Continued Monitoring of the Deep Low Crossing The Olympic Peninsula

December 17, 2012: Monday

00:47 PST: (Vancouver, BC) The La Crosse is now indicating a pressure of 28.94", with the Davis at 97.99 kPa (28.94" Hg) and the aneroid at 28.98". This data yields an average of 28.95" Hg or 98.04 kPa, lowest yet at my home for this storm.

The 06:00 UTC (22:00 PST) HPC surface analysis placed the low near 47.5ºN 127ºW with the cold front just sweeping ashore in Oregon. The indicated central pressure continued at 97.2 kPa. Satellite loops up to 08:30 UTC seem to show the low tracking ashore in the vicinity of Quillayute with the bent-back front nearly upon the coast. The storm appears on track for an early morning crossing of the inland waters.

Figure 1: Enhanced infrared satellite photo taken 09:30 UTC (01:30 PST) 17 Dec 2012. The low center has now landed on the Olympic Peninsula. The associated bent-back front has begun to move ashore behind the low, with the tip apparently aimed at the mouth of the Columbia River. Image courtesy of the US National Weather Service.

Here is the 01:00 roundup:

Location	WX	Temp (ºC)	Dew Point (ºC)	Wind Dir	Wind Spd (km/h)	SL Pres (kPa)	1-h tendency (hPa)
Nth Bend	OVC	11.1	7.2	W	56	100.24	1.4
Astoria	RABR-	8.9	7.8	WSW	45	98.38	1.7
Hoquiam	OVC	8.9	7.2	WSW	27	97.99	1.6
Quillayute	OVC	5.6	5.0	E	11	97.77	0.1
Estevan Pt	RA-	2.8	2.8	NNW	29	98.21	0.9
Salem	RA-	8.3	5.0	SW	27	99.33	1.5
Portland	RA-	8.9	5.0	SW	40	98.88	0.5
Olympia	RABR-	7.8	5.6	S	35	97.96	-1.0
Sea-Tac	RA-	7.2	4.4	S	27	98.10	-0.7
Bellingham	RA-	3.3	2.2	N	14	98.06	-0.8
Abbotsford	SHRA-	2.2	1.1	ENE	11	98.10	-0.4
Vancouver	OVC	2.8	2.2	E	24	98.04	-0.6
Victoria	RABR-	2.2	2.2	WNW	23	97.99	0.3
Comox	SN-	1.1	1.1	NW	13	98.19	0.1
Port Hardy	OVC	2.8	0.0	WSW	10	98.54	0.4
Average		5.6	3.9		26.3	98.39	0.33
3-hr Chg		-0.6	-0.4		1.4	0.02	0.77
High		11.1	7.8		56.4	100.24	1.7
Low		1.1	0.0		9.7	97.77	-1.0
Spread		10.0	7.8		46.7	2.47	2.7

The bent-back front appears to be starting to influence the coast, with W to WSW winds at North Bend, Astoria and Hoquiam and escalating speeds. Barometric pressure at these stations is beginning to rise at a decent clip, too. NNW winds at Estevan Point indicate a low to the SE of this station.

A strong SW breeze is blowing across the Willamette Valley, with Portland's gust reaching a new high of 65 km/h out of 210º at 00:19. The station is also reporting distant lightning to the SE. The pressure at Olympia has fallen to a new low and S winds are on the increase, perhaps a mark of the approaching low and bent-back front. Bellingham, Abbotsford and Vancouver continue to be under a cold surface air layer that is flowing out of the Fraser Valley, depressing temperatures. Victoria, interestingly, is now reporting WNW winds and rising pressure, similar to coastal stations, and likely a reflection of a low center that is moving inland. However, Tatoosh still reported SE winds—though much diminished at 22 km/h 12 kt—as of 01:00, with rising pressure at 97.71 kPa (28.85" Hg). Light snow continues at Comox, with NW winds and a roughly steady pressure.

The average pressure tendency has now barely gone positive, with most stations indicating pressure rises. And the average pressure for all the stations has begun to rise at 98.39 kPa (29.05" Hg). More importantly, the pressure gradient has increased significantly, with a maximum between the stations of 2.47 kPa (24.7 mb), this the result of marked pressure rises at places like Salem and North Bend, whereas locations like Quillayute have only had minor increases so far. The average wind speed is up 26.3 km/h (14.2 kt), again owing to strong wind speeds in the south section and on the coast. The temperature spread is reducing due to a warm sector being demolished by seclusion against the Cascades and incoming cooler air plus a slight temperature rise in the north. 01:46 PST.

06:53 PST: The west wind is now blowing strongly, whistling around the window-frames and throwing rain onto the windows. The patio temp is a cold 3.4ºC with 87% RH and the La Crosse pressure is 29.17", rising. At Vancouver International, the 06:00 report indicated light rain with a temp of 3ºC (37ºF), dew point 2ºC (36ºF), wind W 37 km/h gusting 54 (20 kt G 29) and pressure 98.63 kPa (29.13" Hg), rising 1.7 hPa/hr.

