Shipboard Forecasting Playbook

The shape of a good weather practice

Four layers, each on a different cadence. Skip one and you stop seeing the whole picture:

1. Hourly obs — your eyes + instruments, logged.
2. 6-hourly trend review — what the log is telling you.
3. Daily synoptic check — GRIB, OPC chart, NOAA text, satellite.
4. As-needed decision systems — when you have a question, run the right one.

1. Standing routines

Every hour on passage — the log

Write these into the log. No memory, no estimation — pen and paper (or a digital equivalent you actually look at).

Every 6 hours — the trend review

Open the last 6 log entries and answer five questions in writing:

1. Pressure: steady / falling / rising, and how fast?
2. Wind: backing / veering / steady? Speed building / easing?
3. Cloud sequence: progressing through the warm-front ladder (Ci → Cs → As → Ns), clearing, or convective building?
4. Sea state: building, easing, or new swell direction appearing?
5. Visibility: trending up or down?

Three or more pointing the same way = something real. That's when you move up to the decision systems.

Once a day — synoptic check

Before sleep (or at watch change on a shorthanded passage):

• Pull a fresh surface analysis — Weather Map Atlas.
• Pull 24/48/72/96 hr progs and a 500 mb chart. Is the pattern you're in deepening, filling, or stable?
• Pull a GRIB for your route + 200 nm buffer — GRIB Atlas.
• Pull the NOAA offshore / high-seas text for your zone — NOAA Atlas.
• Check a satellite image — is the forecast low actually where the chart says it is?
• Compare GFS vs ECMWF at 72 h. If they diverge, widen your safety margin.

2. Decision systems — when you have a question

System A — "Where is the low?"

Inputs: pressure trend (3 h), wind direction change over last few hours.

Tool: Buys Ballot's Law. N. hemisphere, face the wind → low is on your right and slightly behind. S. hemisphere → low on your left.

Then read the wind rotation to place it more precisely (N. hemi):

System B — "Is it getting worse, and how fast?"

Inputs: pressure 3-hour rate, cloud sequence direction.

System C — "When will it hit?"

Cloud sequence gives the warm-front lead time, corroborated by pressure:

1. Cirrus (mare's tails) spreading from upstream → system 24–36 h out.
2. Cirrostratus (halo around sun/moon) → 18–24 h out.
3. Altostratus (sun as ground-glass disk) → 6–12 h out.
4. Nimbostratus, continuous rain → front arriving.

Cross-check the lead time against your pressure trace: a falling barometer confirms the cloud sequence isn't just scenery. A steady bar with the cloud ladder means the system is going past, not at you.

Details in the Cloud Atlas.

System D — "How bad will it get at the peak?"

Inputs: isobar spacing on the surface chart, central pressure trend, GRIB max wind/gust, 500 mb.

• Isobar spacing drives surface wind. 4 mb contours crowded tight = gale zone.
• Central pressure deepening 12 mb/24 h = intensifying; 24 mb/24 h at mid-latitudes = bomb.
• 500 mb: look upstream for a digging trough. Deepening surface low almost always has a trough above it.
• GRIB gust field — mean wind × 1.3 is a better planning number than the mean. Look at the actual gust param.
• Sea state peak lags wind peak by 6–12 h in open ocean. Wind easing ≠ sea easing.

System E — "Is this GRIB actually right?"

Before betting on a 72-hr GRIB:

1. Compare GFS vs ECMWF (or ICON) for the same time. If they agree → higher confidence.
2. Pull the nearest NDBC buoy and check: does the current GRIB match the buoy right now? If the model is already 5 kt low today, it'll probably be low tomorrow.
3. Satellite ground-truth: is the forecast low actually present?
4. Sanity-check against the OPC chart — it's human-analyzed, not just model output.

3. Action protocols — triggers & responses

Squall protocol

1. Detect: dark flat-based cloud with anvil, a visible curtain of rain, or a line of darker water ("cat's paws" racing toward you) on the surface.
2. Decide: distance × 1/track = minutes to impact (rough mental estimate — gust fronts travel ~25–40 kt).
3. Reef now. Always earlier than feels necessary. It's cheaper to shake out than to blow out.
4. Bear off slightly to avoid a beam-on knockdown if the squall hits with a big shift.
5. Post-squall: expect 30–90° wind shift. Don't re-trim until pressure + direction stabilize.

Fog protocol

• Risk: warm moist air over cold water, light wind, temp–dewpoint spread < 2°C. Advection fog (N. California coast, NE US, Grand Banks) can persist for days until the air mass changes.
• When it rolls in: reduce speed to match visibility; fog horn per COLREGS; radar on; AIS + VHF monitoring; note a known escape course away from shipping lanes.
• Don't assume clearing. Advection fog only lifts when wind shifts or air mass changes — watch the pressure/wind for that.

Front-passage protocol

Action: for a known cold front passage, prep a reef and trim for a beam reach before the shift. Plot the expected post-front course before it hits.

Storm / rapid-deepening protocol

Trigger: pressure falling > 6 mb / 3 h, or synoptic chart showing 24 mb/24 h deepening within 300 nm.

1. Confirm low position (System A).
2. Decide which semicircle to favour. In the N. hemisphere, the right/dangerous semicircle of a tracking low has stronger winds and a seas build-up from all directions (wind + sea direction aligned with low's motion). Try to stay left/navigable.
3. Consider heaving-to early, before sea state builds past your boat's comfortable range. Heaving-to in 30 kt is fine; in 50 kt with big breaking seas it can be risky to initiate.
4. All hands rested and fed before the peak; in storm conditions you can't make up deficits.
5. Log pressure every 30 minutes through the peak. The turning point (pressure bottom) tells you the closest point of approach and that things are about to improve.

4. How the tools fit together, by time horizon

5. Quick reference — on one screen

Pressure trend (3-h)

• ± < 1 mb = settled · 1–3 mb fall = system approaching · > 3 mb fall = deepening nearby · > 6 mb fall = storm
• > 3 mb rise = cold high pushing in fast, squally leading edge

Beaufort quick recall

• F4 (11–16 kt): small waves, frequent whitecaps
• F6 (22–27 kt): whitecaps everywhere, spray
• F7 (28–33): near gale, foam streaks with wind — reef aggressively
• F8 (34–40): gale · F9 (41–47): strong gale · F10 (48–55): storm

Full table in Sea State Atlas.

Buys Ballot (N. hemi)

Face the wind → low on your right and slightly behind. Falling pressure + backing wind = low going north of you. Falling + veering = low going south of you.

Fog trigger

T − Td < 2 °C and light wind over cold water → fog is possible within hours.

Sig wave height tail

Max single wave in 3 h ≈ 1.6 × Hs. Plan for it.

Where each piece of this came from

• Barometer Atlas — trend rules, Buys Ballot, calibration
• Cloud Atlas — the warm-front ladder, convective cues
• Sea State Atlas — Beaufort, WMO codes, Hs math
• Map Symbols Atlas — fronts, isobars, station models
• Weather Map Atlas — OPC progs + 500 mb
• GRIB Atlas — model choice, Saildocs, gust params
• NOAA Atlas — text forecasts, HF voice, zones
• Live Weather Atlas — buoys, satellite, ASCAT