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There's a buzz going on in weather circles regarding what's known as a stratospheric warming event by Christmas. If you're going huh...just let me explain. While the physics behind sudden stratospheric warming events are complicated, their implications are not: such events are often harbingers of colder weather in the Midwest. A typical warming favors chillier and possibly stormier weather for at least a couple weeks after the event.

Sudden stratospheric warming events take place in about half of all Northern Hemisphere winters, and they have been occurring with increasing frequency during the past decade, possibly related to the loss of Arctic sea ice due to global warming. Arctic sea ice has declined to some of the lowest levels on record.

Sudden stratospheric warming occurs when large atmospheric waves, known as Rossby waves, extend beyond the troposphere where most weather occurs, and into the stratosphere. This vertical transport of energy can set a complex process into motion that leads to the breakdown of the high altitude cold low pressure area that typically spins above the North Pole during the winter, more commonly known as the polar vortex.

The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes where most of us live. When there is a strong polar vortex, cold air tends to stay bottled up in the Arctic. However, when the vortex weakens or is disrupted by warming, like a spinning top that suddenly starts wobbling, it can cause polar air masses to surge south. While we cool, the Arctic experiences milder-than-average temperatures. In essence, it's a total flip-flop.

Tonight evidence is growing that a stratospheric warming event is in the works. Major models are now unanimous displacing the Polar Vortex and spilling plar air into the lower 48 close to Christmas. The colder air could be accompanied by some holiday snow but it all comes down to timing and placement, no way of narrowing down any locations yet as the pieces in the pattern are still moving around. Something we'll keep and eye on in the coming week.

Getting back to the stratospheric warming I just wanted to show you the changes that are being projected in the Arctic. Here's the 10mb temperatures forecast for Sunday December 16th. Minus 82C. up near the pole

By Christmas night the bitter -82 degree cold has been replaced with -16 degree air. Cold by our standards but still a major reversal! The stratospheric warm-up is on. As such, colder air should be forced out of the Arctic and into the Midwest late December into early January.

The next item of business is just how cold. One of the things that bothers me is that the MJO is not forecast to be in its cold phases as it was last Christmas when a similar strat warming produced lows of -22, -24, and -23 December 31st-January 2nd here in Cedar Rapids. This year the MJO is projected by the EURO to be in phase 4 which has a strong tendency to be mild over the country in both December and January.

What this means is that the advertised cold is likely to be far less intense than last year. It may also be short in duration or come in short weaker bursts. If we were in phase 1 or 8 I would be concerned about a major Arctic outbreak. But 4 leaves some significant doubt in my mind as to what the overall impact will be and for how long. Anyway, I thought I would address the stratospheric warming potential before you heard it on the grapevine. Whatever happens, the next 10-12 days look mild. Here's the day 1-5 temperature departures on the GFS.

Days 5-10...out to December 21st.

Now look at the change Christmas Eve. That's a pretty good flip if it comes to pass!

That's all for now. Roll weather...TS

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