DeepRegatta/Guides

Guide · Passage · risk

How do you measure the forecast risk along a route?

Count scenarios, not vibes. One forecast run gives you a single future; an ensemble gives you dozens. Passage checks every ensemble member against your declared limits on every leg and reports the count — in the Cherbourg–Plymouth example, all 51 scenarios exceed the skipper's 28-knot gust limit on three consecutive mid-Channel legs. And when models disagree with each other, that disagreement is itself the finding: wait for the next run.

Worked example · Cherbourg → Plymouth demo briefing
Strongest forecast gust per leg against a declared 28 kt limit
010203040ktdeclared limit · 28 kt gustswithin limitall 51 scenarios above limitL1 Passe de l'Ouest: gusts to 25 kt — within the 28 kt limit25L1L2 Omonville: gusts to 25 kt — within the 28 kt limit25L2L3 Casquets TSS: gusts to 32 kt — all 51 scenarios exceed 28 kt around 16:00 UTC3251/51L3~16:00 UTCL4 Mid-Channel: gusts to 35 kt — all 51 scenarios exceed 28 kt around 19:00 UTC3551/51L4~19:00 UTCL5 Eddystone appr.: gusts to 34 kt — all 51 scenarios exceed 28 kt around 17:00 UTC3451/51L5~17:00 UTCL6 Plymouth W ent.: gusts to 27 kt — within the 28 kt limit27L6
View as table
LegGusts toVs 28 kt limit
L1 · Passe de l'Ouest25 ktwithin limit
L2 · Omonville25 ktwithin limit
L3 · Casquets TSS32 kt51/51 scenarios above, ~16:00 UTC
L4 · Mid-Channel35 kt51/51 scenarios above, ~19:00 UTC
L5 · Eddystone approach34 kt51/51 scenarios above, ~17:00 UTC
L6 · Plymouth entrance27 ktwithin limit
On legs L3, L4 and L5, every one of the 51 ensemble scenarios exceeds the declared gust limit at the times shown — a raw scenario count, not a calibrated probability. This is Passage's built-in demo briefing, a fixed example scenario rather than a live forecast.Source: Passage demo briefing snapshot · route cherbourg-plymouth-v1 · 51-member scenario ensemble.

Three signals, one verdict

Route risk in a Passage briefing reduces to three measurable signals. First, limit exceedance: for each leg and hour, how many ensemble scenarios break a limit you declared in advance — here, 51 of 51 on three legs. Second, model divergence: on the Mid-Channel leg the deterministic models disagree for an hour with a spread of 7.7 knots, and divergence says wait, whatever the average looks like. Third, run age: the briefing names when the next model cycle is expected (~22:30 UTC in the example) so "check again" has a time on it. The demo's overall verdict follows mechanically: the forecasts disagree too much to assess the passage — reassess after the next run.

Where the forecast data comes from

Live Passage briefings draw on openly published model data — NOAA GFS, the GEFS ensemble and GFS-Wave, ECMWF open data, and Copernicus GLO12 currents — ingested on a fixed schedule into pre-processed forecast tiles. The worked example on this page uses a fixed demo scenario so that the arithmetic can be inspected end to end.

Methodology

Every ensemble member is evaluated hourly along the route, course-relative, across each leg's ETA occupancy window, and compared against the skipper's declared limits. Exceedance is reported as a raw member count. Deterministic models are cross-checked against each other, and their divergence is reported with its duration and spread rather than averaged away.

Limitations

A 51-member count is a scenario fraction, not a calibrated probability — 51/51 means "no modeled future stays under your limit," not "100% chance." Agreement between runs is not proof of accuracy. Wave forecasts are deterministic only. The example is a fixed demo scenario; for a live passage, run a current briefing and read the official warnings.

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