Winter is here: like it or not! What might we expect this season?
SCROLL TO BOTTOM TO 'CUT TO THE CHASE' WITH NUMBERS.
Above: Winter Warming since 1970. Minnesota is some of the fastest warming.
First things first, we have to acknowledge the elephant or should I say elephant herd in the room. That's climate change. I know you're probably thinking 'well, duh.' But, I mean when it comes to forecasting longer term patterns. Even many forecasters often get so bogged down by various atmospheric circulations, El Nino or La Nina, etc. that they forget that one of the biggest, consistent influences on month to month, season to season patterns is our warming world.
I break down much more on winter climate statistics (normal, average, and what's changing & how) HERE.
I'll start with one of the main drivers of climate this year (or at least much talked about), La Nina. El Nino is its opposite & warmer cousin. I won't go into El Nino/ La Nina fully here because that's its own story in itself BUT- you supporters can rest assured that in addition to exclusive weather, long range, and climate content, I will be doing weather/climate explainers and certainly one of those will be ENSO (El Nino-Southern Oscillation).
Below is what is often thought of as 'typical' La Nina weather patterns in winter time:
One of the things that should stand out to you is that Minnesota (or at least most of it) really stands in between any blobs of significance. In other words, there's enough statistical significance (running the numbers of La Nina years over time) to say that western Canada & Alaska experience colder conditions with wet conditions in the Pacific Northwest and warm/wet conditions in the mid Atlantic, dry & warm conditions in the south.
Most of Minnesota and specifically the Twin Cities really is a draw when it comes to La Nina years. Let's look at it in some detail.
Below: Each winter season for 30 years (the new 'normal period).
I have the averaged temperature (each high & low for Dec. 1 - Feb. 28/29) and then each year shaded by if it was a neutral, El Nino, or La Nina year and also if it was a weak, moderate, or strong one.
You'll then notice on the right I have the temperature anomalies (difference from the average or normal). The first column is the anomaly compared to the historical (1873-2021) average and the second column is using NORMAL. Our Normal period uses *just* the averages for 1991 to 2020. We calculate a normal to capture what's average in the 'current' climate, using 30 years as the length of a generation.
The thing that should stand out right away is that 20 of the last 30 years are warmer than average right off the bat if we compare winter to the overall 1873-2021 average. That's climate change. If we compare the past 30 winters to 'normal' it's 50/50 as you'd expect. That's because our modern climate is much warmer than the historical average.
Here's a comparison of winter temperature average (again: highs & lows for all 90 (or 91) days of winter):
Back to La Nina... We've had 12 La Nina events since 1991. 7 of them were cooler than normal winters & 5 of them were warmer normal winters. We've had 9 El Nino winters: 5 were warmer than normal, 4 cooler than normal.
When one looks back to the 1970s which is really the earliest were able to decipher ENSO events, you have more data and the correlations with El Nino are even stronger: about 75% of the time in an El Nino event we have a warmer than normal winter, but in La Nina conditions only a slight majority (50-55% ?) are cooler than normal.
To further dive into it: we need to look at the magnitude of those anomalies. Our average La Nina winter temperatures are -6.6° to +2.4° from the normal. That shows a decent correlation toward the cold (negative) end of things.
El Nino have much stronger correlations, however. Our average El Nino winter temperatures are -1.5° to +4.5° from normal.
So in ALL La Nina events: 73% of the temperature spread is cooler than normal and in ALL El Nino events, 75% of the temperature spread is above normal.
We can further filter/break this down into the moderate to strong events. This is where we lose a correlation: in moderate to strong La Nina events only 43% of the temperature spread was cooler than normal where 70% of the temperature spread in stronger El Nino events was warmer.
Translation: El Nino has a much stronger correlation of warm winters than La Nina has on making us colder. The magnitude of such matters and remember there can be several things influencing our weather patterns over a season. ENSO is just ONE of those.
I made a very simplistic box: think of it like 'pros' & 'cons,' except with above (warmer), neutral (average), or below (colder) than normal conditions. I listed the top 6 things I'm basing this winter season's forecast on. These are based on a bunch of statistical analysis & factoring in the longe range computer models, climate change, etc.
The computer models are pretty neutral to slightly above normal for the winter. They're each laid out by column: December, January, February. Note that only 2 of the 9 are cool.
Another statistical element I looked at (& if you've been a supporter for a couple weeks you've seen some of this already) was the correlation between warm Autumns & the following winters. Specifically, I wanted to look at top 10 warmest Autumns: i.e. a VERY warm fall. What happens the following winter?
It was pretty overwhelming. This Autumn was our 4th warmest. When we look at the top 10 Autumns, 80% of them had warmer than normal winters as well & by big margins usually.
As discussed already, La Nina is pretty much a wash, but if we want to give cold an extra point you could barely move it over there.
The polar vortex is another very interesting critter that we're only recently understanding more and more. There's some research out there that seems to suggest that climate change is disrupting the polar vortex more than it used to. It seems counter-intuitive but you WANT a strong polar vortex to keep the coldest air bottled up near the pole. When the Polar Vortex is weak or weakened by a series of blocking/warm waves, it can fall apart or split, allowing brutally cold air to reach North America or Europe and Asia.
