From Almanacs to AccuWeather, the winter outlooks are flying this time of year.
But are they reliable? Or are they just an attempt to grab some headlines?
Today Updraft takes an early look at some of the forecasts and variables that go into seasonal weather forecasting.
Monday’s AccuWeather blog touts “Brutal winter, snow & cold” for Minnesota and the Upper Midwest. That’s a subjective statement, and of course that could be a headline for just about any winter in the Upper Midwest.
But AccuWeather’s idea of a brutally snowy winter this season specifically predicts 56″ snowfall for MSP. That would be a full 30″ less than last winter’s street clogging 86.6″ snow blitz in the Twin Cities! Sounds like a bargain to me.
Accu Weather’s forecast goes on to say “People in Chicago are going to want to move after this winter.” I lived in Chicago twice during my career. Guess what? Some people in Chicago (and Minnesota for that matter) want to move after every winter! AccuWeather then boldly goes on to predict 52″ of snowfall for Chicago this winter….5″ less than last winter.
Seasonal Forecasts: What do we look at?
Seasonal forecasts are a relatively young science.
Getting the 7-day forecast right is a challenge that can turn forecasters prematurely gray. Imagine the odds that go into getting an entire winter a season right.
There are several factors that seasonal forecasters at CPC and other places watch for. Here are a few.
-ENSO cycles: El Nino La Nina phases can tell us a lot about an upcoming winter. Strong El Nino or La Nina years can have as high as a 70% correlation with mild (El Nino) or cold & snow (La Nina) in the Upper Midwest. They can also provide useful “forecast skill” on rainfall patterns in the south.
-NAO, AO, PNA, PDO etc: There are many other indicators that can point to winter weather trends. While each one may suggest a trend…the real problem lies in connecting the dots with reliable accuracy. Which one do we rely on?
Some definitions as pulled form Wikipedia and other sources.
The North Atlantic oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is part of the Arctic oscillation, and varies over time with no particular periodicity.
The Arctic oscillation (AO) is an index (which varies over time with no particular periodicity) of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude.
The Pacific-North American (PNA) teleconnection pattern is a climatological term for a large-scale weather pattern with two modes, denoted positive and negative, and which relates the atmospheric circulation pattern over the North Pacific Ocean with the one over the North American continent.
The “Pacific Decadal Oscillation” (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time.
Watching Siberian snow cover: A new magic bullet?
Seasonal weather forecasters are excited lately about a new indicator that seems to have some skill in predicating the severity of winter and snowfall patterns in the USA. Some forecasters are closely watching Siberian snow cover in September and October.
The theory goes like this. More snow in Siberia deepens atmospheric waves that cause the Polar Vortex to warm and the jet stream to plunge down over the eastern USA…bringing cold and snow.
Figure of relationship between Siberian snow and jet stream pattern courtesy National Science Foundation (NSF). NSF caption excerpt: “When [autumn] snowfall is high in Siberia, the resultant cold air enhances atmospheric disturbances, which propagate into the upper level of the atmosphere, or stratosphere, warming the polar vortex. When the polar vortex warms, the jet stream is pushed south leading to colder winters across the eastern United states and Europe. Conversely, under these conditions the Arctic will have a warmer than average winter.”
But the Capital Weather Gang’s Matt Rogers has a nice write up on why it’s not that simple.
“So is this all we ever need to know about seasonal forecasting? Is this relationship the magic bullet?
Unfortunately, two important caveats limit the technique:
First, the prediction of Siberian snow itself is a challenge. I had the honor of sitting on a seasonal forecast panel with Dr. Cohen in late September 2009. For this WeatherBug Seasonal Conference, Dr. Cohen referenced below normal snow in Siberia to support his very warm 09-10 winter outlook for North America. My Commodity Weather Group team was forecasting a cold and snowy winter due to signs that the North Atlantic pattern (NAO) and the emerging El Niño pattern would be supportive. Within weeks of that conference, it began snowing in Siberia. A lot. And by late October, his very warm winter outlook had switched 180 degrees to a cold forecast. The Siberian snow correlation requires knowing what Siberian snow is going to do.
Second, while I have no doubt that enhanced snow pack in Eurasia is amplifying these waves that warm the Polar stratosphere and creating our mammoth blocking patterns (translation: cold U.S., Europe, and Asia winter), something else is initiating these waves. The reason I believe this is that in October 1998, we had well above normal Eurasian snowfall, but we DID NOT have a cold Eastern U.S. winter. Instead, tropical forcing from a powerful La Niña dominated our warm winter weather. The Siberian snow was there, but the waves were not.
The last two winters have featured relentless waves, triggering cold blocking patterns never before seen in modern times. Last winter’s Arctic Oscillation (AO) was the lowest in the modern record (back to 1950). The North Atlantic Oscillation (NAO) has averaged negative for an unprecedented 15 straight months now. While the Siberian snow is enhancing the waves and blocking, the waves do not start there, and Siberian snow has been heavier than recent years.
So why the record-setting levels? One might argue that the Siberian snow connection is the “elephant’s tail”- an important clue, but not the whole story.”
Why the “crystal ball” may be murky for this winter:
If you’re a seasonal weather forecaster looking for clear indicators of what this winter may look like, you may be disappointed.
The ENSO outlook calls for a weak to moderate La Nina in the tropical Pacific, but not as strong as last winter’s La Nina that was fingerprinted for sending seasonable cold and relentlessly snowy waves into Minnesota.
Siberian snow cover so far is near or slightly below seasonal averages so far in October. There is no clear signal yet of a snowy Siberian Fall that could trigger extreme winter cold and snow in the USA.
The bottom line? Unlike last winter, there may be no clear indicators of just what kind of winter will visit the Upper Midwest this year.
Stay tuned for more…including my own analysis and outlook for the winter of 2011-12 (for what it’s worth) in the coming days!
Here’s a quick preview. No two winters are exactly alike, and there have never been two consecutive winters with 80″+ snowfall in the Twin Cities.