Thursday, November 29, 2012

Why The Pattern Won't Deliver Cold Air


Sometimes when we analyze weather patterns, we tend to look only at the United States or North America.  I want to broaden the perspective with a birds-eye on the northern hemisphere. 

Everyone wants to know when we will get cold and stay cold.  With the current pattern which I will explain below, it doesn't look like it will flip to that anytime soon.  By that I mean within the next 7-14 days.  I do think occasional fronts will bring some rain chances followed by cooler air.  But in most cases, the cooler air will come and go quickly. 

This is the European 168-240 hour mean 500 mb HGT ending late December 8th.  This is off the 0Z Thursday run.  This upper level chart shows several important features one of which I will be watching as we await any sort of pattern change.  Notice the red area in the Pacific.  This MONSTER ridge of high pressure goes up through the Bering Sea.  Also, notice the blue shaded areas.  These are troughs of low pressure.  One is over Asia and the other in the Gulf of Alaska.  This jet stream configuration is known as an "Omega Block" because it resembles the Greek letter "Omega".  The flow of air in the northern hemisphere is from west to east.  This brings the jet stream winds around that Gulf of Alaska low into the western United States.  This is why they are expecting a rather stormy period.  With ridging downstream across the eastern United States, the storms are quickly dampened out and scoot off to the east-northeast.  This only allows for quick moving systems to only tap into small amounts of cold air for a short time, then it's back to warming.  This blocking pattern is very stable and takes time to break down.  It can take weeks for any sort of pattern change.  Will it happen? Of course it will.  When is the big question.  Also, once we undergo a pattern change, what will it look like?  We need that blocking (ridge) over the west coast into Alaska to deliver cold shots into the central and eastern United States.  With all that said, based upon this analysis, I don't see a change in the pattern until the at least the second half of December or later.  I'll keep you updated.

4 comments:

jimmylee42 said...

Todd-Interesting post on the Omega weather phenom. I remember one in Dec. 1984 that gave us one of the warmest Decembers in my memory. We know what happened in January and early February of 85. We had below zero and single digit lows and heavy snow at the end of the month with teens for highs during the snow cover.

Anonymous said...

Please don't let this be a repeat of last winter!

Omarr Wilson said...

I've been looking at that for the last couple of days. I'm thinking early into mid January is when we will start to really feel like Winter. Nothing but hit and miss chances until then. The thought's of snow before Mid January has vanished from my mind. But Like you always say Todd. "Never say Never with Arkansas Weather."

Will said...

Old Man Winter due in mid December as models are beginning to smell out first pattern change of the season. May not be snow / ice but cold is coming. Good snow pack in Canada, unlike last year.

NOAA Winter Guidance