These forecasts are produced
from computer models and may not always reflect official forecasts, especially
in the vicinity of weather fronts, tropical cyclones (tropical depressions,
tropical storms, hurricanes, typhoons, etc.) or in rapidly changing conditions.
As they contain no input from weather forecasters, it is important to check the
official marine forecasts and warnings for your area when strong winds or
tropical cyclones are forecast.
GRIB data is extracted from a
computer forecast model. While such computer data can provide useful guidance
for general wind flow, there are limitations which must be understood. What you
are receiving is a weather prediction generated by a computer run by NOAA/NCEP
(GFS, WW3, NAM and RTOFS models), the US Navy (COAMPS) or the Cyprus
Oceanography Center (WAM4-MED) and downloaded and processed by
PassageWeather.com. The network is complex, and any computer network is subject
to hardware and software failures or human error which can effect accuracy or
availability of data. In particular, if our servers were not able to download a
current data file then the forecast may be based on old data.
Also remember that GRIB data is not reviewed by forecasters before being made
available. You are getting a small part of the raw model data that the
forecasters themselves use when writing a forecast, and it is your
responsibility to make sure that the data is consistent with your local
conditions and with the professionally-generated forecasts (e.g. text bulletins
and weather-fax charts).
GRIB data also has limitations along shore, where local effects often dominate
and may not be adequately modeled. In addition these models cannot provide
adequate prediction for tropical systems, frontal activity or convergence zones.
For example, while global models can provide useful data on the likely track of
hurricanes, they grossly underestimate the strength of hurricanes because of
their small size compared to the model grid. For hurricane/cyclone forecasts,
carefully monitor the appropriate warning messages and do not rely on GRIB data
as your only source of weather information.
That all said, GRIB data can provide useful guidance not available elsewhere.
Understand the limitations and use the data carefully. GRIB data should be
considered supplemental to other forecasts, and not be relied upon in lieu of
professionally-generated charts or forecasts.