Forecast Extratropical Cyclone Still on Track for a Possible Lowland Gale

22:24 PST: With the current front moving inland and fading away, it is time to return focus on the possible Sun-Mon windstorm. The latest WRF-GFS, 00:00 UTC 16 Dec 2012 is available, and I have looked at the output in detail. The forecast track is now further south, cutting across southern Vancouver Island in an eerily familiar way. I am reminded of the Hanukkah Eve storm of 2006, as the low tracks nearly over Vancouver (Steve Pierce, I note, has also noticed this similarity as reported in his weather blog for The Columbian). The modeled system is deeper, with the central pressure reaching 97.2 kPa before landfall. Despite the 97.0 kPa noted on the cyclone-track map in The Storm King, it appears that the 14-15 Dec 2006 windstorm had a minimum central pressure around 97.5 kPa before landfall (based on a more recent analysis that I have done). With terrain-induced isometric flow contributing to some filling as the anticipated 2012 low moves inland, the pressure climbs to about 97.6 kPa by the time the center nears Vancouver. This is still quite deep.

Figure 1. WRF-GFS forecast for 01:00 PST Monday 17 Dec 2012. In this prognostication, a 97.2 kPa low is about to land on Vancouver Island with the center fairly close to Tofino. An intense pressure gradient is evident in the southwest quadrant of the low. Strong west-southwest to northwest winds accompany this feature. Image courtesy of the University of Washington Department of Atmospheric Sciences. 

However, unlike the Hanukkah Eve windstorm, the forecast low originates further north and tracks closer to due east. In this manner, the modeled storm is more similar to the 13-14 Dec 2001 windstorm, though again with a lower central pressure at landfall (97.9 kPa for the 2001 event). A nearly due east track is not as supportive of strong S to SW winds in places like the Puget Lowlands as is a NE to N track. This is in part because upper support is more zonal and also because the entire eastward momentum of the system does not contribute to vectors pointing in a northward direction.

Figure 2. WRF-GFS forecast for 01:00 PST Monday 17 Dec 2012. This shows 50 kPa temp, heights and wind corresponding to lower atmosphere frame in Figure 1 above. At this height, the upper wind support is generally for W to WSW winds over much of Cascadia. This is not the best support for strong southerly winds in interior sections. Upper support is even more strongly for westerly winds in later frames, when the surface isobaric orientation (e.g. pressure slope) is even better aligned for south winds in places like the Puget Lowlands. Image courtesy of the University of Washington Department of Atmospheric Sciences. 

Figure 3. WRF-GFS forecast for 01:00 PST Monday 17 Dec 2012. This shows 85 kPa temp, heights and wind corresponding to Figures 1 and 2 above. At this height, the upper-wind support is still generally for W to WSW winds over much of Cascadia. Even at this level, there is not the best support for strong southerly winds in interior sections. As with the 50 kPa forecast, upper support is even stronger for westerly winds in later frames, when the surface isobaric orientation is even better aligned for south winds in places like the Puget Lowlands. Image courtesy of the University of Washington Department of Atmospheric Sciences. 

A nearly due east track is quite supportive of westerly winds, and the WRF-GFS output indicates that the fastest winds at many locations would be from a W direction—especially those places favored for strong westerly winds. This includes the Strait of Juan de Fuca and also a W to NW surge down the Georgia Strait. Western Vancouver and Southern Victoria are prone to strong winds from this direction. Gonzales Heights (CWLM) is an ideal place for picking up some serious gusts during a westerly surge event. Vancouver International is not far behind. These strong westerly winds are likely coupled with a bent-back front, and could arrive in rather sudden fashion.

WRF-GFS forecast for 10:00 PST Monday 17 Dec 2012. A strong WNW surge is shown in the southern Georgia Strait with event more intense winds over the eastern entrance of the Strait of Juan de Fuca. Image courtesy of the University of Washington Department of Atmospheric Sciences. 

