You hear about the polar vortex every so often and it's namesake is used to refer to any cold, nasty winter weather but the the polar vortex is actually a very complicated phenomena, 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, regularly winter occurrence. If develops late every Autumn near the pole (polar) as a result of the arctic receiving little or no sunlight. This allows cold air to build and descend and as it does it rotates counter clockwise (the vortex). There's some fascinating research that links the warming arctic to the polar vortex; specifically disruptions of it. When large scale warmth builds it can knock the polar vortex off the pole or stretch it or even break it apart. These disruptions then have impacts on weather at mid latitudes such as Minnesota (central North America), Europe, and Asia/Siberia.
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 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. When that happens we get very cold here and that cold reaches places like Texas. 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 not only was the coldest temp of the season colder but we had that streak of days that stayed below zero. 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 (energy).
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, so 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. 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 suddenly 10 degrees warmer. The jet stream and the polar vortex winds are the doors on it.
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 cold lasting a while.
This year (and also in 2018 for example) we had a major disruption- comparable to ripping off the fridge doors abruptly. That means the cold is unleashed 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 atmosphere). Yes the furnace is on 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 pretty quick.
The "this year" example means that once we've finally warmed up all that cold (whether it's this week or next or two more weeks still) temperature are going to likely spike because we've warmed up all the cold there was. This is why I have a hunch (& most official forecast and computer model forecasts) we'll see a hot summer- especially given the background trends in climate change anyway.
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 that April for us: it ended up being our 4th coldest April- MUCH colder than this year (plus two blizzards!). However, we heated up all the cold there was so that May flipped like a light switch (or furnace?) into our 7th hottest on record and that summer ended up being a hot summer also at 1.5 times the standard deviation above normal.
The chart above shows temperatures for spring of 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 blasts that were statistically significant at more than a standard deviation below normal. May then saw three major 'heat waves' that weren't quite a standard deviation above normal except at the end: we hit 100 degrees on Memorial Day.
Now to the forecast
Alright, we know it's going to get hot and we'll be complaining about that instead of the cold- but when?
I did some digging first on the past: statistics. In significantly cold Aprils (defined by one or more standard deviations below normal), there usually is a cool signal persisting into May but at a much lesser extent. 3 out of 4 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 on.
BUT... the opposite is true when looking at summer. After a cold April, 3 out of 4 summers are hotter than normal.
If we plot all the models through May 12th (that's the farthest the operational models go out that we use every day for detailed forecasts): we see a cold trend yet through the first week of May but all of them jump to normal during the second week of the month 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 weeks 2-4 of May.
The longer range models that go out 3 to 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, browns are 70s, 80s and 90s. While for example around May 11th, there are 3 or 4 model scenarios that create a 90 degree high, most of the other 46 model scenarios are a little cooler so the average is 80. Will that specific number happen at that specific date? Probably not but in longe range forecasts it's all about trends. The best way to look at this model is just stare at the colors & it's quite obvious that 'warmer colors' become much more numerous suddenly in the second week of May and beyond, leaving the blue behind.
Again, here's a 2 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:
Will we stay normal (which by that time is in the mid to upper 60s)? Maybe not, but the overall trend will be there and beyond likely. Until then, we'll wait & see on spring 2022.
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