Underway - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 111

Weather Forecasting on Namani
On Namani we had two different routines for receiving and interpreting forecast data, de-
pending on whether we were underway or somewhere with shoreside Internet access.
Underway, we relied on weather fax and email via SSB and a Pactor modem plus laptop
computer combination. Bandwidth is very limited underway, forcing us to restrict data
volumes to the minimum.
We would use weather fax to get surface pressure forecasts ( NOAA [#8] , NZ MetService [#9]
or “BOM”, the Australia Bureau of Meteorology [#10] , depending on location) as well as
NOAA's Streamline Analysis. The surface forecasts were our “bread and butter” in under-
standing the big weather picture up to seventy-two hours out. The Streamline Analysis
helped to identify convergence zones and other tropical trouble spots. Occasionally, we
would also receive NOAA's satellite image for the West Pacific (a grayscale version is
available via weather fax).
We also used the GRIB (“Gridded Binary”) forecast data available from NOAA's GFS
model output. Many boats get GRIB forecasts with wind data (typically displayed as
barbed wind arrows) only for grid points in their immediate vicinity because bandwidth is
at a premium. The problem here is that wind data across a latitude/longitude grid greatly in-
flates the size of GRIB files (two vector components for each grid point and time step,
stored at full floating point precision). Instead of wind forecast GRIB, we would get two
separate files via email:
1. A GRIB file over a relatively large area with only surface pressure (plus rain if
tropical convergence zones are a concern). With twenty-four hour intervals and a
2° x 2° grid size, a ten-day forecast over the entire southwest Pacific will still be
under 10KB in file size. For example, an email to [e-mail] with
the following line in the message body will provide isobars and precipitation in
twenty-four hour intervals up to ten days out from central Australia to west of New
Zealand between 5° and 45° south:
send GFS:05S,45S,140E,170W|2,2|24,48..240|PRMSL,RAIN
2. A Spot Forecast that lists a set of forecast parameters for a specific position in a
tabular text format. While underway, we would request a “moving forecast” for our