Thursday, September 16, 2004

Sue posted a comment that points out that west coast weather forecasts are as often wrong as they are right. There are several reasons for this.

First of all weather on the coast is local. VERY local. So what might be happening in Vancouver, less than 15 kilometers away as the raven flies could be totally different here. Two obvious examples are the rain and wind. Bowen lies nestled in the mouth of Howe Sound. When clouds are driven into the sound, they pile up against the North Shore mountains. Naturally they drop rain there, as the air loses density and temperature as it rises to flow over the mountains. It can be pouring for days on the north shore and be dry in Vancouver. And at the same time it can be sunny for days on end in White Rock, 50 kilometers to the south.

We have unique winds in Howe Sound. Typically in the fall, we get the southeasterly winds as the low pressure systems move in from the Pacific. These winds swirl around and deliver massive amounts of rain to our doorsteps. That's the kind of wind we're getting now. THese systems tail off a bit in the winter, but they are still the source of our heaviest rainfalls. They are known here on the coast as "The Pineapple Express" because the warm, wet air they funnel up here originates in Hawaii. Some days you would swear that you smell flowers on the wind.

When the systems pass over us, the winds back to northwesterly, coming down the Georgia Strait. This brings clear skies and cooler weather and big waves to the west side of the island, and to the beaches in downtown Vancouver. In Vancouver on stormy, rainy and windy days you would be surprised how calm it is on the water. Once the winds back and the rain stops, the big rollers make their way into English Bay, slopping over the seawall and bring great windsurfing conditions to the city.

Here in Howe Sound we also get katabatic winds which are caused by huge masses of air cooling over the mountains and flowing downhill (cold air is heavier, remember). These winds, called "Squamish winds" rip through Howe Sound in the winter, bringing bitter windchills on winter days, toppling trees and bringing down branches on the north and exposed eastern sides of the island. Typically the Squamish blows like a narrow river down the Queen Charlotte Channel between Bowen and the mainland, and can be so strong that it whooshes clear across the Strait of Georgia and right into Nanaimo. Next time it's blowing really hard I'll post a picture. In 1990 a sustained Squamish wind toppled hundreds of trees on Bowen and knocked the power out for a week. It was one of those seminal events in Bowen history for which you were either here or you weren't.

When the Squamish outflow wind is howling, it'll be calm and pleasant in Vancouver. The Squamish is so consistent at the head of the Sound that the town of Squamish has become a major destination for windsurfers. The winds blow all day long during the summer and let up around 5:00pm. Most days, if it's hot at sea level, the glacier cooled air can rush down hill at speeds of 40km/h or more making for great windsurfing conditions. In summer these winds, despite their speed, generally dissipate before they get to Bowen in any kind of force.

Bowen is also subjected to anabatic winds, which are inflow winds caused by the air rising over the land at the head of the Sound and drawing in cooler air from the Strait. These winds are not so strong right on our island, but the can get up to speed further down the Sound at Pam Rocks and Anvil Island. They aren't cold like the Squamish winds.

To make matters more confusing, the east side of Bowen gets a little more rain that the west side, and the north side gets different weather than the south. So if you live on Hood Point, you might be bracing through a cold and wild squamish wind, dodging falling branches and stopping leaks in your roof while your fellow islanders in Tunstall Bay are remarking on how pleasent the wether is. And vice versa, when the northwesterly is blowing.

For this reason, my weather links contain reports from many different stations. The Point Atkinson and Gibson's stations give me accurate weather when the southeasterlies are blowing. The Pam Rocks and Squamish stations are good for gauging the strength of the Squamish winds, and the Gibson's station is my "go to" for the weather produced by northwesterlies, although I know that I won't suffer the winds here on the east side of Bowen.

For regular weather, a combination of the Squamish and Vancouver forecasts generally gives me an accurate temperature and rainfall forecast. I usually split the difference between the two.

By far the best general source for weather information is the surface analysis maps produced by the NOAA in the States. The north Pacific weather maps shows the major systems, where they are coming from and what they are doing. Generally I can predict our weather from these alone. Low pressure systems passing over head will bring southeast backed by northwest winds and lot of rain. If the system passes to the north, pleasant weather will ensue and if they pass to the south, it'll be cool and a little wet. High pressure is nice, but will also produce the inflow and outflow winds. In the winter the Squamish will be cold, and in the summer the temperature will be hot and the air reasonably still.

So there you go Sue. That's what I have learned from watching the skies around here for three winters. Your mileage may vary, and you only live a couple of kilometers away.