Judging by the 12:00 UTC HPC surface analysis, it looks like the low center very nearly tracked right over the Lower Mainland, or just to the south. At that time, the weakening 98.0 kPa center sat on top of the San Juans.

Figure 2: Surface analysis at 09:00 UTC (01:00 PST) 17 Dec 2012 overlain satellite imagery.  The cyclone center has landed on the Olympic Peninsula at this time, near Quillayute, with a 97.7 kPa central pressure. Image courtesy of the UC Hydrometeorological Prediction Center.

Figure 2: Surface analysis at 12:00 UTC (04:00 PST) 17 Dec 2012 overlain satellite imagery.  The weakening 98.0 kPa cyclone center has moved over the San Juan Islands. Image courtesy of the UC Hydrometeorological Prediction Center.

Here is 04:00 roundup:

Location	WX	Temp (ºC)	Dew Point (ºC)	Wind Dir	Wind Spd (km/h)	SL Pres (kPa)	1-h tendency (hPa)
Nth Bend	OVC	8.9	5.0	W	50	100.71	1.7
Astoria	OVC	7.8	3.9	W	32	99.41	2.2
Hoquiam	RA-	7.2	4.4	W	53	99.00	2.9
Quillayute	RABR-	6.1	4.4	W	23	98.64	3.3
Estevan Pt	MM	5.6	5.0	WNW	45	98.76	1.7
Salem	RA-	7.2	5.0	SSW	21	99.89	1.6
Portland	OVC	8.3	4.4	WSW	16	99.58	2.7
Olympia	RA-	6.7	3.9	SW	19	98.79	3.1
Sea-Tac	OVC	6.7	3.3	SSW	48	98.42	1.8
Bellingham	OVC	3.3	2.2	NNW	10	98.20	1.0
Abbotsford	RABR-	2.2	2.2	ENE	11	98.23	0.6
Vancouver	RA-	2.2	2.2	WNW	37	98.32	1.8
Victoria	RABR-	2.8	2.8	W	11	98.32	2.2
Comox	RASN-	2.2	1.1	NW	23	98.51	1.3
Port Hardy	SHRA-	2.8	2.2	WSW	14	98.84	1.1
Average		5.3	3.5		27.6	98.91	1.93
3-hr Chg		-0.3	-0.4		1.3	0.52	1.60
High		8.9	5.0		53.1	100.71	3.3
Low		2.2	1.1		9.7	98.20	0.6
Spread		6.7	3.9		43.5	2.51	2.7

Strong westerly winds continue along the coast behind the bent-back front that has now become rather indistinct as the low tracks into the Cascades. Astoria reported a peak gust of 85 km/h (46 kt) out of 280º at 03:08, accompanied by brief heavy rain and fast pressure rises. In the hour ending 01:55, the pressure jumped +3.7 hPa. Hoquiam had a high-wind peak gust of 96 km/h (52 kt) out of 260º at 03:40, just behind a +4.3 hPa pressure surge in the hour ending 02:53. Quillayute joined in with a sudden wind shift from E 7 km/h (4 kt) at 02:01 to WNW 30 km/h gusting 61 (16 kt G 33) by 02:12, a dramatic 180º+ swing in about 10 minutes. WNW winds were just picking up at Estevan Point during this hour. Temperatures had dropped at most locations as more moderate air behind the low poured in off of the Pacific, displacing warm-sector air in the south and colder air in the north, evening out the latitudinal temperature differences. The zone of marked baroclinicity had moved away from the region, partly the result the low occluding. This is even more apparent during the 07:00 hour.

Willamette Valley locations had received their maximum gusts by 04:00. Portland reported 78 km/h (42 kt) out of 210º at 02:24. Salem's already described 74 km/h (40 kt) occurred before midnight. Eugene had a maximum gust magnitude similar to Portland with 78 km/h out of 220º at 21:27. Looking north, Olympia had a peak gust of 59 km/h (32 kt) out of 240º at 02:50. Sea-Tac had a serious case of bent-back attack. Indeed, the wind pattern is rather reminiscent of the Hanukkah Eve storm of 2006, though weaker in magnitude, with a period of strong southerly winds gusting to 74 km/h (40 kt) ahead of the bent-back front then shifting to southwest as the wraparound swept through and climbing toward a peak gust of 94 km/h (51 kt) out of 220º at 04:49, this followed by a second gust of 89 km/h (48 kt) out of 220º at 05:19. High-wind criteria reached Sea-Tac. Gusts at other Seattle-area locations were not so dramatic, with Renton reporting 70 km/h (38 kt) out of 200º at 04:49 and Boeing Field 67 km/h (36 kt) out of 190º at 04:37. Sea-Tac is a bit favored for high winds from a SW direction relative to the other locations and may not be the best indicator of a area wind speeds. Sea-Tac also reported a +3.5 hPa pressure surge in the hour ending 05:53, right after maximum winds. Tacoma, McChord reported a peak gust of 74 km/h (40 kt) out of 220º at 03:20.

