Marine Weather Without the Nonsense
Why forecasts often feel like a foreign language
Weather forecasts are written by meteorologists for meteorologists, then handed to sailors. The result is sentences like: "Seas 4 to 6 feet occasionally 8, with a southeasterly swell 3 to 5 feet at 12 seconds, building to 5 to 7 feet through the afternoon."
That sentence contains at least five distinct concepts compressed into one. If any of them is unfamiliar, you'll misread the whole picture. Let's unpack each one properly.
The essential terms
If forecast says "15 knots," expect gusts of 20–22 regularly. If it says "15, gusting 25," you're in lumpier air with uneven loading on the rig and helmsman.
You can have flat local conditions with 8-foot swell rolling through from a three-day-old Atlantic storm. You can also have 12-knot local wind with no swell — a beautiful sailing day.
Here's the surprise: a 6-foot wave at 5 seconds is far worse than a 6-foot wave at 12 seconds. Short period means the waves are steep and close together — the boat pitches violently, dishes slide, crew gets sick. Long period means the waves are gentle rollers — the boat rises and falls slowly, often perfectly sailable.
The North Atlantic has essentially unlimited fetch for northeasterly storms. A gale sitting off Iceland for three days has been building waves across 2,000 miles of open ocean before that swell arrives at the Mona Passage. This is why the Mona can be violent even when the local wind is calm.
Backing: wind rotating counter-clockwise — NE → N → NW. This is what happens as a cold front approaches and passes. Often signals an opening window for eastbound travel.
Trough: an elongated area of low pressure. Associated with unsettled weather, wind shifts, and cloud development. A trough moving through can temporarily disrupt the trades — or bring squalls.
Squall line: a line of intense convective storms, often preceding a cold front. Fast-moving, brings brief violent winds (sometimes 30+ knots for 20–30 minutes) followed by relative calm. Not pleasant to be caught in, but they pass.
"Subsiding": conditions improving. But the ocean lags — swell subsides slowly over 24–36 hours after wind drops.
"Occasionally to": this is the sneaky one. "Seas 4 to 6 feet, occasionally to 8" means 8-foot waves will happen regularly. It's not a rare exception. The "occasionally" is statistical — roughly 10% of waves will hit that height. In a two-hour window, you'll feel several of them.
In light wind with leftover swell, your sails flap, the boat rolls uncomfortably, and the motion is often worse than in moderate wind — at least then the boat is pressed over and tracking steadily. In a light-air slop, you're a cork.
Putting it together: reading a real-style forecast
What this actually means: Trades are up. Swell is from NE (the same direction you're trying to go). You'll see 8-foot seas regularly, not occasionally. Gusts will hit 28–30 near squalls. This is a classic "do not depart" day for an eastbound boat. Wait.
The Big Engine of the Route
The trade wind problem
The Caribbean trade winds blow from the northeast, roughly 10–25 knots, for most of the year. They exist because the Bermuda High — a massive, semi-permanent high-pressure system sitting over the subtropical Atlantic — forces air clockwise and southward. The Earth's spin (Coriolis effect) bends that southward flow toward the west, giving you steady NE winds across the Caribbean basin.
For a boat trying to go east or southeast from Florida toward Puerto Rico, this is the core problem. The wind is coming from exactly where you want to go. You can't sail directly into it. You tack — but with islands, shoals, and current against you, every degree off your intended course costs miles. This is the Thorny Path.
How winter cold fronts change everything
Between November and April, cold fronts sweep down from North America every 7–14 days. Each front temporarily disrupts the trade wind pattern. As a low-pressure system tracks east across the continent, it pulls warm southern air northward — suppressing or reversing the trades in its wake.
The sequence: before the front arrives, southerly flow often produces light, variable winds — a potential window. During the front, line squalls and shifting winds — stay put. After the front passes, winds back to N or NW and often go light. This is the window. It typically lasts 18–36 hours before the trades reestablish.
Why the route gets harder as you go
The trade winds strengthen and become more consistent as you move south and east. Florida to the Bahamas is the easiest stretch — you're in the transition zone. The Bahamas to the Dominican Republic is harder. The north coast of Hispaniola is a sustained windward grind. The Mona Passage is the most demanding single crossing on the route.
