How long is a standard SIGMET valid?

7-1-6. Inflight Aviation Weather Advisories
a. Inflight Aviation Weather Advisories are forecasts to advise en route aircraft of development of potentially
hazardous weather. Inflight aviation weather advisories in the conterminous U.S. are issued by the Aviation
Weather Center (AWC) in Kansas City, MO, as well as 20 Center Weather Service Units (CWSU) associated with
ARTCCs. AWC also issues advisories for portions of the Gulf of America, Atlantic and Pacific Oceans, which
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are under the control of ARTCCs with Oceanic flight information regions (FIRs). The Weather Forecast Office
(WFO) in Honolulu issues advisories for the Hawaiian Islands and a large portion of the Pacific Ocean. In Alaska,
the Alaska Aviation Weather Unit (AAWU) issues inflight aviation weather advisories along with the Anchorage
CWSU. All heights are referenced MSL, except in the case of ceilings (CIG) which indicate AGL.
b. There are four types of inflight aviation weather advisories: the SIGMET, the Convective SIGMET, the
AIRMET, and the Center Weather Advisory (CWA). All of these advisories use VORs, airports, or well-known
geographic areas to describe the hazardous weather areas.
c. The Severe Weather Watch Bulletins (WWs), (with associated Alert Messages) (AWW) supplements these
Inflight Aviation Weather Advisories.
d. SIGMET. A SIGMET is a concise description of the occurrence or expected occurrence of specified en
route weather phenomena which is expected to affect the safety of aircraft operations.
1. SIGMETs:
(a) Are intended for dissemination to all pilots in flight to enhance safety.
(b) Are issued by the responsible MWO as soon as it is practical to alert operators and aircrews of
hazardous en route conditions.
(c) Are unscheduled products that are valid for 4 hours; except SIGMETs associated with tropical
cyclones and volcanic ash clouds are valid for 6 hours. Unscheduled updates and corrections are issued as
necessary.
(d) Use geographical points to describe the hazardous weather areas. These points can reference either
VORs, airports, or latitude-longitude, depending on SIGMET location. If the total area to be affected during the
forecast period is very large, it could be that, in actuality, only a small portion of this total area would be affected
at any one 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.
2. SIGMETs over the contiguous U.S.:
(a) Are issued corresponding to the areas described in FIG 7-1-5. and are only for non-convective
weather. The U.S. issues a special category of SIGMETs for convective weather called Convective SIGMETs.
(b) Are identified by an alphabetic designator from November through Yankee, excluding Sierra and
Tango. 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) area (reference  FIG 7-1-5) for
phenomenon moving from the Salt Lake City (SLC) area will be SIGMET Papa 3, if the previous two issuances,
Papa 1 and Papa 2, had been in the SLC area. Note that no two different phenomena across the country can have
the same alphabetic designator at the same time.
(c) Use location identifiers (either VORs or airports) to describe the hazardous weather areas.
(d) Are issued when the following phenomena occur or are expected to occur:
(1) Severe icing not associated with thunderstorms.
(2) Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms.
(3) Widespread dust storms or sandstorms lowering surface visibilities to below 3 miles.
(4) V olcanic ash.
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3. SIGMETs over Alaska:
(a) Are issued for the Anchorage FIR including Alaska and nearby coastal waters corresponding to the
areas described in FIG 7-1-4. and are only for non-convective weather. The U.S. issues a special category of
SIGMETs for convective weather called Convective SIGMETs.
(b) Use location identifiers (either VORs or airports) to describe the hazardous weather areas.
(c) Use points of latitude and longitude over the ocean areas of the Alaska FIR.
(d) Are identified by an alphabetic designator from India through Mike.
(e) In addition to the phenomenon applicable to SIGMETs over the contiguous U.S., SIGMETs over
Alaska are also issued for:
(1) Tornadoes.
(2) Lines of thunderstorms.
(3) Embedded thunderstorms.
(4) Hail greater than or equal to ¾ inch in diameter.
