Saturday 31 March 2012

Another Southern Extratropical Cyclone

A 98.0 kPa low had reached the 130ºW line at 43ºN as of 1400 PDT today, 31 Mar 2012. This maturing extratropical cyclone is expected to move NE and weaken before making landfall this evening. Image courtesy of the US National Weather Service.

On this final day of a very busy March for weather, a strong 98.0 kPa low has developed off the Southwest Oregon coast and is now near 43ºN 131ºW, at least as of 1200 UTC. Looking at the upper-air information available from the US NWS, the system is nearly vertically stacked up to 50 kPa, but is not cut off from a decent 130 knot (240 km/h) jet stream. Therefore, it appears that this system is maturing, but had not reached a fully vertical state as of 0500 PDT this morning.

Both the MM5-NAM and the WRF-GFS model runs, courtesy of the University of Washington, for 00:00 UTC 31 Mar 2012 show the low weakening and elongating on a north-south axis as its moves ashore in the evening, with the remains of the center moving into Northwest Oregon or Southwest Washington. A system being stretched in this manner—akin to de-focusing the storm's energy—significantly lessens the chance of a major windstorm for this extratropical cyclone. However, the low is on an interesting track, and bears watching. There is the possibility of a decent westerly surge behind the low in places prone to west to northwest winds, like the Strait of Juan de Fuca and Vancouver, BC. However, currently, the WRF-GFS is only indicating maximum winds of 20-25 knots (37-46 km/h) in the Georgia Strait in the vicinity of Vancouver and 25-35 knots (46-65 km/h) in the eastern Strait of Juan de Fuca near Victoria. These are not particularly high values compared to many westerly surges.

Looking at the METAR, the leading warm front appears to have reached Astoria. The station reported a wind shift from ESE 15 km/h (8 kt) at 0638 to S 19 km/h (10 kt) gusting 33 (18) by 0655, accompanied by moderate rain. A pressure minimum of 99.17 kPa (29.28" Hg), quite low for so late in the season, occurred at 0555 just ahead of the warm front. The temp rose from 5.0ºC (41ºF) at 0355 to 7.8ºC (46ºF) by 07:21. Winds as of the most recent report, 0755, were S 20 km/h (11 kt) gusting 35 (22). Not particularly strong.

The surface warm front had reached Newport by 0455, with a high temp of 10.0ºC (50ºF). E winds 19 km/h (10 kt) at 0415 shifted to SE 26 km/h (14 kt) gusting 37 (20) by 0435, then S 50 km/h (27 kt) gusting 61 (33) by 0455, the highest speeds associated with the warm front. As with Astoria, moderate rain accompanied the arrival of the warm boundary. The cold front, right on the heels of the narrow warm sector, reached Newport by 0535, with winds shifting to SSW. By 0635, peak post-cold-front speeds were reached with SW 39 km/h (21 kt) gusting 54 (29 kt). The temp had fallen to 7.2ºC (45ºF) by this time. As of 0755, the temp stood at 6.1ºC (43ºF), with winds S 20 km/h (11 kt) gusting 30 (16 kt). The pressure, having gone through a minimum of 29.31" (altimeter) at 0435, climbed to 29.43" by 07:15 as the two frontal systems went through, and now appears to be falling again as the low center nears.

The cold front has yet to reach Astoria at this time. Or—perhaps quite likely—the triple point passed south of Astoria. Even Tillamook reported a maximum of 10.0ºC (50ºF) at 0555 with the leading warm front. This warmth has since tapered off. But the temperature has so far failed to get as high in Astoria. This suggests that Astoria experienced an occluded front, while places immediately south received the warm-front cold-front combo.

Update at 1211 PDT:

NDBC Buoy 46002, some 500 km (275 nautical miles) west of Coos Bay, OR, reported a low pressure of 97.74 kPa (28.86" Hg) at 0650 PDT. The pressure then underwent a slow rise over the next two observations. Then, between 0850 and 0950, the pressure jumped 3.9 hPa to 98.46 kPa (29.08" Hg). In the next hour, the pressure climbed even faster, increasing by 4.4 hPa. The anemometers at this station failed some time ago, and there is no wind data. However, seas have climbed to 7.2 m (24 ft), indicating strong winds. The cyclone's bent-back front appears to have passed Buoy 46002, and has probably triggered some significant W to NW winds in the area. The bent-back feature is typically associated with the upper-trough behind the surface low. With the cyclone becoming stacked, bringing the upper trough right over the low center, I expect the bent-back front to weaken before landfall.

In Northern California, wind gusts became fairly strong in the vicinity of the cold front. METAR reports from Arcata indicate a peak gust of 76 km/h (41 kt) out of 170º at 0152. Then temp fell from 15ºC (59ºF) at 0153 to 8.9ºC (48ºF) by 0425 as the cold front moved through. At Crescent City, wind gusts reached 80 km/h (43 kt) out of 200º at 0409. Currently, southeast winds are again on the increase at this location, this time due to the approaching low pressure center. A wind gust of 78 km/h (42 kt) occurred out of 140º at 1149.