As is typical for tracks of this type, Bellingham and Abbotsford have only had rather light winds so far. The WNW surge arrived in Vancouver right around the 04:00 hour. At 03:00, winds were E 7 km/h (4 kt). Westerly winds remained rather light at the Victoria Airport. Even well-exposed Gonzales Heights reported just NW 20 km/h gusting 33 (11 kt G 18) at 04:00. A slight warming at Comox changed the snow into a mix by 04:00.

Strong upward pressure tendencies were evident this hour, with an average of +1.94 hPa/hr. The average pressure had risen to 98.91 kPa (29.21" Hg). And the maximum gradient between stations reached its approximate maximum of 2.51 kPa (25.1 mb), this between Bellingham, which sat nearly under the low center, and North Bend. Average wind speed climbed to 27.6 km/h (14.9 kt), this in large part due to westerly winds increasing at west-wind-prone locations. With moderate Pacific air invading most of the region behind the low, the temp spread has lowered to 6.7ºC (12ºF).

Here is the 07:00 roundup:

Location	WX	Temp (ºC)	Dew Point (ºC)	Wind Dir	Wind Spd (km/h)	SL Pres (kPa)	1-h tendency (hPa)
Nth Bend	BKN	7.8	2.8	W	39	101.08	1.4
Astoria	OVC	7.2	1.1	WNW	42	100.08	2.1
Hoquiam	RA	6.1	2.8	WSW	47	99.76	2.5
Quillayute	RA-	5.6	2.2	W	32	99.48	2.6
Estevan Pt	MM	6.1	3.3	W	76	99.33	2.0
Salem	RA	6.1	3.9	SSW	21	100.47	1.7
Portland	RA-	6.1	3.9	WSW	16	100.23	2.5
Olympia	RA-	5.6	3.3	SW	21	99.65	3.1
Sea-Tac	OVC	6.7	1.1	SW	42	99.31	3.1
Bellingham	RABR-	3.3	2.2	VRB	11	99.02	3.2
Abbotsford	RASN-	1.1	0.0	S	16	98.90	2.9
Vancouver	RA-	2.8	2.2	W	48	98.84	2.1
Victoria	OVC	2.2	2.2	W	14	99.06	2.4
Comox	OVC	2.2	1.1	W	5	98.97	2.0
Port Hardy	OVC	2.8	1.1	SW	19	99.29	1.5
Average		4.8	2.2		30.0	99.56	2.34
3-hr Chg		-0.6	-1.3		2.4	0.66	0.40
High		7.8	3.9		75.7	101.08	3.2
Low		1.1	0.0		4.8	98.84	1.4
Spread		6.7	3.9		70.9	2.24	1.8

During the 07:00 timeframe, westerly winds continued strong along the coast. Quillayute reported a peak gust of 65 km/h (35 kt) out of 260º at 05:18. Estevan Point's maximum gust so far has been 91 km/h (49 kt) out of the W at 07:00. Winds were winding down in the Willamette Valley and also in the Puget Lowlands. Winds at Bellingham could pick up a bit at some point, but right now the location is caught up in an eddy, probably related to the low center having passed nearly over the station, that is resulting in variable and light wind direction. Wind direction has shifted to S at Abbotsford and likely reflects the low moving inland to the ENE. Light rain/snow mix will likely switch to pure rain as slightly warmer air eventually moves into the Fraser Valley. Westerly winds at Vancouver, now hitting 59 km/h (32 kt) in gusts, are probably still accelerating. Winds at the Victoria Airport have failed to pick up significantly. A gust to 35 km/h out of the W at 04:43 is the highest so far from a westerly direction. Gonzales Heights, however, is showing signs of an accelerating surge in recent observations, with WSW 48 km/h gusting 70 (26 kt G 38) at 09:00. At Comox, winds remained light and precipitation, frozen or not, had ceased.

Upward pressure tendencies were quite dramatic by the 07:00 hour, with an average of +2.34 hPa/hr. The average pressure jumped to 99.56 kPa (29.40" Hg). And the maximum pressure gradient had begun to relax at 2.24 kPa (22.4 mb). All strong indicators that of a low that had moved inland while weakening and is now departing the region. Average wind speed nevertheless still climbed to a new high of 30.0 km/h (16.2 kt) as westerly winds continuted to accelerate in prone areas. The temp spread remained the same as during 04:00.

