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.