stronger the surface system because large are winds within 2,000 feet of the earth’s sur-
amplitudes indicate that more cold dry air is face. This derived data is used for areas from
mixing with warm moist air. Strong North- which there is sparse information from ships.
ern Hemisphere upper-level ridges with ex- Streamline charts use symbols to indicate the
tensive north-south development (high am- stages of development of adverse conditions
plitude) are called “Omega blocks” because that originate in the tropics.
their shape on a chart resembles the Greek
Sea-State Charts
letter. Omega blocks will often divert or stall
approaching low-pressure systems and per-
The charts described above provide analy-
mit high pressure zones to predominate.
ses and forecasts for the atmosphere. Sea-
The NWS generates 500 Mb analysis charts
state analysis and prognosis charts provide
for the Atlantic and Pacific twice daily at
conditions on the sea surface and valuable
0000Z and 1200Z; 48-hour 500 Mb forecasts
information about wave height and direc-
are also issued at those times. A 96-hour 500
tion. Sea-state analyses based on actual ship’s
Mb prognosis is issued once a day at 0000Z.
reports are issued once a day by the Marine
Twelve-, 60-, and 120-hour 500 Mb fore-
Prediction Center (MPC). Contour lines
casts may be issued when the normal 48- or
indicate wave height in feet or one-meter
96-hour forecasts are not available.
increments; arrows can be used to show the
Be aware that forecast data on surface
primary direction of the swells. In addi-
charts is a blending or consensus derived
tion, maximum and minimum combined
from the forecast models of several national
wind, wave, and swell heights are noted by
and international weather agencies and
the abbreviations MAX and MIN next to
from forecasters’ personal experience. Up-
the average wave height amounts. Analysts
per-air charts are computer generated and
derive the combined wave height by adding
released without alteration. Because they
approximately one-third the height of the
are generated differently, these two types
wind wave to the height of the swell. During
of charts may not coincide exactly in their
the winter, the edge of the ice is depicted by a
analyses or forecasts.
bold, jagged line. r
In addition to the daily analyses, the MPC
Streamline Charts
issues 48-hour sea state forecasts each day at
In mid-latitudes, winds are usually caused 0000Z and 1200Z. These forecasts are based
e
athe
by the differences in pressure between east- on ships’ reporting and the short-range sig-
W
ward-moving highs and lows. Below these nificant wave height forecast from the U.S.
latitudes in the tropics you’re not likely, in Navy and the National Center for Environ-
most seasons, to find a succession of such mental Prediction. These prognosis charts
moving highs and lows. Variations in pressure use the same contour lines and symbols as
r
are normally slight. Consistent winds in this the analysis charts.
region are attributed more to the semi-per- If you use sea-state charts in conjunction
e
athe
manent subtropical high to the north, around with surface charts and 500 Mb upper-level
which air flows in a large-circular clockwise charts, you will have a very complete weather
pattern (eastward flowing south of the high). picture. A version of each of these three charts
In mid and high latitudes, winds swirling is issued at 0000Z and 1200Z. Using them to-
around highs and lows blow parallel to isobar gether will help you locate and possibly avoid
linesÖor nearly so. Since tropical regions gen- the worst weather and the largest seas.
erally do not have well-formed high and low
Single Observer Forecasting
r
standing W
patterns, isobar lines are not useful indicators
of tropical wind flow. Therefore, meteorolo-
When you don’t have access to professional nde
gists use streamlines to show wind patterns
analyses and forecasts, you can use, with a
U
in tropic areas. Streamlines are solid, bold,
good measure of success, the six basic indica-
curved lines plotted parallel to surface airflow.
tors—barometric pressure, wind direction/
Arrows indicate the wind direction.
strength, cloud characteristics, temperature,
Wind information on a streamline analysis
humidity, and sea and wave characteristics—
chart is obtained from ships’ reports and
to make short-term marine forecasts for
from boundary layer analysis derived from
your location. You can also use your local
satellite information. Boundary layer winds
observations to augment coastal forecasts
and identify micro phenomenon.
791