Thursday 20 December 2012

Vancouver, BC, Snowstorm of 19 Dec 2012

When I think of snow situations, I do not typically think of southeast winds. However, on 19 Dec 2012, a snowstorm developed during a SE gale at many places along the Georgia Strait.

For Vancouver, BC, as storms track toward the region, a cold, dense surface layer typically pours westward on strong E to NE flow out of the Fraser Valley. SE to S surface winds then tend to dam the air against the high and steep North Shore Mountains. When warm sector air being advected northward encounters this cold offshore flow, the lower-density air simply rides over the colder denser layer. Vancouver is then shielded from strong southerly winds. Temperatures will rise significantly up at Cypress and other ski areas while Vancouver Metro stays cold (e.g. temperature relatively unchanged). Depending on the situation, this cold air dam can completely protect Vancouver for the duration of a southerly wind event. It does not always happen this way. If the easterly flow is cut off at any point, the cold surface layer erodes away fairly quickly.

This is what happened on 19 Dec 2012 (yesterday). The result: Steady snow in Vancouver, up to moderate intensity at times, with cold E winds while a SE near-gale to gale raged in places like Bellingham, Victoria and Comox. Locations that received the SE winds tended to have temperatures a bit too high for snow. For one example, look at these reports from 12:00 PST yesterday:

Victoria: Heavy rain, temp 5ºC (41ºF), wind SE 48 km/h gusting 61 (26 kt G33)
Vancouver: Moderate snow, temp 0ºC (32ºF), wind E 26 km/h (14 kt)

At the above 12:00 observing time, a band of moderate to heavy precipitation stretched from Victoria to Vancouver and beyond, marking the frontal boundary.

To make things even more interesting:

Sand Heads: 4.5ºC (40ºF), wind SSE 65 km/h gusting 76 (35 kt G 41)

Sand Heads is approximately 10 km SW of Vancouver International. Quite a contrast over such a short distance. 

Vancouver International reported snow, generally light but sometimes moderate, from 03:34 to 12:46 before the cold air layer became shallow enough to for the precipitation to fall as rain. The transition was abrupt, with the snow going to rain in about 8 minutes. The entire snow event at Vancouver occurred with temperatures right on the margin, ranging from 0-1ºC (32-34ºF), and classic "Cascade concrete" accumulated. A weather observer friend of mine reported a 2.5:1 snow water equivalent at his home in Vancouver. This made shovelling the soggy white stuff off of sidewalks quite difficult.

The above observer measured about 10 cm (4") of snow at his house. Shortley after the snow changed to rain, I took a series of measurements in Tisdall Park near my home, shown in the table below:

Sample # Depth (cm)
1 8.4
2 7.0
3 7.9
4 7.8
5 6.0
6 7.8
7 7.2
8 7.3
9 8.2
10 6.0
Average 7.4
Max 8.4
Min 6.0

As I took the measurements between 13:24 and 13:36, a steady, thudding moderate rain pummelled the accumulated snow and I. The snow gradually melted even as I collected my data, but the liquid assault probably only reduced the totals by a small amount. With the temperature just above freezing during much of the snow event, melting likely had been occurring for some time. Had the temp been cooler by a degree or two, the accumulation would have been deeper, perhaps 10-12 cm. The ongoing melting and high water-content snow prevented that beautiful "winter wonderland" appearance from lasting very long. The snow thudded and dripped from the trees at a rapid enough pace reduce the white frosting to a minimal amount between bands of moderate snow. The gusty east wind helped shake the slush from the branches, though earlier in the morning it contributed to a beautiful white pasting on the east side of trees and light poles.

My average of 7.4 cm (2.9") makes the 19 Dec 2012 snowstorm the deepest event for 2012. This number reflects snow measurements largely on hard surfaces such as asphalt and wood. I also took a series of measurements across the grass field at the park and returned an average of 10.2 cm (4"), which agrees well with the depth at my friend's home. This event, at my location, outclassed the series of light snowstorms we had between 14 and 20 Jan. Yesterday's storm reversed the situation entirely: Back in January, Abbotsford and Victoria received significant snowfalls while Vancouver received little. This time around, Vancouver Metro received a much bigger dose than the other two locations.

