Possible Windstorm For Vancouver Metro and Surrounds on Monday 17 Dec 2012

NWS Seattle is still watching a potential windstorm situation for Monday. One model brought the low over Portland, which would have a completely different outcome, but the other models continue bringing a strong low across Vancouver Island. The latest WRF-GFS, 12:00 UTC 14 Dec 2012, brings a 97.7 kPa (28.85" Hg) cyclone ashore very near Estevan Point. Interestingly the gradient near the center is not very strong—probably indicating a mature low—but is quite tight further south and west. The strongest winds for Vancouver may be to the rear of the low as a westerly surge. This scenario appears somewhat similar to 25 Sep 1999, though the modeled low is deeper in this case suggesting that the potential for damaging winds is a bit higher. 08:38 PST.

WRF-GFS output of sea-level pressure, temperature at 92.5 kPa and wind at 10 m for 04:00 PST 17 Dec 2012. This image shows a strong, but mature, 97.7 kPa cyclone landing near Estevan Point on Vancouver Island. This is a classic setup for high winds in the Lower Mainland and Greater Victoria, and also the Oregon and Washington coast, Northwest Interior and possibly the Puget Lowlands. Image courtesy of the University of Washington Department of Atmospheric Sciences.

09:34 PST: I took a closer look at the WRF-GFS model and my own regression modeling. Note that my regression model is preliminary. If we assume the low has a central pressure around 97.5 kPa and tracks across northern Vancouver Island, then my model predicts an average peak gust for Victoria, Vancouver and Abbotsford of 41 kt (76 km/h). Since peak gusts tend to vary between the stations, the average could contain an instance of 48 kt (89 km/h) or higher, my own minimum cutoff for "high wind" that allows for some error in measurement. Some examples of high-wind generating cyclones with a central pressure at landfall around 97.5 kPa include 04 Feb 2006 with an 3-station average peak gust of 42 kt (78 km/h) and 11 Dec 2006 with 49 kt (91 km/h). Both these storms produced high-wind criteria gusts in the Lower Mainland and/or Greater Victoria.

The WRF-GFS indicates that the highest winds for Vancouver may occur as the low passes inland to the north of the station in a classic westerly surge event that may climb to 35 kt (65 km/h), which may bring gusts upwards of 45 kt (83 km/h), arriving around 15:00 PST on 17 Dec. Again, quite similar to 25 Sep 1999. This westerly surge may be linked to the passage of a well-developed bent-back front. As the low tracks toward The Island, easterly winds could climb to 25 kt (46 km/h) Sunday night on the 16 Dec.

Interestingly, the NWP model suggests that the pressure in Vancouver could fall as low as 98.0 kPa (28.94" Hg) around 04:00 on Monday. My own regression model predicts a minimum around 99.0 kPa (29.23" Hg). Some of the difference may be due to the track, which is nearly due east in the case of the expected storm and generally ENE to NE on average which is reflected in the regression model. Probability, however, is on the side of the higher minimum-pressure prediction, and perhaps one might expect a minimum at Vancouver between these two values. 09:56 PST.

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.

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.