FIG 7-1-4
Alaska SIGMET and Area Forecast Zones
4. SIGMETs over oceanic regions (New York Oceanic FIR, Oakland Oceanic FIR including Hawaii,
Houston Oceanic FIR, Miami Oceanic FIR, San Juan FIR), points of latitude and longitude are used to describe
the hazard area.
(a) SIGMETs over the Oakland Oceanic FIR west of 140 west and south of 30 north (including the
Hawaiian Islands) are identified by an alphabetic designator from November through Zulu.
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(b) SIGMETs over the Oakland Oceanic FIR east of 140 west and north of 30 north are identified by an
alphabetic designator from Alpha through Mike.
(c) SIGMETs over the New York Oceanic FIR, Houston Oceanic FIR, Miami Oceanic FIR, and San Juan
FIR are identified by an alphabetic designator from Alpha through Mike.
(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-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 obse rved 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
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FIG 7-1-5
SIGMET Locations - Contiguous U.S.
FIG 7-1-6
Hawaii Area Forecast Locations
f. AIRMET. AIRMETs are a concise description of the occurrence or expected occurrence of specified en
route weather phenomena that may affect the safety of aircraft operations, but at intensities lower than those
which require the issuance of a SIGMET.
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1. AIRMETs contain details about IFR conditions, extensive mountain obscuration, turbulence, strong
surface winds, icing, and freezing levels. Unscheduled updates and corrections are issued as necessary.
2. AIRMETs:
(a) Are intended to inform all pilots, but especially Visual Flight Rules pilots and operators of sensitive
aircraft, of potentially hazardous weather phenomena.
(b) Are issued on a scheduled basis every 6 hours, except every 8 hours in Alaska. Unscheduled updates
and corrections are issued as necessary.
(c) Are intended for dissemination to all pilots in the preflight and en route phase of flight to enhance
safety. En route AIRMETs are available over flight service frequencies. Over the contiguous U.S., AIRMETs
are also available on equipment intended to display weather and other non -air traffic control -related flight
information to pilots using the Flight Information Service-Broadcast (FIS-B). In Alaska and Hawaii, AIRMETs
are broadcast on air traffic frequencies.
(d) Are issued for the contiguous U.S., Alaska, and Hawaii. No AIRMETs are issued for U.S. Oceanic
FIRs in the Gulf of America, Caribbean, Western Atlantic, and Pacific Oceans.
TBL 7-1-2
U. S. AIRMET Issuance Time and Frequency
Product Type Issuance Time Issuance Frequency
AIRMETs over the Contiguous U.S. 0245, 0845, 1445, 2045 UTC Every 6 hours
AIRMETs over Alaska
0515, 1315, 2115 UTC
(standard time)
0415, 1215, 2015 UTC
(Daylight savings time)
Every 8 hours
AIRMETs over Hawaii 0400, 1000, 1600, 2200 UTC Every 6 hours
3. AIRMETs over the Contiguous U.S.:
(a) Are displayed graphically on websites, such as, aviationweather.gov and 1800wxbrief.com, and
equipment receiving FIS-B information.
(b) Provide a higher forecast resolution than AIRMETs issued in text format.
(c) Are valid at discrete times no more than 3 hours apart for a period of up to 12 hours into the future
(for example, 00, 03, 06, 09, and 12 hours). Additional forecasts may be inserted during the first 6 hours (for
example; 01, 02, 04, and 05). 00-hour represents the initial conditions, and the subsequent graphics depict the
area affected by the particular hazard at that valid time. Forecasts valid at 00 through 06 hours correspond to the
text AIRMET bulletin.
(d) Depict the following en route aviation weather hazards:
(1) Instrument flight rule conditions (ceiling < 1000' and/or surface visibility < 3 miles).
(2) Widespread mountain obscuration.
(3) Moderate icing.
(4) Freezing levels.
(5) Moderate turbulence.
(6) Non-convective low-level wind shear potential below 2,000 feet AGL.
