Vancouver, BC, Enjoys Weekend Warmth While Thunderstorms Strike Portland, OR

Friday and Saturday turned out warm, sunny and generally lovely in Southwest BC. At Vancouver International, the temp climbed to 20.3ºC (69ºF) by 1600 on 25 May, even despite a strong WNW to NW wind up to (17 kt) at 1300. On 26 May, the temp climbed to the highest of the year so far, reaching 23.5ºC (74ºF) by 1900 (quite late!), with light SW to WSW winds. The low between these two highs occurred at 0500 with a reading of 8.5ºC (47ºF). Interestingly, our high on Saturday proved warmer than at Sea-Tac with a report of 22ºC (72ºF). Portland also reported a relatively cool high of 20ºC (68ºF) on the same day.

Diurnal thunderstorms afflicted Portland on 25-26 May, with the event of the second day quite spectacular. The Friday event occurred between 1828 and 2010, with light rain amounting to 2.2 mm (0.09"). Maximum winds were E 30 km/h (16 kt) at 1828. The following day at nearly the same time, between 1917 and 2004, a thunderstorm brought with it very heavy rain (for the region). Some 25.9 mm (1.02") fell in the hour ending 1953. NE winds reached 30 km/h (16 kt) gusting 56 (30 kt) at 1932. Vancouver, WA, reported heavy rain from 1929 to 1946, with 21.1 mm (0.83") in the hour ending 1953. According to NWS Portland, both Portland International and Vancouver Pearson set new record daily maximum rainfall due to this convective activity, with the old records of 23.6 mm (0.93") and 16.5 mm (0.65"), respectively, standing since 1953 and 1909. These thunderstorms did not show up in places west, such as Hillsboro. Sea-Tac reported a thunderstorm with light rain between 1851 and 1853 on 25 May, with just a trace of precipitation measured.

The weather up here in Vancouver, BC, changed noticeably on Sunday, 27 May, as a Pacific frontal system approached. Easterly winds picked up dramatically in the morning even as a deep marine layer moved ashore and brought a solid, gloomy stratus deck inland, apparently overriding the offshore surface flow. Winds reached E 37 km/h (20 kt) gusting 54 km/h (25 kt) at 0300, then slowly tapered off into midmorning, and shifting direction to ESE-SE. The clouds fragmented somewhat during the afternoon, allowing a peeks of sunshine at times and helping the temp warm to 18ºC (64ºF). The clouds thickened overnight, and light rain showers began around 0451, continuing until 0900 with the culmination of steady light rain. The temp hovered around 12ºC (54ºF) during the precipitation. Now a gray stratus deck dominates, with light southerly winds and fairly high humidity. In association with the incoming front, the pressure fell to 100.92 kPa (29.80" Hg) at 2000 on 26 May, even with the boundary well offshore. The atmospheric pressure slowly rose afterward as the steadily weakening front approached. During the rain, the barometer indicated 101.75 kPa (30.05" Hg) at 0900 this morning.

A First Real Taste of the Summer Dry Season in 2012

High pressure built in on Friday and Saturday, 11-12 May 2012, bringing with it dry and relatively sunny weather. With limited large-scale circulation in the Vancouver Metro area, pollution, pollen and other particulates quickly contributed to a haze that obscured distant objects, such as the under-construction Port Mann Bridge seen here. Photo taken from Minnekhada Regional Park near Coquitlam, BC, 12 May 2012.

11:55 PDT: Despite an upper trough that is currently passing through the region, the weather continues sunny and relatively warm, though for much of yesterday high clouds obscured the sky. At Vancouver International, W to WNW sea breezes, triggered by higher surface heating over land than over the cool Georgia Strait, have dominated during the warm spell, typically peaking in the afternoon and early evening, with speeds of 17-30 km/h (9-16 kt). The onshore wind has had the effect of keeping the airport temperature readings much lower than places inland. Indeed, even locations only a few km from the shore tend to report significantly warmer readings than along the runways of Vancouver International.

In any event, with the bright sun high in the powder blue sky, a general and persistent lack of precipitation and comfortable temperatures outdoors throughout much of Cascadia, it has been real easy to think Summer has arrived. In fact, some places in the southern part of the region have had temperatures that could be described of as hot.

High pressure aloft left skies clear from southern California all the way into the central British Columbia interior, as shown in this visible satellite image taken at 1730 PDT on 13 May 2012. A Pacific frontal system, stalled by the high pressure ridge, can be seen streaming into Haida Gwaii and southern Alaska, missing Cascadia entirely. Image courtesy of the NOAA National Weather Service.

Here are some high temps from Saturday through Tuesday, capturing the first part of this spring heat-wave:

High Temperatures ºC or (ºF) Mid-May 2012 Warm and Dry Spell
Location	12 May	13 May	14 May	15 May
Comox	17.4 (63)	22.6 (73)	22.8 (73)	23.5 (74)
Vancouver	18.3 (65)	18.6 (65)	18.9 (66)	17.3 (63)
Abbotsford	25.5 (78)	26.4 (80)	27.1 (81)	18.9 (66)
Victoria	21.6 (71)	22.1 (72)	23.8 (75)	18.7 (66)
Bellingham	22.8 (73)	23.3 (74)	23.3 (74)	21.1 (70)
Everett	21.1 (70)	21.7 (71)	23.3 (74)	21.7 (71)
Sea-Tac	24.4 (76)	25.5 (78)	26.7 (80)	24.4 (76)
Olympia	26.1 (79)	27.8 (82)	29.4 (85)	25.5 (78)
Portland	28.9 (84)	30.6 (87)	31.1 (88)	25.5 (78)
Salem	28.3 (83)	30.6 (87)	26.7 (80)	26.1 (79)
Eugene	26.1 (79)	28.3 (83)	21.1 (70)	23.9 (75)
Medford	31.7 (89)	35.0 (95)	28.3 (83)	31.1 (88)
Redding	35.0 (95)	34.4 (94)	25.5 (78)	29.4 (85)
Tofino	19.3 (67)	21.5 (71)	23.3 (74)	17 (63)
Quillayute	23.9 (75)	24.4 (76)	25.5 (78)	21.1 (70)
Hoquiam	24.4 (76)	24.4 (76)	22.8 (73)	21.1 (70)
Astoria	24.4 (76)	23.3 (74)	14.4 (58)	17.8 (64)
Arcata	13.3 (56)	11.7 (53)	15.0 (59)	13.3 (56)

A decent onshore push cooled off coastal stations on Monday, and also invaded parts of the Willamette Valley. Arcata, in typical fashion, missed the heat wave entirely and remained under a cool marine layer with plenty of low clouds and fog. The strong contrast in temperature between Vancouver and Abbotsford is also a common outcome and, as noted above, reflects the cooling power of the still winter-chilled waters of the Georgia Strait.

The dry weather is expected by forecasters to continue through Saturday, though an upper trough could bring a little precipitation late on Thursday. Rain may return on Sunday, at least in the north section, heralding a sustained period of cooler, damp weather for next week. 

Not just humans, but many creatures appeared to enjoy the spell of warm, dry weather. Here a Margined White Butterfly (Pieris marginalia) sips nectar under the warming light of the sun at Minnekhada Regional Park on 12 May 2012.

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.