I have repeatedly written that when we are analyzing weather forecasts (GRIB-based included) and making decisions based on them, we must always remember that:
A weather forecast is only the most likely of several possible outcomes.
Me
For example, in this chapter in our Online Book on weather, I relate the story of how post-Hurricane Maria produced far more violent conditions than those being forecast just a day before and, worse still, from the opposite direction than predicted.
At the time, three yachts, including us on Morgan’s Cloud, were transiting southern Labrador and the Straits of Belle Isle in September, no place to get caught out in unexpected storm force conditions, let alone from a different direction than forecast. Luckily, I had recognized this possibility a full five days before it happened, giving all three boats plenty of time to find shelter from the actual wind direction.
I was warned of the potential danger because I was getting and analyzing 500 mb charts. But the problem is that it takes a lot of study and practice to learn this skill and many cruisers won’t have the time or inclination. (For those of you who are interested, we have a study guide.)
It’s important to understand that I was not any smarter than the professional forecasters. Rather, all I did was recognize that a deep trough at the upper level had the potential to combine with Maria as she went extra tropical and produce those unforecast conditions.
The point being that I’m sure the professionals knew that this potential existed too, but they had no way to warn anyone since they were constrained to one forecast.
But now the folks at the US Ocean Prediction Centre have come up with an excellent graphic way to forecast multiple wind speed probabilities, instead of just one.
This is a huge breakthrough. To see why, have a look at the chart at the top of the post that shows the probability of sustained winds equal to or greater than 34 knots (gale force).