The 2020-21 snow and ice season has started off very quickly across the central United States and Northeast.
An early season significant winter storm first brought widespread heavy snow to Montana Oct. 17-19, followed by widespread heavy snow in the Dakotas, Minnesota, and Wisconsin Oct. 20-22. The Rockies in southern Wyoming, Colorado, and into northern New Mexico then saw a winter storm Oct. 26-28 that resulted in heavy snow in the mountains and a significant ice storm in central Oklahoma. Finally, the Northeast saw their first snowstorm of the year on Oct. 30th, which brought a large swath of 4-7 inches of snow that impacted Boston.
To the left is a map from the National Weather Service that shows the October snowfall events, with select records and dates of occurrence as mentioned above. So far, we’re off to a more active start with winter weather nationwide than we saw in 2019. However, the question remains: Will it last?
Seasonal forecasts are a bear to take on- we’re creating a single outlook that summarizes at least 90 days’ worth of weather for the entire nation, which is far more difficult than preparing a local 7-day forecast. While specific details aren’t possible to predict weeks and months ahead of time, meteorologists can look at several pieces of data to get an idea of what a season may bring. In winter (December-January-February), we consider several factors, including the following:
The El Nino Southern Oscillation, or “ENSO”, is one of the strongest pattern drivers for winter weather in the United States. You’re likely more familiar with the names of two of its phases: the warm phase (El Nino) and the cool phase (La Nina). These phases are characterized by sea surface temperatures in the Equatorial Pacific, as well as the strength and direction of the trade winds along the equator.
We discussed this in depth back in August when NOAA issued a La Nina Watch. Since then, sea surface temperatures in the Equatorial Pacific have cooled significantly, and strong easterly trades have persisted, plunging us into a moderate La Nina. These conditions are expected to remain into winter, with a strong to near-record La Nina possible by mid-December to early January.
Moderate to strong La Nina years have historically brought an active Polar Front Jet (polar jet stream) to the United States, with wet conditions in the Pacific Northwest and Great Lakes region, and dry conditions across southern portions of the country. Additionally, temperatures have been above normal south with below normal temperatures in the north-central United States.
Moderate to strong La Nina’s also favor heavy snowfall northwest and below normal snowfall for much of the Plains and Mid-Atlantic into the Northeast, but that doesn’t mean big snows aren’t still possible in those areas.
Other factors that we look at to prepare our winter outlook include the solar (sun spot) cycle, the location and strength of the Polar Vortex, custom analogs for years with similar late Summer and Fall conditions, global sea surface temperatures, sea ice extent, and several teleconnections. These pieces of data are less known to the general public, but still play an important role in the weather patterns we see through the winter months.
We created custom analog data using these parameters, and found that they generally support the same pattern- cold in the north-central United States and dry conditions south.
Since the Polar Vortex has been such a “hot” topic in recent years, I’ll briefly discuss its current state and where it looks to be headed for this winter. The Polar Vortex is not a new concept, it’s just become sensationalized in news media the last few years. The Polar Vortex is just a large area of low pressure and very cold temperatures that typically resides at the poles (the North Pole for reference in my discussion here). When strong, this cold air resides at the pole. When the Polar Vortex weakens, it buckles toward the equator, bringing well below normal temperatures to the affected areas of Canada and the United States.
Over the past few years, we’ve seen this cold bottled up at the North Pole for most of the winter season, with few visits further south. This has contributed, in part, to the above normal temperatures during these months. We’re not currently seeing indications that the Polar Vortex will behave significantly different this year.
Taking into consideration all the factors above, we created our winter outlook. It favors the typical moderate to strong La Nina pattern with the potential for below normal temperatures in the north-central U.S and above normal temperatures south. Additionally, wet conditions can be expected in the Pacific Northwest and Great Lakes region, with dryness persisting south and southwest.
As far as snowfall is concerned, the Pacific Northwest into Montana likely cashes in on big events, with above normal snowfall also expected for the northern Great Lakes and near the Canadian border in the Northeast. The possibility for below normal snowfall is greatest in the Four Corners Region and in the Mid-Atlantic to southern Ohio Valley.
Elsewhere, there are no strong signals either direction. The greatest uncertainty to the forecast is how much warmth the later winter southeast ridge will pump into the eastern United States. We’ve seen this dominate the last few years.