La Nina and winter 

Posted: 11:47 am Sunday, September 17th, 2017

By Kirk Mellish

Its getting a bit hot and humid again, but many are thinking ahead to what follows the active hurricane season.

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The expectation had been we would have a neutral to weakly cool ENSO signature for the coming autumn and winter. But odds of that are declining with all indicators now pointing to a La Nina or -ENSO Pacific Ocean sea-surface temperature anomaly.

The first thing to remind you about though, is inevitably any LA NINA will not be the only factor that influences fall and winter weather patterns.

This is NOT a forecast for the coming winter, because an August or September winter outlook is too early to be taken seriously in my opinion. We all know long-range forecasting is still developing science and even forecasts made at the start of a season fail or at least come up short often. They are better than a coin-flip but not by all that much.

What all of that means for the month of October is an active tropical cyclone risk in the Atlantic Basin through the middle of the month, as potential for southwesterly wind shear over the Caribbean Sea and Gulf of Mexico will likely drop. This scenario makes the threat from Maria ever higher. The ECMWF weeklies show a +PNA type response (warm West vs. cool Central/East), something along the lines of  early autumn of 1995. The eastern third of the country could have to deal with a combined tropical feature with or without an incoming strong cold front before mid-October.

LA NINA WATCH:

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OTHER PRELIMINARY ANALOG YEARS:

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I have some missing data for the oldest years, but what I have shows an average of 1.2 inches snow in Atlanta.

PREVIOUSLY the expectation was more NEUTRAL:

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But NOW it’s for a negative ENSO/La Nina, which on average typically yields:

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It’s also interesting to see what’s happened in the 2000s.

Does a cool summer suggest a cool winter on a national basis? It has in the 2000s:

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And there has been a pattern of winter types in pairs in the 2000s:

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So for now just food for thought, as we wait for critical signs to come from the data of Sept-November. I typically make a preliminary winter outlook by Halloween and a final by December 1st.

More to come, watch this space and FOLLOW ME on Twitter @MellishMeterWSB

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