Polar vortex are two words Minnesotans don't want to hear. The name is often used to refer to any cold, nasty winter weather, but the polar vortex is actually a very complicated phenomenon, even to meteorologists and climate scientists.
Despite a chilly January and February, this year's winter was not defined as a "polar vortex" winter. The polar vortex is also a normal, regular winter occurrence. It develops late every Autumn near the pole (polar) because of the Arctic receiving little or no sunlight. This allows cold air to build and descend and as it does it rotates counterclockwise (the vortex).
There's some fascinating research that links the rapidly warming Arctic to disruptions of the polar vortex. The idea is that as we warm the Arctic faster than the rest of the planet, the temperature contrasts are lessened, weakening the jet stream. This, combined with a reduction in sea ice means more heat is being released into the air, influencing a domino effect of conditions leading to large scale warmth that builds (into a sort of wave) and can knock the polar vortex off the pole, stretch it, or even break it apart.
These disruptions then have particular impacts on the weather at mid latitudes, such as Minnesota (central North America), Europe, and Asia/Siberia. So, ironically, global warming is leading to some short-term cold outbreaks for us. Keep in mind though that overall we’re much warmer than in the past and these are shorter-term weather patterns.
This year was a strong (fast) polar vortex, which meant the coldest air largely stayed bottled up near the pole. The reason we had bouts of cold was because the vortex was stretched by being bombarded by warm air out of Siberia and Asia, but because it was strong, it didn't get knocked off the pole or broken apart.
When we have a major disruption, very cold air spills farther south than it would normally. This is what happened last February (2021). When that happens, we get very cold here and that cold reaches places like Texas, which resulted in record coldand devastating power outages. But in a stretched year like this one we get smacked by lobes of cold but are spared from the worst. That's why this year we actually didn't have any cold extremes whereas last February we had that streak of days that stayed below zero.
We were just consistently cool this winter (more on that in a La Nina discussion ahead). We watch for these events in the winter because they have major impacts within weeks or a month after a disruption. The vortex usually regroups after such events during the winter because it's still dark with a net loss of heat.
A similar thing can happen in the spring. Most years the polar vortex dies a gradual death as sunlight returns to the arctic. This allows chunks of cold to break off slowly, creating storm systems and normal spring cold snaps. Some years however, a major and abrupt disruption can occur in March. When this happens the vortex cannot recover because sunlight is increasing rapidly at the pole.
This year we had a strong vortex recall. It had a lot of core cold air contained so when it was broken up by a major wave of warmth in March it unleashed a full pool of cold that also splintered off all over the place. Our frequent storms this spring have been a result of these chunks of cold being thrown around and a lot of cold air to circulate, particularly in North America and Europe. A normal spring sees a gradual dissolution.
Polar vortex in your kitchen as an example?
You can think of it this way: Let's say the Arctic is an industrial sized fridge/freezer. And say the return of spring is the equivalent to 'pulling the plug' on the fridge (i.e. it cannot rebuild cold air) and you set the thermostat 10 degrees warmer. The jet stream and the polar vortex winds are the opened doors on your fridge.
In a normal year the doors flutter open/closed/open/closed releasing cold in waves that are warmed by the ambient kitchen air. In a normal year you'd feel the quick breeze of a door opening and closing but not so dramatically and the room would warm slowly as the puffs of occasional cold from the fridge act to slow the warming. This will help regulate the temperatures and have a reservoir of some lasting cold.
This year (and also in 2018 for a recent example) we had a major disruption, which is comparable to ripping off the fridge doors abruptly. That means the cold is unleashed all at once into a big blob and splinters through the kitchen. You'll definitely notice the blast of cold until the room warms it up. This could take minutes (or 4 to 8 weeks in the atmospheric time frame).
Yes, the furnace is trying to warm the room to that setting you entered but you won't notice the warmth yet because it will take time to counter that blast of cold. But once that cold is finally warmed up, there's nothing left in that fridge to hold back the temperature and the room will then heat up rapidly, just like we experienced in May 2018 when it was 100 degrees on Memorial Day weekend.
