Sunday, May 1, 2016

"What do you use for weather?" (Update: Ocean Swell & NHC)

This should really be called "The Weather Cycle."

NERD ALERT!  This blog isn't for everyone.  I'll update as our offshore weather knowledge evolves.

We thought, as pilots, we'd have an advantage in this department and maybe we do, but offshore weather is different. You are not only concerned about wind direction and speed (and thunderstorms or "squalls") but wave direction, height and period. Our previous weather sources weren't cutting it on the Chesapeake. One thing for sure is most good weather info is free!  NOAA & the NWS are government-funded and, as it turns out, will carry you pretty far down the Caribbean. It's always a good idea to consult at least two sources of info.  You'll learn which one is more accurate in your area.  Here's what we are currently using:

PocketGrib app, map view

PocketGrib app, Meteogram view

GRIB: NOAA automated, computer-generated forecast usually GFS but also WW3, US Navy, or Euro, for example.  I'm not going to explain all of that here, but know what you are looking at.  Different GRIB viewers have different default settings. You also need to know a human does not review this information.  You need to use your noggin (or pay Chris Parker see below).  Our preferred GRIB viewer is the app, PocketGrib (Not user-friendly, so you may need to find a fellow sailor. Thanks Deb Akey, for the tutorial).  Again, GRIB's are free information, so do your research before paying for this info or a viewer!

Windfinder app

WindFinder app: This is the third most common source of weather info we hear about from other cruisers (Chris Parker is second to GRIB). We paid for the pro-version to get Euro & the extended forecast (also uses less data since there are no ads). A cool trick you can do is watch conditions at the ports before you, ie. if you're in Georgetown, watch Staniel Cay for the wind shift.  In St. Louis, we would watch Jefferson City or Columbia.)  UPDATE:  Unfortunately, this seems to underestimate the winds on the south coast of Puerto Rico.

NOAA Marine Forecast

NWS NOAA Offshore Report by Tropical Prediction Center in Miami:  I believe the SSB broadcast has been cancelled, but you can get this on the internet ( or by email ie. Iridium (see SailDocs). We get the Tropical N Atlantic & Caribbean Sea.  Don't let the clunky, government website deter you--there is some good stuff here (check out the tide tables while you're browsing:  Blog "When is high tide in Georgetown?"  This report is a little daunting to read. It used to make my eyes glaze over, not unlike an electrical schematic.  First, figure out which area you need (see outline below) or start with "condensed for radio broadcast" until you get the hang of it.  It's very general (broad time frames over a large area) but like the GRIB, it is part of the source info used by Chris Parker and other paid sources. 

  • Report Name (FZNT23.KNHC, you'll need this for SailDocs)
  • Check the time to see how old the report is (1114 AM EDT SUN MAR 27 2016)
  • Synopsis

(For a reference lat/long map, try Bahamas Explorer Chartbook weather page or

AMZ011:  N of 18N, W of 85W (including yucatan basin)
AMZ013:  N of 18N, 76W-85W (including cayman basin)
AMZ015:  Caribbean approaches to the windward passage
AMZ017:  Gulf of Honduras
AMZ019:  15N-18N, 80W-85W
AMZ021:  15N-18N, 72W-80W
AMZ023:  N of 15N, 64W-72W (DR, PR, Virgins)
AMZ025:  Offshore waters Leeward Islands (Lesser Antilles, St. Martin-Dominica)
AMZ027:  15N-19N, 55W-60W
AMZ029:  11N-15N, W of 80 W 
AMZ031:  11N-15N, 72W-80W (including Colombia basin)
AMZ033:  S of 15N, 64W-72W (including Venezuela basin)
AMZ035:  Offshore waters windward islands (Martinique through Grenada) including Trinidad & Tobago
AMZ037:  Tropical n atlantic from 07n to 15n between 55w and 60w
AMZ039:  SW caribbean S of 11N (including approaches to Panama Canal)
AMZ101:  Synopsis for the SW N atlantic including the bahamas
AMZ111:  Atlantic from 27N to 31N w of 77W
AMZ113:  Atlantic from 27N to 31N between 70W and 77W
AMZ115:  Atlantic from 27N to 31N between 65W and 70W
AMZ117:  Bahamas including Cay Sal bank
AMZ119:  Atlantic from 22N to 27N (E of Bahamas) to 70W 
AMZ121:  Atlantic from 22N to 27N between 65W and 70W
AMZ123:  Atlantic S of 22N W of 70W (including approaches to the Windward Passage)
AMZ125:  Atlantic S of 22N between 65W and 70W (including Puerto Rico Trench)
AMZ127:  Atlantic from 19N to 22N between 55W and 65W

If you fell asleep in the the middle of the Offshore Report, WAKE UP!

Buoys:  Another original source of weather info is buoys. You can got to NOAA Data Buoy Center or download apps to see live information, particularly helpful while we were in the Chesapeake. (They always seemed to be out of service when we really needed them). Good for a day sail when you have cellular. 

