Fall Transition 2012: Deep Cyclone Skirts Haida Gwaii

The fall transition of 2012 occurred during the period of 12-14 Oct 2012, just after the 50th anniversary of the Columbus Day Storm (AKA Typhoon Freda), with the arrival of two moist frontal systems, followed by a strong extratropical cyclone after the weekend closed:

15 Oct 2012: Monday

1540 PDT: A short-term forecast issued by the National Weather Service, Seattle, at 1533 indicates thunderstorms moving east into the Olympics, Strait of Juan de Fuca and the Chehalis Valley. They expect the thunderstorms to reach Seattle within an hour. Small hail, heavy rain and gusty winds may accompany these showers. Environment Canada indicates the possibility of thunderstorms here in Vancouver this evening, so it appears we are under the lightning gun as the vigorous cold front moves ashore. There is also a wind warning for Victoria, for strong SE winds shifting to W after frontal passage, with the possibility of gusts to 90 km/h, perhaps even up to 100 km/h. Here in Vancouver, SE winds are expected to reach 60 km/h at times, before shifting to west 50-70 km/h.

Based on the latest Water Vapor (WV) satellite loop, the low pressure center is just south of the southern tip of Haida Gwaii, with the leading front now coming ashore on Vancouver Island. The most enhanced cloud-tops, which have a granular, almost ropy structure, indicate the presence of numerous convective cells reaching down the coast from Vancouver Island to Southern Oregon.

Visible satellite photo of the developing extratropical cyclone, center just south of Haida Gwaii. Time 22:30 UTC on 15 Oct 2012. Image courtesy of the National Weather Service.

Quillayute reported a thunderstorm with heavy rain at 15:18, and some 5.3 mm (0.21") of precipitation fell between 1453 and 1537. Winds were not particularly strong during the thunderstorm, with gusts to 29 km/h (16 kt). Hoquiam has not reported a thunderstorm, but heavy rain occurred around 14:50, after which a wind shift occurred: from ESE 16 km/h (9 kt) at 14:53 to SSE 34 km/h (18 kt) by 15:01. Gusts climbed to SSE 53 km/h (29 kt) by 1539. A thunderstorm with light rain struck Astoria around 1509, followed by S gusts to 69 km/h (37 kt) by 1509.

This infrared image from the same time as the visible photo above reveals strongly enhanced cloud tops, stretching SE over Cascadia like giant fingers, associated with deep convection (about 10 km, or 35,000 feet) ahead of the extratropical cyclone's cold front. Most of the thunderstorm activity was confined to the coast, though heavy showers did strike some inland stations. For example, both Eugene and Salem reported heavy rain, with hourly totals around 5-6 mm (0.20-0.25"). Image courtesy of the National Weather Service.

We have a dark gray sky with light rain falling right now, with a patio temp of 13.8ºC, RH 83%, unsettled winds perhaps up to 15-20 km/h at times and a pressure of 29.63" (100.34 kPa), falling at a decent clip. Vancouver International began reporting light rain showers around 1345, with E winds of 16 km/h gusting 27 (9 kt G 15). The 1500 report indicated light rain showers with temp 13ºC (55ºF), dew point 11ºC (52ºF), wind ENE 11 km/h (6 kt) and pressure 100.63 kPa (29.72" Hg), falling 1.9 hPa/hr. Victoria at 1500 reported moderate rain and mist, with temp 12ºC (54ºF), dew point 11ºC (52ºF), wind NNE 10 km/h (5 kt) and pressure 100.55 kPa (29.69"), falling 1.9 hPa/hr.

The 2100 UTC HPC surface analysis indicated a 98.0 kPa central pressure for the extratropical cyclone, with a position almost due south of Haida Gwaii at latitude 51ºN. The weather situation is evolving rapidly. 1605 PDT.

1902 PDT: Not a whole lot of excitement happening here so far. Just a steady light rain outside, with somewhat unsettled winds. The patio temp is 13.2ºC. The most exciting thing, perhaps, is the pressure fall, which has been fairly fast at times.

Vancouver International reported a drop of 2.7 hPa to 100.13 kPa (29.57" Hg) in the hour ending 1700, followed by an additional 1.8 hPa in the next hour. East winds picked up to 24 km/h gusting 34 at 1800 (13 kt G 18). Moderate rain occurred at 1748, dropping back to light by 18:00. It appears that the threat of a thunderstorm is probably over, with the more enhanced cloud tops off to the east, based on the WV satellite loop.

