Southern Vancouver Island is recovering from a fairly substantial wind event this morning (Sunday). Although the ferocious winds have eased, I've received a couple questions about why Nanaimo and other parts of the Inner South Coast didn't see similar wind velocities to Victoria.
Or re-phrased, why was this wind event so localized?
A couple factors come into play here. When people think of a damaging wind event, they picture a deep low pressure system, but wind events in southern B.C. can get a surprising added boost – but how isolated were the winds?
Peak wind gusts Sunday:
The secret ingredient which can turn a mundane blustery day, into a damaging wind storm...
The Answer: Topography
You can think of topography as steroids for winds.
It's a common myth that topography and the mountains make B.C. weather forecasting extra challenging, but I'd argue it makes weather forecasting MORE reliable – Mountains don't move, and patterns begin to develop with experience.
Earlier this morning, a cold front pushed through the region and the strong winds formed post-frontal (after the front passed).
Not all weather models are created equal; some of the global models failed to pick up this damaging wind event as their resolution is too coarse to accurately model the detailed, intricate interactions of the atmosphere with the topography.
Below is an example of some of the highest resolution weather models available in 2015:
Another zoomed in version of the Victoria region by another high resolution model indicates with near perfect precision that extent of the strong winds along with the intensity nearing 55 knots offshore.
Now, it must be said all models have their strengths and weaknesses, and if there's inaccuracies in the main parent models these errors will be greatly magnified in the higher resolution models; consequently, forecasters are always are a little bit cautious when interpreting these types of weather models.
This situation brings up another oddity in the Pacific Northwest along with such extreme topographic influences that ultimately gives meteorology in British Columbia a unique flavour.
The Cold Front Paradox
Definition: A cold front is typically a region that has a distinct boundary between warmer and colder air. Air behind a cold front is typically drier than the warm air, so the front can usher in fair, although initially unsettled weather along the coast. Most precipitation is ahead of the cold front or right along the region of the boundary where atmospheric lift is highest.
- B.C. cold fronts are nearly always connected to mid-latitude cyclones. You won't often see a rogue cold front meandering in the middle of the Pacific Ocean. If you do, it's misguided and lost.
- Exception: Arctic cold fronts and maritime arctic fronts are the exception; the cold air advection (movement of colder air) often comes from the continent rather than the mild Pacific Ocean.
- Look for a mid-latitude cyclone, or a textbook comma shape (see tweet below). If the cyclone hasn't fully occluded (matured and the cold front has overtaken the warm sector), you'll often see a trailing cold front. Once the comma shape is in play, the cyclone is reaching maturity and peak strength as it goes through the weakening process.
After the passage of the frontal system early this morning, the atmosphere was inherently unstable, as the cold front worked wonders to destabilize the atmosphere.
Basic parcel theory dictates that air would descend following the dry adiabatic lapse rate (DALR), which would effectively mix down the faster upper level winds down to the surface, albeit a little slower due to frictional forces. In a few locations in B.C. wintertime inversions take place where the temperature increases with height. When a cold front barrels through, warmer temperatures aloft can mix down to the surface, increasing temperatures near the ground.
In fact, winds at approximately 1500 metres above sea level were screaming at over 100 km/h (see image below). The instability triggered by the cold front was enough to mix down some stronger gusts than what's typically seen.
But first, what's downsloping?
Downsloping is after all responsible for Alberta's extreme temperature fluctuations and world-famous Chinooks:
Things get a little more complicated when meteorologists attempt to calculate and figure out exactly how strong the winds will be when they mix down to the surface, but recent enhancements and the development of the technique called momentum transfer is painting a clearer picture (see right image) in the often tricky gust forecast.
This morning, models were hinting at a substantial amount of momentum transfer from the higher levels of the atmosphere that would imply gusts over 90 km/h weren't just possible but likely.
Another factor diminishing the temperature effect the cold front brings to the Pacific Northwest involves what's known as the source region (or where the air is originating from).
You can think of an air mass like a chameleon.
Air temperatures loves to mimic the region of the planet where the air-mass originates from.
I'll give you a little hint of where the majority of British Columbia's air comes from:
Cold air that moves over the Pacific Ocean naturally becomes unstable (the aforementioned notion that the instability promotes the mixing (overturning) of the atmosphere).
The following radar simulation highlights the post-frontal instability and convective-type showers that are often such a signature after the storm.
An update to the long range and the next parade of storms (may arrive by next weekend!) is due soon.
Victoria, I hope all your power is restored in a timely fashion! Cheers!