Weather, Part 7

Marine Forecasts

Previously in this BLOG I talked about weather services we use while cruising. The most basic service is the marine weather forecasts from NOAA’s National Weather Service. The marine forecasts are categorized as Coastal/Great Lakes, Offshore and High Seas. These are further subdivided into areas and zones. Each zone has a unique Zone ID. Coastal/Great Lakes forecasts are broadcast on VHF radio and Offshore and High Seas forecasts are broadcast on HF radio. All of them are also available from the internet and email as well.

In The Gentleman’s Guide to Passages South: The Thornless Path to Windward, by Bruce Van Sant, he talks about using the NWS offshore report exclusively to watch and plan for weather. He recommends a short hand notation for copying the HF broadcasts, but we find it easier to have them emailed to us. The NOAA website explains how to get the email, but we have found an easier way using SailMail.

Finding the marine forecast when connected to the internet is pretty straight forward. You just enter a location or click on a map. When you are planning to cruise offshore, you need to know which zones you will be cruising in. It’s a good idea to start looking at the forecasts before leaving so you are familiar with the conditions.

Marine Forecast Zone IDs

I will explain how we used marine forecasts for our recent Bahamas cruise. We started by finding the zone IDs for the Bahamas by going to NWS Offshore Marine Forecasts page and clicking on the Tropical Atlantic link to get a clickable map.

Tropical Atlantic Zones
Tropical Atlantic Zones

Zone IDs are six characters; two letters, the letter “z” and three digits. The numbers for each area are closely grouped. In this case 111 to 127. Each area has a synopsis, which is not shown. The synopsis is the lowest number in the group. In this case it is amz101.

The Bahamas zone ID is amz117. Clicking on it will give you page http://forecast.weather.gov/shmrn.php?mz=amz117&syn=amz101. It has a synopsis for the Tropical Atlantic area and a five-day forecast for the Bahamas including Cay Sal Bank.

You might notice that the offshore zones don’t cover the Gulf Stream crossing that we need to cross from Molasses Reef to Gun Cay. In this case we go to the Coastal/Great Lakes forecast by Zone page and click on South, followed by Miami to get the following clickable map.

Miami Forecast Zones
Miami Forecast Zones

Click on amz671 Waters from Deerfield Beach to Ocean Reef FL from 20 to 60 NM excluding the territorial waters of the Bahamas and you will get to http://forecast.weather.gov/shmrn.php?mz=amz671&syn=amz600. It has a synopsis of amz600 and a five-day forecast.

Now that we know the zone IDs we can get access to the data directly. Each forecast and synopsis is a .txt file stored on the NWS website. Start by going to http://tgftp.nws.noaa.gov/.  Click on data, then forecasts, marine and either coastal or offshore.  Next click on the directory with the two letter name that matches your zone ID, such as am. You will see a list of .txt files. Click on amz101.txt to see the synopsis and amz117.txt to see the five-day forecast for the Bahamas.

These files are updated periodically to contain the most current forecast. We like to look at them each day at about the same time. It’s a good idea to look over previous forecasts to see how they change. We could make copies each day, but we find it easier to write it down in a small notebook. Also make sure to check the date and time in the file.

Getting Forecasts by email

You can have the forecasts and synopses emailed to you. Here is the way as described in the NWS website. Address your email to nws.ftpmail.ops@noaa.gov.  The subject is ignored and you put commands in the body similar to below:

!REM send to nws.ftpmail.ops@noaa.gov
open
cd data/forecasts/marine/coastal/am
!REM Atlantic Coastal Waters Synopsis
get amz600.txt
!REM Atlantic Coastal Waters Forecast
get amz671.txt
cd ../../offshore/am
!REM Bahamas Synopsis
get amz101.txt
!REM Bahamas Offshore Waters Forecast
get amz117.txt
quit

It uses the FTP protocol. The lines starting with “!REM ” are ignored and can be used as comments in a file that you copy and paste into your email. The biggest disadvantage with this is that it is very easy to make an error and all or part of your request will fail without explaining why.

An easier way is to use SailDocs . Send your email to query@saildocs.com. The subject is ignored and the body looks as follows:

send amz600
send amz671
send amz101
send amz117

This format is easier to write and you can change the word “send” to “subscribe” to get an email everyday. The default subscription is for 14 days, but you can change that.

subscribe amz600 days=7
subscribe amz671 days=7
subscribe amz101
subscribe amz117

Some of the zones are not supported by SailDocs. In that case you can format your email as a webpage request.

Send http://tgftp.nws.noaa.gov/data/forecasts/marine/coastal/an/anz152.txt

Note: The above email is formatted as a single line. You can substitute “Subscribe” for “Send” and get a file everyday for two weeks.

We use AirMail, with SailMail on our SSB while cruising, but you can use any email service to get these forecasts from SailMail.

Using Marine Forecasts

The format of the marine zone forecast is pretty obvious. It goes out for five-days, or ten 12 hour periods. We like to compare them to other weather resources that are labeled by hour so we note the days and hours when we copy it. Times are noted in local time of the reporting office, which is EST or EDT for all Atlantic zones. Some other weather resources are in UTC (Universal Coordinated Time), but we have gotten used to converting.

The format of the forecast is straightforward. Both the synopsis and five-day forecast  have dates and times, descriptions of what they are and the area covered. They use a limited set of characters – all uppercase letters, numbers and very few punctuation marks. This is a holdover from years ago when there were devices that could not display all the characters.

The synopsis has a few sentences, describing current patterns and what’s expected in the next coming days.

Synopsis amz101
Synopsis amz101

We use the synopsis to verify our analysis of weather faxes and other weather resources. The weather faxes are like snapshots that are 24 hours apart, the synopsis explains what happens in between. If you want more detailed information about the forecast, look at the discussion for the area.

Five-day Forecast amz117
Five-day Forecast amz117

The five-day forecast has wind direction, wind speed, sea height and precipitation forecasts for five days. Wind speeds are in knots and seas are in feet. We copy the forecast to a small notebook along with notes from weather faxes and other resources. We have one page a day and generally go back to see how forecasts change. We also compare actual conditions to what was forecast.

Weather Log
Weather Log

Here’s a page of our weather log. It starts out with the date and location. Next are notes from the Surface Analysis and 24, 48, 72 and 96 hour Surface Forecast weather faxes. Refer to Weather, Part 6 for more information about weather faxes.  Next is the five-day forecast. When we are actually cruising there will be more on this page, such as observed conditions and notes from Chris Parker and other weather resources. I will discuss our weather log in more detail in future posts.

Summary

In this post I discussed different ways to get marine forecasts. The method we use depends on whether we have access to the internet. When we are cruising in the Bahamas we use SailMail to get our forecasts by email, otherwise we access them directly. Some cruisers prefer to listen to the broadcasts on their HF or VHF radio. Whatever you decide, you can’t say that marine forecasts are not a valuable weather service when cruising.

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