Additionally, the Caribbean topography creates funneling effects. Wind accelerates through inter-island passages and around capes. What's 18 knots in open water can be 28 in the Windward Passage between Cuba and Haiti. Plan for this — always.
Reading the Forecast Stack
Why one source is never enough
Every weather model has a resolution, a refresh cycle, and a set of assumptions baked in. A single source gives you a single model's best guess. Where models agree, you can trust the forecast. Where they disagree, there's genuine uncertainty — and you should know that before leaving the dock.
Van Sant famously consulted multiple sources before every passage. His hierarchy: buoy observations (truth), then text forecast, then graphics, then model output. He treated GRIB files as a planning tool, not a go/no-go decision.
Your forecast stack
The triangulation rule
Weather Windows for Going to Windward
The paradigm shift
Recreational sailors think about departing when the weather is "nice." Van Sant thinks about departing when the weather is less bad than it usually is for an eastbound boat. These are not the same thing.
Normal trade wind conditions — 15–20 knots from the NE, 4–6 foot seas — are perfectly pleasant sailing weather if you're going downwind. They are miserable, slow, and potentially dangerous for a boat trying to make easting. A "nice day" for a Bahamas beach trip is a "no-go" day for the Thorny Path.
What makes a window
A valid eastbound window has several components working together:
Wind direction: N, NE, or NW is manageable. E or SE means you're headed directly into it — wait. NW is often the best possible direction for an eastbound boat.
Wind speed: Under 15 knots sustained is comfortable. 15–20 is workable with good sea state. Over 20 on the nose with swell — wait.
Swell height and period: Under 6 feet at 10+ second period is generally acceptable. Over 6 feet at short period — wait, even if wind looks fine.
Duration: The window needs to last long enough for your leg. A 40nm night hop needs 8–10 hours. The Mona Passage needs 18–24 hours of good conditions. Don't start a long leg into a closing window.
Interactive: Is this a window?
Seven-day forecast for a boat staged in Luperon, DR, planning to cross the Mona Passage. Tap each day for the call.
The night departure principle
Van Sant almost always departs at or just before midnight. This is not arbitrary. It's because the night lee effect — the calming of wind near large landmasses as the land cools — is strongest from roughly midnight to 9am. Departing at midnight lets you:
1. Motor or motorsail in the calm zone during the best conditions
2. Be well offshore by dawn when any wind acceleration begins
3. Arrive at your destination by mid-morning, before afternoon trades build
4. Anchor and rest while the afternoon trades blow past you
Landfall determines departure, not the other way around. Work backward from your destination anchorage. When do you need to arrive? That tells you when to leave. Van Sant never departs based on a calendar — only on the physics of the leg.
Island and Coastal Effects
The night lee effect
This is Van Sant's signature technique, and the physics behind it are straightforward once explained.
During the day, the sun heats the land. Warm land heats the air above it, which rises. The trade winds rush in to fill the low-pressure gap — amplifying in the process. Wind near shore is stronger during the day than offshore because of this thermal pumping.
At night, the land cools rapidly. The thermal pump reverses or stops. In the lee of large landmasses — particularly mountainous ones like Hispaniola (peaks over 10,000 feet) and Puerto Rico — the wind calms significantly in a band roughly 5–15 miles offshore. Often 5–12 knots when the offshore forecast shows 18–22.
Katabatic winds — the mountain gift
Katabatic means "flowing downhill." At night, air near mountain peaks cools and becomes denser than the surrounding air. That cold, dense air drains down mountain slopes toward the sea — creating an offshore wind that can reach 15–25 knots near the coast of high islands like Hispaniola.
This sounds counterproductive — but it's actually what creates the night lee. The katabatic outflow pushes against the trades near the coast and suppresses them. It also means that sailing very close inshore at night on the north coast of the DR, you may find the katabatic flow actually filling your sails from behind, helping you east. Van Sant exploits this directly.
Wind acceleration zones — the dangerous flip side
The same geography that creates calm lees also creates dangerous acceleration zones. Wind squirts through inter-island channels and around headlands, sometimes at 150–200% of the open-ocean speed. The Windward Passage between Cuba and Haiti, the gap near Cape Engaño on the DR's northeast tip, and the Anegada Passage further south all funnel and accelerate wind dramatically.
