2022-2023 Winter Outlook
Winter Seasonal Forecast By Kyle Elliott
* Third-year, "Triple-Dip" La Nina to Reduce Major Snowstorm Risk This Winter *
Wednesday, November 2, 2022:
After a rather mild and benign 2021-2022 winter season, snow-lovers may face frustration and disappointment yet again this year. One of the main factors that influence both the weather and climate in the winter are ocean sea-surface temperature (SST) anomalies, or how much above- or below-normal the surface water temperature is, in the Pacific Ocean. When SST anomalies in the equatorial Pacific are below normal, we are in a state of La Nina. For the last two years, La Nina has played a major role in dictating winter weather patterns across the contiguous U.S., and it's still going strong. Climate models are in good agreement that a moderate La Nina will persist through the winter. Thus, we'll be dealing with a third-year, or “triple-dip,” La Nina for only the third time in the last 40 years. Despite La Nina being present for the past two winter season, there were significant differences between the winter of 2020-2021 and last winter. Part of this can be attributed to the fact that large-scale weather patterns in first-year La Nina winters often have not yet fully responded to its influence. Another key factor to consider is the strength of the Stratospheric Polar Vortex (SPV). The SPV can play a major role in preventing or unleashing Arctic outbreaks across the central and eastern United States. When the SPV is strong and centered near the North Pole (as was the case last winter), cold air tends to remain bottled up in Arctic regions. When the SPV weakens or splits into multiple pieces, bitter cold may plunge into the eastern U.S. and/or Europe just a few weeks later. One such split occurred in January 2021 and was at least partially responsible for the crippling ice storm and record cold that gripped the southern Plains (particularly Texas) in February. Although the SPV may be weaker in December than it was last year, its overall strength should more closely resemble last winter than the 2020-2021 winter season.
The phase of the Pacific Decadal Oscillation (PDO) is yet another primary winter climate influencer. When SSTs are above normal in the interior North Pacific and below normal along the North American coast, the PDO has a negative value. The opposite is true for a positive PDO. Last winter, the PDO was in a strong negative phase and is expected to remain strongly negative again this season. In negative PDO winters, the coldest winter weather (relative to normal) often occurs in the Pacific Northwest, northern Rockies, and northern Plains.
The last factor that I'll highlight is the Madden-Julian Oscillation (MJO). The MJO, which can be thought of as an eastward-moving area of convection between the Indian and central Pacific Oceans, can be active in 8 different phases. Each phase corresponds to a specific location of this tropical convection. Which phase the MJO is active in at any given time can alter weather patterns across North America on a week-to-week basis. Based on the location of highest SST anomalies between the Indian Ocean and central Pacific, a meteorologist can make a reasonable guess as to which phases the MJO should be most active in during a particular winter season. This year, there is a pronounced warm blob just to the north and east of Australia favoring MJO phases 4-7 (see below). Although these are generally "warm phases" for the eastern U.S. during the second half of a La Nina winter, phases 6 and 7, in particular, can have the opposite effect from December into January.
(1/4) Here we go, weather fans! The moment you've all been waiting for has finally arrived: my 2022-2023 Winter Outlook! Two primary winter-season influences will be our third-year, "triple-dip" La Nina and "warm blob" to the north/east of Australia.. pic.twitter.com/VmNO9hg3UP— MU Weather Center (@MUweather) November 1, 2022
For those of you wondering how I create a winter forecast, the recipe is actually quite simple. I look at the distribution of temperature and precipitation patterns across the U.S. in past winters with similar conditions to those I expect in the upcoming winter. These are called “analog years,” and they’re a vital part of seasonal forecasting. In second- and third-year La Nina winters with a negative PDO and strong Polar Vortex (what I'm expecting this year), there tends to be an active, Pacific storm track and lack of big, East Coast snowstorms, or Nor’easters. Because most storm systems should be associated with the northern branch of the Jet Stream this winter, they’ll often track to our north and west and be unable to tap into rich, Gulf of Mexico moisture. As a result, I expect frequent light precipitation events this winter with "moisture-starved systems" being the norm. We'll also experience several “changeover storms” where precipitation starts as snow but quickly turns over to ice or rain. Any bigger storms will tend to cut into the Great Lakes region or up the Appalachians, placing the Lower Susquehanna Valley on the "warm side" of these systems. The Polar Vortex may wobble, or weaken, at times during December and the first half of January, allowing some shots of colder weather to drop into the Northeast. Even if this doesn't happen, high-latitude blocking could develop and force colder air into the eastern U.S. at times. However, this solution is highly dependent on the MJO remaining active in phases 6 and/or 7 through mid-January. Along these lines, I can't rule out a significant ice storm this winter, especially between Dec. 15 and Jan. 15. However, this "battleground" pattern featuring alternating cold and mild shots should gradually give way to more persistently above-normal temperatures during the second half of January and February. Temperatures in both December and January should be "near normal" before averaging 2-4 degrees above normal in February. As a whole, I am expecting 15-25” of snow this season in most of Lancaster County, with a below-normal chance of a major, double-digit snowstorm. Total precipitation (rain, sleet, ice, and snow) will likely be near- to slightly below-normal.. -- Elliott