Saturday 15 December 2012

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.

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