Underway - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 112

average course and speed. A file with a few days' worth of forecast data will be
small (<2KB) even at six or three-hour intervals.
The large scale overview (point 1, above) provides the big picture. In most cases the wind
can be estimated accurately enough from isobar contours - with all the usual warnings
that this is raw computer model output. We used this data to complement the surface fore-
casts and get an idea what might happen beyond seventy-two hours in the future. Obvi-
ously, there is a great deal of uncertainty the more you advance the forecast horizon.
Nevertheless, the longer-range model predictions help alert us to potential trouble.
Continuity between subsequent forecasts is often a good indicator of how confident the
models are in their prediction.
The short-term detailed data (point 2, above) is helpful at a micro-level: for example to
pick out changes in wind direction along our route (or at anchor without Internet access).
We would send an email to [e-mail] with one line in the message body:
send Spot:18.3S,168.5E|3,3|PRMSL,WIND,WAVES,RAIN,LFTX,CAPE
This means “send me surface pressure, wind speed and direction, wave height and direc-
tion, precipitation, Lifted Index and CAPE [1] for position 18°18'S 168°30'E over the
coming three days in three-hour intervals.” The ensuing email response would look like
this:
Data extracted from file gfs150117-06z.grb dated 2015/01/17
11:08:02
Data extracted from file ww3-20150117-06z.grb dated 2015/01/
17 10:50:42
request code:
Spot:18.3S,168.5E|4,3|PRMSL,WIND,WAVES,RAIN,LFTX,CAPE
Forecast for 18°18S 168°30E (see notes below)
Date Time PRESS WIND DIR WAVES DIR PER RAIN LFTX CAPE
utc hPa kts deg mtrs deg sec mm/h °C
----------- ------ ----- --- ----- --- ---- ---- ---- ----
01-17 06:00 1009.5 6.9 232 0.9 139 8.1 0.1 49.2
01-17 09:00 1010.8 4.4 215 0.9 144 8.3 0.0 -0.4 53.2
01-17 12:00 1010.6 2.6 209 0.9 149 8.4 0.0 -1.1 76.0
01-17 15:00 1009.7 2.4 219 0.8 135 8.6 0.0 -1.4 101
01-17 18:00 1009.6 1.1 206 0.8 146 8.8 0.0 -1.2 108