Update at 18:08 PDT:

Wow, Crescent City took a beating from the southern extratropical cyclone. A gust of 106 km/h (57 kt) occurred out of 170º at 1520. Two-minute average winds climbed to 76 km/h (41 kt) out of 170º at 1544. These are significant wind speeds, reflecting a sharp pressure gradient that has set up in the base of the low. To the north of the centre, the pressure gradient has been far weaker. The low is very elongated on a north-south axis, as expected. North Bend, OR, for example reported maximum gust of 52 km/h (28 kt) at 1255, out of the SE, in the more gentle pressure slope.

The main low-pressure-centre spiral appears to be heading right for the Coos Bay area. But it is losing definition in the satellite loops and is clearly weakening. In fact, the pressure has been rising at North Bend: 29.47" (altimeter) at 1755, up from 29.41" at 1435, indicative of a fading storm. The barometric pressure is also showing signs of rising up north in Astoria.

Update 23:08 PDT:

Looking at the METAR, the bent-back trough has clearly reached the Pacific coast during the past hour. North Bend, OR, for example, reported winds SSE 9 km/h (5 kt) at 1955, shifting sharply to W 26 km/h (14 kt) by 2015,  the first indication that the low centre had moved inland. Winds shifted to the NW and accelerated to 46 km/h (25 kt) gusting 57 (31) by 2115, with gusts to 65 km/h (35 kt) at 2135 and 2155. At the same time, the pressure began rising dramatically. The altimeter jumped from 29.61" at 2055 to 29.74" by 2155, a full 4.4 hPa in one hour. The NW winds and rapidly rising pressure are classic signatures of the bent-back front.

Crescent City also reported a decent wind strike out of the NW octant. By 2154, WNW winds had climbed to 52 km/h (28 kt) gusting 65 (35). The maximum gust associated with the bent-back feature--so far--touched 70 km/h (38 kt) out of 300º at 2135. The pressure in one hour jumped 3.2 hPa to 101.08 kPa (29.85" Hg) by 2156. Arcata reported a gust to 72 km/h (39 kt) out of 310º at 2134. Newport reported winds NW 39 km/h (21 kt) gusting 54 (29) at 2135 and Astoria WNW 33 km/h (18 kt) gusting 57 (31) at 2155. Astoria also reported a squall at 2153, marking the sudden shift in wind from SW 15 km/h (8 kt) to NW 30 km/h (16 kt).

The timing of the maximum wind speeds up-and-down the coast pins the arrival of the bent-back front right around 2200 PDT--for just about everywhere along the shore. As expected, the wind speeds have not reached the magnitude of more intense storms in the past, like the Hanukkah Eve windstorm of 2006, or 07 Feb 2002. Soon the cyclonic tail will sweep across the Willamette Valley and other interior sections.

The north end of the wraparound band is just now reaching the Lower Mainland of British Columbia. We will probably get around round of cold, winter-like rain, with some west to west-northwest winds to follow. Currently, Sheringham, in the Strait of Juan de Fuca, is still reporting winds out of the ENE, so the shift to westerly has not occurred. The speed has slowed down from E 35 km/h (19 kt) at 1900 to 11 km/h (6 kt) by 2200. And the pressure has risen slightly, climbing a mere 0.2 hPa to 99.75 kPa (29.46" Hg) during the 2200 observation. So perhaps the expected westerly surge is about to arrive. Tatoosh Island is also still reporting E winds, at 35 km/h (19 kt) gusting 43 (23 kt). Interestingly, winds climbed to a powerful E 85 km/h (46 kt) gusting 94 (51) at 0900, as the low approached the coast far to the southwest, but at near-peak intensity.

At Vancouver International, close to my home, the 2200 report indicated a stratus overcast accompanied by light rain showers, a temp of 5.0ºC (41ºF), dew point 2.2ºC (36ºF), winds E 9 km/h (5 kt) and a pressure of 99.86 kPa (29.49" Hg), down 0.4 hPa in the past hour. Quiet, but rather chilly conditions.

Friday 30 March 2012

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.

Thursday 29 March 2012

Late March Bluster

The frontal system of a deep and mature cyclone reaches the Pacific coast of North America. Image time is 0100 PDT 29 Mar 2012. Infrared satellite photo courtesy of the US National Weather Service.

At 0130 PDT, the sound of the wind awoke me, and I listened to the trees roar under periodic heavy gusts. As with yesterday's bluster, the noise did not quite reach the crescendo of the 12 Mar 2012 windstorm, but did have some force. This latest front brought stronger winds to Vancouver, BC, than yesterday's shortwave trough (more on this below).