There is indication of the possible development of a lee-low east of the Olympic Mountains, especially evident on hour 39, or around 07:00 PST Monday 17 Dec. Enhanced gradients to the south of this low could contribute to faster wind speeds especially along the east slopes of the Olympics and in the vicinity of the Hood Canal.

In any event this forecast storm is still not looking as strong as the 2006 Hanukkah Eve tempest even despite a deeper central pressure. Prognosticated maximum winds for Vancouver are around 35 kt (65 km/h) with higher gusts. This has been consistent for many model runs and is significantly short of the 45 kt (83 km/h) that occurred in the early morning hours of 15 Dec 2006. Some favored places in the Strait of Juan de Fuca may receive westerly winds of 45 kt, and perhaps just off the Washington and Vancouver Island coasts. Winds in the Puget Lowlands are forecast to perhaps reach 25 kt (46 km/h), maybe somewhat higher at times, well short of what occurred in 2006 and looks more like the 13-14 Dec 2001 storm.

This is not to say that whatever arrives on Sun and Mon cannot deliver a decent wallop. This is a dynamic situation and every storm is unique in its own way, meaning that the potential to surprise is always there. The actual track location will be key to determining outcomes. The models seem to be trending southward with each run. A path much further south, say a landfall between Hoquiam and Quillayute, would likely spare Vancouver, and perhaps even Victoria, a serious gale, while at the same time bring stronger winds to the Puget Lowlands and the Willamette Valley.

Of note, the time period 11-17 Dec is quite notorious for big storms. For that date range, eight high-wind events have occurred in the Grater Victoria and Lower Mainland in the 19 years 1994-2012.

Also, soils are quite saturated, with more rain on the way. In local parks here in Vancouver, whenever there is even a little rain, to step on the fields is to step in mud, or standing water. Soil-root cohesion is weakened when soils are water-logged. This means that there will likely be a greater risk of tree-falls during whatever winds may arrive. The frontal system that moved through today brought wind gusts in the 50-60 km/h (27-33 kt) range in in Vancouver Metro, and 80 km/h (43 kt) around Victoria and up in Comox. Toppled trees caused scattered power outages during this event, one indication of the potential for windthrow. 

Finally, if this WRF-GFS forecast verifies, barometric pressure could get quite low in Vancouver, perhaps the lowest in approximately the past five years. Barometer needles may approach 97.5 kPa (28.79" Hg). We shall see. 23:06 PST.

California's Storm

November 30, 2012: Friday

08:19 PST: (Vancouver, BC) On this last day of November, a steady rain falls from a leaden sky. The ground is soaked, street and sidewalks gleaming with wetness. A decent easterly wind shakes the leaf-bare trees in Tisdall Park. I estimate 20-30 km/h at times in gusts. The patio temp is a still warm 10.8ºC, with 80% RH and a still quite low pressure of 29.33".  Vancouver International at 08:00 reported light rain showers and mist with a temp of 9ºC (48ºF), dew point 7ºC (45ºF), wind ESE 22 km/h and pressure 99.34 kPa (29.34" Hg), up 0.4 hPa from a recent pressure minimum at 07:00.

The broad, stacked and semi-permanent 97.0 kPa low still sits over the Pacific off of Vancouver Island. Bands of moisture continue to swing around the base of this low and inland—such as the current rain producer.

Far to the south, this system has produced a decent storm in the San Francisco Bay Area. Strong winds, heavy rain and even thunder have occurred as the trailing cold front interacts with Northern California. The San Francisco Airport reported a maximum gust so far of 87 km/h (47 kt) out of 180º at 02:58. This gust occurred in association with a thunderstorm reported at 03:02. Low pressure also occurred at this time, with an altimeter of 29.77". Short period rainfall reached 6.1 mm (0.24") in the hour ending 04:56, with a total of 29.0 mm (1.14") between 10:56 yesterday and 06:56 this morning. The temp has hovered around 17ºC (63ºF) with a dew point of 16ºC (61ºF) during the main heavy rain episode—a remember those beautifully warm rains. They made for great walks. Winds at Oakland were not as strong, with a peak gust of 54 km/h (29 kt) out of 140º at 03:29. No thunderstorm has been reported there. Low altimeter occurred at 03:10 with a reading of 29.81", probably as the same feature that produced the thunder in San Francisco moved through Oakland. Heavy rain is continuing currently at both stations. Oakland reported 5.6 mm (0.22") in the hour ending 07:53 for the highest so far. However, the same amount, 5.6 mm, has fallen between 07:53 and 08:09, reflecting a very heavy dose of rain. Since 18:53 yesterday, some 36.83 mm (1.45") has fallen at Oakland.

