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.
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.
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.
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.
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