Monday, March 05, 2012

The Fail Of Winter -- Where Was The Block?

Over the next couple of weeks, I'll write some occasional musings about why this winter failed for cold and snow lovers and how badly the fail was. I think we all know that most everyone's seasonal forecasts for the coming winter will go in the ash heap of history's fails...along side bell bottoms, leisure suits, and the Kid N' Play fade haircut.  What were a couple of the reasons why those forecasts of cold and snow failed?

First, let's look at the upper atmosphere from a polar bird's eye view.  This shows the mid level (aka the 500 mb level of the atmosphere) pattern's anomaly (how different to normal things ended up being) for both this winter (first map below) and last winter (second map below).
If you look at the two maps, you see things are almost completely opposite from last winter to this winter.  The reds on the map show "ridging" or high pressure systems aloft whereas the blues and purples show where troughs reside.  This past winter featured a quasi-permanent cold vortex over Alaska and also over Arctic Canada.  These features helped to bring extreme cold and snow to Alaska and also kept the cold bottled up far to the north, in general this winter.  The only couple of forays of arctic cold into lower latitudes were either fleeting in North America's case or went over to Europe, where they had a three week stretch of epic winter.

Last winter was different.  Ridging in the higher latitudes blocked in place much of the early half of winter, which set us up for a cold December-January locally and then set us up for the icy start to February as this pattern began to break down.  However, the blocking was strong enough to provide an eight week stretch of rather significant cold for our part of the world.   You can see the ridge over Greenland and the trough look from the East Coast of the US to Spain...if there were ever a textbook -NAO look, that would be it.
This resulted in a cold pattern for the East last winter...but the opposite look this winter has yielded a milder look for the East Coast.

The other pattern kicker...the one that prevented the arctic cold that built over Alaska from making any significant inroads to the US was the strength of the Pacific jet stream this winter.   The two maps below show the jet stream in this past winter (first map below) and in last winter (second map).  The brighter the color, the stronger the wind in the general jet pattern.
Three things stand out here.  First, the Pacific jet was stronger overall this past winter compared to the prior winter.  The stronger jet stream prevented cold block patterns from really establishing themselves over North America.   Second, that jet's mean push of energy was taking storm tracks into British Columbia, Yukon, and a lesser extent Washington.  The mean storm track being into Western Canada allowed for warmth to flood into the US but also pwned Alaska with snow.

Third, look at the difference in wind speeds and track for the jet stream over the Eastern US from last winter to this.  The jet was ripping across the mid Atlantic and was rather potent at our latitude the prior winter while this winter was much more relaxed.  The weaker jet this winter was also positioned in a southwest to northeast orientation along the East Coast, meaning there was a stronger ridge of high pressure in the Atlantic at play that prevented storm systems from taking off the coast and sent the mean storm track farther west.  The stronger jet last winter was oriented farther south last winter (weaker ridge of high pressure in the Western Atlantic) and allowed for the two big snowstorms that developed -- the Boxing Day event and the Thundersnowstorm, as well as the icy events in early February.

All this means there were three factors that screwed the East Coast from cold and snow.  The lack of high latitude (Arctic and Greenland) blocking patterns (no -AO nor -NAO), a stronger Pacific jet stream that was farther north and keeping the cold bottled in Alaska, and a stronger ridge of high pressure in the Western Atlantic this winter all played a pivotal role in making the winter a failure from a snow and cold standpoint.

I'll continue to muse on some information about what all went down and throw some data at you over the coming days.