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Quick Discussion of Storm Outcomes:

Clearly maximum winds have been reached at most locations by 07:00, with a few areas like Vancouver and parts of Victoria probably still awaiting their highest wind speeds. Peak winds at most locations in the interior generally did not exceed 80 km/h (43 kt), with many stations having maximums below 70 km/h (38 kt). Sea-Tac is one exception, and as stated earlier this station is prone to strong wind readings from a SW direction--stations less well exposed like Renton and Boeing Field did not receive gusts nearly as strong. Many coastal stations did reach high-wind criteria, though typically not strongly so, staying within about 90-110 km/h (~50-60 kt).

The maximum gradient in the Willamette Valley approached +10 hPa around 01:00, certainly strong enough for a major gale. And at this time isobars were arranged in a decent south pressure slope. Two ingredients were in place for a significant Valley windstorm. However, as discussed in previous posts, with the strong easterly track for this storm, indicating westerly winds higher up, the upper support was not in place for a major southerly gale. The coast is well exposed to westerly winds, with an excellent overwater fetch, and felt the brunt of the surge that arrived with the bent-back front. The mountainous terrain, the coast ranges, impeded strong westerly winds from migrating inland in many locations.

Relative the 03 Nov 1958 storm, which tracked nearly due east and produced intense winds in some interior sections, the 16-17 Dec 2012 low was occluded as it moved inland. In 1958, the cyclone appears to have remained largely an open wave even as it nearly tracked over Olympia. This kept the warm sector very close to the low center and the region of maximum pressure gradient. The result: Major winds despite a nearly due east track. During the 16-17 Dec 2012 storm, the warm sector was vacating the area as the gradient peaked and reached its best orientation in the Valley. Not the best recipe for extreme winds.

I suggest, when a model forecast shows a low-probability event like a major high-wind storm in the Willamette Valley, keeping a healthy skepticism and carefully considering how the situation may not support the forecast scenario.10:02 PST.

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10:42 PST: According to the 15:00 UTC HPC surface analysis, the low is well inland now, following an ENE track and continuing to weaken at 98.6 kPa.

Here are the 10:00 weather reports:

Location	WX	Temp (ºC)	Dew Point (ºC)	Wind Dir	Wind Spd (km/h)	SL Pres (kPa)	1-h tendency (hPa)
Nth Bend	BKN	7.8	2.8	W	58	101.46	1.0
Astoria	OVC	7.2	0.0	W	34	100.64	1.9
Hoquiam	BKN	6.7	1.1	WSW	34	100.36	1.6
Quillayute	HZ	5.6	-1.1	W	24	100.10	1.8
Estevan Pt	RA-	3.9	2.2	WNW	60	99.90	1.8
Salem	BKN	5.0	1.7	SW	8	101.02	2.4
Portland	RA-	6.1	2.2	SW	14	100.78	1.8
Olympia	RABR-	5.0	2.8	WSW	27	100.30	2.1
Sea-Tac	BKN	5.6	0.6	SW	42	99.99	1.9
Bellingham	RA-	5.0	2.2	SSE	19	99.74	2.2
Abbotsford	RA-	2.8	2.2	S	26	99.66	2.6
Vancouver	OVC	5.0	2.2	WNW	23	99.60	2.4
Victoria	BKN	2.8	2.2	SW	13	99.73	2.1
Comox	OVC	2.2	1.1	CALM	0	99.55	2.1
Port Hardy	SHRA-	2.2	0.0	WSW	14	99.77	1.8
Average		4.9	1.5		26.4	100.17	1.97
3-hr Chg		0.1	-0.7		-3.5	0.61	-0.37
High		7.8	2.8		59.6	101.46	2.6
Low		2.2	-1.1		0.0	99.55	1.0
Spread		5.6	3.9		59.6	1.91	1.6

Winds continue to wind down along the coast. This has generally been the case in the interior, too. Southerly winds have finally arrived in Bellingham well behind the low, and the temp at Abbotsford has climbed high enough in the southerly flow to change the precipitation to rain. Vancouver received a maximum gust of 65 km/h (35 kt) out of the W at 08:00. Winds have calmed considerably since, but sometimes a second round of westerlies arrives in this kind of situation and high pressure builds in behind the passing low. Winds remain light at the Victoria Airport but at Gonzales Heights have reached WSW 54 km/h gusting 76 (29 kt G 41) at 10:00.

Upward pressure tendencies are slowing on average, with a rate of +1.97 hPa/hr as of 10:00. The average pressure has risen to 100.17 kPa (29.58" Hg) and the maximum gradient between stations has slacked off to 1.91 kPa (19.1 mb), this between North Bend and Comox. Average wind speed has diminished to 26.4 km/h (14.3 kt). The temperature spread has diminished even further to 5.6ºC (10ºF). All these indicators point toward a departing storm. 11:28 PST.