A typical Midwestern winter had to start at some part, right?
Up until January 12th, I’m sure plenty of snowmobiles, sleds, and skis were collecting dust instead of powder.
Figure 1: 4.3″ of snow fell last month, which was 7″ below average, and at least 20″ behind the previous 4 Decembers.
December had an average high temperature of 35° and a whopping 4.3″ (Fig. 1) of snow for the entire month. This month has had two record-setting 53° days, an average high of 41° until Tuesday, and just a trace of snow through Jan. 11th.
Why has this winter been so mild and relatively snow-free, despite predictions in November saying the opposite?
Let’s start with La Niña, which is influencing our weather right now. La Niña may sound familiar, as last winter was a La Niña winter. We were definitely wetter than average, especially last December.
Figure 2: La Niña reduces weakens the polar jet stream and causes altered weather patterns over North America.
During a La Niña event, cooler than average waters occur in the Pacific near South America (Fig. 2). The jet stream that drives our weather gets its strength from the temperature contrast between the Equator and the North Pole, and when that contrast is weakened by a cooler Pacific, the jet sinks south. In turn, weather patterns in North America are affected by the altered path of the polar jet (Fig. 3).
Figure 3: Usual impacts of La Niña on U.S. winters
We’ve been anything but cooler and wetter than average, so far. Here’s the next step: another disturbance called the Madden-Julien Oscillation (MJO) has come into play this winter (see how complex weather can be!). I won’t bore you with the details, but the MJO can modify the jet stream further, and push it north into Canada.
Figure 4: The MJO alters the jet stream, and keeps precipitation and cold air north of the Midwest.
This matches the pattern we where in for the start of this winter (Fig. 4), with the polar jet keeping cold Arctic air bottled up in Canada (keeping us warmer than average), and keeping the main storm track north of the Midwest (less chances for snow).
Looking ahead, now that there is a decent amount of snow pack after Thursday’s storm, temperatures will stay colder than where we’ve been lately for a while (snow reflects sunlight, so the sun cannot heat up the air as easily and temperatures stay colder).
Figure 5: The CPC's forecast for the next 3 months
The Climate Prediction Center is predicting temperatures to stay near or slightly above average for the rest of winter, which may be MJO’s doing. La Niña will still be playing a role this winter. The CPC also has above average chances for precipitation for the Midwest (Fig. 5). I guess that means we may have a chance at a true winter full of snow after all! Stay tuned!
News 19 Meteorologist Alex Kirchner
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This post was written by akirchner on January 14, 2012