Drought is the Watchword as Winter 2016-17 Approaches

After a year with a record number of billion-dollar flood disasters, the United States is now heading into a period where drought may be the leading concern, according to forecasters behind NOAA’s initial winter outlook for 2016-17 that was released Thursday. “The winter forecast doesn’t bode well for [many] areas around the nation experiencing drought,” said Mike Halpert, deputy director of NOAA’s Climate Prediction Center, in a press conference Thursday. The most confident signal in the outlooks (see Figures 2 and 3) is for warmer-than-average conditions across the Sun Belt, from California to Florida, and for drier-than-average conditions across the southern tier of states, especially from the Southern Plains to the Southeast.

A five-year drought continues to grip central and southern California, and a rapidly intensifying drought now stretches from Alabama to the western Carolinas, as evident in this week’s U.S. Drought Monitor. Parts of northern Alabama and Georgia have only recorded about half of their average rainfall over the last six months, according to the Drought Monitor. New England and New York are also grappling with serious, months-long drought, although heavy rains this weekend may provide some relief.


Figure 1. Sunlight reflects on the surface of Lake Purdy in Birmingham, AL, on Tuesday, October 11, 2016. The lake's water levels have dropped several feet due to a severe drought. Image credit: AP Photo/Brynn Anderson.

More room for surprises
The winter of 2015-16 largely followed expectations for a strong El Niño event, with two big exceptions: persistent dryness in the drought-plagued Southwest and unusual warmth that prevailed across nearly all of the U.S., including areas such as the Gulf Coast that trend cool during El Niño winters. This winter, we don’t have a strong El Niño or La Niña event shaping North American climate, so there is even more room for natural variability and the potential for surprises in the mix. In its most recent monthly advisory, issued on October 13, NOAA deemed it likely that a La Niña will develop by late autumn, but odds are just slightly better than even that it will persist through winter, and computer models agree that it should be a weak event if it does develop. Overall, NOAA’s winter outlook for the contiguous U.S. largely follows the playbook for La Niña, which typically favors relatively wet, cool conditions toward the north and relatively warm, dry weather toward the south.


Figure 2. NOAA precipitation outlook for winter 2016-17, expressed as the probabilities for wetter- or dryer-than-average conditions for the winter as a whole. The probabilities are expressed in thirds, so a region with 40% odds of an outcome has a better-than-average chance of that outcome. Image credit: NOAA/NWS.


Figure 3. NOAA temperature outlook for winter 2016-17, expressed as the probabilities for warmer- or colder-than-average conditions for the winter as a whole. Image credit: NOAA/NWS.

Could another cold Midwest/East winter be in the cards?
One line of research suggests that several winters of the 2010s that featured intense cold across parts of the U.S. Midwest and Northeast, such as 2013-14 and 2014-15, may be related to a chain of events that begins with above-average October snow cover in Siberia (facilitated in part by recent losses of Arctic sea ice in autumn north of Siberia). In this view, above-average snow cover in autumn fosters high atmospheric pressure over the region. In turn, this deflects the jet stream and eventually disrupts the circulation over the Arctic, allowing cold air masses to pour southward more easily by winter.

Judah Cohen, one of the most prominent exponents of this hypothesis, leads an effort at Atmospheric and Environmental Research to predict winter temperatures across the Northern Hemisphere. Snow cover has been advancing at a faster-than-usual pace this autumn in Siberia, according to Cohen. Based on this, as well as below-average sea ice extent across the Barents and Kara Seas and the lack of a strong ENSO signal in the mix, AER issued a preliminary winter 2016-17 outlook on October 19. The outlook favors warmer-than-average conditions across the U.S. Southwest and colder-than-average conditions from central Canada to the southeastern U.S., including most areas east of the Great Plains except for Maine (see Figure 4). Last winter, AER called for most of North America to be mild, in keeping with the strong El Niño that developed, but it expected below-average readings over the eastern U.S., where they didn’t materialize.


Figure 4. The outlook for departures from average temperature for the contiguous U.S. issued by Atmospheric and Environmental Research on October 19, 2016. An update will be issued in November. The model uses October Siberian snow cover, sea level pressure anomalies, predicted El Niño/Southern Oscillation anomalies, and observed September Arctic sea ice anomalies. October Siberian snow cover has so far this month advanced at an above normal rate. This is an indication of an increased probability of a weakened polar vortex or a sudden stratospheric warming, and a predominantly negative Arctic Oscillation during the winter and cold temperatures--especially east of the Mississippi. Image credit: Judah Cohen, AER.