December 1, 2012: Saturday

08:43 PST: The NWP models continue to show a developing cyclone move into Southwest Oregon overnight. The track is too far south for a serious high-wind threat up here. But I plan to follow the low closely. The water vapor satellite loop reveals an enhancing baroclinic leaf inside of 140ºW and along 40ºN currently.

21:07 PST: The expected low is developing well to the south. The 03:00 UTC HPC surface analysis indicated a 99.8 kPa open wave centered near 40ºN 129ºW. The water vapor satellite loop shows an enhanced dry slot and a rapidly ballooning baroclinic leaf in association with the surface low. A very fast jet stream is clearly associated with the cyclone, carrying upper-level cloudiness rapidly to the NE. Barometers all over the west coast seem to be responding to the developing wave, with downward trends even this far north.

Water vapour satellite view showing conditions over the NE Pacific at 20:30 PST 02 Dec 2012.  A marked baroclinic leaf is evident, with a mass of enhanced clouds moving into SW Oregon and NW California. An enhanced dry slot trails right behind the clouds--this is a well-defined an strong weather system. Image courtesy of the National Weather Service.

Crescent City has had some interesting weather over the past few days—not just the Bay Area received a good storm from this persistent system. On 29 Nov, during period with strong S winds gusting up to 67 km/h (16:44), extremely heavy rain arrived. A whopping 21.6 mm (0.85") fell in the hour ending 16:56, followed by 18.3 mm (0.72") in the next. However, the rain intensity reports ceased during that first hour, as of 16:52, with just mist being reported afterward, so this makes the data at lease somewhat suspect to me. Yesterday, 30 Nov, a thunderstorm with heavy rain occurred between 22:52 and 22:59, followed by high winds. The peak occurred at 00:42 this morning with S 67 km/h gusting 85 (36 kt G 46). This is probably the response of the wet and blustery wave that moved through here a few hours later. The thunderstorm dropped 7.4 mm (0.29") of rain in the hour ending 22:56. During the morning of 29 Nov, Arcata received 11.7 mm (0.46") of rain in the hour ending 09:53, but only had light rain during Crescent City's supposed deluge. Winds have not been as strong, with a peak gust of 59 km/h (32 kt) out of 200º at 23:55 yesterday, and a gust to 67 km/h (36 kt) out of 120º at 07:53 on November 28^th. 21:34 PST.

December 2, 2012: Sunday

08:13 PST: (Vancouver, BC) The southwest Oregon low did not quite get fully developed as it moved ashore, remaining an open wave. Northern California has received, and is still receiving in places, a strong storm from this system. Further north, plenty of rain moved through Oregon, Washington and Southwest British Columbia, but winds were significantly slower.

The strong frontal boundary is moving through N California as of the 07:30 PST 02 Dec 2012 time of this water vapour satellite image. The boundary has not yet reached the Bay Area at the time of this image. A large mass of upper-level moisture has moved over Idaho and Montana--the storm is heading into the Intermountain Empire. Strong southerly winds have picked up in the Snake River Plain area in response to the approaching system. Image courtesy of the National Weather Service. 

