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.

Spring Interlude

Over the long Easter Weekend, mid to high clouds from a nearly stationary weather system just offshore visited the Cascadia region. Between patches of blue, wispy cirrus, and thicker blankets of cirrostratus drifted overhead, giving the skies a milky cast, such as shown in the above photo, taken at Alice Lake north of Squamish, BC. Though the persistent clouds reduced the intensity of the sunshine at times, temperatures in many areas climbed to their greatest values for the year to date. Photo taken on 08 Apr 2012.

Sunday and Monday turned out rather warm and quite spring-like. At times, altostratus and cirrostratus from the stalled front off the coast obscured the sun. However, with a tendency for offshore flow, and a southerly airflow around the semi-stationary low off of Northern California, the temperature warmed nicely in many locations.

After spending all weekend off of the Northern California coast, a cutoff low still remains offshore today, and appears to now be slowly drifting SE from its former quasi-stationary position. The system is very much weakened, with a 100.2 kPa central pressure as of the 1500 UTC Hydrometeorological Prediction Center surface analysis. The associated weak frontal system has now reached the Pacific Coast, with moisture streaming due north over Cascadia. However, only light precipitation is associated with the band and among somewhat scattered locations.

For the Easter weekend, a quasi-stationary low set up off of Northern California, quite easy to pick out in this visible satellite photo. The associated frontal band virtually stalled just offshore for a number of days, with mid and high clouds streaming over Cascade country. The southerly flow along the front, carrying air from the subtropics, contributed to the warmth. Also, a tendency for offshore surface flow, with E to NE winds, also contributed to warmer temperatures than during the week previous. Image taken on 09 Apr 2012 at 1130 PDT (1830 UTC) and is courtesy of the US National Weather Service.

Despite the frequent presence of clouds similar to those depicted in the Alice Lake photo above, the temperature at the Squamish Airport reached 19.4ºC (67ºF) at 1400 PDT on Sunday, and then bumped up to 20.6ºC (69ºF) on Monday, 09 Apr.

I heard from a local weather observer that his thermometer in Vancouver, BC, north of the airport, reached 20ºC, the highest so far this year. Last year, the first 20ºC reading did not occur until 13 May. So, it appears that this spring will not be quite as cool as that in 2011. Though, one brief warm spell may not be indicative of a longer-term trend.

Vancouver International had much cooler highs temps than places even a short distance from the water, almost certainly due to a tendency for a WNW to NW breeze to set up during the afternoon, at times reaching speeds of 28 km/h (15 kt) on Sunday, and 17 km/h (9 kt) on Monday. The highs were 12ºC (54ºF) and 15ºC (59ºF) respectively.

The thermometer at Abbotsford, however, more closely reflected the temperature that the weather observer reported, with a high of 19ºC (66ºF) on Sunday and 21ºC (70ºF) yesterday. In 2011, the temperature in Abbotsford did not reach or exceed 21ºC until 13 May. Places south, like Sea-Tac and Portland, had highs similar to those at Abbotsford and Squamish this past weekend. The former reported 21ºC (70ºF) yesterday, and the latter 20ºC (68ºF) on Sunday for the toastiest highs of this spring warm spell (so far).