The slow-moving frontal system (depicted below) responsible for the wind, rain and snow brought a hefty dose of precipitation to Vancouver. The airport reported a soaking 43.8 mm (1.72") for 18 Dec. The effects could be seen today: Gigantic puddles, soil saturated to mud, water streaming over park paths, sidewalks and ponding in roads in places. Slush plugging up storm drains contributed to the latter.

Figure 1: Visible satellite view taken at 19:30 UTC (11:30 PST) 19 Dec 2012. A moisture-rich frontal system associated with a mature low in the Gulf of Alaska brought wind, rain and snow to Cascadia. Image courtesy of the US National Weather Service.
The Hood Canal area of Washington also received some decent snows from this storm. This is another region that experiences the damming of cold air against a mountain range. In this case, offshore winds shove colder, denser air against the east slopes of the Olympics, leaving the Hood Canal vicinity well-chilled. Yesterday, 15 cm (6") of snow accumulated at Seabeck. At least 1-2" (2-5 cm) accumulated around Shelton and Bremerton. Ahead of the warm front, the freezing level hovered around 150 m (500 ft) or even less over much of the region. Some hilltop locations in Southwest Washington, around 200-250 m (700-800 ft) reported 12-14 cm (5-5.5") of snow.

Looking at a few peak gusts at locations that experienced the strong SE wind:

Victoria: SE 70 km/h (38 kt) 08:15 & 07:40
Comox: SE 85 km/h (46 kt) 09:14 & 11:18
Saturna Island: S 100 km/h (54 kt) 14:53
Sand Heads: SSE 85 km/h (46 kt) 12:53 & 13:26
Bellingham: SE 78 km/h (42 kt) 19:44
Navy Whidbey: SE 96 km/h (52 kt) 17:22
Friday Harbor: SE 83 km/h (45 kt) 09:32
Hoquiam: SE 67 km/h (36 kt) 12:11
Astoria: S 87 km/h (47 kt) 13:54

Underachievers:

Quillayute: SE 52 km/h (28 kt) 08:30
Vancouver: E 52 km/h (28 kt) 13:14 & the same speed from the ESE at 16:00

Winds were not particularly strong at Quillayute, but they did receive a thunderstorm with heavy rain at 09:11. This with a temp of 4ºC (39ºF). The station reported 6.9 mm (0.27") of rain in the hour ending 09:53, some of which fell during the lightning show. A juicy 63.5 mm (2.50") of rain fell in the 24-hr ending 03:53 on 20 Dec at this coastal station.

BC Hydro reported numerous outages from both strong winds and snow, often from toppled trees hitting power lines, on both sides of the Georgia Strait, and inland. Some of these outages involved thousands of customers. A power outage at the University of British Columbia, likely due to a snow-laden tree hitting the vulnerable transmission line that cuts right through well-forested Pacific Spirit Park, contributed to the closure of the campus and the cancellation of numerous finals.

In closing here are some photos taken in and around Tisdall Park, in the Oakridge area of Vancouver:

Moderate snow at high noon (12:01 PST).

No baseball today.

Building snow-folk and throwing snowballs proved more interesting to children than this play area.

New park visitors.

Walking to school. Most public schools stayed open despite the slick, wintery conditions. Many private schools and some university campuses such as Simon Fraser University did close.
Cascade concrete on Ash St. A sloppy, slippery mess.








2 comments:

  1. Very insightful analysis. Can we conclude that for Vancouver in these situations the switch in wind direction from NE/E to SE/S is the indication that milder air is about to invade and snow is about to become a mix or rain?

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    Replies
    1. Yes, basically that is what I would expect. However, the wind direction change from E to SE (or S) may actually occur after the precipitation changes over to rain (this happened on 19 Dec). As the cold surface air layer erodes, it becomes shallow enough to allow rain to reach the ground before the SE winds finally surface. When the cold air layer is frigid enough, this can result in a period of freezing rain before the warmer air moves in on the SE winds. In the case of 19 Dec, the magnitude of cold barely supported snow--hovering around 0 to 2ºC--keeping freezing rain to a minimum as the cold air layer reduced in depth.

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