Winter Outlook 2020-21

El Nino Southern Oscillation (ENSO) “La Nina” phase this winter

In trying to make a forecast for a season ahead we look to identify the primary atmospheric drivers or what will be the main forcing mechanism or mechanisms in the months ahead.

After a couple years in the warm water “El Nino” Pacific Ocean phase we have transitioned into a cool water “La Nina” phase in the Pacific Ocean. El Nino and La Nina have been around forever and have been widely discussed across all media forever so it would be hard to not know about it. I’ve certainly blogged about it for decades, including over the past couple months But google if you need a refresher.

In addition to the Pacific Ocean ENSO. We also check around the globe for teleconnections which are patterns in the atmosphere and oceans worldwide that have shown statistical or dynamic connections to future behavior of jet streams and subsequent prevailing weather patterns. Forecasters use those in shorter term forecasts as well for periods of a couple weeks in advance.

CURRENT PACIFIC OCEAN SEA SURFACE TEMPERATURE ANOMALIES (SSTA):

MULTI-MODEL PREDICTED SST FOR WINTER:

An El Nino that lasts a couple years has only happened roughly six times since 1950, so we look at those years and add them to the analog list for the coming winter and see what they suggest could happen this winter given what happened with past two-year El Ninos that switched to a La Nina:

The reason for the pattern is the primary jet stream is up North as usual and there tends to be a LACK of a sub-tropical jet stream across the South.

Record low Arctic Sea ice coverage impacts the Polar Vortex in the Stratosphere in a way unfavorable for cold-air build-up (source region) and polar plunges of the jet stream (lack of high latitude blocking) as a result of impacts on the AO/NAO/Polar Vortex and Sudden Stratospheric Warming Events similar to this depiction:

which results in a snowfall anomaly pattern similar to the past 5 winters AND similar to Strong La Nina years:


Research shared by the WeatherBell company by the Climate Prediction Center of NOAA shows the prevailing pressure pattern and jet stream pattern in winters with a low solar cycle and a Westerly stratospheric winds (+QBO):

The behavior or weather so far this Autumn and the behavior of the hurricane season also allow for a comparison test to find matching analog years from past hyper active seasons.

Clippers and “Miller B” types are favored in La Nina winters.

Last winter ended up quite different from predictions because a number of things changed from expectation. The QBO ended up different more positive in a way that only happened one other time in history, the IOD unexpectedly ended up record strong and changed things, warm water off the West Coast and North Pacific unexpectedly backed off and changed configuration, expected SSW events held off until winter was almost over, and the actual atmospheric behavior in general never really coupled with the ENSO signature in the waters. LAST WINTER we had a “Modoki El Nino” but the atmosphere did not respond to it as it has in the past.

Long-range forecasting is still in its infancy and we have a lot to learn.

I note that for quite some time now Mother Nature has been “out of wack” compared to past history, (I mentioned the QBO behavior happened only once before and current MJO behavior and weather behavior has never happened before on record) with things happening outside the record books with our climate regime shift. So this raises a red flag for potentially seeing these wild cards come to play in the winter outside of current expectations which are based on the past. In other words a shift in the underlying base state climate leads to “weather weirding” and thus outcomes that fall outside the normal bellcurve distribution throwing forecasts off more than usual.

Just applying short-term climate trends for the past decade there have been more warmer than normal winters than cold and more wet winters than dry in the Southeast U.S. and likewise snowfall for much of the nation has been less than normal more often than not the past 5-10 years, so that is a weight factor for analogs as well taking the climate regime shift into account for a base state.

The past 5-10 years while averaging warmer than normal, have averaged wetter than normal in much of the South including NW Georgia.

I must remind you that no two El Nino or any two La Nina winters are ever exactly alike, mother nature does repeat but more often it rhymes so you get similarities most of the time but with occasional wild variance. Similar does not mean same.

How strong the LA NINA ends up being during the winter months will also have an impact. Currently I am assuming a MODERATE La Nina. Other forecasters think it will be strong or very strong and a few think it will be weak. ( I am not an expert on it).

In fact this La Nina is already unlike any in the past because except for the La Nina cool pool much of the globe has warmer than average sea-surface temperatures and this includes a warm pool in the Northeast Pacific Ocean, which is not normal for a La Nina. Thus we have a conflict in signals from the Pacific one warm the other cold:

Likewise the colder than normal waters Northwest of Hawaii if it grows larger and stronger would be a cold teleconnection as well.

Meanwhile the SST pattern in the Atlantic favors a weak NAO which is a warmer and drier signal.

So the preponderance of evidence suggests a milder than normal winter across much of the South and on the dry side, but that DOES NOT preclude some cold waves and snow or ice:

LIST OF FACTORS CHECKED FOR LONG-RANGE WINTER OUTLOOK:

I also look at the extent and expansion of snow cover over the Northern Hemisphere (Eurasia) in October and as previously mentioned the hurricane season and September/October U.S. weather patterns. The October Eurasian snow cover extent for 2020 is a half standard deviation above the mean according to Dr. Cohen and a close match to 2010.


HIGH IMPACT HURRICANE SEASON ANALOGS:

I also look at long-range computer guidance for what they show for the coming winter:

There are many many other models that I am not showing because none of them are that different from those shown.

Analog year listing (in no particular order):

1952,1955,1959,1961,1970,1971,1972,1973,1978,1983,1988,1995,1996,1998,1999,2002,2005,2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018,2019.

I have a sneaky feeling that this La Nina will end up NOT BEING TYPICAL, at least at times, and this will result in a cooler winter than the past five winters and a bit cooler and snowier than a typical La Nina on a national basis. One reason for my thinking is how weather patterns have been out of whack from traditional evolution (for example MJO and QBO) and the warm water off the Pacific NW Coast is strong and should lead to some tall ridges at times out West which leads to cooler troughs in the East (+PNA). So while the majority of data indicates a mild and dry winter with little or no snow, there is still hope for those who like cold and snow in Georgia.

This is a first year La Nina following two years of El Nino with La Nina SSTA basin wide. As of now the La Nina this year looks to start moderate to strong and end weak in the Spring.

ANALOG RESULTS WINTER TEMPERATURE (first map) AND PRECIPITATION (second map) ANOMALY:

So the full analog list results in the map above project a warmer than normal winter for most of Metro Atlanta and near-normal precipitation.

A STRONGER La Nina usually increases the odds of a Southeast Ridge and warm and dry so we will monitor it in the months ahead! As we get into actual winter it should matter if the La Nina gets strong or trends weak.

As of November 2nd the WEEKLY ENSO 3.4 REGION is -1.7 on track to tie 1973 and 1988, here is what that combo looked like:

Culling from the full list to pick BEST FIT TOP ANALOGS:

SO MY WINTER OUTLOOK FOR 2020-2021 is for the 3-month period (DJF) to have temperatures a little warmer than normal on average with precipitation near-normal on average, with odds for snow near-normal to below-normal. Remember this is the average of the entire winter NOT what every day, every week, or every month will be like.

Even in a “mild” winter it only takes one or two wild swings of the jet stream and just the right timing of cold and moisture to get snow, but such perfect timing is much harder to get in our part of the country.

Of course it usually goes like this:

For daily weather info follow me on Twitter @MellishMeterWSB.

Kirk Mellish

Meteorologist

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