“It is the life of the crystal, the architect of the flake, the fire of the frost, the soul of the sunbeam. This crisp winter air is full of it.”
The re-emergence of La Nina sends us once more unto the breach, changing the game from earlier in the year when the transition to a weak El Nino seemed as if it could be underway. This suggests some similarities to last year which is likely to stir varying sentiments depending on location across the country. Let’s exercise some caution in drawing too close a parallel as there are a variety of factors in play that imply this season won’t necessarily tread that same path. We’ve already seen a stark shift in outcomes between November 2016 and 2017 which may help to offer clues as to what could be in store.
It still seems prudent to use a typical La Nina base state as the primary building block, mixing and matching other components as best we’re able. Whereas last winter featured a fleeting & weak central/west-based La Nina this year has taken on a more traditional look with added gusto and thus perhaps enhanced staying power. A more robust La Nina which may approach or even achieve moderate status and isn’t restricted to the central and western ENSO regions is a key difference.
There’s also the establishment of the negative/easterly phase of the Quasi-biennial oscillation which was one of the expectations last year that failed to materialize as a bizarre return to the positive/westerly helped to throw things for a loop. The subsequent lack of high-latitude blocking along with the busy Pacific jet hamstrung the pattern at points, particularly beyond December. A -QBO can be correlated to an increased chance of blocking actually materializing, something that has already been in play prior to the beginning of met winter and is forecast to take hold again heading into mid-December.
We’re a long way removed from the apparition of El Nino behind the curtain of the spring predictability barrier and although a weak Nino may not have been entirely unfavourable, the return to a cool ENSO state leans all the more in our favour. So long as the other indices don’t turn exceptionally hostile we should be able to maneuver through some tough spells on the way to a solid winter season. If they function collaboratively then the upside is all the greater.
Although incorporated, I’ve adjusted the composite look based on the mitigating elements and the progression we’ve seen to date.
Analog composite 500mb / temps / precip
Removed from the initial list but maybe still worth a mention were 1964-65, 1996-97 and 2011-12. Other years that have been under consideration at times were 2000-01 and 2010-11 which happened to be in last year’s selections as well.
Beyond the aforementioned weak-to-moderate La Nina, My overall expectations are for:
1) A modestly negative AO & NAO in the means with a high level of variability between positive and more deeply negative periods. This could be augmented by a lull in solar activity and the increased chance for stratospheric warming.
2) A mostly negative PNA outside of December where the positive phase could hold sway, trending downwards through January and February.
3) A neutral PDO that plays into the above variability in the PNA
4) A neutral-positive EPO which is occasionally negative depending on the exact position of the NPAC ridge which is common in La Nina
5) A negative WPO which speaks to that NPAC ridge flexing poleward west of AK. If this ridge ends up flatter-than-anticipated it will have knockdown consequences on the downstream pattern.
BC bore the brunt of winter’s wrath last time around as the west often does in La Nina conditions. Shades of this pattern could be repeated this winter but aren’t likely to be quite as harsh or as prolonged, particularly in December where more of a +PNA could take hold and deliver a milder/drier outcome before the mean trough position migrates west later in the season and returns shades of a typical Nina pattern with wetter & cooler weather making more extended appearances. If the PDO sinks into negative territory it will help to push things further towards cold & unsettled conditions and vice-versa if it winds up biased positive.
The best chance for a sustained cold pattern looks to set up across this region with waves of arctic air outperforming warmer streaks, albeit with an eastward deviated compared to last winter when AB was more involved in the surprisingly restricted western cold and warmth was able to gain more of a foothold through parts of SK & MB. With periods of strong ridging both up and downstream the mean trough axis should largely embed itself across the region and allow for a diverse but active look as both clipper-type systems and storms slicing towards the GL can have impacts.
An active storm track should yield numerous precip opportunities but the question of where exactly the dividing line between snow and ice/rain is a crucial one as we’ve recently learned firsthand. A gradient between precip zones across the region is often a component of La Nina with an upper ridge anchoring over the SE USA that tends to keep lows tracking out of the western/central trough close-at-hand. When cold highs set up shop across QC it can serve up additional hazards as low-level cold stubbornly lodges in place and leads to the potential for dangerous freezing rain through adjacent areas in NB and E ON (another contemporary echo). Better odds for January to be more cooperative this go around, though February can be another toss-up before a potential rebound for March which can occasionally perform quite well in a degrading Nina.
More at the mercy of the nearby Atlantic than influence from the relatively far-flung Pacific, the east coast is again the greatest wild card. Mild & somewhat wet conditions seem likely, though not to the extent of last season as the mean trough position is allowed to carve farther east and assist in prompting some coastal lows beyond the more commonplace inland/Appalachian runner style storms ejecting from the SW USA. Much depends on the state of the NAO since a strong west-based blocking ridge can be troublesome if it takes hold for extended periods (moreso for temps than precip).
I’m tempted to say there aren’t too many but with every year there are inevitably some curveballs and hopefully lessons learned down the road. The states of the AO and particularly the NAO are nearly always tough to predict at seasonal lead times, although we have managed to work with their positive phases when the upstream Pacific domains are performing. The PDO could potentially fluctuate into a more genuinely positive or negative phase which would tend to migrate the continental trough axis either farther east or west, causing some havoc to the forecast as it does and in which case the feed of expected cold & associated storm track(s) go with it. Also wise to keep an eye on the often volatile Nino 1+2 region hugging the coast of South America as this was another monkey wrench last year when highly anomalous warming took hold during the latter half.
High stakes raise the tension as La Nina returns in force. Historically she has delivered the goods and kept the reputation of the Great White North intact more often than not, particularly when in a weak/moderate state unhindered by destructive interference. With an appreciable improvement in signalling from 2016-17’s motley mixture we’ll boldly go once more and play what we’re dealt – as is tradition.