The Vancouver Island Windstorm of 12 Mar 2012

Visible satellite photo of the 12 Mar 2012 extratropical cyclone taken at 1030 PST, just after peak winds at many locations. The centre is right over the northern tip of Vancouver Island. Image courtesy of the US National Weather Service.

A factor that makes the 12 Mar 2012 extratropical cyclone stand out is that it landed at, or very near, peak intensity. Many cyclones arrive on Vancouver Island in a mature and fading state. These dying lows can still deliver a good gale, but cyclones at peak intensity have a greater capacity to deliver a major windstorm.

Using hourly pressure data from Estevan Point (CWEB), Solander Island (CWRU) and Port Hardy (CYZT), changes in the sea-level pressure gradient can be followed as the storm swept over Solander Island. As the low neared, the gradient rapidly escalated to 18.3 hPa/100 km by 0900 PDT, with a pressure slope orientation of 76º.

This magnitude of gradient is phenomenal. Many storms of history--such as 03 Mar 1999, 14 Dec 2001, 15 Nov 2006, 15 Dec 2006 and 02 Apr 2010--have not produced a gradient at this level. The steep pressure slope of the 12 Mar 2012 storm is very reminiscent of Freda (AKA the 1962 Columbus Day Storm). For regions affected by a given windstorm, most appear to generate maximum gradients in the range of 5-10 hPa/100 km, and a gradient of 4-6 hPa/100 km is often enough to deliver a gale. The next strongest event behind the 12 Mar 2012 storm, 03 Mar 1999, produced a 14.7 hPa/100 km maximum gradient along the North Washington coast; the 15 Dec 2006 event brought a peak 12.2 hPa/100 km gradient to the same region.

An 18.3 hPa/100 km gradient at the latitude of North Vancouver Island corresponds to a powerful geostrophic wind magnitude of 470 km/h. Of course, at the surface, this kind of speed is not likely to be realized due to turbulent drag effects (a key assumption of the geostrophic wind is that it is non-accelerating and largely free from friction). However, if the surface wind speed reaches just 25% of the geostrophic potential, it is still an incredible 118 km/h.

The intense gradient only extended for a short distance, say approximately 100 km from the centre of the extratropical cyclone. Outside of this, a strong gradient existed for a few hundred km more. Using pressure data from Victoria (CYYJ), Comox (CYQQ) and Squamish (CWSK) on 12 Mar 2012, the gradient peaked at 0800 with a value of 7.2 hPa/100 km. Nothing like the intense reading near the storm center, but enough to deliver a memorable windstorm.

Of interest is that the timing of peak winds at a number of locations corresponds well to the time of peak gradient.

In response to the diminishing pressure gradient southward from the low centre, the peak wind magnitude of the storm diminishes relative to the climatology. For example, at some selected stations here is the time interval between the 12 Mar 2012 windstorm and the last time winds of comparable magnitude occurred:

• Campbell River: ~40 years [1]

• Comox: ~27 years [2]

• Victoria: ~5 years

• Vancouver: ~2 years

[1] Station operates from 0600-2100 LST, so some big storms may have been missed.
[2] A peak gust of 133 km/h occurred on 23 Mar 1989. Probably measured on an analog U2A anemometer when "instant gusts" (highest value indicated on a direct-reading dial or strip-chart) were recorded, and the indicated speed is probably comparable to the 117 km/h 5-second average measured on 12 Mar 2012.

Here are the corresponding peak wind, a 2-minute average, and gust, a 5-second average, speeds during the 12 Mar 2012 windstorm:

• Campbell River: 74 km/h (40 knots) gusting 106 km/h (57 knots)

• Comox: 96 km/h (52 knots) gusting 117 km/h (63 knots)

• Victoria Int'l: 67 km/h (36 knots) gusting 95 km/h (51 knots)

• Vancouver Int'l: 65 km/h (35 knots ) gusting 89 km/h (48 knots)

The maximum wind speed at Comox is incredible. Winds of this magnitude can have a terrific effect on trees, with great potential for swathing forests in a phenomenon termed windthrow. Structures can be physically damaged by the raw wind force: shingles peeled from rooves, awnings hammered apart, garage doors punched-in. Interestingly, the ratio of gust/wind is pretty high for most of the listed stations, with the exception of Comox. This kind of response is, like the intense pressure gradient, reminiscent of Freda.