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.

Late Season Cyclone Lands in Northwest Oregon, Bringing Blustery Winds and Moderate to Heavy Rain

As of 1200 UTC (0500 PDT) on 03 May 2012, a fairly strong 99.4 kPa surface low neared the Oregon coast. This storm brought moderate rain to a large swath of Western Oregon and into Southwest Washington. Image courtesy of the NOAA Hydrometeorological Prediction Center.

0815 PDT: A 99.4 kPa extratropical cyclone that developed along 42ºN yesterday is making a somewhat unusual late-season visit to Northwest Oregon this morning. A strong, winter-like, 130-140 kt zonal jet stream supported the development of this cyclone. As of 1200 UTC, the low sat just offshore of Newport, OR, with the associated frontal system stretching southward nearly upon the coast bringing light to moderate rain.

At 0655 PDT, Astoria reported a pressure of 99.95 kPa (29.52" Hg), lowest so far with this event, but not falling particularly fast at 0.5 hPa/hr. Winds remain light and easterly. Further south, at Newport, the altimeter reported a minimum of 29.54" (100.00 kPa) at 0635, with the readings slowly climbing afterward. South winds picked up after the pressure minimum, and had escalated to SSW 44 km/h (24 kt) gusting 63 (34 kt) by 0735. Newport reported a pressure (altimeter) minimum of 29.61" (100.27 kPa) at 0535. Southerlies picked up shortly afterward and climbed to SSW 37 km/h (20 kt) gusting 56 km/h (30 kt) at 0715. Temperatures have been relatively cool with this system, only climbing to 11ºC (52ºF) at North Bend during the onset of the southerly winds, and 10ºC (50ºF) at Newport.

So far, in the Willamette Valley, winds have been rather light, with Salem and Eugene reporting SSE 13-15 km/h (7-8 kt). However, moderate rain in associated with the storm's front swept through the entire Willamette Valley from Eugene to Portland, bringing a good soaking. Eugene reported the heaviest short-duration reading (so far), with 3.0 mm (0.12") in the hour ending 0254.

As of 1430 UTC (0730 PDT), the cloud shield of the extratropical cyclone had reached Southwest British Columbia, bringing light rain northward to the Lower Mainland, South Vancouver Island and the San Juans. At this time, the low centre appears to be onshore over Northwest Oregon. Image courtesy of the National Weather Service.

Update 2250 PDT:

The extratropical cyclone tracked northeast right over Astoria with a 99.8 kPa central pressure around 0900 this morning. After landing, the low slowly progressed inland and gradually filled as it encountered the rough coastal terrain. The path put the low nearly due north of the Willamette Valley during the afternoon. As a result, the pressure slope became nearly due south for a few hours, a perfect orientation to support south winds up the natural wind funnel made by Oregon's Coast Range and the Cascades. Coinciding with the good isobaric alignment, the pressure gradient increased dramatically. The Eugene-Portland (KEUG-KPDX) measure climbed to +7.0 hPa by 1500, a fairly strong reading that is just a few hPa short of many windstorms in history. The graph below shows the sea-level pressure trend in the Willamette Valley for this storm (many thanks to Richard Halter and Steve Pierce for first bringing this information to my attention):

Sea-level pressure trends in the Willamette Valley on 03 May 2012. A +7.0 hPa gradient is quite strong, though short of many historic windstorms. 

     As a result of this good setup for wind, a rare May windstorm developed, whipping trees draped in fresh green spring-time leaves with gale-force gusts.


     Here are some peak wind and gust reports:

Peak Wind and Gust: km/h (knots) For OR and WA Locations Northwest Oregon Cyclone 03 May 2012 (PDT)
Loc	Wind	Dir (º)	Time	Gust	Dir (º)	Time
Eugene	35 (19)	180	0954	52 (28)	240	1134
Corvallis	44 (24)	210	1235	59 (32)	210	1235
Salem	41 (22)	170	0956	65 (35)	230	1344
McMinnville	41 (22)	200	1253	57 (31)	190	1345
Aurora	31 (17)	190	1153	69 (37)	200	1409
Hillsboro	35 (19)	230	1653	61 (33)	230	1545
Portland	41 (22)	240	1753	72 (39)	210	1519
Troutdale	37 (20)	230	1553	65 (35)	240	1548
Vancouver	30 (16)	210	1453	59 (32)	190	1435
North Bend	37 (20)	200	0815	57 (31)	200	0635
Newport	44 (24)	210	1135	63 (34)	200	0735
Tillamook	33 (18)	190	1315	41 (22)	250	1415
Astoria	31 (17)	300	1455	54 (29)	310	1458

     Clearly this event is a minor windstorm, with peak gusts barely making it to gale force at a few stations. The WSW peak gust at Tillamook and NW peak at Astoria reflect the near passage of the low center to these stations. Wind speeds north of the low center, such as at Hoquiam and Olympia, were generally from the NE through NW and typically lighter than the speeds witnessed in the Willamette Valley. It is not uncommon for lows to have an asymmetrical structure with much weaker pressure gradients, and reduced wind speeds, on the north side of the low.

     The timing of peak winds in the Valley appears to have been a few hours after the front moved through. This is probably the result of the pressure slope swinging to a favorable orientation for south winds behind the front as the low slowly tracked to the due north. An easterly pressure slope, not favorable for high winds in the Valley, had set up over the region ahead of the low in the northeast quadrant. This did support some blustery E to ESE winds through the Columbia Gorge and in the Portland Metro area.

A decent, but not exceptionally strong pressure surge occurred throughout Northwest Oregon as the low moved inland. Astoria reported +3.6 hPa/hr at 1600 and 1200, Salem +2.6 hPa/hr at 1500 and Portland 2.7 hPa/hr at 1600. In the Valley, the strongest gusts are decently correlated with the maximum pressure tendencies. The late timing of the pressure surge at Astoria is probably due to the passage of a weakly-defined bent-back front. A rapid wind shift from SSW to WNW between 1412 and 1423, well after the leading frontal system moved through, with precipitation tapering off after 1453, supports this conclusion.

Gust maxima are also well correlated with the peak pressure gradients. The Eugene-Salem measure reached +3.9 hPa around 1300, and the Salem-Portland measure hit +4.1 hPa around 1500, both coinciding closely with the peak gust times for Salem and Portland evident in the table. Pressure gradient verses wind speed for these locations is shown in the two graphs below.

Pressure gradient and associated 2-minute average wind speed (not 3-second average gust) for key Willamette Valley stations. Maximum winds were generally associated with peak gradients. However, at Salem the wind spiked well ahead of the gradient peak. The escalation in wind may have been due to the mixing of upper-air wind momentum down to the surface in downdrafts associated with the leading frontal system. 

Finally, as the low tracked to the north, carrying the associated frontal band through the Willamette Valley, heavy rain visited Portland, with 3.6 mm (0.14") falling in the hour ending 1153.