Add the Venturi effect (fluid speeds up when forced through a narrowing) to background trade winds already 20 knots, and you can encounter 35–40 knots in passages where the NWS offshore forecast shows 20. This is not a forecast error. It's local physics that no model resolves well at the island scale.
Route Segments & Decision Rules
The route overview
The Thorny Path is not one passage — it's a series of distinct legs, each with its own character. What works for the Bahamas doesn't necessarily work for the Mona. Click each leg on the route below for its specific go/wait/divert criteria.
Leg-by-leg decision framework
Florida East Coast & Gulf Stream Crossing
The Gulf Stream runs north at 2–4 knots between Florida and the Bahamas (roughly 50nm). Wind against current — any northerly component to the wind — creates steep, dangerous breaking seas in the Stream. This is the first serious commitment of the voyage.
Distance: ~50nm to Bimini or ~90nm to Nassau area. Typically motored or motorsailed.
The Skipper's Weather Routine
Why process beats judgment alone
Good judgment comes from experience. But on a passage you haven't made before, experience is exactly what you lack. A repeatable process fills that gap. It forces you to check all the sources, ask all the questions, and make the decision systematically — not based on optimism, impatience, or a vague feeling that "it looks okay."
Van Sant's philosophy, decoded: he doesn't just wait for weather. He watches weather with a specific process, on a specific schedule, until the pattern he's looking for emerges. Then he moves immediately.
The daily weather cycle
| Horizon | What you're doing | Sources | Decision |
|---|---|---|---|
| 72-hour outlook | Watching the pattern. Where are the fronts? Is a window forming or closing? This is planning, not committing. | NHC graphics, Windy (GFS + ECMWF comparison), AFD for uncertainty language | Tentative go/no-go for the leg. Alert crew. |
| 48-hour check | First real commit decision. Models should be aligning if the window is real. Diverging models = uncertainty = wait. | All sources. Compare models. Read the buoy trends for your area. | Commit or don't commit. Provision and fuel if committing. Rest. |
| 24-hour update | Verify the window is still there and hasn't shifted. Check swell specifically — especially for the Mona. | NWS text forecast (most current), PredictWind wave detail, buoys. | Final go/wait call. Departure time confirmed. |
| Departure check | 30–60 minutes before casting off. Final state of the atmosphere. Any squalls in the area? Barometer trend? | VHF WX, visual sky assessment, barometer reading and recent trend. | Go or hold. No pride here — a hold is free. A mistake is expensive. |
| Underway (every 4–6 hrs) | Update against unfolding reality. Are conditions matching forecast? Is swell building faster than expected? | VHF offshore forecast, visual sea state assessment, Windy if offshore connectivity. | Continue, modify speed/course, or identify bail-out harbor. |
| Bail-out criteria | Pre-defined before departure. If conditions exceed X, you go to Y harbor. Decided with clear head, not in the middle of a nasty night. | Chart — pre-marked bail-out anchorages for each leg. | No discussion required. If criteria met, you go. Period. |
The skipper's pre-departure checklist
1. What is the wind direction and speed for the full duration of my leg?
2. What is the swell height and period — and what was the Atlantic weather 48 hours ago?
3. Are there fronts within 36 hours that could change conditions mid-passage?
4. Do the GFS and ECMWF models agree on the forecast?
5. What does the nearest buoy currently show?
6. Where is my bail-out harbor if conditions deteriorate?
7. Am I departing at the right time to maximize the night lee?
If you can't answer all seven confidently, you're not ready to go.
The mindset: patience is strategy
Van Sant's most important lesson is not meteorological — it's philosophical. The sailors who get hurt on the Thorny Path are almost always on a schedule. A flight home. A haul-out appointment. A rental car return. The weather doesn't care.
The sailors who arrive safely — and have a good time doing it — have built genuine flexibility into the passage. They budget 3–6 weeks for what might take 3 days in ideal conditions. They enjoy the anchorages. They treat a 10-day wait in Luperon as part of the cruise, not a failure.