NOAA's Winter Forecast: Drought and Warmth in the South

Expect increased chances of a warmer than average winter across much of the Southern U.S. and New England, and a cooler than average winter across portions of the Northern Plains near the Canadian border, said NOAA in their annual Winter Outlook, released on November 21. The forecast also calls for drought to persist and intensify over much of the Southwest U.S., and to develop over the Southeast U.S. This year's forecast was more difficult than usual to make, said Mike Halpert, deputy director of NOAA’s Climate Prediction Center, due to the lack of an El Niño or La Niña event. When these patterns of above average or below average ocean temperatures in the Eastern Pacific are present, they strongly impact winter weather patterns by altering the path of the jet stream and the associated winter storms that travel along the axis of the jet stream. We currently have neutral El Niño conditions over the tropical Pacific Ocean, which means that ocean temperatures are near average along the Equator from the coast of South America to the Date Line, putting more subtle, harder-to-predict factors in control of this year's winter weather. NOAA relied heavily on climate trends over the past fifteen years and long-range computer models such as their CFS forecast model to predict this year's winter weather.


Figure 1. Forecast temperature and precipitation for the U.S. for the upcoming winter, as predicted in the NOAA Winter Outlook, released on November 21, 2013.


Figure 2. NOAA's seasonal drought outlook, issued on November 21, 2013, calls for drought to persist and intensify over much of the Southwest U.S. this winter, and to develop over the Southeast U.S.

What will the Arctic Oscillation and North Atlantic Oscillation do?
While El Niño is usually a key factor controlling winter weather patterns, it is often overshadowed by the North Atlantic Oscillation (NAO)--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. 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 and Europe more readily. This pattern is kind of like leaving the refrigerator door ajar--the Arctic refrigerator warms up, but all the cold air spills out into the house where people live. The NAO is a close cousin of the Arctic Oscillation (AO), and can be thought of as the North Atlantic component of the larger-scale Arctic Oscillation. Since the AO is a larger-scale pattern, scientists refer to the AO instead of the NAO when discussing large-scale winter circulation patterns. The winter of 2009 - 2010 had the most extremely negative NAO pattern (and AO pattern) since record keeping began in 1950. Vicious "Snowmageddon" winter storms occurred in both the U.K. and the United States that winter, as both Europe and North America suffered though an unusually cold and snowy winter. Thus, the phase and strength of the AO/NAO pattern is a key factor controlling winter weather. Unfortunately, this pattern is not predictable more than about two weeks in advance, and thus was not considered by NOAA in their forecast for the upcoming winter.

In my April 2, 2012 blog post, Arctic sea ice loss tied to unusual jet stream patterns, I discuss research that argues that Arctic sea ice loss may cause an increase in the probability of cold, negative-AO winters in the U.S. and Northern Europe. The warm phase of the decades-long pattern of warming and cooling of Atlantic Ocean waters known as the Atlantic Multidecadal Oscillation (AMO), which we've been stuck in since 1995, may also increase the odds of negative-AO winters. Both of these factors may act to increase the odds of a cold winter this year. Climate change wunderblogger Ricky Rood notes in the just-posted final installment of his 7-part series on Climate Change and the Arctic Ocsillation that the AO has shown an increasing trend since 1960 towards getting stuck in the same phase for multiple years in row. Since last year was dominated by negative AO conditions, we might expect increased chances of another predominantly negative-AO winter this year, with increased odds of colder than average conditions in the Eastern U.S. and Northern Europe. Note, though, that last winter was a bit of an oddball. The AO was predominately negative, and we saw cold and snowy weather in Northern Europe, as expected. However, the AO and NAO were out of phase, which only happens about 10% of the time, and the U.S. had a much warmer than average winter (19th warmest on record.)


Figure 3. Last year's NOAA forecast for the winter of 2012 - 2013 (top two panels), made in October 2012, called for increased chances of warm weather over the Western U.S. and cool weather over Florida. The contiguous U.S. ended up having its 19th warmest winter in the 118-year record. Florida recorded a top-ten warmest winter, and most of the Western U.S. ended up near average or cooler than average (bottom left panel.) Last year's winter precipitation forecast fared well for the Southeast U.S., which had a top-ten wettest winter on record (see bottom right panel for what actually happened.) The predicted dry winter for the Pacific Northwest also materialized. However, the Upper Midwest was unusually wet, in contradiction to predictions for above-average chances of a dry winter.

Winter weather and the sunspot cycle
Another major influence on the AO and winter circulation patterns may be the 11-year solar cycle. Recent satellite measurements of ultraviolet light changes due to the 11-year sunspot cycle show that these variations are larger than previously thought, and may have major impacts on winter circulation patterns. A climate model study published in Nature Geosciences by Ineson et al. (2011) concluded that during the minimum of the 11-year sunspot cycle, the sharp drop in UV light can drive a strongly negative AO pattern, resulting in "cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada, with little direct change in globally averaged temperature." The winters of 2009 - 2010 and 2010 - 2011 both occurred during a minimum in the 11-year sunspot cycle and fit this pattern, with strongly negative AO conditions leading to cold and snowy weather in the Eastern U.S. (15th coldest and 37th coldest winters in U.S. history, respectively.) There was more solar activity during the winters of 2011 - 2012 and 2012 - 2013, which had more positive AO conditions than the previous two winters. The coming winter of 2013 - 2014 will have about the same level of solar activity as last winter (Figure 3).


Figure 4. The number of sunspots from 2000 - 2013 shows that solar minimum occurred during the winter of 2008 - 2009, and that solar activity is now near a peak. Image credit: NOAA Space Weather Prediction Center.

Summary: flip a coin or catch a woolly worm
I'm often asked by friends and neighbors what my forecast for the coming winter is, but I tell them to flip a coin, or catch some woolly bear caterpillars for me so I can count their stripes and make a woolly bear winter forecast. I never make seasonal winter forecasts for the same reason I never make seasonal hurricane forecasts--why make a forecast that will be wrong nearly half of the time? Seasonal forecasts have some skill, and you'll come out ahead if you bet on their accuracy year after year. But forecast busts are common, and you'll often be remembered for your most recent terrible seasonal forecast. Making an accurate winter forecast is very difficult, as the interplay between El Niño, the AO/NAO, the AMO, Arctic sea ice loss, and the 11-year sunspot cycle is complex and poorly understood. I've learned to expect the unexpected and unprecedented from our weather over the past few winters; perhaps the most unexpected thing would be a very average winter during 2013 - 2014. Note that this year's Woolley Worm Festival held in October in Banner Elk, North Carolina crowned as champion a woolley bear caterpillar named "Fuzz" with no black bands on it. Local folklore has it that the woolly worm’s 13 body segments, which are brown or black in color, represent the 13 weeks of winter. More black segments mean a harsher winter; a majority of brown segments represent a milder winter. Fuzz's lack of black bands implies an unusually warm winter for U.S. East Coast, and this winter's official woolly worm forecast, beginning December 21, looks like this:

Week 1: Average Cold Wet Snow and Rain
Week 2: Average Cold Wet Snow and Rain
Week 3: Average Cold Wet Snow and Rain
Week 4: Above Average Temperatures
Week 5: Above Average Temperatures
Week 6: Above Average Temperatures
Week 7: Above Average Temperatures
Week 8: Above Average Temperatures
Week 9: Warm Weather
Week 10: Warm Weather
Week 11: Warm Weather
Week 12: Warm Weather
Week 13: Warm Weather

Jeff Masters