Weak Front Brings Moderate Westerly Surge to SW British Columbia and NW Washington

After a pleasant weekend and rather warm Monday with highs exceeding 20ºC (68ºF) at many locations and 25ºC (77ºF) at some of the southern stations, a slow moving front finally swept ashore on Tuesday, bringing cooler weather.

I noted yesterday, 08 May 2012 at 0805:

Sunshine pokes between a rather thick assemblage of mid and high clouds, including some altocumulus in the western quadrant. Seeing the bright yellow light paint Tisdall Park is a bit surprising given that the incoming cold front is now quite close. Tofino at 0600 reported E winds of 11 km/h (6 kt) with light rain. By 0700, the wind had shifted to NW 13 km/h (7 kt) and at 07:48 a special observation indicated NW 20 km/h (11 kt) gusting 31 (17 kt). The wind shift indicates frontal passage. The pressure also began to rise, some 0.5 hPa from the pressure minimum of 102.02 kPa (30.02" Hg). Temp at 0700: 8ºC (46ºF). Clearly this front is not particularly strong or moisture rich.

Indeed, looking at the satellite photos, this front does not have much in the way of enhanced cloud tops in the infrared, and appears rather narrow in the visible. The band is very slowly moving eastward. Maybe with enough sunshine this morning adding to some instability, we could experience a thunderstorm with this frontal band.

The temperature remained rather warm overnight, with a low at Vancouver International of 11ºC (52ºF) around 0300 to 0400, and again at 0700. Winds have been pre-frontal-classic offshore, E to ESE 11 to 22 km/h (6 to 10 kt) since about 0100. However, our pressure minimum appears to have already occurred, with a reading of 101.85 kPa (30.08" Hg) at 0300. As of 0700, the pressure had climbed to 101.95 kPa (30.11" Hg). This could be an indication of a weakening front. Or perhaps the rising barometer has something to do with a diminishing thermally-induced trough from yesterday's rather warm temperatures.

Today, I note:

Yesterday's front moved through during the evening. At Vancouver International, the wind shifted from SSW 13 km/h (7 kt) at 1800 to WNW 13 km/h (7 kt) by 1900, with the temp falling from 17ºC (63ºF) to 14ºC (57ºF). The sea-level pressure minimum occurred during the wind shift, with a reading of 101.81 kPa (30.06" Hg) at 1900. Though an array of clouds covered the sky in multiple broken decks, no precipitation fell. The airport only reported light rain showers during the morning, around 1044, an area of precipitation that did not appear to show up in my Oakridge Neighbourhood.

After the evening wind shift, the westerly winds slowly escalated through the night, accompanied by a gradual rise in pressure. By 0100, westerly wind gusts at the airport began to exceed 45 km/h (24 kt). And by 0300, maximum winds were reached with a reading of WNW 48 km/h (26 kt) gusting 59 (32 kt). Cooler, drier air arrived in these post-frontal conditions, with the dew point falling to 0ºC (32ºF) by 0300, and the temp falling to 8ºC (46ºF) by 0600. Even after the peak, westerly winds continued strong, with 2-minute average speeds of 35-46 km/h (19-25 kt) and gusts over 50 km/h (27 kt) up to the most recent report at the time of this writing, 1000.

What has occurred at Vancouver International is a classic "westerly surge" event, with a strong W to WNW airflow down the Georgia Strait and through the Strait of Juan de Fuca. These events are a regular feature of the Georgia Strait climate and as a result will often be discussed here. One of the key requirements for such an event is to have a southwest to west pressure slope in place, not an uncommon setup behind frontal systems. And, indeed, such a pressure slope did set up overnight (see graphic below). West wind speeds tend to be particularly fast at places with a long westerly overwater fetch, such as Vancouver International. These winds tend to slow down considerably as they move inland due to enhanced turbulent drag from frictional effects caused by objects such as trees and houses. For W to NW winds, it is a good rule of thumb to reduce the wind speeds measured at Vancouver International by a factor of 2 (50%) to estimate the magnitude being experienced throughout much of the Greater Vancouver Metro Area, say inland of 2-4 km from the exposed coast. There are exceptions to this rule, but for most of the events (say greater that 90%), this reduction in wind magnitude holds true.

By 0100 PDT, a southwest pressure slope, with a fairly strong gradient, had set up over Southwest British Columbia. When the isobars roughly parallel the length of Vancouver Island with low pressure inland over British Columbia, conditions are ripe for a westerly surge down the Georgia Strait and Strait of Juan de Fuca. Image courtesy of the NOAA Hydrometeorological Prediction Center.

Westerly surges can be much stronger than the one that occurred today, with one example from history being the major windstorm of 14-15 Dec 2006. Five-second average gusts reached 96 km/h (52 kt) at Vancouver International during that storm.

Here are some selected maximum wind and gust speeds from around the region:

Peak Wind and Gust, km/h (knots) Westerly Surge Event of 09 May 2012 (PDT)
*****Location*****	**Wind**	Dir	Time	**Gust**	Dir	Time
Vancouver Int'l	48 (26)	290	0300	59 (32)	290	0300
Abbotsford	22 (12)	190	0900	33 (18)	190	0900
Bellingham	20 (11)	160	2153	33 (18)	160	2153
Sand Heads	48 (26)	300	0300	57 (31)	300	0246
Saturna I.	26 (14)	220	2200	39 (21)	230	2107
Victoria Int'l	30 (16)	250	2200	39 (21)	250	2200
Victoria Gonzales	44 (24)	250	2000	63 (34)	250	2010
Race Rocks	74 (40)	260	2200	83 (45)	260	2223
Sheringhm	54 (29)	290	2000
Tatoosh I	48 (26)	291	1940	54 (29)	290	1934

    Bellingham and Abbotsford are rarely affected strongly by westerly surge events, and Race Rocks often has the highest readings among official station networks. In this regard, today's winds behaved as expected.

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.