Is there a way to winter snow even with La Nina? 

Posted: 6:01 am Tuesday, October 3rd, 2017

By Kirk Mellish

The short answer is yes. The ENSO, solar cycles and QBO all work in tandem along with the AMO and PDO etc.

Last winter the nature of the warm and cold pools in the Pacific changed during the winter in ways unexpected, that can always happen again.

Even in a winter that averages warmer than normal and drier than normal a single system with the right timing can still bring a winter event of some kind.

On the other hand, a warm winter also increases the risk of fall and winter severe weather.

This is NOT a forecast but an explanation.

A weak La Nina or near-neutral Pacific Ocean sea surface temperature pattern in the equatorial region that leans cool can lead to more areas of the country being cold and snowy.

Keep in mind that a warm start in October across the East half of the nation is not necessarily an indicator of what the rest of autumn or the coming winter will turn out to be. I am leaning in the cool but not cold direction for October through November on average but with come and go warm spells. One issue which will be looked at is if there are any signs of high-latitude blocking.
Typically, if we are going to see colder changes for the latter half of the season, ridging in the EPO, PNA, AO, and NAO positions will start to show up in computer model forecasts in the second or third week of October. Also watch  California. If warmth and dryness keeps repeating over the West Coast in the tenth month, chances for cold, ice, and snow will grow from the Rocky Mountains to the East Coast (above Interstate 20 of course….) as we advance through winter.
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Research shows that winters with the least amount of variability are a strong El Nino and a strong La Nina:

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As of now we are in the zone between neutral (La Nada) and a weak La Nina, which if it were to maintain that, implies greater fluctuation in the winter jet streams providing at least some opportunity for cold and moisture to combine in more places in the country.

A southeast ridge is still evident, but the storm track gets closer when polar jet plunges giving some chance:

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Past recent winters without a strong Pacific Ocean signal:

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Yes, the average or mean of the winter is still above-normal temperatures in the Southeast, but not by much. This suggests that it will not be as warm as last winter, thus giving a little more chance that at least one winter system could occur in an otherwise benign pattern.

Also, the state of the QBO and the AO/NAO are always very important in week to week and month to month variations in winter weather. They modulate the ENSO (El Nino+La Nina oscillation) by impacting high latitude jet stream blocking around Greenland or lack thereof (lower right panel):

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Notice above the left side was last winter with a +westQBO strong upper ridge over the Southeast. On the right, is the projected -eastQBO with a weaker SE upper ridge more suppressed with upper level trough over northern plains upper Midwest and a blocking ridge around Greenland. Again, suggesting chances for polar jet plunges.

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So you can see the models say cool neutral or weak La Nina this winter, indicating as per the upper chart, that IF the La Nina stays weak or neutral with a -east QBO there is more of a chance for a neutral or -NAO/AO and some cold shots.

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Since we are in a low solar cycle period and expect an east-QBO, it lines up with what a La Nina with an east-QBO shows, i.e. they are consistent (upper left panel):Q3

Again showing less of a ridge over the Southeast.

Also, as pointed out in the prior blog post on winter, -easterlyQBO states are more favorable for Sudden Stratospheric Warming (SSW) events which can cause jet stream blocking in the AO/NAO regions:Screen Shot 2017-09-30 at 12.24.30 PM

And that favors more cold air intrusion into Eastern U.S. IF that happens during winter.

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Although we are in a solar minimum cycle they do vary daily weekly and monthly. A “quiet sun” does favorable things to the other factors we’ve talked about or modulates them but they all work together to impact seasonal outcomes.

IF the sun geomagnetic index declines this winter it would add to the case for more high latitude -AO blocking and a colder winter outcome in at least more places east of the Rockies.

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But an uptick would move the needle in a warmer direction.

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The analog of winters after a pair of warm ones this decade shows this:

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Another thing we look at over the next 60 days is Eurasian snow cover for clues to the coming winter. When snowfall is high in Siberia and snow cover grows rapidly, the resultant cold air enhances atmospheric disturbances, which propagate into the upper level of the atmosphere, or stratosphere, warming the polar vortex. When the polar vortex warms, the jet stream is pushed south leading to colder winters across the eastern United states and Europe.  The speed or rate at which snow cover in those locations grows in October-November is important.

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Snow there normal to above but too early to be meaningful as we have just started October monitoring:



Volcanic ash is yet another factor that can and has impacted summer and winter averages. Currently there are insufficient volcanic aerosols in the stratosphere to be a factor one way or another. But one big eruption or multiple smaller ones in the more northern reaches of the hemisphere could change that.


So while there are no strong indications for cold and snow in the south, these indications point out that it only takes the right timing to get something, even in a generally mild winter.

Monitoring all these factors, and the ones mentioned in the previous two posts on the coming winter La Nina and Winter or Early thoughts on Winter 2017-18 over the next 8 weeks will determine the final forecast.

Stay tuned to WSB Radio on your favorite device for future announcements and updates.

FOLLOW ME on Twitter @MellishMeterWSB.