AIM Ch.7-1
c. SIGMET (WS)
1. A SIGMET advises of nonconvective weather that is potentially hazardous to all aircraft. SIGMETs are unscheduled products that are valid for 4 hours. However, conditions that are associated with hurricanes are valid for 6 hours. Unscheduled updates and corrections are issued as necessary. In the conterminous U.S., SIGMETs are issued when the following phenomena occur or are expected to occur:
(a) Severe icing not associated with thunderstorms.
(b) Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms.
(c) Dust storms or sandstorms lowering surface or inflight visibilities to below 3 miles.
(d) Volcanic ash.
2. In Alaska and Hawaii, SIGMETs are also issued for:
(a) Tornadoes.
(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 excluding Sierra and Tango. (Sierra, Tango, and Zulu are reserved for AIRMETs.) The first issuance of a SIGMET will be labeled as UWS (Urgent Weather SIGMET). Subsequent issuances are at the forecaster's discretion. 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
MOD TO OCNL SEV TURB BLW 080 EXP DUE TO STG NWLY FLOW. CONDS CONTG BYD
1000Z.
Sigmets are issued for Severe Icing. And part 121 flys into them all the time. So, Yes to your Question.