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AIM

10/12/17

7−1−11

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−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−

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−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−

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

2/28/19

AIM