Friday, February 5, 2021

It is Winter After All - update Feb 5, 2021

For the most part it's been an abnormally mild winter - so far.   With the weather pattern that was established during the fall, the initial thought was that any brutally cold periods could be somewhat infrequent.  Without knowing what the main influence on the weather pattern was, confidence on temperature and precipitation patterns was not very high.  

First - back to the post I did on the 22nd.  I've been discussing a mid-month chance of the next significant precipitation event (for the High Plains), specifically a day or two around the 15th. What is currently going on across the northern hemisphere (and on our side - as opposed to the Asian continent) may mitigate what happens around the 15th.  

First things first.  Those that have read this blog for many years have read about the Arctic Oscillation (AO) and North Atlantic Oscillation being a contributor to our weather, especially during the colder months of the year.  A very high percentage of the time when the index of the AO becomes negative for a period of time, high latitude (particularly Arctic) airmasses are dislodged into the U.S.. Look at the AO for the past 3 months and what is expected for much of February (the gray shade)....

The sigma numbers on the left scale indicate the Standard Deviation from normal (SDN).  A 2 to 3 SDN is extremely significant and for the index to be that low/strong for that long is mind blowing.  Sometimes this means the cold is the other side of the hemisphere but other times it is on our side. But, the cold usually oscillates from "that side" to "our side".  That hasn't happened until currently.  We're about to pay for that relatively mild winter!

Here was this mornings upper air chart....

The big red L up across the Canada is the "polar vortex" (strong upper level low) that has been displaced south and west from a typical winter location.  This is in a perfect position to allow surface high pressure to get very high and move the Arctic air south.  BTW, this setup is a direct result of something called Sudden Stratospheric Warming (Google that if you want to know more) that occurred a couple of weeks ago.  The red X near the Canada/Washington border is a fast moving upper level disturbance that will help spread snow across Nebraska and the northeast half of Kansas late tonight and Friday.  Behind this system will be the first surge of Arctic air into the central U.S..  

But there are other disturbances that will be sliding southeast across the central U.S. into the first of the week.  The Arctic air will mitigate very briefly but then come in with a vengeance Sunday night and stick around through next weekend (the 13-14), at least.  I would not be surprised to see single digit highs later next week with a possibility of low temperatures falling well below zero (-10 to -15 depending on clouds clearing out and any snow cover).  This is for the high plains.  Farther northeast temperatures will be even colder!  

As far as precipitation next week, there should be several rounds of snow that eventually will add up to several inches.  But, it will be a very dry snow with very little moisture with it.  As far as a significant storm around the 15th as indicated in the past few blog posts - there could still very well be one but with the airmass being so cold and eventually very deep (the cold air), an organized storm does not seem likely.  It's still possible, just not likely.  But, there should still be a period or two of snow.  By the 15th and after several rounds of snow, there could be snow depths of at least six inches across some areas of the high plains.  Here is the precipitation outlook through next Friday (from the Weather Prediction Center)...

Since any snow would be dry in nature, it won't help much with the drought.  Here is the latest map valid for February 2.

I'll try and update next week and discuss the trend through the rest of February and into March.