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.