Instant Low

In this water vapour satellite photo, taken at 1700 PDT on 29 Mar 2012, a strong "dry slot" is evident moving ashore over Western Washington and extreme Northwest Oregon. This is one key signature of a developing open wave. An extratropical cyclone had begun to form in the base of an old, mature Gulf of Alaska low, in an area with good jet-stream support and diffluence aloft. Image courtesy of the US National Weather Service.

Yesterday afternoon, an open wave initiated along the section of the a cold front draped over Oregon southwestward out over the Pacific, then moved inland over Western Washington. This wave had a very distinct dry slot and had the strong appearance of early cyclogenesis as it moved ashore. With a narrow high pressure ridge over northern California, the pressure gradient tightened considerably south of the open wave. In essence, Western Oregon sat under a strong pinch between high pressure and low pressure, a classic setup for strong wind in the Willamette Valley, and also on the Oregon coast. With a steep SSE pressure slope, strong winds indeed developed. Also, a modest warm sector visited Oregon. For example, the temperature stood at 12ºC (54ºF) in Salem as the strongest southerly winds arrived, compared to 9ºC (48ºF) at Olympia with light winds shifting from south to west then north as the wave moved inland right over the city, keeping the warmer air just to the south.

Peak Wind and Gust: km/h (knots) Sudden Open Wave 29 Mar 2012 (PDT)
Loc	Wind	Dir	Time	Gust	Dir	Time
Kelso	28 (15)	SSE	1715	44 (24)	SE	1355
Hillsboro	30 (16)	S	1600	48 (26)	SSW	1606
Portland	37 (20)	S	1728	59 (32)	S	1724
Troutdale	33 (18)	SSW	2053	54 (29)	S	1727
Aurora	37 (20)	SSW	1753	69 (37)	S	1611
McMinnville	41 (22)	S	1453	54 (29)	S	1532
Salem	48 (26)	S	1555	72 (39)	S	1649
Corvallis	33 (18)	SSE	1435	50 (27)	SSE	1435
Eugene	46 (25)	SSW	1254	76 (41)	S	1326
Roseburg	31 (17)	S	1153	56 (30)	S	1309
Astoria	24 (13)	SW	1535	35 (19)	SW	1555
Tillamook	52 (28)	S	1515	80 (43)	S	1515
Newport	69 (37)	S	1355	100(54)	S	1355
North Bend	54 (29)	S	1435	87 (47)	SSW	1615

The tight pressure gradient developed just south of Astoria, resulting in a moderate breeze at the typically windy location. Newport, right in the area of strongest pressure gradient, reported high-wind criteria gusts of 58-62 mph from 13:55 to 15:35. In the Willamette Valley, the fastest winds appear to have struck right up the middle—places like Eugene and Salem—with only a strong breeze at stations nearer to the east and west valley margins. For wind-sheltered Roseburg, wind gusts in excess of 50 km/h marks a decent showing. North of Kelso, in the more relaxed gradient near the center of the open wave, winds were rather light.

Moderate to heavy rain accompanied this open wave at a number of stations. For example, maximum hourly rates:

• Portland: 5.6 mm/hr (0.22"/hr) at 2053
• Olympia: 4.3 mm/hr (0.17"/hr) at 1654
• Astoria: 8.1 mm/hr (0.32"/hr) at 1455

Up where I live, Vancouver, BC, became caught under the developing wraparound band with steady light rain from 16:00 to 21:00 and light easterly winds. The pressure fell to 99.61 kPa (29.41" Hg) at 20:00 as the wave continued shifting inland.

Low pressure at Oregon locations included:

• Portland: 99.90 kPa (29.50" Hg) at 16:53
• Salem: 99.94 kPa (29.51" Hg) at 15:56
• Eugene: 100.22 kPa (29.59" Hg) at 14:54
• Astoria: 99.65 kPa (29.42") at 15:55

Note: The Salem station went "twitchy" and started periodically reporting unrealistic rainfall and pressure values, so the data are questionable. Though, the wind observations look reasonable.