You may recall in one of my posts about a week ago that I stated, "that until I saw some change in the depth and position of of a frigid air mass in the western Arctic, I saw no reason why our mild weather would not continue". Well, the signs are there now and that means spring could face a challenge from time to time in the next couple of weeks.
Let's start with the stratosphere at 10mb. That's the highest levels of our atmosphere where temperatures are known to be crazy cold. Here's today's departures, you can see the frigid blob positioned west of the pole in purple. Temperatures there about -70C. Plenty of cold air over the rest of the 10mb stratospheric layer of the Arctic.
In just 10 days look what happens.The stratosphere is dramatically warming near the pole and into much of Canada. Some places have warmed 40 degrees C to -30. That's still dang cold but it's a massive change.
It's not unusual at this time of year for winds in the polar vortex to weaken. The cold air then descends very rapidly within the polar vortex and this causes the temperature in the stratosphere to rapidly rise, as much as 50°C over only a few days; hence the term sudden stratospheric warming.
As the cold air from high up in the stratosphere disperses, it can affect the shape of the jet stream as the cold air sinks from the stratosphere into the troposphere where we exist. It is this change in the jet stream that causes our weather to change. It can take a couple weeks to kick in but it increases the likelihood of colder weather in the Upper Midwest.
Several other factors are starting to indicate colder weather as we get into mid and perhaps late March. Not the least of which is the EURO MJO (Madden Julien Oscillation). Since early December we've been cycling through warm phases 3 through 6. Look what's projected going forward.
The dotted green lines are the daily movement from March 10-24th. The MJO is showing tropical convection in the regions 1, 2, and 3 of the tropical Pacific. In March that's associated with below normal temperatures across the Midwest as you will see in the phase analogs below. 2 and 3 are particularly chilly.
I'm also seeing significant movement by the EURO and GFS to send the EPO (eastern Pacific Oscillation) into strongly negative phases. That hasn't happened to this degree in months. The EPO is big driver during the cold months and I suspect this is in reaction to all the events previously discussed above.
See what the negative EPO is known for.
Even the AO (Arctic Oscillation) is shown nearing neutral, a region its not been close to in ages running strongly positive since early December. If the strat, warm occurs as projected, the AO may go into negative territory as the polar jet weakens allowing strong highs and polar air masses access to the U.S. later in the period.
How all this manifests itself is hard to say but these are 3 significant teleconnections for late season chill. (better now than January when it could be really ugly). The EURO shows this 500mb jet stream pattern March 25th. That ridge poking into Alaska is formidable and that downstream central U.S. trough looks cold.
So cold that it leads to temperature departures as low as 35 degrees below normal over central Kansas. My area is more like 12-18 degrees below what's typical. I think the most likely period for colder than normal readings to consistently show up would be after March 19th. By then normal highs are in the upper 40s so even if you are 15 below normal you are at least above freezing for a high....small consolation!
There should also be an active storm track along the thermal gradient but where that sets up is a tad difficult to see at this range. The EURO has it from my area south and east.. These are the 15 day precipitation departures.
I'm thinking that's enough conjecture for now. The first pop of cold doesn't show up until the end of the week and it could lead to some light snow over far southern Iowa, northern Missouri, and central Illinois Saturday. Most of that should stay south of my area. Maybe a dusting in a few spots south of I-80 if the current trends hold. Saturday will be crisp with a raw E/NE wind and highs in the upper 30s to near 40. Chillier where any snow falls to the south. Gotta run. Roll weather...TS
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