2008 REED’s East Coast Edition - Weather
Understanding Weather
General Weather Strategies
your voyage will be coastal (within 25 miles
Understanding weather, recognizing
of land) or offshore (more than 25 miles from
weather patterns and using weather data
land). This distinction is important because
are critical skills for the offshore cruisers. A
weather occurring where land and water
competent mariner should be able to read,
meet can be quite different from open-ocean
understand and analyze the many sources of
weather, even when both locations are under
professional weather data available via ra-
the influence of the same general weather
dio, satellite and the Internet. But forecasts
pattern. These weather variations occur be-
and professional analysis can still fail to rec-
cause land and water adsorb and radiate heat
ognize localized conditions. Self-sufficiency
at different rates. Heat exchange plays a very
and safety at sea also depends on being able
important role in the formation of weather
to observe the conditions around you and
conditions. Marine weather forecasts often
interpret their meanings.
specify whether they’re for coastal or offshore
Meteorology is divided into two main dis-
waters. Be careful to use the right one.
ciplines: analysis (observation) and progno-
Also, beware that even the best coastal
sis (forecasting). Forecasts are based on the
forecasts can’t include the effects of local
data collected during analysis.
terrain—for instance, a cliff-lined shore or a
Analysis: Just by observing local weather
narrow current-filled river mouth—that can
conditions, you can generate useful short-
have powerful influences over local condi-
term local forecasts. Whenever available, also
tions. Use your local observations to aug-
employ the complex analytical data collected
ment a coastal forecast.
by the National Weather Service (NWS) and Phases of a Voyage
other government agencies from sophisti-
When planning your weather strategy, it is
cated sources, including automated offshore
helpful to divide your trip into three phases:
weather buoys and earth-orbiting satellites.
pre-departure, en route, and pre-arrival.
This far-reaching data is used for both local
Pre-departure: During the pre-departure
forecasts and synoptic (large area) forecasts.
phase, the key decisions are whether and
Forecasts: Weather forecasts cover specific
when to leave port. For a daytrip, this deci-
geographic areas and project conditions to
sion is based in large part on current local
specified times in the future. Short-term ma-
conditions such as wind and seas at a harbor
rine forecasts cover a six- to 24-hour period
entrance; wind strength and direction rela-
and do not provide an adequate basis for
tive to the coast; and tides and currents. You
long-term plans. Long-term forecasts cover
can use this data to gauge your ability to
from 24 hours to 10 days and occasionally
gain sea room and to maneuver safely. When
longer. Forecast accuracy decreases with
departing for a new destination, consider the
time, so if you were given the choice of rely-
reported conditions and local forecasts for
ing on one 48-hour forecast or two successive
waters or harbors you expect to reach a few
24-hour forecasts, the two 24-hour forecasts
hours to a day later.
would generally be the better choice.
Prior to departure on a voyage of more
The NWS can estimate only the likelihood
than a day’s duration, you should also obtain
of future conditions; they can’t predict with
24-, 48-, 72-, and 96-hour projections and
certainty. The farther out in time a computer
review various sources of weather infor-
model extrapolates, the larger the uncertain-
mation including broadcasts, surface and
ty. You should use long-term forecasts be-
upper-air weather charts and, if possible,
yond 72 hours to indicate the general trend
satellite images. (A discussion of the various
of weather patterns rather than specific con-
types of weather charts follows later in this
ditions in your location. Sources of weather
chapter.) Remember that weather is likely
forecasts begin on page 805.
to be more dynamic during the change of
Inshore/Offshore
seasons. Conditions can evolve and intensify
The weather information you choose will
more rapidly in the fall, when boats along
depend on whether a substantial portion of
the Eastern Seaboard are often heading south
for the winter, and in the spring, when boats
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