Underway - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 113

01-17 21:00 1010.3 0.8 052 0.8 75 9.0 0.0 -0.9 68.8
...
When looking at this GFS forecast, we assumed that actual sustained winds would vary by
plus or minus five knots from the forecast, plus 40% in gusts. A forecast wind speed of
less than ten knots typically meant “light and variable.” For a moving forecast, we would
add boat speed in knots and true course over ground in degrees to the request (note the
5.5,215 at the end of the line):
send
Spot:18.3S,168.5E|3,3|PRMSL,WIND,WAVES,RAIN,LFTX,CAPE|5.5,215
When we have shoreside Internet access, things are easier because bandwidth becomes
less of a restriction. We would still look at surface charts and satellite images and also use
the GFS GRIB data.
In addition, we found the following two sources of weather forecast data very useful, es-
pecially when looking for longer weather windows between the tropics and New Zealand
or Australia:
1. MetVUW [#11] , the University of Wellington (New Zealand) Meteorology Depart-
ment. They compare US, Euro and UK model outputs and either pick and chose or
blend them into their own seven-day forecasts for the Southwest Pacific.
2. ECMWF [#12] , the European Center for Medium Range Weather Forecasting. This
is the European version of the US GFS model and provides a surface pressure
forecast for ten days. For longer-range forecasts, we found it very useful to check
whether the ECMWF and GFS models disagreed significantly. If so, we knew that
anything might happen. Otherwise, we would have reasonable confidence in the
basic longer-term outlook beyond seventy-two hours.