La Niña returns? rain, wind and lower elevation snow

Friday, September 15, 2017, 11:20 AM - Thousands of kilometres away, a weak pool of abnormally cool ocean water is starting to surface near the equatorial region west of South America — the beginnings of La Niña.  

In particular, temperature anomalies lurking beneath the surface were more impressive. Although not officially declared, computer models and long term climate computer models are suggesting that a weak La Niña may persist through the winter months.

FALL IS HERE: After a summer that varied from coast to coast, what can Canadians expect from spring? Find out with The Weather Network’s 2017 Spring Forecast | FORECAST & MAPS HERE

If you recall, last fall featured a La Niña which had significant ramifications for the season to follow. By last November, it became more likely we would be in for a significant departure from the many mild seasons Vancouver had been accustomed to.  

The Climate Prediction Center is now giving a probability of roughly 60 per cent that La Niña will be officially proclaimed… 

Q: What does this mean for weather in the Pacific Northwest?  

A: Weather forecasters and climatologists like to use analog years and often group years by similar sea surface temperature (SST) configurations to better understand what the future state of the atmosphere may resemble. 

Ironically, the best analog years for this upcoming year is Fall/Winter 2016.  

But, let’s go back further.  

Looking at the past few decades shows a very distinct and predictable pattern for snow lovers in the Pacific Northwest.  

Want to ward off significant snow events for Vancouver? The best weapon to utilize would be El Niño… 

Above, courtesy of NCEP, is a summary of the state of the waters in the Nino 3.4 region (a common region to observe sea surface temperature departures). The official threshold for La Niña or El Niño is set where 5 consecutive months have an average 90 day mean 0.5 °C off the benchmark neutral state. 

Local weather nerds will quickly recognize the seasons highlighted in yellow—they feature ample lowland snow around the Lower Mainland. Those specific years are strongly correlated with a neutral or weak La Niña.  

This is an important clue as we enter the fall and winter, as this particular temperature anomaly will likely be a primary driver of our weather patterns during the next several months. 

Other factors that need to be watched include sea surface temperatures adjacent to British Columbia; subsequently, temperatures and weather patterns can be greatly influenced by ‘the blob’ of anomalous sea surface temperatures which has been featured in past years.  

Q: Is a snowy season a certainty or a high confidence forecast? 

A: No, not as of yet.  Forecasters play the odds and can only rely on computer models, analog years, and current observations. For long range forecasting, the probabilistic approach is best and prudent to follow. As of mid-September, there’s a higher probability B.C. will see above average snowfall for lower elevations when compared to 1980-2010 climate normals—but this forecast will evolve and become more precise over the coming months. 

Q: Is the South Coast of B.C. at risk for a damaging wind storm this fall/winter?  

A: There is some correlation between equatorial sea surface temperature states and threatening storms. Last October, abnormally deep low pressure systems explosively developed southwest of British Columbia, giving several bouts of damaging winds and heavy rain. We are once again at risk of more damaging wind storms this upcoming storm season. 

Cliff Mass, a University of Washington climate scientist, illustrates using the above plot how the majority of our historic wind events tend to occur within a relatively narrow window of sea surface temperature anomaly, and often this major storm development avoids the extreme La Niña or El Niño scenario. 

Q: Can you guarantee an excellent, above average ski season in British Columbia? 

A: No, as often is the case with La Niña a persistent ridge of high pressure may develop over the North Pacific and nose into parts of Alaska. This just stacks the deck toward the favour of the ski hills, but by no means is plentiful snowfall a guarantee. This crucial player allows for the polar jet (upper level winds) to dive south across parts of western Canada, allowing pulses of Arctic air and below seasonal temperatures to migrate across the continent. The primary risk to hamper the ski season is the ridge of high pressure setting up too far east which can shut the storm track down entirely, limiting the amount of snow that can accumulate at higher elevations.  

This is often a setup where an inversion (increase in temperature with height) can occur, featuring warm temperatures aloft; therefore, this traps the moist, dense air at the surface producing fog, low cloud and drizzle along coastal sections and valleys. It’s also a situation that can see deteriorating air quality with such a stagnant weather pattern.  

We’ll carefully monitor this developing situation and long term weather pattern throughout the rest of fall and provide updates as necessary.