Wednesday, April 4, 2012

Southern Straits of Georgia Wind Forecast

This exercise would apply to the Southern Straits of Georgia Yacht Race or any other sailing or rowing event in these waters. This is a preliminary look at the winds, made at noon on Wed for a race starting Friday morning.

We are close enough now (72 hours) to start getting good data The course is 135 nmi. With a SE  wind there is one weather leg of 30 nmi which adds another 15, so we are roughly 150 nmi. other wind directions make the race longer. Fast boats in good wind will finish Saturday morning in good condition, but many boats will race throughout Sat as well.

This note is intended as a learning process, so you can make similar forecasts on your own.


Start with the official Environment Canada forecast page:

Here we see a forecast (from wed noon... this might be different when you look later on) for the whole race course (Strait of Georgia, north of Nanaimo) of

Friday Wind variable 5 to 15 knots.
Saturday Wind light becoming easterly 10 to 15 knots.

This makes it an absolutely bueautiful practice exercise, because variable means they do not know the direction, and 5 to 15 is essentially the same as 0 to 15 meaning they do not know the speed.

Also note that "Str. of Georgia, North of Nanaimo" zone goes all the way north to Cape Mudge, so the race takes place only in the southern half of this northern zone.

But they do have a clear trend forecasted, going easterly 10 to 15 on sat, but they do not say what time sat.

A fast boat will be done early sat, daylight or so in good wind, definitely later in the day if light air.

Our job now is to see if we can do any better.  Maybe yes, maybe no, but we do not know yet.

We rely on the model calculations from UW Atmospheric Science dept, from the team of Cliff Mass, whose weather blog we link to here on the side. It is a great source of local weather and much general information as well.

As far as i can tell, the only way we can see the UW forecasts for 84 hr is use their 4-km data, which we can read from this page:

Then go to 4 km Surface, then to Western WA 10m Wind Speed  and click the loop, then stop the loop and step through the maps. From these pages we can get a small peek into the region west of Vancouver, marked here with a red box. (I tried posting direct links but they are not dependable.)

Then stepping through you can see the predictions for wind in the Georgia Basin. We have cut out and pasted the region of the race course. Then I have read the map west of Vancouver to get the wind predictions marked in blue.

In principle, we could get a nice summary of these observations from the meteograms of the same data.

These are online at

to use them, click a spot in the middle of the region marked with a red box and ask for the meteogram and you get something like this,

These only go out 72 hr, so we do not see the race yet.  Here we can only see the hour just before the maps above, which for the center of the region is about right.

Once you are sure the meteograms are doing the job of depicting the maps, and you get closer to race time, you can print a meteogram for key points in the course, such as Pt Atkinson, Halibut Bank, Balennas and Entrance Island and in principle have a nice forcast.

Then of course listen to the reports when underway. The Canadian Marine Weather Guide tells where you have reports. In principle once you get close to live times, the local VHF radio weather should be pretty close.

To my knowledge, this procedure here would be the best way to make your own forecast.  In this example we see slight variations in the details over the EC forecasts, ie we see a prediction for going NW later on Sat, but the wed forecasts from EC predicts only E on Sat.

We also see that these predictions go easterly 11pm Fri night, so we do not have to guess when on Sat will they go east.  We still have to guess this is right, but you start out with more specific guesses, which are in fact based on the best science for such predictions that is available and we are grateful for the good work of the UW meteorology dept  in providing these.

On race day one could  switch to this source of data which has higher resolution. Go to the UW models link above, but then selection 1.33-km Surface winds for Northern Waters. This will  be higher resolution data on a larger scale chart, but it only goes out 6 hr.


Anonymous said...

"....because variable means they do not know the direction, and 5 to 15 is essentially the same as 0 to 15 meaning they do not know the speed......"

It really means that it's difficult to discern an accurate average wind direction and speed over a time period and the area being forecast for (the larger the area, the difficult the task. This could be a result of the synoptic situation and/or terrain effects, shower or thunderstorm activity. In light winds, say less than 6kn, one can see the direction wander around the compass. It's not that forecasters don't know.
Sorry David but I have to protect we forecasters.

kenn batt said...

I posted this David I don't understand how Anonymous got there.
Kenn Batt

david burch said...

OK, i think i see the problem, i cannot comment on my own blog when i am logged in... at least that seems to be the issue. so i will try.

As a response to Anonymous.

I see your point and apologize for the wording. My goal was to comment on the wording of the forecast itself, not on the forecast, and especailly not on the talented folks who make these forecasts.

My use of the word "they" over personalized this in a way not intended. I am the first one in line to praise Pacific NW forecasters and their model work. In fact, my goal here is to show that they are doing a better job than we can get from more conventional public sources.

My description would have been better put as "the direction could not be predicted for the present conditions."

my observation that 5 to 15 means in practice 0 to 15 I think is based on my own experience. I am not sure if the intention was to imply the winds will not be calm, but a sailor who sees winds going from N to S in minutes, staying under a few knots, tends to think of parts of this transition as effectively calm.

In passing, we have found a better way to present this data and are working on it now.