Weather Routing Software for Ocean Passages: PredictWind vs Windy vs NOAA GRIB Files

Forecasts are reasonably accurate maybe five days out, less reliable beyond that, and the only honest job of weather software is to tell you when you don't know what's coming. That's the frame. PredictWind, Windy, and raw NOAA GRIB files are not interchangeable, and picking the wrong one for the wrong passage costs you either money you don't need to spend or confidence you haven't earned.

I'm working through this question for the 50-foot cat we're researching. The plan is a five-year family circumnavigation with four kids on board, and the routing software sits in the same risk-management stack as the EPIRB, the satellite phone, and the Jordan Series drogue — tools that exist so the day you actually need them, you're not improvising. So I've taken the question more seriously than the marketing copy on any of these products wants me to.

What weather software actually does

Strip the dashboards away and you're paying these tools to do three different jobs:

1. Show current conditions. Is that low actually 200nm away? Is the wind at this waypoint what was forecast 24 hours ago?
2. Forecast along your route. What does day three look like at 25°N, 40°W?
3. Suggest optimal routing. Given my polars, which line minimizes time and rough hours?

No single tool does all three perfectly. The cheap ones don't route. The expensive ones bury you in interface. The free ones make you do the synthesis yourself. Knowing which job you actually need is most of the decision.

PredictWind

PredictWind is what serious offshore crews use. Commercial shipping, ocean racers, and cruisers planning real passages all converge on it for the same reason: ensemble modeling.

Instead of one forecast, you see a range. Eight of ten model runs agreeing? That's a confident forecast. Six and four? That's uncertainty, and uncertainty is the entire point — it tells you to wait, hedge, or reroute. The departure planning tool runs your route through multiple weather scenarios and tells you which days are viable and which will have you beating into 30-knot headwinds for 36 hours straight.

You also get GRIB downloads at higher resolution than the public NOAA feeds, integrated into the routing interface, plus access to ECMWF model data the US-only GFS feed can't match.

The downside is the subscription — $99/month for the full package — and a learning curve that assumes you already understand what ensemble dispersion and isochrone routing mean. It's not a casual tool.

Worth subscribing if: you're doing real ocean passages, can use ensemble data competently, and want professional-grade decision support. For our circumnavigation budget it's a line item I'm planning to fund.

Windy

Windy is the most beautiful weather visualization on the internet, full stop. The animated wind maps communicate weather systems the way a table of numbers never will. You can layer ECMWF, GFS, ICON, and a few others and watch them agree or diverge in real time. The free tier is generous; paid tiers extend the forecast horizon.

What Windy isn't: a routing tool. It does not run your boat's polars against the forecast and produce optimal isochrones. The route-planning feature is fine for coastal hops, marginal for ocean work. Windy is a comprehension layer, not a decision tool.

Use it for: understanding atmospheric patterns, learning to read weather systems, comparing models. Pair it with something that actually routes.

NOAA GRIB files

GRIB (GRIdded Binary) files are the raw underlying data — the same NOAA GFS output that feeds half the consumer weather products on the internet. NOAA gives them away for free.

If you run OpenCPN, Expedition, or Adrena and you can read a wind barb, you can pull the same model data the professionals use and overlay it directly on your chartplotter route. That's powerful. It's also, by itself, not enough for an ocean passage. There's no ensemble. There's no routing calculation. You're looking at one model run from one provider, and you're doing all the synthesis yourself.

Use it for: backup, supplement, and offline access. Don't lean on it as your only weather decision tool unless you can read raw GRIB the way other sailors read a chartplotter.

How I'd layer them

Two to three weeks before a real passage: PredictWind departure planning. Which weather windows look viable? How tight is the ensemble?

One week out: switch to Windy. Watch the systems develop. See what's actually forming where the models predicted it would.

During the passage: daily GRIB updates pulled via PredictWind or directly from NOAA. Compare predicted vs. observed at every waypoint. When they diverge, the forecast is decaying — slow down, change tactics, or alter the plan.

The discipline that ties this together is simple: never trust a single weather model. Tight ensemble means confidence. Loose ensemble means uncertainty. Uncertainty should make you more conservative, never less. That's the part the tools can't do for you.

Bottom line

For ocean passages, PredictWind earns its subscription — the ensemble modeling and routing logic alone justify the cost for anyone crossing oceans on a schedule. Windy is a free supplement that makes you better at reading weather, full stop. GRIB files are the raw substrate, useful if you have the software and the skill, insufficient if you don't.

The goal isn't a perfect forecast. There isn't one. The goal is making better decisions than you'd make without the tools, and recognizing when the data is uncertain enough to wait. If you're working through gear and ownership decisions for your own offshore plan, the boat-selection worksheet walks through the same thinking applied to hulls, systems, and budget.