Even though the Arctic is MUCH warmer than it used to be, if you get that air directly overhead in mid latitudes it's COLD (think of Texas last February!).
This year, so far at least, the Polar Vortex is strong. Last year it showed signs of collapse by late December which allowed two big cold waves: one in Europe late January & then here in early-mid February.
Below is a discussion/tweet from Dr. Simon Lee on the strong polar vortex this season.
A Polar Vortex disruption can have big consequences on a winter. Let's look at last year. It was VERY mild December & January, but then that Polar Vortex disruption yielded a cold outbreak February 5th-20th.
Here's an example, visually from Milan, Minnesota in the winter temperature pattern:
In the Twin Cities, we were running +4.9° above normal Dec. 1- January 31, which is a HUGE margin. Then, that February cold hit. It broke almost NO records here: which is a sign of climate change really: that arctic air isn't as cold as it was a century ago, but it did take a big bite out of that winter surplus of temperature. Those two weeks in February averaged 20° below normal, so that our net winter temperature averaged to be 'just' +0.7° above normal. Had we not seen that brutal cold snap, the winter probably would have ended up around +4° above normal.
The only correlation I could find to support some cold was when I looked at the top 10 hottest summers. Recall this year we narrowly took the #1 spot beating out 1988. Interestingly, when you look at the top 10 hottest summers, on average, they saw cooler than normal winters that followed. A lot of this is skewed by the odd cold winters of the 1930s however and, one of the big drivers of our hot summer was definitely climate change driven. That was evident by the consistent warmth and not really a lot of extremes (like in 1988 or the 1930s).
And finally, Climate Change. Almost any long range forecast MUST include the fact that we're in a 'new normal.' Especially in a place like Minnesota. Our winters are warming faster than at any other period in our year and we're seeing the biggest increase in winter temps of anywhere in the lower 48. To get a legit cold winter anymore is hard to do.
These are the new 30-year normals (1991-2020) compared to the previous ones we used (1981-2010). And just in that minor decade change you can see what's happening:
Just by shaving off the 1980s & adding the 1990s, all but two months saw temperature increases: February & April. To go back to our earlier discussion: this may very well be a result of the increased frequency of Polar Vortex disruptions. There's debate as to whether there's an increase in them or not and a lot about how Sudden Stratospheric Warmings (of the Stratospheric Polar Vortex: which sits above the tropospheric one- the weather portion of our atmosphere) that we just don't understand. It is plausible however that decreasing arctic sea ice and anomalous warmth is pushing it out of control more than it used to.
So now, let's dive into the numbers. First let's look at what happened last winter:
We discussed last winter's temperature trend already: very mild for the first two thirds then that blast of cold blew away (quite literally) that +4.9° surplus and dwindled it to +0.7° overall.
For snowfall, the season was almost normal. We had 48.7" compared to the normal of 52.0". Of note, while temperatures are quite obviously warming in the winter- snowfall is also increasing. We get almost a half foot more snow in a winter now than we used to historically. This is mainly because a warmer atmosphere holds more water vapor, and even though Minnesota winters are more than 2° warmer than they used to be (& increasing), it's still cold enough, most of the time to snow rather than rain... for now, at least.
THIS WINTER: I think snowfall will fall short, only partly because it will be a slightly more milder than normal winter, but mainly because the drought persists. After a drought like we had last summer, that continued into the Autumn, it's hard to see precipitation suddenly reaching normal levels abruptly.
Just like with temperatures, a much more simplistic box of wet versus dry can be made:
The computer models are actually forecasting above normal precipitation this winter, which boggles my mind really. There's almost nothing to support that. La Nina has pretty neutral impact on snowfall and the drought definitely favors less snow, but you can see at least in this demonstration, it's 'a wash.'
My forecast starts us out on a drier than normal path toward close to normal snowfall by February. Snow depths will likely be below normal because of the milder temps (at times) & below normal snowfall. I break down the climatological norms at the bottom of this page much more in depth.
Keep in mind the numbers above reflect just Dec-Jan-Feb (meteorological winter), but I've put the seasonal total overall in the lower left of 42", a full 10" below normal.
This will be reflected in the snow depths as I mentioned:
And finally, last but certainly not least, the temperature outlook.
So, in conclusion: the winter will be overall slightly mild compared to normal, but not off the charts (like summer & fall were). If you're very observant, I'm forecasting a slightly milder winter than last- that's because I'm hedging my bets that we either won't see a Polar Vortex disruption like last winter or it will be more brief. Think more along the lines of January of 2019 where we saw brutally cold air for several days but not two weeks. That was also the last time it was colder than -20°, an increasingly rare feat in recent years.
Our normal coldest winter temperature now is about -16°. I would be shocked if we saw that. Last February we got to -19°. Of course all these figures are based on MSP airport (Twin Cities) climatology.
Again, for reference it's worth checking out my separate page on winter climate. I discuss what's normal, average, and how it's all changing with climate change. There are very interesting statistics.
- Meteorologist, Sven Sundgaard
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