Tough. That’s how I’d describe the 2021-22 winter outlook in one word. It’s a process we go through every year looking at analogs, teleconnections, along with a variety of other tools. What makes this year so difficult is the lack of any strong signals from the past or future. In my opinion, being honest and transparent is the best way to set realistic expectations. With that said, we are still going to answer the basic questions that we address each year. Will it be warm or cold? How much snow if any will fall? You can always skip ahead to the end if you just want the summary. Look for the section titled "So, what do we think?"
Ask any meteorologist, when it comes to putting together a winter outlook, the very first thing to investigate is the current state, as well as projected sea surface temperatures over the Eastern Equatorial Pacific known as ENSO. ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe.
Even though ENSO is a single climate phenomenon, it has three states, or phases, it can be in. The two opposite phases, “El Niño” and “La Niña,” refer to the warming and cooling of sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean. Don’t forget about the third phase known as “Neutral” where tropical Pacific SSTs are generally close to average. In the world of ENSO, double-dipping is not a party foul and it’s actually quite common for La Niña to occur in consecutive winters. Well, that’s the phase we are in right now and there's a 90% chance it will last through the winter.
Image Credit: NOAA
However, no two La Niña winters will have identical temperature and precipitation patterns across the United States. This series of maps shows temperature patterns across the continental United States compared to the 1981-2010 average for every winter season—December through February—since 1950 that coincided with La Niña conditions in the equatorial Pacific Ocean. The years are ranked by how far below average the temperatures were in the central/eastern topical Pacific: strong (at least -1.5° Celsius colder than average), moderate (between -1° and -1.5°C), and weak (between -0.5° and -1°C colder-than-average.
In general, the stronger the La Niña, the more reliable the impacts on the United States. The current La Niña is even weaker than compared to this time last year! The typical temperature impacts are warmer than-average conditions across the southern tier of the United States and colder-than-average conditions across the north-central Plains. Now, I want you to look at the maps below and try to identify similarities between them. You’re probably having a tough time because there is a great deal of variability, even among strong La Niña years events.
Image Credit: NOAA
Last winter, also La Niña, we used analogs for composite temperature anomalies during all of the ENSO-negative years to guide our forecast. November was running warmer than average (4.2 degrees) and it matched up perfectly with the analogs. That strong signal increased our confidence moving forward. This year, November is colder than average (-1.5 Nov. 22) so we have to throw the entire technique in the garbage.
In situations like this with a lack of reliable analogs and a relatively weak La Niña, you need to find answers in places you normally wouldn’t look. That place is the Bering Sea, Gulf of Alaska and coastal waters off the Pacific Northwest. In recent years, scientists began referring to unusually warm water in this area as “The Warm Blob” covering more than 600,000 square miles. Well, this year we might actually have the opposite on our hands with colder water gathering in that location! (I've also pointed out the current cooling of sea surface temperatures in the central and eastern tropical Pacific Ocean on the map below)
Image Credit: NOAA
Colder ocean temperatures promote more of a trough in the atmosphere meaning the southern jet stream would be aimed at the West Coast to Pacific Northwest. The polar jet stream will vary, but have the tendency to dip into the Central United States. It's not a perfect match by any means, but this has similarities to the classic pattern for La Niña...
Image Credit: NOAA
The theme this year is to live in the present! We used analogs to explore the past, and teleconnections for the future when in reality, the most important thing is what's happening now. Taking into account the current pattern, the chill has been a dominating factor. Without any major shifts visible in our near future, I would expect this trend to continue. One effective way to predict frigid outbreaks is by inspecting temperatures in the stratosphere, which is located in the layer that is 5-30 miles above Earth’s surface. It's simple, when the stratosphere warms, we turn colder. Winds that normally flow from west to east around the pole weaken dramatically and even reverse direction, corresponding to a breakdown of the polar vortex. These Sudden Stratospheric Warmings (SSWs) can eventually cause the jet stream to weaken, which allows cold air bottled up near the polar cap to escape and expand into the middle latitudes, including us here in the Ohio Valley.
Image Credit: Weather Bell
So, what do we think?
I want to begin by stating the obvious. Seasonal forecasts are not the most accurate thing we do because our weather models essentially become useless after 10 days out. In regard to temperatures, I have low confidence in a "slightly below normal to near normal" winter by about 0 - 2 degrees. We've been chilling out this November, December offers up more cold, and we aren't even officially into the climatology coldest part of the season. Moving on, I also have low confidence in an "above normal precip/near normal snow" season. For reference, Louisville receives around 13.4 inches of snow on average each winter season. The chart below shows snow from all recorded La Nina years...
For the 2021 - 2022 winter season, I would expect an active pattern along with potential for very cold intrusions at times. In order to get snow, we would need both of these elements (moisture & cold) to sync up! As always, the WDRB weather team will be here for you every step of the way so please check back with us on a daily basis.