AIM

8/15/19

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Meteorology

(b)

Lines of thunderstorms.

(c)

Embedded thunderstorms.

(d)

Hail greater than or equal to 

3

/

4

 inch in

diameter.

3.

SIGMETs are identified by an alphabetic

designator from November through Yankee exclud-
ing Sierra and Tango. (Sierra, Tango, and Zulu are
reserved for AIRMET text [WA] products;
G

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AIRMETS do not use the Sierra, Tango, or Zulu

designators.) The first issuance of a SIGMET will be
labeled as UWS (Urgent Weather SIGMET).
Subsequent issuances are at the forecaster’s discre-
tion. Issuance for the same phenomenon will be
sequentially numbered, using the original designator
until the phenomenon ends. For example, the first
issuance in the Chicago (CHI) FA area for
phenomenon moving from the Salt Lake City (SLC)
FA area will be SIGMET Papa 3, if the previous two
issuances, Papa 1 and Papa 2, had been in the SLC FA
area. Note that no two different phenomena across the
country can have the same alphabetic designator at
the same time.

EXAMPLE

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Example of a SIGMET:

BOSR WS 050600
SIGMET ROMEO 2 VALID UNTIL 051000
ME NH VT
FROM CAR TO YSJ TO CON TO MPV TO CAR
OCNL SEV TURB BLW 080 EXP DUE TO STG NWLY
FLOW. CONDS CONTG BYD 1000Z.

d. Convective SIGMET (WST)

1.

Convective SIGMETs are issued in the

conterminous U.S. for any of the following:

(a)

Severe thunderstorm due to:

(1)

Surface winds greater than or equal to

50 knots.

(2)

Hail at the surface greater than or equal

to 

3

/

4

 inches in diameter.

(3)

Tornadoes.

(b)

Embedded thunderstorms.

(c)

A line of thunderstorms.

(d)

Thunderstorms producing precipitation

greater than or equal to heavy precipitation affecting
40 percent or more of an area at least 3,000 square
miles.

2.

Any convective SIGMET implies severe or

greater turbulence, severe icing, and low

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level wind

shear. A convective SIGMET may be issued for any
convective situation that the forecaster feels is
hazardous to all categories of aircraft.

3.

Convective SIGMET bulletins are issued for

the western (W), central (C), and eastern (E) United
States. (Convective SIGMETs are not issued for
Alaska or Hawaii.) The areas are separated at 87 and
107 degrees west longitude with sufficient overlap to
cover most cases when the phenomenon crosses the
boundaries. Bulletins are issued hourly at H+55.
Special bulletins are issued at any time as required
and updated at H+55. If no criteria meeting
convective SIGMET requirements are observed or
forecasted, the message “CONVECTIVE SIGMET...
NONE” will be issued for each area at H+55.
Individual convective SIGMETs for each area (W, C,
E) are numbered sequentially from number one each
day, beginning at 00Z. A convective SIGMET for a
continuing phenomenon will be reissued every hour
at H+55 with a new number. The text of the bulletin
consists of either an observation and a forecast or just
a forecast. The forecast is valid for up to 2 hours.

EXAMPLE

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CONVECTIVE SIGMET 44C
VALID UNTIL 1455Z
AR TX OK
FROM 40NE ADM-40ESE MLC-10W TXK-50WNW
LFK-40ENE SJT-40NE ADM
AREA TS MOV FROM 26025KT. TOPS ABV FL450.
OUTLOOK VALID 061455-061855
FROM 60WSW OKC-MLC-40N TXK-40WSW
IGB-VUZ-MGM-HRV-60S BTR-40N
IAH-60SW SJT-40ENE LBB-60WSW OKC
WST ISSUANCES EXPD. REFER TO MOST RECENT
ACUS01 KWNS FROM STORM PREDICTION CENTER
FOR SYNOPSIS AND METEOROLOGICAL DETAILS