Backcasting

We’re starting to give up on Environment Canada forecasts. It is likely that staffing and budgets are at play so I’m not blaming the organization but depending on the region and the time of year the forecasts are either consistently under or over forecast.

In our experience, the winds in the Gulf Islands are consistently over forecast in the summer. When they predict 25 knot winds, we assume we’ll see 15 or less. We always assumed this was to reduce the number of new/infrequent or charter sailors from getting themselves in trouble by going out in winds that were too strong for themselves.

When we arrived at the North tip of Vancouver Island we found that the winds were not extremely accurately forecast but at least the errors were on either side of the true wind – sometimes stronger, sometimes weaker. Normal forecasting errors.

So far, none of this is frustrating at all. Weather is difficult to forecast and the idea of protecting boaters from themselves (and thus protecting the poor underfunded Coast Guard who has to go rescue them) is probably for the best.

What is a weird is that regularly this winter they have been *under* forecasting the strength of the winds, often substantially and then amending their “forecast” to represent actual conditions. For example, recently they were forecasting SE winds in the region we are in at 20-25 knots. Between the morning forecast and the afternoon forecast, the winds were 30-35 knot and then increased to 35-40 knots. As the wind increased, they amended their prediction to 30-35 and then 35-40. For vessels that are comfortable in 25 knots but not 40 this is quite a big change in forecast and it would occur while they were already underway. This happened regularly while we were in Victoria as well.

Again, this would be only mildly frustrating to us as we always assume the forecast is only a guess anyways, however we have been watching the grib files as part of our own weather self-education and time after time we can tell that the winds coming in will be stronger than the forecast. The Environment Canada have access to at least as much information as we do, and they are “experts”, so the underforecasting and then backcasting is just bizarre.

On both the W Coast of Van Isle and our winter sailing trips we have found the grib files to be generally better predictors of actual conditions than the forecasts. - Livia

The gribs are better on the West side than on the inside because of the funneling effects of the land masses which the grib files don’t show. This is where the VHF forecasts really add valuable information. Grib files show systems coming through (lows/highs/troughs/ridges) but Environment Canada does a better job of predicting the direction of the wind between the island and the mainland than gribs because of their local knowledge and it is also important to understand that local physical land masses will funnel the wind in some regions into higher speeds than the grib will show.

Also, it is important to look at the strength of the low or high pressure system. One example of where Environment Canada was underforecasting is when a low pressure system around 980 millibars comes through just South of Alaska. When those come through, the isobars get tight, the grib shows strong winds, and, no surprise, we get strong winds.  Every time this happened you could add 10-15 knots to the Environment Canada forecast. - Carol

3 comments:

  1. We agree! Just yesterday we took advantage of a sunny afternoon (and Carol's sister visiting from Manitoba) to head out from Nanaimo for a little sail... our first time under sail. Sunshine and the forecasted 10-15kts was perfect for our novice venture. What we got was whitecaps and 20-25kts! While plenty of fun, there were moments when we would have really appreciated at least a bit more knowledge.

    Carol and Lance

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  2. Here's how I interpolate wind forecasts:

    Less than 10 kts means-- No Wind Today
    10-15 kts means--Maybe some wind but not much
    10-20 kts means--You'll have wind but nobody knows whether it'll be a lot or just right
    15-25 kts means-- Someplaces will have a bit too much and others won't have much at all so you'll be reefing and unreefing the main today
    20-30 kts means-- Go see a movie or bake cuz it's probably more wind than you want
    25-35 kts (or more) means you don't really want to leave the boat cuz getting back in the dinghy won't be fun or alternatively, put out extra dock lines

    As for wind directions- well it's either got a N'ly component or a S'ly component but may vary by up to 90 degrees!

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  3. @Carol&Lance - That is a fair amount of wind for a first sail!
    @Dryfeet - we *love* it. Both laughed out loud while reading it. Too true.

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