Aeronautical Information Manual (AIM), page 512

Index   511 -- Page 512 -- 513


vertical visibility is "VV" followed by a three-digit
height in hundreds of feet.

As 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 nonconvective 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
2,000 feet. 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.


d. 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% and the PROB40 group is

used when the occurrence of thunderstorms or

precipitation is 40-49%. This is followed by two

four-digit groups separated by a "/", giving the

beginning date and hour, and the ending date and hour

of the time period during which the thunderstorms or

precipitation are expected.

NWS does not use PROB 40 in the TAF. However U.S.
Military generated TAFS may include PROB40. PROB30
will not be shown during the first nine hours of a NWS

PROB40 2221/2302 1/2 SM +TSRA "chance between
2100Z and 0200Z of
visibility one-half
statute mile in
thunderstorms and
heavy rain."
PROB30 3010/3014 1SM RASN . "chance between
1000Z and 1400Z of
visibility one statute
mile in mixed rain
and snow."

e. 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
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 six-digit date, hour, and minute the change is
expected to begin and continues until the next change
group or until the end of the current forecast. A "FM"
group will mark the beginning of a new line in a TAF
report (indented 5 spaces). Each "FM" group
contains all the required elements-wind, visibility,
weather, and sky condition. Weather will be omitted
in "FM" groups when it is not significant to aviation.
FM groups will not include the contraction NSW.
FM210100 14010KT P6SM SKC - "after 0100Z on the
21st, wind one four zero at one zero, visibility more than six,
sky clear."

2. Becoming (BECMG) group. The BECMG

group is used when a gradual change in conditions is

expected over a longer time period, usually two

hours. The time period when the change is expected

is two four-digit groups separated by a "/", with the

beginning date and hour, and ending date and hour of

the change period which follows the BECMG
indicator. The gradual change will occur at an
unspecified time within this time period. Only the
changing forecast meteorological conditions are
included in BECMG groups. The omitted conditions
are carried over from the previous time group.

7-1-68 Meteorology

Page 512 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   511 -- Page 512 -- 513