Consider our first bout of modified arctic air last week a test run or a warm-up because the real deal is looking to transpire next week.
But I must mention one thing. This post won't be about snow. As you know if you're an avid reader of this blog, snow is notoriously hard to predict even a couple days out for Vancouver. The set-up I'm about to describe in detail is one of the most common ways for which it has the potential to snow in Vancouver.
A MONSTER ridge is poised to develop over 140W and Alaska next Monday. This big ridge in the north Pacific is similar to the historical October dry streak that also featured an omega blocking pattern as well, except this time it's located over the Pacific. This blocking pattern features a broad area of high pressure sandwiched between two smaller troughs of low pressure. The normal atmospheric river pattern has a very hard time continuing it's usual zonal (west to east flow), so air flow is forced over the massive high pressure ridge. The right side of an omega block typically has plenty of cold air advection and below normal temperatures.
Sure enough, Environment Canada's temperature anomaly prediction maps paint a frigid painting of western Canada next week. I hold these maps to a high degree of accuracy, since they are produced from the North American Ensemble Forecasting System (NAEFS). The dark blues below indicate a greater than 90% chance British Columbia will see below normal temperatures between December 4th-December 11th. In other words, over 90% of the ensemble members predict below normal temperatures for the period.
The omega block has been a feature of YVR's coldest arctic air outbreaks, and I'm going to take the time now to highlight a couple historic cold snaps and show the similarities of this upcoming pattern shift.
Notice the typical omega blocking pattern, even further west than the one forecasted for next week. The advection of cold air was unprecedented for Vancouver International Airport that month, with a mean daily high temperature of -2.9°C with an extreme record low of -17.8°C. Absolutely astounding, considering the weather station is at sea level and several hundred meters away from the warm waters of the Pacific Ocean.
The NCEP reanalysis maps indicate yet another omega block over the Pacific Ocean with the jet stream diving south from the Canadian Arctic in 1968. Temperatures the morning of December 29th, 1968 tied the all time low at YVR once again reaching a bone-chilling -17.8°C. During the peak of the cold snap, daytime highs stayed below double digit negative temperatures at the airport!
An important parameter for measuring the intensity of cold is the 500 mb heights of the atmosphere (typically about 5500 meters above sea level). During the most severe cold snaps the 510 dam (decameters) line will shift as far west as Tofino, so this can be used as a measuring stick for sequential cold snaps.
Our most recent significant arctic air outbreak for YVR is relatively fresh in our minds, as for nearly two weeks in December temperatures remained below zero, with an extreme low of -15.2°C at YVR the morning of December 20th. This cold air outbreak wasn't just known for it's frigid temperatures, but also its ample amount of snow, with nearly 90 cm recorded in the Vancouver region in December alone.
Guess what feature was once again prominent?
Yep. Omega for the win.
And, since the last significant cold snap happened in 2008, we have at our disposal state of the art mesoscale computer models run by the University of Washington that provides high resolution weather prediction data.
The comparison below should be interpreted with some caution, as the model has been tweaked since 2013 including changes in domain configuration with the expectation to improve the model.
Snow pack and cover looks fairly healthy throughout BC with an increase in both snow cover and sea ice when compared to 2012. This will help to limit the amount of arctic air modification when the cold, dense air travels over snow and ice in the Arctic and through central BC.
But what are the trusty ensembles hinting at for the severity of the cold snap?
Two words. Cold and dry.
Not what you want to hear snow lovers.
Vancouver, prepare for sub zero temperatures for at least a few days and possibly longer with the greatest chance of snow when the arrival of the arctic air coincides with a front Sunday/Monday. The cold front associated with the arctic invasion will drop from the north on Sunday and a brief transition to snow showers is possible, as the air mass becomes cold enough and a continental arctic high builds into the region.
If you compare next week's omega block with some historical ones, it's further east which heightens the risk of the cold air shifting more towards Alberta and Saskatchewan and away from the BC coast. This is something to keep our eye on during the next few model runs.
I assure you Vancouver's all time low temperature record will be safe, with the severity of this cold snap projected being most similar to 2008, and maybe a little shorter in duration with temperatures dipping as low as -10 depending on if some reinforcing arctic air pushes in later next week.
A little speculation, I know, but the event is still too far away, and I rarely like talking about anything past 7 days, but I think this warrants your attention.
Thanks for reading.