SIGMETs (WSs) are broadcasted for hazardous weather that is considered of
extreme importance to all aircraft. SIGMETs (acronym for "SIGnificant
METeorological information") warn of the following weather hazards: severe
icing, severe and extreme turbulence, dust storms, sandstorms, or volcanic ash
lowering visibility to less than 3 miles. A Convective SIGMET (WST) is issued
for hazardous convective weather (such as tornadoes, thunderstorms, hail) and
covers severe or great turbulence, severe icing, and low-level wind
shear.
A SIGMET (WS) example is shown below.
DFWA UWS 051710
SIGMET ALFA 1 VALID UNTIL 052110
AR LA MS
FROM MEM TO JAN
OCNL SVR ICING ABV FRZLVL EXPCD.
FRZLVL 080 E TO 120 W.
CONDS CONTIG BYD 2100Z.
Line 1 This is an urgent weather SIGMET (UWS) for the Dallas Fort Worth
area issued on the 5th day of the month at 1900.
Line 2 This is the
first issuance of this SIGMET (ALFA 1 and is valid until the 5th day of the
month at 2110 (Note: The first report of a different weather hazard is noted by
ALFA 1, with its second notice given the notation ALFA 2. The first report of a
weather hazard different from the first one would have been labelled BRAVO with
its second notice given the notation BRAVO 2.; The first report of a weather
hazard different from the first two would have been labelled CHARLEY with its
second notice given the notation CHARLEY 2.; that is alphabetical order
beginning with A-B-C;
Line 3 This weather report is good for the
Arkansas, Louisiana and Mississippi area.
Line 4 From Memphis,
Tennessee to Jackson, Mississippi
Line 5 Occasional (OCNL) severe (SVR)
icing above (ABV) freezing level (FRZLVL) expected (EXPCD)
Line
6 Conditions (COND) continuing (CONTG) beyond (BYD) 2100 Zulu time