This year's example means that temperatures are going to likely spike because we've warmed up all the cold there was. This is why I have a hunch (and most official NOAA forecast and computer model forecasts agree) we'll see a hot summer, especially given the background trends in climate change, which of course has made it pretty hard to get a cool summer these days unless it’s really rainy. It’s much easier to have a cooler winter sometimes, with La Nina patterns and polar vortex disruptions described above.
A great example of this crazy scenario is 2018 because it was more exaggerated than usual. We had a major disruption of the polar vortex in late March and that translated to severe cold in April for Minnesota. It ended up being our fourth coldest April, much colder than this year and there were two blizzards. But we heated up all the cold there was so temps in May flipped like a light switch and was the seventh hottest May on record.
The chart above shows temperatures for spring 2018. The brown shaded region is the normal temperature range (+/-1 standard deviation: i.e. 68% of data falls within that range in a 'normal' year). April 2018 had two major cold (record breaking) blasts that were statistically significant at more than a standard deviation below normal. This April, despite being cold, hasn’t really broken any records of significance. May 2018 then saw three major heat waves that weren't quite a standard deviation above normal except at the end when we hit 100 degrees on Memorial Day.
What to blame for the cold April and mid to late winter?
Large scale weather patterns are very complicated. It’s naïve to think that only one thing influences a month or season. Alas, while we can definitely blame the death blow to the polar vortex for April cold, the pattern that allowed it to remain so consistently over us was largely influenced by the winter La Nina pattern which kept us chilly January through February (and to a lesser degree March). In a La Nina pattern we tend to see a big ridge (high pressure) develop in the eastern Pacific.
This is a result of air that rises over the tropical western Pacific (what goes up must come down) and descends east (thanks to the jet stream). That upper level ridge pattern to the west means we are funneling air from the north and northwest consistently.
La Nina is the opposite of El Nino, when waters are warmer in the eastern Pacific. Where waters are warmest is where there’s the most thunderstorm activity and that influences air flow substantially.
So in short, the winter La Nina pattern in tandem with a big disruption to the polar vortex created unusual cold for April and likely into the start of May, along with an active storm (mid latitude cyclones/low pressure storm systems, not thunderstorms per say) season.
It's probably going to be a hot summer
Get ready to get hot. La Nina has little influence on our summer pattern because the bigger influence now becomes the sun and massive heating of the continents, whereas in the winter the biggest source of heat are the oceans, which are influenced by El Nino/La Nina. The polar vortex is also not a thing anymore because we spent all spring heating it up, much to our displeasure.
So when will it get hot?
I did some digging on past springs. In significantly cold Aprils (defined by one or more standard deviations below normal), there usually is a cool signal persisting into May (which makes sense) but at a much lesser extent. Three out of four Mays that follow a really cold April are also cool. That tells us that after a particularly cold April, some of that cold lasts into May enough to keep the whole month's average down even if it's hot later in the month.
BUT... the opposite is true when looking at summer. After a cold April, three of four summers are hotter than normal.
If we plot all the models through May 12 (that's the farthest the operational models go out that we use every day for detailed forecasts), we see a cold trend through the first week of May. But then all of the models jump to normal during the second week of May.
This would be pretty typical for scenarios like we've been discussing. It doesn't mean there won't be another chill. In fact there likely will be another mid or late month cool blast, but the overall pattern turns around toward normal and even above in the second half of May. The simple fact is that the remnant cold of the polar vortex is no match for spring and summer sunshine.
The longer range models that go out 3-6 weeks give us another good view. These are the long range versions of both the American and European models as ensembles. Ensembles are literally just different scenarios of the same model (i.e. we fudge the initial data a little this way and that to come up with 50 different forecasts from the same model to see if there's a consensus or what the average is.)
Oranges, reds and browns are 70s, 80s and 90s. For example, around May 11 there are 3-4 model scenarios that create a 90-degree high in the Twin Cities, while most of the other 46 model scenarios are a little cooler. The average of the models is 80 degrees.
Will that specific number happen at that specific date? Probably not, but in longer range forecasts it's all about trends and patterns. The best way to look at this model is just stare at the colors and it's quite obvious that warmer colors become much more numerous in the second week of May and beyond, leaving the blue (cooler temps) behind.
Again, here's a two-week forecast using the average of the models rather than all the different model lines at once to give you a smoother example of the trends. We are hanging out below normal until temps spike after the first week of May.