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.