Meanwhile, The Weather Company called for readings to be above average for most of the contiguous U.S. during the winter of 2016-17 in an outlook released on September 23 (see Figure 5 below). Referring to the switch from El Niño to a weak La Niña, TWC chief meteorologist Dr. Todd Crawford said: "The reversal of tropical forcing suggests that the coldest weather in the eastern U.S. may occur earlier in the winter, with increasing chances for warmth during the late winter.” Crawford added: “Climate model forecasts for winter are unusually warm, likely reflecting the excess post-El-Nino global warmth, and another very warm winter is not out of the question due to this factor alone."


Figure 5. Temperature outlook for winter 2016-17 released by The Weather Company on September 23. Image credit: weather.com.


The cold winters of the 2010s in eastern North America may also have a link to the Pacific. Dennis Hartmann (University of Washington) emphasizes the role of warmth across parts of the tropical Pacific in generating an atmospheric “bridge” that can extend to cold, snowy conditions in the eastern U.S. and Canada. Hartmann and colleagues have investigated a pattern called the North Pacific Mode (NPM), which is distinct from the El Niño/Southern Oscillation (ENSO). The NPM’s positive phase features warmer-than-average SSTs extending from the western Pacific across to much of the north and northeast Pacific, where a recurrent area of warm water dubbed “The Blob” has returned this fall. This positive phase of the NPM in wintertime tends to favor a strong upper-level ridge from the U.S. West Coast to Alaska (reducing the likelihood of storms affecting California) and a deep, cold trough across eastern Canada and the U.S. Midwest and Northeast, favoring intrusions of Arctic air. Hartmann discusses the NPM in more detail in a March 2015 post at climate.gov.

Hartmann has not yet analyzed the NPM for recent weeks, but he told me in an email: “The [current] tropical SST pattern is such as to force a high anomaly in the pressure in the midlatitude Pacific this winter, and that would give more ‘blob’ and a downstream anomaly like that of January 2014, all else being equal. So dry in California and cold in the East seems like a reasonable prediction, although the uncertainty is high because the atmosphere generates a lot of random variability unrelated to tropical SST.  It will be interesting to see what develops.”


Figure 6. The two most common sea surface temperature (SST) correlation patterns in Pacific Ocean north of 30°S over time, based on EOF (empirical orthogonal function) analysis, a technique used to break down the role of multiple potential factors. The first is the classic signal of El Niño and La Niña (together referred to as the El Niño-Southern Oscillation, or ENSO), while the second is the North Pacific Mode (NPM). The contour interval is 0.1, and the zero contour is white. Red and blue show correlations between anomalies of opposite sign. When red areas have above-average SSTs, blue areas have below-average SSTs, and vice versa. Image credit: Dennis Hartmann, University of Washington, and climate.gov.


Figure 7. Departure from average in sea surface temperature (SST, shown in degrees C) for the period October 9 through October 15, 2016. Image credit: NOAA/ESRL/PSD.

What makes a season?
It’s important to note that even a winter that’s overall warmer than average in the Midwest or Northeast may include a couple of fierce cold blasts and/or major snowstorms. Take 2015-16: even though it was the warmest winter on record for the contiguous U.S. as a whole, and a top-ten warmest winter from New Jersey through New England--including the warmest Christmas Day ever experienced by millions of East Coast residents--it also included the epic blizzard of January 20-22, 2016, which dumped more than two feet of snow from West Virginia to New York, and the brief but intense cold wave of mid-February 2016, which produced Boston’s coldest weather since the 1950s. It’s an open question whether people will remember the East Coast’s winter of 2015-16 as brutal or balmy. When asked at Thursday’s press conference whether we are likely to see a “memorable” winter, Mike Halpert responded: “That’s not really what our outlook is about.”

We’ll be back with out next post on Friday, including a wrap-up on Typhoon Haima, which was heading toward the southeast coast of China on Thursday after ravaging parts of the Northern Philippines. We are also keeping an eye on Invest 99L, the long-simmering system east of the Bahamas. The National Hurricane Center gives 99L a 50% chance of briefly developing into a depression, and perhaps a subtropical or tropical storm, as it moves northward, eventually merging with a frontal system off the U.S. East Coast.

Bob Henson