NAO winters tend to bring cold winters to Europe, and the prevailing storm track moves south towards the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.
The winter of 2009 - 2010 had the most extreme negative NAO since record keeping began in 1950. The NAO index was -1.67, beating the previous record of -1.47 set in the winter of 1962 - 1963. The record negative NAO was responsible for unusual cold weather and snows over Eastern North America and Europe, and resulted in an upside-down winter: coldest in 25 years in the U.S., and warmest on record in Canada, with snow needing to be trucked in for the Winter Olympics in Vancouver. This "Warm Arctic-Cold Continents pattern" had occurred previously only three times in the past 160 years. If a strong negative NAO establishes itself this winter, we could have a winter like 1995 - 1996, which featured a weak La Niña and a strongly negative NAO. That winter featured many cold air outbreaks across the Eastern U.S., resulting in fifteen major cities setting new all-time seasonal snowfall total, including 75.6" at New York City's Central Park. Unfortunately, the NAO is not predictable more than about two weeks in advance.
Wildcard number 2: How will Arctic sea ice loss affect the winter?
NOAA issued their annual Arctic Report Card last month, and discussed the fact that recent record sea ice loss in the summer in the Arctic is having major impacts on winter weather over the continents of the Northern Hemisphere. The Report Card states, "There continues to be significant excess heat storage in the Arctic Ocean at the end of summer due to continued near-record sea ice loss. There is evidence that the effect of higher air temperatures in the lower Arctic atmosphere in fall is contributing to changes in the atmospheric circulation in both the Arctic and northern mid-latitudes. Winter 2009-2010 showed a new connectivity between mid-latitude extreme cold and snowy weather events and changes in the wind patterns of the Arctic; the so-called Warm Arctic-Cold Continents pattern...With future loss of sea ice, such conditions as winter 2009-2010 could happen more often. Thus we have a potential climate change paradox. Rather than a general warming everywhere, the loss of sea ice and a warmer Arctic can increase the impact of the Arctic on lower latitudes, bringing colder weather to southern locations." As a specific example of what the Report Card is talking about, Francis et al. (2009) found that during 1979 - 2006, years that had unusually low summertime Arctic sea ice had a 10 - 20% reduction in the temperature difference between the Equator and North Pole. This resulted in a weaker jet stream with slower winds that lasted a full six months, through fall and winter. The weaker jet caused a weaker Aleutian Low and Icelandic Low during the winter, resulting in a more negative North Atlantic Oscillation (NAO), allowing cold air to spill out of the Arctic and into Europe and the Eastern U.S. Thus, Arctic sea ice loss may have been partially responsible for the record negative NAO observed during the winter of 2009 - 2010, and the emergence of the "Warm Arctic-Cold Continents pattern." This pattern is kind of like leaving the refrigerator door ajar--the refrigerator warm up, but all the cold air spills out into the house. If the Arctic Report Card is right, we'll be seeing more of this pattern during coming winters--possibly even during the winter of 2010 - 2011.
I'm often asked by friends and neighbors what my forecast for the coming winter is. My reply is usually, "Flip a coin. We don't have the capability to make very skillful predictions of the coming winter." I'll share with you my hunch for this winter, though--we are due for a rather ordinary La Niña winter like we had in 2007 - 2008. After a year of some extraordinary extreme weather, we are overdue for a relatively quiet season or two of weather.
For more information
Golden Gate Weather has a nice set of imagery showing historic La Niña winter impacts, based on whether it was a "weak", "moderate", or "strong" event.
Francis, J. A., W. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron, 2009: Winter northern hemisphere weather patterns remember summer Arctic sea-ice extent. Geophys. Res. Lett., 36, L07503, doi:10.1029/2009GL037274.
Honda, M., J. Inoue, and S. Yamane, 2009: Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett., 36, L08707, doi:10.1029/2008GL037079.
Overland, J. E., and M. Wang, 2010: Large-scale atmospheric circulation changes associated with the recent loss of Arctic sea ice. Tellus, 62A, 1.9.
Petoukhov, V., and V. Semenov, 2010: A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. J. Geophys. Res.-Atmos., ISSN 0148-0227.
Seager, R., Y. Kushnir, J. Nakamura, M. Ting, and N. Naik (2010), Northern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10, Geophys. Res. Lett., 37, L14703, doi:10.1029/2010GL043830.
Thanksgiving break is at hand, and I plan to spend it enjoying family and friends, eating far too much delicious food, and watching the invincible juggernaut that is my favorite football team, the Detroit Lions, demolish yet another hapless opponent on Thanksgiving Day (not!) I'm also looking forward to seeing the season's first snowflakes here in Michigan on Friday--winter has been late arriving here this year. I'll be back with a new post on Monday. Have a great Thanksgiving, everyone!
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