LA NINA HAS WINTER IMPLICATIONS...

October 4, 2017

By all accounts it appears the coming winter will not be influenced by an El Nino as it was 2 years ago. Currently the water in the tropical Pacific is actually cooler than normal and that constitutes La Nina conditions...just the opposite.

The cooling has expanded during the past 2 months.

This late in the year it's a near certainty there will be no El Nino, which often leads to a mild Midwest winter. Here you can see the CPC model indicates a 4% chance of El Nino in the December-February period.

The CFSv2 member plumes are consistent in showing the La Nina reaching -1 to 1.5, nearing moderate strength.

The other dynamical models are not as aggressive with a combined mean closer to -.5. That's a weak La Nina. It borders on La Nada which is more of a neutral phase.

The fate of our winter will likely be determined by the amount of blocking that takes place at high latitudes. It's been proven that high latitude blocking makes all ENSO states colder, especially La Nada (the neutral phase). You can see in this graphic from the Climate Prediction Center that the coldest winters are found when blocking establishes the negative phase of the NAO (North Atlantic Oscillation) in conjunction with the negative phase of the AO (Arctic Oscillation)...bottom left.

One thing I've noticed about weak La Nina's is that they have a propensity to produce extreme variations in temperature, especially when high latitude blocking is minimal, During periods when the upper level flow is amplified (or buckled) Arctic air can bring bitter and often snowy periods.

Conversely, when de-amplification takes place the zonal flow off the Pacific can kick in and that often results in some healthy but generally short lived warm-ups.

So, the question in my mind is how much blocking materializes and where. That combined with the ultimate strength of the La Nina will determine how much shoveling takes place across the Midwest. I still think the north central United States has the best chance of seeing the coldest winter temperatures. 

 

Just for kicks here's the CFSv2 winter outlook. Remember this model has the strongest La Nina compared to the other dynamical models. It shows the cold a little further northeast than what I'm expecting.

Precipitation is generous over the central Midwest and Great Lakes. This would imply above normal snowfall.

For now, we wait and see. Roll weather...TS

 

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© 2019 Terry Swails