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AIM

10/12/17

7

−1−70

Meteorology

of no significant weather phenomena occurs, the
contraction NSW (No Significant Weather) will
appear as the weather group in the new time period.
(NSW is included only in TEMPO groups).

NOTE

It is very important that pilots understand that NSW only
refers to weather phenomena, i.e., rain, snow, drizzle, etc.
Omitted conditions, such as sky conditions, visibility,
winds, etc., are carried over from the previous time group.

(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
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.

NOTE

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.

WS010/18040KT 

− “LOW LEVEL WIND SHEAR

AT ONE THOUSAND, WIND ONE EIGHT ZERO
AT FOUR ZERO”

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.

NOTE

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
forecast.

EXAMPLE

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
report.

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.