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GEN 3.5-68
15 MAR 07
Nineteenth Edition Federal Aviation Administration
c) Weather. The expected weather phenomena is
coded in TAF reports using the same format,
qualifiers, and phenomena contractions as METAR
reports (except UP).
Obscurations to vision will be forecast whenever the
prevailing visibility is forecast to be 6 statute miles or
less.
If no significant weather is expected to occur during
a specific time period in the forecast, the weather
group is omitted for that time period. If, after a time
period in which significant weather has been forecast,
a change to a forecast of no significant weather
occurs, the contraction NSW (no significant weather)
will appear as the weather group in the new time
period. (NSW is included only in becoming
(BECMG) or temporary (TEMPO) groups.)
d) Sky Condition. TAF sky condition forecasts
use the METAR format described in the METAR
section. Cumulonimbus clouds (CB) are the only
cloud type forecast in TAFs.
When clear skies are forecast, the contraction “SKC”
will always be used. The contraction “CLR” is never
used in the aerodrome forecast (TAF).
When the sky is obscured due to a surface-based
phenomenon, vertical visibility (VV) into the
obscuration is forecast. The format for vertical
visibility is “VV” followed by a three-digit height in
hundreds of feet.
NOTEAs
in METAR, ceiling layers are not designated in the TAF
code. For aviation purposes, the ceiling is the lowest
broken or overcast layer or vertical visibility into a
complete obscuration.
SKC “sky clear”
SCT005 BKN025CB “five hundred scattered,
ceiling two thousand five
hundred broken
cumulonimbus clouds”
VV008 “indefinite ceiling eight
hundred”
e) Optional Data (Wind Shear). Wind Shear is
the forecast of non-convective, low-level winds (up
to 2,000 feet). The forecast includes the letters “WS”
followed by the height of the wind shear, the wind
direction and wind speed at the indicated height and
the ending letters “KT” (knots). Height is given in
hundreds of feet (AGL) up to and including
2,000feet. Wind shear is encoded with the
contraction “WS” followed by a three-digit height,
slant character “/” and winds at the height indicated
in the same format as surface winds. The wind shear
element is omitted if not expected to occur.
WS010/18040KT “low level wind shear at one
thousand, wind one eight
zero at four zero”
30.5 Probability Forecast. The probability or
chance of thunderstorms or other precipitation events
occurring, along with associated weather conditions
(wind, visibility, and sky conditions).
The PROB30 group is used when the occurrence of
thunderstorms or precipitation is 30-39 percent and
the PROB40 group is used when the occurrence of
thunderstorms or precipitation is 40-49 percent. This
is followed by a four-digit group giving the
beginning hour and ending hour of the time period
during which the thunderstorms or precipitation are
expected.
NOTENeither
PROB30 nor PROB40 will be shown during the
first six hours of a forecast.
EXAMPLEPROB40
2102 1/2SM +TSRA - “chance between 2100Z
and 0200Z of visibility one-half thunderstorm, heavy
rain.”
PROB30 1014 1SM RASN - “chance between 1000Z and
1400Z of visibility one rain and snow.”
30.6 Forecast Change Indicators. The following
change indicators are used when either a rapid,
gradual, or temporary change is expected in some or
all of the forecast meteorological conditions. Each
change indicator marks a time group within the TAF
report.
30.6.1 From (FM) Group. The FM Group is used
when a rapid change, usually occurring in less than
one hour, in prevailing conditions is expected.
Typically, a rapid change of prevailing conditions to
more or less a completely new set of prevailing
conditions is associated with a synoptic feature
passing through the terminal area (cold or warm
frontal passage). Appended to the “FM” indicator is
the four-digit hour and minute the change is expected
to begin and continues until the next change group or
until the end of the current forecast.
30 AUG 07
AIP
United States of America
GEN 3.5-69
15 MAR 07
Federal Aviation Administration Nineteenth Edition
An “FM” group marks the beginning of a new line in
a TAF report (indented 5 spaces). Each “FM” group
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