Here in Vancouver, we had a brief period of moderate rain at 04:00, just ahead of pressure minimum, a quite low 98.73 kPa (29.15" Hg) at 05:00. The low barometric readings are probably the most interesting aspect of this storm up here. Winds have not been much, with a maximum of E 22 km/h gusting 31 (12 kt G 17) at the time of lowest pressure, accompanied by light rain. We recently had a wind shift, to light ESE at 06:22, then S 17 km/h (9 kt) at 07:00, followed by SW 17 km/h at 07:40 and SSW 19 km/h gusting 31 (10 kt G 17) during the most recent observation at 08:00. Looks like the rather weak occluded front moved through. The pressure had climbed to 99.0 kPa (29.23" Hg) as of the latest report, up a strong 1.5 hPa/hr. The temperature hovered around 7ºC (45ºF) for much of the night and is still at this level. The dew point fell a degree to 5ºC (41ºF) by 08:00. Vancouver reported 10.8 mm (0.43") of rain yesterday. Westerly winds are expected to pick up today, but I am not expecting them to be very strong. Our semi-permanent stacked low, now 98.0 kPa and centered right over Haida Gwaii, is currently supporting a SE pressure slope over the area, not the SW slope that is very conducive to westerly surges.

Looking at California, Crescent City reported a peak wind gust of 85 km/h (46 kt) out of 170º at 23:41. During the peak wind phase, where 2-minute speeds ranged from S to SSE 50 to 61 km/h (27 to 33 kt) between 11:22 and 11:52, light to moderate rain fell. Just ahead of the strongest winds, a period of heavy rain brought 7.4 mm (0.29") in the hour ending 09:56. Low pressure occurred at 21:56 with a reading of 99.51 kPa (29.38" alt). Winds shifted to W by midnight and diminished considerably, accompanied by a temperature drop from 14.5ºC (58ºF) at 23:56 to 12ºC (54ºF) at 12:01—just five minutes. Clearly marking frontal passage. A soaking 48.5 mm (1.91") of rain fell in the 24-hr ending 03:56.

Arcata had 56.1 mm (2.21") of rain in the same 24-hr (ending 03:53 in the case of this station). Periods of moderate to heavy rain left as much as 8.6 mm (0.34") in the gauge in the hour ending 01:53. Low pressure of 99.76 kPa (29.45" alt) occurred at 00:53, right at the time of maximum winds: SW 43 km/h gusting 67 (23 kt G 36). Heavy rain immediately followed by 01:06 and winds had shifted to WNW 30 km/h gusting 43 (16 kt G 23). The temp fell from a lofty 15ºC (59ºF) to 12ºC (54ºF) during those same 13 minutes. Frontal passage. Further south, Ukiah had a good dose of heavy rain with 11.7 mm (0.46") in the hour ending 04:56. In the 24-hr ending 03:56, 52.3 mm (2.06") fell. Note that an additional 30.5 mm (1.20") fell after this up to 07:56. The strongest winds occurred during frontal passage with WNW 33 km/h gusting 72 (18 kt G 27) at 04:45. The temp fell from 17ºC (63ºF) to 12.5ºC (53ºF) between 04:26 and 05:56.

Red Bluff reported 17.5 mm (0.69") of rain in the 24-hr period ending 03:54, though the bulk of precipitation associated with the front fell after this time. A soaking 13.2 mm (0.52") fell in the hour ending 05:54, in association with the frontal boundary. Low pressure occurred at 03:54 with a reading of 99.82 kPa (29.48" alt). Ahead of this front, winds climbed to SSE 72 km/h gusting 83 (39 kt G 45). A powerful peak gust of 100 km/h (54 kt) occurred out of 140º at 03:40. Frontal passage happened between 04:54 and 05:54 with the wind shifting from S to SW then WSW and diminishing rapidly. Nearby Redding had 12.2 mm (0.48") of rain in the hour ending 04:53 and wind gusts up to 106 km/h (57 kt) out of 170º at 03:00. Low pressure of 99.49 kPa (29.39" alt) occurred at 03:53. Note the strong 3.3 hPa gradient between the two stations.

The front is just now sweeping through the Bay Area, and I plan on reporting some of the results later. Southerly wind gusts at San Francisco have exceeded 80 km/h (43 kt) at times.
Some other 24-hr rainfall totals for the 24-hr period ending approximately 04:00 include: Eugene 31.5 mm (1.24"), Portland 15.5 mm (0.61"), Olympia 7.4 mm (0.29"), Sea-Tac 8.1 mm (0.32"), Bellingham a mere 1.0 mm (0.04"), Astoria 25.4 mm (1.00"), Hoquiam 33.0 mm (1.30") and Quillayute 24.4 mm (0.96"). 

