Friday 14 December 2012

Possible Windstorm For Vancouver Metro and Surrounds on Monday 17 Dec 2012


NWS Seattle is still watching a potential windstorm situation for Monday. One model brought the low over Portland, which would have a completely different outcome, but the other models continue bringing a strong low across Vancouver Island. The latest WRF-GFS, 12:00 UTC 14 Dec 2012, brings a 97.7 kPa (28.85" Hg) cyclone ashore very near Estevan Point. Interestingly the gradient near the center is not very strong—probably indicating a mature low—but is quite tight further south and west. The strongest winds for Vancouver may be to the rear of the low as a westerly surge. This scenario appears somewhat similar to 25 Sep 1999, though the modeled low is deeper in this case suggesting that the potential for damaging winds is a bit higher. 08:38 PST.

WRF-GFS output of sea-level pressure, temperature at 92.5 kPa and wind at 10 m for 04:00 PST 17 Dec 2012. This image shows a strong, but mature, 97.7 kPa cyclone landing near Estevan Point on Vancouver Island. This is a classic setup for high winds in the Lower Mainland and Greater Victoria, and also the Oregon and Washington coast, Northwest Interior and possibly the Puget Lowlands. Image courtesy of the University of Washington Department of Atmospheric Sciences.

09:34 PST: I took a closer look at the WRF-GFS model and my own regression modeling. Note that my regression model is preliminary. If we assume the low has a central pressure around 97.5 kPa and tracks across northern Vancouver Island, then my model predicts an average peak gust for Victoria, Vancouver and Abbotsford of 41 kt (76 km/h). Since peak gusts tend to vary between the stations, the average could contain an instance of 48 kt (89 km/h) or higher, my own minimum cutoff for "high wind" that allows for some error in measurement. Some examples of high-wind generating cyclones with a central pressure at landfall around 97.5 kPa include 04 Feb 2006 with an 3-station average peak gust of 42 kt (78 km/h) and 11 Dec 2006 with 49 kt (91 km/h). Both these storms produced high-wind criteria gusts in the Lower Mainland and/or Greater Victoria.

The WRF-GFS indicates that the highest winds for Vancouver may occur as the low passes inland to the north of the station in a classic westerly surge event that may climb to 35 kt (65 km/h), which may bring gusts upwards of 45 kt (83 km/h), arriving around 15:00 PST on 17 Dec. Again, quite similar to 25 Sep 1999. This westerly surge may be linked to the passage of a well-developed bent-back front. As the low tracks toward The Island, easterly winds could climb to 25 kt (46 km/h) Sunday night on the 16 Dec.

Interestingly, the NWP model suggests that the pressure in Vancouver could fall as low as 98.0 kPa (28.94" Hg) around 04:00 on Monday. My own regression model predicts a minimum around 99.0 kPa (29.23" Hg). Some of the difference may be due to the track, which is nearly due east in the case of the expected storm and generally ENE to NE on average which is reflected in the regression model. Probability, however, is on the side of the higher minimum-pressure prediction, and perhaps one might expect a minimum at Vancouver between these two values. 09:56 PST.

6 comments:

  1. Very interesting--this bears watching as more info becomes available. I am glad the overhanging tree beside me was recently cut down.

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    1. One thing that I forgot to mention in the post--I am a bit concerned that the ground here in Vancouver is quite saturated, with the soil gone to mud and standing water in places, and more rain on the way for tonight. With water-logged soils, trees are more likely to uproot in a strong wind.

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  2. Great blog post! Very interested from Federal Way, Washington. I couldn't seem to find the info anywhere, but is this blogspot authored by Wolf Read?

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    1. Yep, that is me. Thanks for the kind words. I am planning a new post on this upcoming storm either later tonight, or more likely tomorrow morning.

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    2. Ah, very cool; huge fan of your Storm King page. I still remember the day I stumbled on it almost a decade ago and have had it bookmarked on every browser on every computer since. Can't wait for the blog post!

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    3. Thanks much. I am always glad to hear that someone is using the information of The Storm King.

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