Terminal forecasts for the world follow an
internationally accepted format. The TAFs are issued
four times daily for 24 hour periods beginning at 00Z,
06Z, 12Z, and 18Z. In the United States, TAFs are issued
three times each day at roughly 08-10Z, 15-17Z and
Each TAF is a series of time segment forecasts. The
number of time segments varies from forecast to
forecast, depending upon the expected weather
conditions. A new time segment signifies a change in the
weather conditions that is significant to aviation
operations. These conditions are related to ceiling
heights, visibilities, precipitation and other
obstructions to visibility, and wind speed and
The generic format for a TAF is:
TAF CCCC YYGGggZ YYHHHH dddff(f)GffKT VVVVSM
[ww NNNhhh] [Wshhh/dddffKT] [TTTTT xxxx] repeated as
Where: CCCC is the ICAO identifier
YYGGggZ is the issue time
YYHHHH is the start and end time of the forecast
dddff(f)GffKT is the wind direction and speed/gusts
VVVVSM is the visibility in statute miles (or M for
ww is significant weather [if present]
NNN is the cloud coverage [if present]
hhh is the cloud height [if present]
TTTTT is the indicator for a change in condition [if
xxxx is the start and end time of the change in [if
needed] conditions. This group is followed by the group
in brackets giving the change in the wind, visibility,
weather and cloud types.
The format for decoding TAFs is therefore as
A. Station identifier
B. Issue time of
C. Valid time of forecast
direction and speed (knots)
E. Visibility in statute
F. Significant weather (from list below)
Cloud coverage and height
H. Non-convective low level
I. Forecast Change Indicators
Expected change in prevailing conditions
Expanding on these items:
A. Station identifiers are all ICAO identifiers.
B. Issue time is a six-digit group ending with Z. The
first two digits represent the date, while the last four
numbers reflect the hour and minute, using a 24-hour
clock. The time is in UTC.
C. Valid time is always a 24-hour period with the
first two numbers the start time, and the second two
numbers the ending time of the forecast. Thus, 1212
means a forecast valid from 12Z today to 12Z
D. Wind is a five (or six) digit group with the first
three numbers the direction in degrees and the last two
(three) numbers the speed in knots. When wind gusts are
expected, the gusts are listed with a G after the
average wind speed forecast. For example, 33020G35KT
means wind direction from 330 degrees with an average
speed of 20 kts and gusts to 35 kts.
E. Visibility is in statute miles. Any visibility
more than 6 miles (10 km) is coded as P6SM.
International TAF's will list the visibility in meters.
An easy way to convert meters into miles is to divide by
1600, the result being in miles. For example, a
visibility of 300 meters is 300/1600 or 3/16 of a mile.
A visibility of 2600 meters is 2600/1600 miles which is
1 and 1000/1600 miles, or 1 5/8 miles.
F. Significant weather is taken from a listing of
Qualifiers and Weather Phenomena. Combinations from this
list are possible.
Qualifier for Intensity:
Moderate (no sign)
VC In the vicinity (0SM to 10 SM for precipitation,
and 5SM to 10 SM for non-precipitation)
||Small hail/Snow pellets|
||Mist (visibility 5/8 statute miles or
||Fog (visibility less than 5/8 statute
||Well developed Dust/Sand Whirls|
||Funnel Cloud, (+FC for Tornado, or
G. Cloud coverage and height are listed if they are
expected to occur during the forecast period. The first
three characters relate to the amount of the sky that is
covered by clouds. The following codes and coverage
amounts, in eights of the sky are the same as used in
SKC or CLR 0/8
FEW >0 - 2/8
SCT 3/8 - 4/8
BKN 5/8 - <8/8
VV (vertical vsby) 8/8
The height of the clouds, in hundreds of feet,
immediately follows the code for cloud amount.
H. Non-convective low level wind shear will appear in
the TAF when low level wind shear (up to 2000 feet above
the ground), is expected. In Canadian TAF's, this group
will appear immediately after the wind, while it is not
given for other international locations.
The group is coded as follows:
where WS is the indicator for the LLWS group
hhh is the forecast height of the shear, in hundreds
of feet AGL
ddd is the forecast wind direction above the
ff is the forecast wind speed in knots above the
KT is the units indicator for knots
I. Forecast Change Indicators including:
FMxxxx meaning FroM to indicate a significant change
from prevailing conditions. First 2-digits are the
beginning hour of the period and the last two are the
TEMPO meaning TEMPOrary, with changes expected for
xxxx and in total, < half of the 2-digit beginning
and 2-digit ending period.
PROBpp meaning PROBability and 2-digit percent chance
of occurrence during 2-digit hour beginning and 2-digit
ending time period.
BECMG BECoMinG meaning a change expected during the 2
digit beginning and 2-digit ending time period.
J. After a forecast change indicator, there will be a
listing of the weather elements that are expected to
change during that period. Only the elements that are
expected to change are listed, otherwise any remaining
elements are expected to be the same as the previous
Other abbreviations used in some international
WX NIL..the end of thunderstorms or freezing
CAVOK...no clouds under 5000 ft, no thunder, no
precipitation and visibility 6 miles or greater
NOSIG...no elements are expected to change in such a
way as to require a change to be indicated.
KSEA 121733Z 121818 16006KT P6SM SCT035 BKN045
TEMPO 1821 4SM -SHRA BR BKN030
20006KT P6SM -RA BKN035 OVC060
TEMPO 2202 5SM RA BR
FM0400 19006KT P6SM -SHRA BKN022 OVC045=
The above forecast for Seattle was issued on the 12th
of the month at 1733Z. The valid period runs from 18Z on
the 12th to 18Z on the 13th.
At 18Z, the wind is from 160 degrees at 6 knots, the
visibility is greater than 6 statute miles, there is a
scattered cloud layer at 3500 feet, a broken layer at
4500 feet and an overcast layer at 6000 feet.
Temporarily between 18Z and 21Z, the visibility will
drop to 4 statute miles in light rain showers and mist
with a broken cloud layer at 3000 feet. From 2100Z, the
wind will be from 200 degrees at 6 knots, the visibility
will be greater than 6 statute miles in light rain, with
a broken cloud layer at 3500 feet and an overcast layer
at 6000 feet.
Temporarily between 22Z and 02Z, the visibility will
drop to 5 statute miles in moderate rain and mist with a
broken layer at 2500 feet.
From 0400Z the wind will be from 190 degrees at 6
knots, the visibility will be greater than 6 statute
miles, with light rain showers. There will be a broken
layer at 2200 feet and an overcast layer at 4500