Wednesday, October 31, 2012

Winter Forecast 2012-13: From Russia With Love

This is not the easiest of winter forecasts -- heck, no winter forecast is really easy since you're trying to pin the weather several months out. It's rare that one can hit home run after home run in seasonal forecasting -- some do pretty well...others have their share of hits and misses...and others out there, well, are blooper reel quality.

We tend to switch between pretty well and blooper reel.  Such is life in the Mid Atlantic...and life in the uniqueness of our climate.  This winter, with a Nino that can't fully get itself going, and a mixed bag of signals, provides an increased level of difficulty.

We'll tackle Nino tomorrow...first, the NAO and snow and ice.  There are some pretty clear signs that arctic and high latitude blocking may be a prominent feature this winter.  The problem is that the blocking may not align perfectly for us the entire winter.  The result will be a mixed bag of various signals -- warmth for stretches, cold for others.

First, the NAO.  October's NAO has generally aligned on the negative side of the fence.  Historically, October NAO state has not agreed with the wintertime average about 60 percent of the time, meaning that if we have a negative NAO in October the following winter is positive about three out of five years.

Why are there mixed signals?  The last five Octobers have had agreement -- if it was negative in October, it was negative through winter.  Whether this makes the sixth October in a row to stay in alignment or not remains to be seen.

What contributes to that mixed signal is a rather healthy and increasing snowpack across the Asian parts of Russia.  Snow cover growth during the month has been impressive, with much of Central and Eastern Russia above average on snow cover at this point.  This is critical for winter cold and snow as snow build up helps establish cold airmasses over Siberia and the Arctic, helping us down the line as these airmasses chill and push across Alaska and down into North America.

Compared to other years (see below), we're doing rather well at this point.  Snow is ahead of the last three winters across the northern hemisphere as of October 19th.  It's not a bad place to be if you want a colder winter.

Snow cover rate of growth in October in Eurasia is important -- the faster the rate of growth the better the chances of a colder, snowier winter.  It's not a guarantee but it's a good positive.  Snow cover, through October 19th, is ahead of 2009 across this area.  2009's last ten days were explosive in growth and helped fuel a cold winter for the East Coast.

Snow cover growth in Eurasia -- graphic courtesy of

One of the contributors to the blocking regime is the relative dearth of sea ice across the Arctic -- there have been a few studies published over the past few months that have suggested that the diminishing of sea ice in the Arctic has been a contributor for the extremely blocky feature of the past several winters.  This year's sea ice was the lowest since measurements began in the late 70's.  As we approach winter, we are seeing sea ice catch up but still remain a fair bit below average for late October (second graphic below).

The good news is we are starting to pass 2007, the former record holder for least sea ice.  However, it's important to note that we're still significantly below the realm of average and have a ways to go on this to get closer to average.  Increased sea ice coverage will help increase cold air masses over the North Pole and high arctic as albedo (reflective nature of snow/ice) will bounce back what little solar heating there is in the arctic winter back into space.

Putting all of these factors together and you have a bevy of mixed bags.  Snow cover is up and ahead of 2009 at this point.  Sea ice is gaining ground but still a ways behind where we should be in late October.  The NAO state is negative...which if persistence in recent years is the trend to follow is not a bad thing but since 1950 odds favor a +NAO for the winter based on an October -NAO.  The one thing that does seem a bit likely is a blocky winter overall.  The general lack of sea ice compared to average is something that should lend credence to a winter that will feature some blocks in the overall winter pattern -- the question is whether we end up on the positive or negative side of the ledger!