Peak Wind and Gust: km/h (knots)
Minor Windstorm 28-29 Mar 2012 (PDT)
Location
Wind
Dir
Time
Gust
Dir
Time
Vancouver
50 (27)
SE
0300
67 (36)
SE
0200
Abbotsford
44 (24)
SSE
0400
59 (32)
SSE
0400
Bellingham
48 (26)
SSE
0353
85 (46)
S
0358
Victoria
50 (27)
SE
0151
65 (35)
SE
0151
Comox
59 (32)
SE
0200
70 (38)
SE
0000
Port Hardy
28 (15)
ESE
0100
37 (20)
ESE
0100
Navy Whid
56 (30)
SSE
0422
81 (44)
SSE
0412
Everett
30 (16)
S
0653
54 (29)
S
0641
Sea-Tac
33 (18)
S
0153
52 (28)
S
0215
Portland
24 (13)
SSW
1053
43 (23)
SSW
1053
Solander I
100(54)
ESE
0000
137(74)
ESE
2300
Estevan Pt
56 (30)
ESE
2200
89 (48)
ESE
2200
Quillayute
30 (16)
SE
0053
70 (38)
SE
2233
Hoquiam
46 (25)
SSE
0053
72 (39)
SE
0054
Astoria
46 (25)
S
0055
72 (39)
SSW
0133
Newport
57 (31)
S
0235
80 (43)
S
0335
North Bend
44 (24)
S
0535
65 (35)
S
0555


Interestingly, save for the usual spot—Solander Island—the coast did not receive a particularly strong wind relative to much of the north part of the interior. The storm has not completely departed the region at the time of this writing, and I may have to update this table as winds could climb higher in places where the front has yet to pass through, such as the southern section. Plus, the broad, mature 96.9 kPa (at 12:00 UTC) low center is only slowly departing toward the north, moving to the west of Haida Gwaii. The region will be under the influence of this system for much of the day, though it is likely maximum winds have been achieved here in Southwest British Columbia.

Yesterday, a shortwave trough moved through the region in the late morning and early afternoon, triggering strong SE to S winds in places. Here in Vancouver, a very dark sky preceded the feature, with periodic waves of rain that climbed to moderate intensity at times. The precipitation did not persist long enough to produce major accumulations, with 6.8 mm (0.26") in total for the entire day at Vancouver International. Wind speeds and direction were more interesting.


Peak Wind and Gust (mph)
Minor Windstorm 28 Mar 2012 (PDT)
Loc
Wind
Dir
Time
Gust
Dir
Time
Vancouver
44 (24)
S
1400
59 (32)
S
1400
Abbotsford
50 (27)
S
1404
65 (35)
S
1404
Bellingham
46 (25)
SSE
1353
76 (41)
SSE
1408
Victoria
24 (13)
SW
1339
37 (20)
S
1512
Comox
33 (18)
SSE
1500
56 (30)
SSE
1500
Port Hardy
28 (15)
ESE
1600
41 (22)
ESE
1600
Navy Whid
61 (33)
SSE
1356
87 (47)
SSE
1405
Everett
50 (27)
S
1453
74 (40)
S
1347
Sea-Tac
39 (21)
S
1853
56 (30)
S
1142
Portland
48 (26)
SSW
1353
65 (35)
SSW
1505
Solander I
67 (36)
SSE
1700
80 (43)
SSE
1700
Estevan Pt
48 (26)
ESE
1200
69 (37)
ESE
1200
Quillayute
31 (17)
S
1453
54 (29)
SSW
1531
Hoquiam
43 (23)
SSE
1253
72 (39)
S
1728
Astoria
MM
MM
MM
MM
MM
MM
Newport
50 (27)
SSW
1255
74 (40)
SSW
1355
North Bend
44 (24)
SSW
1615
65 (35)
SSW
1555


Astoria had a reporting outage between 0655 and 1355 (not inclusive) on 28 Mar 2012, missing the shortwave trough response entirely.

Vancouver International reported peak winds out of the south. This is unusual. More typically, when strong winds arrive, Vancouver is visited by one of two classes of windstorm: the westerly surge with winds from a W-NW direction, or the southeaster with winds from a ESE to SSE direction. This is a product of the terrain surrounding Vancouver Metro, with favours winds from the ~NW down the Georgia Strait, or the opposite direction of ~SE up the Georgia Strait. East is also a favoured direction at Vancouver, BC, due to a strong tendency for offshore flow from the Fraser Valley in the cold season, but winds speeds rarely reach gale force from this direction. Yesterday's shortwave trough brought unusual strong southerly winds to Vancouver, a direction more typically seen at places like Seattle and Portland due to North-South terrain channeling by the Cascades and coast ranges. Strong south winds at Abbotsford are not unusual.

These two weather systems also exemplify that strong wind events can sweep through the region back-to-back in 12 hours or less. In a rare 24 hour period, three separate systems can affect the area, keeping winds going strongly for a long period of time--in other words, the lulls between systems may not get all that low. This can confound statistical techniques for calculating high-wind return intervals, such as the "method of independent storms".