The Cliff's Notes version of this post:
- Milder temperatures
- Fewer snow events (but not necessarily less snow than normal)
- More ice events
- A brown Christmas
- An active February
El Nino is basically a warming of surface water temperatures in the eastern and central Pacific near the equator. This phenomenon results in some pretty solid weather trends for North America. Among those is a drier and warmer winter in the Midwestern section of the United States.
At first glance, those "warmer and drier" deviations may seem small. Generally, El Nino would mean our average temperatures during the December/January/February "meteorological winter" might be 2-4 degrees above normal...and snowfall might come in 3-5 inches under seasonal averages. But in 1997-98, El Nino produced a greater disparity for southern Wisconsin -- with temperatures averaging 7-8 degrees above normal, and snowfall averaging as much as 10 inches below normal.
Here's a great summary of the '97-'98 El Nino from the National Weather Service out of Milwaukee.
The "nuance" of El Nino (for us, anyway) is that for warmer temperatures to occur, you need a greater frequency of south/southwesterly wind patterns. And a few degrees can make a huge difference.
In the Madison area, the lowest average temperature in winter is 27 degrees. That occurs in mid-January. Add a few degrees to that average, and you're nearer the freezing mark. Add a southerly wind flow, and you have the perfect recipe for ice storms, as warmer air overrides colder temps at the surface -- hence my prediction of more icing events above.
That more frequent southerly flow also diminishes, in theory, anyway, the frequency of Alberta Clipper snowstorms -- those fast-moving lows from the northwest that dump moderate amounts of drier, fluffier snow. Hence my prediction of fewer snow events.
But if you start at the source of El Nino...add a south/southwesterly wind flow...mix it with intense low pressure in the New Mexico/Texas area, you increase the chances of the snowstorms that dump the most snow on southern Wisconsin -- panhandle hooks. If you want to learn more about this specific storm track, read my post from this exact date last year. So, while we may have fewer snowstorms, those that we do get may dump a lot of snow. Hence the prediction that our average snowfall -- usually around 4 feet for the season -- may still materialize.
So what about the February prediction? Every El Nino computer model, and there are dozens, predicts a moderating of water temperatures as we go through the next 6 months or so...to the point where water temperatures return to near-normal by next May. That moderation will make its greatest swing around February and March, which could lead to a more active weather pattern that feels more like a typical Wisconsin winter. If Vegas had betting odds on this sort of thing, I'd put money down that February will be our most active weather month here in Wisconsin. Thankfully, I'll be in Key West for the first half of it.
In the short term, look for higher than average temperatures and more rain events through December. If it snows before Christmas, and it probably will, chances are good that temps warm enough to melt it. Hence the brown Christmas.
That's my story, and I'm sticking to it. Leave any questions in the comments.
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