Chris Parker:  The Marine Weather Center ( Chris is a meteorologist who specializes in marine Caribbean forecasts. He is good at interpreting and explaining the big picture and trends. He also combines a lot of the above information into a more readable format.  You will learn about weather patterns, so you can also track them on other sources (above) when out of range. Read his FAQ including philosophy (he gives "worst case" which is good, but don't let it scare you from ever leaving port) & "How to read email forecasts".  Make a copy of the Explorer Chart Weather Plotting chart and copy down an outline of the email/broadcast so you don't get overwhelmed.
If you have SSB, you can listen to his broadcasts for free, if you know the time and frequency (check with other cruisers in the area or link frequency list). His broadcasts tend to be more detailed and up-to-date (sometimes better if your eyes gloss over while reading the email, but my wanders during the long broadcast--try having a printed format in front of you to help follow along. See below). 
We receive his email forecasts: We can get a copy on the Iridium when wifi isn't available. We like having a printed copy to study. We get it the night before.  We can discuss without missing any of the broadcast. It also gives us access to his online simulcast & recordings.  We use this free resource to get NOAA weather on the Iridium GO!  Practice before you need it!  Go to for more information.  I found their email instructions tedious.  Our friends on Kelly Nicole helped me set this up.  Caution:  Delete your “signature” or “email-footer” or Saildocs may not respond.  It may be best to copy & paste the request.  SailDocs is very picky about capital letters, punctuation and spacing.
  • OffShore Report: To receive the full SW & Tropical North Atlantic & Caribbean Sea, send an email to, (subject blank), body of email:  FZNT23.KNHC  (FZNT31.KNHC for condensed).  
    • To subscribe (receive once per day for 14 days): sub FZNT23.KNHC
    • To receive every 6 hours:  sub FZNT23.KNHC time=00:00 interval=6
    • To receive for more than 14 days: sub FZNT23.KNHC time=00:00 interval=6 days=90
  • Grib (beware of data usage, no spaces—adjust lat/long for your area.  When you receive the email, it will ask how you want to open it.  Select your preferred grab viewer):  send gfs:22N,19N,72W,69W
  • Request text-only web page (beware of data usage)
Local net:  It's always better to go to the source (and have a written copy to review at your leisure--another reason we like emails), but if you don't have internet or SSB, the local net will often read Chris Parker emails or WindFinder info (ask the source if they don't disclose).

Still here?  Here's some new toys we've been playing with: showing "island lee" in the Mona Passage This is a great graphic website if the Offshore Report makes your eyes glaze over.  The teacher in me thinks this would be a great way to teach island lee or the relationship between wind & waves.  Try playing with wind and waves in the Mona Passage.  It's a bandwidth hog, so this isn't our primary source of weather.  (I think it works better on the laptop than the iPad.) showing the current through the Mona Passage  A similar website to, but also allows you to view current.  Helpful and educational!

Beaufort table explains how wind affects wave height

What I've learned about waves:
Waves are complicated.  They are mostly caused by the the wind.  The Beaufort table above explains the wave heights you should expect based on wind strength.
The depth of water also affects waves.  A shallower body of water can be "choppier" with shorter period waves, ie. Chesapeake Bay and Bahia de Samana in the Dominican Republic.
Fetch is how far a wave has travelled.  The longer the fetch, the bigger the waves, ie. any large body of water, but in particular, the Atlantic Ocean!

An NOAA article, "A closer look at wave forecasts: force Wind & wave table
After a blow, waiting 1-2 days for waves to "lay down" will give us the best ride.

What is ocean swell? Swell is a collection of waves moving away from distant weather systems.  They can travel halfway around the world!   An excellent explanation of these waves comes from surfers.  Check out's "Wave Basics" for a more detailed explanation.  Being aware of a north swell is important on the north shore of the Dominican Republic, northern anchorages in the BVI and in the more exposed anchorages of the southern Caribbean. I've added the graphic "Peak wave period, primary swell direction" to our email weather list. request, email
For 48 HR, current:  send PJEI11.TIF
Subscribe to 48 hr, 0000Z: sub PJEI88.TIF
Subscribe to 48 hr, 1200Z: sub PJEI89.TIF
Get/subscribe 72 HR, 0000Z:  send/sub PKEK88.TIF

Learn more about this graphic forecast on the National Hurricane Center's Marine Product Descriptions page:


  • Track the weather daily, so you know which of your sources is most accurate in your area and currently functional.  {cough, cough}  Saildocs.  {cough, cough}  Iridium.
  • Know the source and time stamp of the data.  Don't assume.  Ask if it's not obvious.
  • Use multiple sources of info.  Cross reference.
  • Have a written copy--either that you transcribed, email or saved from app/website.  Take a screenshot if necessary, for when the internet is not available.  The Captain frequently refers to PocketGrib throughout the day.
  • Discuss!  No one loves to talk about the weather more than cruisers.  "Hey, did you check the weather this morning?"  (Like we didn't!  Ha, ha).  "What do you think?"  "That Crazy Chris Parker!"  "THAT wasn't in the forecast!" 

Don't forget to bookmark the mobile versions of these websites:

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