I have often expressed my worry about the number of cruisers who are overly relying on forecast tools that present raw weather-model output in cool ways:
- And many others
All these sources use the same underlying government sources. Each is just a different way to make those data sets look pretty.
To present a different view, I wrote an entire Online Book on how to reduce the risk from unexpected weather outcomes by adding in, and gaining understanding of, other weather sources.
But it's some time since we have updated said book—on the schedule for this winter—so here's a new chapter with a concise approach to assessing forecast accuracy. Think of it is a kind of Coles Notes to the Online Book.
Why Does It Matter?
But first, why does this matter? Modern models are quite accurate, so surely all we have to do is look at the GRIBs outputted by them, in whatever viewer we like best (see above), and all is good?
No, here's why:
A weather forecast (or model output) is only the most likely of several possible scenarios.
For example, a purely GRIB-based passage plan might forecast a voyage to Bermuda with no more than 30 knots of sustained wind, even when there's a 33% chance of sustained storm-force winds—the former a boisterous sail in a well-found boat, the latter verging on survival conditions in any boat, particularly one with a crew who had not planned for that eventuality.
And making this effort is worth it even if we decide to make the passage despite the threat, as I probably would if trying to get south in the fall with the winter fast approaching, given that at that time of year a 2-in-3 chance of getting to Bermuda without a gale or worse is probably as good as it gets1.
If we actually want to make voyages, we can't be too timid either.
Anyway, whether we go or not, knowing that two of the six chambers (33%) in the weather revolver are loaded with storm-force winds will make us far safer mariners than unrealistically assuming that the revolver is not loaded, based on a single forecast or routing source, particularly a raw-from-computer source.
Forecast-Error Risk Assessment
So let's dig into tools we can use to assess the chances of more severe weather than that forecast. The good news is that the process is easier and less time consuming than it was just a few years ago.