GRADING THE COMING WINTER...
We've all seen our first snowflakes (in some cases far more than that) so it's no surprise that winter is fast approaching. Most of you weather junkies have heard that there is a La Nina developing with NOAA (The National Oceanic and Atmospheric Administration), laying 85% odds that La Nina conditions will persist through at least February with the odds at 60% that it will linger through spring 2021. The World Meteorological Organization just issued their assessment and they have upped the La Nina chances to 90%. Most indicators are pointing to a moderate intensity event.
So now that we know it's coming, what's a La Nina? The very basic definition is this: La Nina is the cold phase of the El Nino - Southern Oscillation cycle, which is a naturally occurring fluctuation in ocean - atmosphere temperatures in the east central equatorial Pacific Ocean. Cooler than average sea surfaces temperatures in the east central Pacific characterize a La Nina (warmer than average is an El Nino) with episodes lasting anywhere from 9-12 months on average and recurring at 2-7 year intervals. The warm phase, (or El Nino), typically occurs more frequently. The strength of a La Nina is determined by the amount of cooling that takes place with sea surface temperatures. Those which are below average by a large margin rank a La Nina as strong, vs. sea surface temperatures that are slightly below average, which would rank a La Nina as weak. The El Nino and La Nina are important because of the impacts to the global moisture and circulation patterns, especially those that impact the northern hemisphere in winter.
Here are the seas surface departures as of November 1st clearly showing the colder waters of the tropical Pacific in the blue-green shades.
The "Typical" La Nina jet stream pattern over North America features a blocking high pressure system aloft over the North Pacific ocean which diverts the polar jet northward toward Alaska and then down around a western U.S. ridge. A subsequent trough of low pressure, or dip in the jet stream, is then favored across the north central part of the United States. The resultant fast flow often drives numerous vigorous, but generally moisture starved clipper systems through the northern U.S. with occasional buckles that favor more robust storms. The strength of the La Nina will influence the intensity and position of the key North American winter weather players in any given La Nina year.
Typically La Nina winters feature a lot of weather and temperature variability with large swings from mild to very cold and from tranquil to quite stormy. An average La Nina pattern will favor warmer and drier than average weather over the southern United States and Southeast, with colder than typical weather over the north central part of the country and wetter than typical conditions across the Northwest and Great Lakes states. No La Nina year is the same and often weather conditions can vary considerably from the conceptual model shown in the NOAA graphic below.
So, what does La Nina mean for us in the central Midwest this winter? The honest answer is not much in itself. La Nina is just one member of a team of players comprised of the Madden Julien Oscillation, (MJO), the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), The Eastern Pacific Oscillation (EPO), the Stratospheric Polar Vortex, as well as the Pacific Decadal Oscillation (PDO), among others, that all interact creating the weather we experience in the winter. Some of these players have long term (multiple month) predictability like La Nina and the PDO, whereas other important players like the MJO and AO have only short term predictability of up to ten days or so. Due to that, it should be clear why a long term multi-month seasonal outlook is challenging when some of the key players on the field only have predictability out ten days. Seeing the whole picture is imperative to a successful outlook and that's virtually impossible to achieve. A little luck, good or bad always goes a long way in the final outcome.
One of the trends I have noticed with our La Nina's over the years with some regularity is variability. I've already seen this trend rear its head in recent weeks where we have pattern amplification that is followed by de-amplification and zonal flow. This has led to wild temperature extremes from mild to cold and back again. That is a trait I expect to see plenty of this winter. I also look for significant stormy stretches followed by what could be long 6-10 day periods of inactive weather.
Below you can see the type of amplified La Nina pattern that would bring us cold and snow.
Then there are the quiet nice periods where the zonal flow brings mild Pacific air into play.
There is also a known correlation between the strength of a La Nina and the amount of snow it produces in the central Midwest. The stronger the La Nina, the greater the chances for a warmer and less snowy winter. Here are some snowfall projections from weak, average, and strong La Nina's.
The weaker the La Nina the better the chances for a snowy winter in my area. This year with a moderate to perhaps strong event I think the snow track may be far enough north that much of my area ends up with near to below normal snowfall. Chances for a more normal snow season are greater the further north you go.
Should this evolve into a strong La Nina I could certainly see above normal average temperatures. However, as it stands now moderate is the call and that points to temperatures that average closer to normal. That's deceiving though as the major fluctuations I spoke of will bring some healthy cold shots, they just wont hit and hold.
Of course there are all those other players that can come into play that are unknown variables with game changing ability. I will be paying close attention to the MJO, AO, and EPO going forward. If blocking can send them into cold phases for extended periods of time that could certainly change the face of winter. Alas, I have no way of knowing how those cards will fall. If I was to grade winter out for overall severity potential with (A) being nasty, cold, and snowy and (F) being mild and snow free, I would assign this year a C+ to B-. (in other words not far from average).
On that note, I've had my say on winter in the central Midwest and will watch it all unfold in the weeks ahead. Happy hump day and roll weather...TS
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