The extratropical cyclone center has passed the southern end of Haida Gwaii and is now racing ENE toward the mainland. Buoy 46147, near Haida Gwaii's Khungit Island, reported a wind shift in the past few hours, from SE 58 km/h (31 kt) at 1500 Alaska Daylight Time to W 47 km/h gusting 87  (25 kt G 47) by 1600 and then WNW 67 km/h gusting 90 (36.5 kt G 49) by 1700, with a pressure jump from 97.15 kPa (lowest pressure; 28.69" Hg) to 97.65 kPa (28.84" Hg), or 5.0 hPa, in the hour ending 17:00. Out in the Hecate Strait, buoy 46185 reported SE winds of 76 km/h gusting 101 (41 kt G 55) at 1700 Pacific Daylight Time, followed by SE 72 km/h gusting 93 (39 kt G 50) by 1800. The pressure continued to show a falling trend, down 3.1 hPa/hr to 97.28 kPa (28.73" Hg), as of the most recent observation, 1800. Note the SE winds as well: The low center has not yet passed north of Buoy 46185.

Sandspit had winds shifting from NNE around 1737-1807 to N by 1815 and then NNW by 1832 finally to NW by 1900 as the low tracked south of the station. Wind speeds have generally been in the 40-50 km/h (22-27 kt) range, with gusts as high as 66 km/h (36 kt). A steady heavy rain fell during the entire wind shift, with the temp slowly falling from 11ºC (52ºF) to 9ºC (48ºF). 1928 PDT.

16 Oct 2012: Tuesday

0823 PDT: (Vancouver, BC) Buoy 46185 had a classic bent-back attack after my last entry yesterday. At 1900 PDT, wind became SSE 50 km/h gusting 67 (27 kt G 36.5), with the temperature climbing to 12ºC (54ºF) from 11ºC (52ºF). The pressure reached its minimum of 97.14 kPa (28.69" Hg), quite low. Then, by 2000, the wind had shifted to W, with a speed of 72 km/h gusting 96 (39 kt G 52). The temp dropped back to 11ºC (52ºF) and would fall further in the next hour to 10.5ºC (51ºF). More importantly, the pressure jumped 3.5 hPa to 98.17 kPa (28.79" Hg) in the hour ending 2000, to be followed by an even stronger climb of 6.8 hPa to 98.77 kPa (28.99" Hg) in the next hour. Winds tapered off after the initial surge. This is a classic pattern for a station that falls under the tip of a extratropical cyclone's bent-back occlusion. Sometimes a city like Victoria or Vancouver is on the receiving end of a strong wraparound band. For many stations, the strongest wind speeds on record have occurred in association with this feature. Here is the wind, gust and sea-level pressure plot for 46185:

Wind speed in knots (blue line), gust (red line) and sea-level pressure in inches (green line) for Buoy 46185. The signature of the 15-16 Oc 2012 extratropical cyclone is quite evident in the sharp wind and pressure response on the right-hand side of the chart. The first escalation in wind occurred out of the SE as the low center approached the station. The second surge, not quite as dramatic, arrived out of the W and is associated with the storm's strong bent-back front. Extremely fast pressure rises accompanied this feature, resulting in a V-shaped pressure trace that is reminiscent of a hurricane. Chart courtesy of the National Data Buoy Center.

1040 PDT: Looking locally, the storm did not quite meet forecast expectations. At Vancouver International, peak winds reached W 47 km/h gusting 56 (25 kt G 30) at 0700, with a maximum gust of 58 km/h (31 kt) reported at 0600. The westerly surge arrived around 0300, and slowly escalated. These wind speeds were not even close to the maximum 61 km/h gusting 74 (33 kt G 40) that occurred after a dry upper trough moved through overnight on 01-02 Oct 2012.

Victoria International reported maximum winds of W 34 km/h gusting 45 (18 kt G 24) at 03:00, nothing like the forecast 90 km/h (even 100 km/h in one warning). Even wind-favored Gonzales Heights only managed WSW 52 km/h gusting 69 (28 kt G 37) at 2300 with a peak gust of 76 km/h (41 kt) at midnight.

Abbotsford reported peak winds of SSW 34 km/h (18 kt) at 0500, with a maximum gust of 40 km/h (22 kt) at 0500 and 0700.

Bellingham reported SSE 39 km/h gusting 52 (21 kt G 28) at 2053, with a maximum gust to 58 km/h (36 kt) at 2019.

Nothing really spectacular happened rainfall-wise with this storm. Bellingham reported 5.6 mm (0.22") in the 24 hours ending 0453. Vancouver had 3.6 mm (0.14") yesterday, with Victoria reporting 10.1 mm (0.40") and Abbotsford a soaking 17.0 mm (0.67"). Quillayute had 25.9 mm (1.02") in the 24 hours ending 0453, a good soaker but still far from any record. Sea-Tac had 7.9 mm (0.31") in the same time period, and Portland 11.2 mm (0.44").

The 15-16 Oct 2012 extratropical cyclone, though it did not bring a big storm to the major population-centers of Cascadia, nevertheless made for an interesting opening to the 2012-13 storm season. 1104 PDT.    

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.