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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.