The sun is shining right now, with much clear sky showing between broken stratus clouds. The yellow low-angle light paints Tisdall Park in a beautiful light. A waning gibbous moon hangs low to the west, a crisp bluish-white orb hanging in the clean blue sky. The wind is calm and the patio temperature 8.5ºC with a 83% RH and pressure of 29.27", rising. 09:04 PST.

Surface analysis overlaid on a satellite photo composite for 04:00 PST 02 Dec 2012. The frontal wave is moving through Oregon at this time, with a strong cold front sweeping into N California. Image courtesy of the Hydrometeorological Prediction Center.

16:45 PST: The frontal boundary and associated wave has moved well inland and to the south now. Even Los Angeles reported some rain early this afternoon as the boundary moved through.

Some of the Bay Area outcomes with this storm include: Heavy rain moved through San Francisco with 7.1 mm (0.28") falling in the hour ending 07:56 this morning. Wind speeds climbed to S 63 km/h gusting 81 (34 kt G 44) at 06:26, with a peak gust of 85 km/h (46 kt) out of 180º at 07:00. Low pressure occurred at 07:56 with 100.80 kPa (29.77" alt). In the 120 hours ending 15:56 this afternoon, San Francisco has had 103.9 mm (4.09") of rain.

Oakland reported 6.6 mm (0.26") of rain in the hour ending 07:53, accompanied by SSW winds of 52 km/h gusting 70 (28 kt G 38) and a mild 16.5ºC (64ºF) temperature. Winds shifted to W by 08:35 and had diminished to 20 km/h (11 kt), with the temp falling to 16ºC (61ºF), all the mark of frontal passage. Oakland had a peak gust of 72 km/h (39 kt) out of 190º at 05:54. In the 120-hrs ending 15:53, this station received 87.9 mm (3.46" of rain).

San Jose had a soaker with 13.2 mm (0.52") in the hour ending 09:53. Maximum winds were not particularly strong with a peak gust of 56 km/h (30 kt) out of 130º at 06:13. Low pressure occurred at 07:53 with a reading of 101.00 kPa (29.83" alt): that makes a decent 2.0 hPa gradient between San Jose and San Francisco at this time. In the 120-hrs ending 03:53, 62.2 mm (2.45") of rain fell at San Jose.

Santa Rosa received a real deluge with 21.3 mm (0.84") of rain in the hour ending 05:53, of which 9.9 mm (0.39") fell in the last 13 minutes. This occurred right at frontal passage, with the wind shifting from S to W and the temp falling from 16ºC (61ºF) to 13ºC (55ºF). Low pressure occurred at 04:53 with a reading of 100.34 kPa (29.65" alt). Winds climbed to a peak of S 48 km/h gusting 74 (26 kt G 35) at the same time. Some 25.7 mm (1.01") of rain fell in the 24-hr ending 03:53, with an additional 37.3 mm (1.47") afterward. Santa Rosa has certainly received a soaking, with a whopping 196.1 mm (7.72") in the past 120-hr ending 03:53. This is 179% of their November normal precipitation (1971-2000) and 171% of the December normal—take your pick, as the past 120 hours occurred in both months. This comparison provides some idea about the magnitude of precipitation in N California.

Ukiah, incidentally, had a little bit more precipitation than Santa Rosa with 202.9 mm (7.99") in the same time period. Arcata received 150.6 mm (5.93") and Crescent City 190.2 mm (7.49").

Looking quickly at Idaho, Pocatello reported a peak gust of 106 km/h (57 kt) out of 160º at 10:35 as the front approached. That about wraps up this storm.

As of the 21:00 UTC HPC surface analysis, the long-lived stacked 98.0 kPa low was centered NW of Haida Gwaii. Seems to be slowly moving away from here. 17:27 PST.