AIM 

4/20/23

 

7

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1

−

14

Meteorology

(d)

In addition to SIGMETs issued for the phenomenon for the contiguous U.S., SIGMETs in the oceanic

regions are also issued for:

(1)

Tornadoes.

(2)

Lines of thunderstorms.

(3)

Embedded thunderstorms.

(4)

Hail greater than or equal to 

¾

 inch in diameter.

e. Convective SIGMET

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