MIDWEST LOOKS AHEAD TO LIKELY SNOWIER THAN AVERAGE WINTER, ESPECIALLY IN OHIO VALLEY…
- Using four summer and fall datapoints (ENSO, Atlantic tropical cyclone activity, Northern Hemisphere snowpack, and fall longwave pattern), an updated look at the severity, aggregate storm track, and snowfall anomalies for the winter of 2016-2017 is constructed.
- A weak La Nina has developed in the central Pacific, but will have trouble influencing the North American winter this year, as it is forecast to weaken over the course of the next few months. Still, the analog years arising from this methodology shows that above normal snow and ice is likely for the Ohio Valley and Upper Midwest, and with below average precip in the southern tier.
- Overall, temperatures are expected to be somewhat above average across the southern Plains and Southeast U.S., near normal in the Ohio Valley, and slightly below average in the Northern Plains.
Outlook and Discussion
The last several winters have established a trend of wild regional swings from one extreme of temperature and precipitation to the other. While the winters of 2013-14 and 2014-15 saw numerous all-time temperature and snowfall records fall, particularly in the Ohio Valley and New England, in 2015-16 a major El Nino event brought extreme warmth to the Midwest and East, which was punctuated by several very powerful winter storms. So, the natural question is whether this pattern of extremes will repeat yet again in 2016-17. In general, I believe seasonal forecasting is done best when it is simplest, so this look ahead at winter 2016-17 will only consider a few variables: ENSO state, Atlantic hurricane activity, Northern Hemisphere snowpack, and fall longwave pattern. Potential trends with significant predictive power for the winter ahead reveal themselves in these predictors.
The first step is to identify cases in the historical record in which the primary driver of global weather patterns—the El Nino/Southern Oscillation, or ENSO— is a reasonable match for this year’s Pacific sea surface temperature anomalies. In 2016, this is a reasonably easy exercise in pattern matching, because this year’s La Nina event has slowly come to fruition over the fall. As shown below left, the central Pacific is seeing -0.5 to -0.75C or so SST anomalies (at left, above).
Additionally, the uncertainty regarding the future evolution of this –ENSO event is declining; most models, including WeatherTiger’s proprietary “ENSO Whisperer” shown above right now suggest little additional intensification over the next couple months, with weakening back to neutral-negative conditions by early 2017 continuing into spring. Thus, the set of potential analog winters begins with years in which the late fall Multivariate ENSO Index was somewhat negative and not trending much stronger.
The next step is to eliminate years in which Northern Hemisphere tropical cyclone activity is not aligned with the somewhat above normal (~130%) Atlantic hurricane activity observed in 2016 (left). As tropical cyclones balance energy flow from the Equator to the Poles, less busy than usual Atlantic hurricane seasons usually prefigure active winters. This was the case in 2013, 2014, and 2015 when well below normal activity preceded severe U.S. winters. Practically, this meant throwing very inactive hurricane seasons (like 2013) out of the potential analog set. Additionally, above average Siberian snowpack in fall often precedes more Arctic airmass outbreaks in the winter, which is the case this year (right) despite a scorching November and depleted snowpack over much of North America and record low sea ice in the Arctic. Finally, jet patterns for the remaining analog years are individually compared to that observed in the fall of 2016, with poor matches thrown out. This process results in five historical years corresponding to 2016 conditions: 1955-56, 1966-67, 1975-76, 1998-99, and 2000-01.
Average December through February anomalies for temperature (left) and precipitation (right) for these years are shown above. The analog method suggests that a weak (and, eventually, weakening) La Nina will struggle to apply too strong of an influence to mid-latitude weather in the Northern Hemisphere this winter, which will result in a rather variable pattern over the Plains and Midwest. Currently, a strong Pacific jet is exercising a powerful warming influence on U.S. weather, which is likely to persist into December and perhaps beyond. This influence may keep deep troughing from being too persistent, particularly in the first half of winter, leading to a drier and warmer than average skew over the southern tier.
However, a tendency for east central U.S. troughing (though likely NOT a persistent polar vortex displacement) is likely to bring near normal to slightly cooler than average temperatures to the northern Plains and western Great Lakes, with a band of near normal conditions across the northern Ohio Valley. The favoured storm track tends to be along the line of delineation between warmer and cooler than average airmasses, which this year would generally favour above average snow and ice from the south central Plains, north and east into the central and eastern Great Lakes, including the Ohio Valley. Another factor in play supporting above average Great Lakes snowfall are well above average lake temperatures, which will fuel a powerful lake-effect snow machine into January in Michigan, Ohio, western Pennsylvania, and western New York.
Estimated winter 2016-17 temperature and snow/ice anomalies are given for major U.S. regions in the table below:
Overall, the volatility of this winter’s pattern is likely to lead to enhanced storminess in the east central U.S., with particularly enhanced seasonal snowfall totals in the central and eastern Great Lakes regions. Thus, while the overall pattern in the continental U.S. is likely to tend towards warmth, especially south, historically this is not necessarily an impediment to significant snow and ice, particularly in the transitional zone between warmer and more seasonable conditions. Winter 2016-17 is unlikely to be a historic winter, but there are signals that it is likely to be a relatively active one nonetheless from a snow/ice perspective, particularly in the Great Lakes.