The Polar Vortex is not a NEW thing nor is it a term from “the media”, meteorologists have followed it for decades and it got a lot of attention 5 years ago and because some people had never heard of it they jumped to the conclusion that it was new or made up. It likely has existed as long as the earth has had an atmosphere. It was first mentioned in public writings in 1853!
It exists at various levels of the atmosphere and is NOT a storm or a cold wave. However, it can influence both storms and air masses when it behaves in different ways related to “Sudden Stratospheric Warming” (SSW) events.
Not every SSW and Polar Vortex disruption results in a big storm or big cold wave in the U.S. but sometimes they do, other times it occurs in Europe or some other part of the world depending on the configuration of the polar vortex collapse (no two events are ever the same).
Polar Vortex disruptions and major mid-winter SSW warming at 10mb does not guarantee a cold December or a cold winter as sometimes the coldest air is deflected elsewhere, as Dr. Judah Cohen graphics show no two are alike:
Research by Karpechko .et .al show only about half of past SSW events actually propagated to the surface.
So we can have a SSW event but unless the Polar Vortex rotates or splits in the right manner it won’t result in a cold blast to Europe or America.
Also a -AO/NAO SSW Polar Vortex disruption is not the only way it can get cold and stormy in the Eastern U.S. as I discussed in my earlier winter outlook blog post.
Vortex disruptions and SSW events are related to the AO and NAO jet stream blocking patterns (Wxbell graphic).
A vortex exists over BOTH poles of the earth North and South, we only focus on the one that impacts weather in the Northern Hemisphere where we live.
When it brings a cold wave it’s usually not the true vortex itself that comes to the U.S. but rather a reflection of it or a piece or “lobe” of it extending from the stratosphere into the troposphere impacting the jet stream and thus our weather. The SSW and vortex disruption start way up in the atmosphere close to outer space then “work their way down” to weather levels (nexusenergy graphic).
Some years it stays in place all year long or all winter long, other times it shifts periodically onto one side of the hemisphere or another.
The stratosphere ranges from 6 to 31 miles high (33,000-164,000 feet) at mid-latitude.
I mention it now because the action of the PV is something long-range forecasters ATTEMPT to estimate when making a winter forecast but it’s predictability remains elusive.
Years with a lot of SSW PV disruptions tend to be colder winters and those with few tend to be less so.
It is a little more predictable in the short-term rather than months in advance.
I bring it up because we are seeing signs of such an “event” in current data observations and in some computer model simulations.
Notice that over the area above the North Pole the pressure pattern splits as temperatures high up warm up.
POLAR VORTEX CONFIGURATION NOW (33,000 Feet):
GFS ENSEMBLE MODEL SIMULATION FORECAST DECEMBER:
So clearly the model is projecting a significant weakening of the polar vortex and getting into SSW territory that could cause the type of split that could send colder than normal air South again middle or end of next month.
Keep in mind the effects on the weather are not instantaneous, it takes time for the lower atmosphere to respond to changes at the top (downward propagation) so there is a delay in any impacts of a couple weeks on average after a SSW displacement event occurs.
I’ll continue to monitor to see if real-world observations show the stratospheric polar vortex weakening or not.
As I’ve shown in previous blogs this October and November have been a lot like last year, which was followed by a WARM December and a mild winter.
Commodity Weather Group Research has shown this is typical of most December cases with an El Nino like ocean state but a La Nina type atmospheric signature (as we have now). They found just 3 out of 10 historic examples that were cold while the mean of such Decembers is above-normal:
As it is the probability that this December will be as warm as last December looks fairly low, and if the PV disruption being projected is real it COULD even end up cooler than average.
As it is, I have UPDATED MY WINTER OUTLOOK ANALOGS and they are not as warm as the first set and also indicate the snow/ice odds are normal to above normal for the coming winter in North Georgia.
NEW ANALOG YEARS WINTER 2019-20:
Reminder the above maps for winter is the average for the 3-month period NOT every day or every week.
The recent trend of the greater winter feel and impacts to come later in the winter and extend into March is likely to repeat for the coming winter.
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