(7) Sustained surface winds greater than 30 knots.
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4. Interpolation of time periods between AIRMETs over the contiguous U.S. valid times: Users must keep
in mind when using the AIRMET over the contiguous U.S. that if a 00 -hour forecast shows no significant
weather and a 03-hour forecast shows hazardous weather, they must assume a change is occurring during the
period between the two forecasts. It should be taken into consideration that the hazardous weather starts
immediately after the 00 -hour forecast unless there is a defined initiation or ending time for the hazardous
weather. The same would apply after the 03 -hour forecast. The user should assume the hazardous weather
condition is occurring between the snap shots unless informed otherwise. For example, if a 00 -hour forecast
shows no hazard, a 03-hour forecast shows the presence of hazardous weather, and a 06-hour forecast shows
no hazard, the user should assume the hazard exists from the 0001 hour to the 0559 hour time period.
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FIG 7-1-7
AIRMET over the Contiguous U.S.
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5. AIRMETs over Alaska and Hawaii:
(a) AIRMETs over Alaska and Hawaii are in text format. The hazard areas are described using
well-known geographical areas. AIRMETs over Alaska are issued for three Alaskan regions corresponding to
Alaska area forecasts (See FIG 7-1-4).
(b) AIRMETs over Alaska are valid up to eight hours. AIRMETs over Hawaii are valid up to six hours.
Unscheduled issuances contain an update number for easier identification.
(c) AIRMET Zulu describes moderate icing and provides freezing level heights.
EXAMPLE-
Example of AIRMET Sierra issued for the Southeast Alaska area:
WAAK47 P AWU 241324
WA7O
JNUS WA 241315
AIRMET SIERRA FOR IFR AND MT OBSC VALID UNTIL 242115
LYNN CANAL AND GLACIER BAY JB
MTS OBSC BY CLDS/ISOL PCPN. NC.
CNTRL SE AK JC
MTS OCNL OBSC IN CLDS. NC.
SRN SE AK JD
PAWG-P AKT LN W OCNL CIGS BLW 010/VIS BLW 3SM BR. IMPR.
ERN GLF CST JE
OCNL CIGS BLW 010/VIS BLW 3SM BR/-RA BR. DTRT.
=JNUT WA 241315
AIRMET TANGO FOR TURB/STG SFC WINDS VALID UNTIL 242115
ERN GLF CST JE
OFSHR ICY BAY W SUSTAINED SFC WND 30 KTS
OR GTR. SPRDG E. INTSF .
=JNUZ WA 241315
AIRMET ZULU FOR ICING VALID UNTIL 242115
ERN GLF CST JE
16Z TO 19Z ALG CST W ICY BAY OCNL MOD ICEIC 080-160.
FZLVL 045 EXC 015 INLAND. WKN.
EXAMPLE-
Example of AIRMET Tango issued for Hawaii F A area:
WAHW31 PHFO 241529
WA0HI
HNLS WA 241600
AIRMET SIERRA UPDATE 2 FOR IFR VALID UNTIL 242200
NO SIGNIFICANT IFR EXP .
=HNLT WA 241600
AIRMET TANGO UPDATE 3 FOR TURB VALID UNTIL 242200
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AIRMET TURB...HI
OVER AMD IMT S THRU W OF MTN.
TEMPO MOD TURB BLW 070.
COND CONT BEYOND 2200Z.
=HNLZ WA 241600
AIRMET ZULU UPDATE 2 FOR ICE AND FZLVL VALID UNTIL 242200
NO SIGNIFICANT ICE EXP
g. Watch Notification Messages
The Storm Prediction Center (SPC) in Norman, OK, issues Watch Notification Messages to provide an area threat
alert for forecast organized severe thunderstorms that may produce tornadoes, large hail, and/or convective
damaging winds within the CONUS. SPC issues three types of watch notification messages: Aviation Watch
Notification Messages, Public Severe Thunderstorm Watch Notification Messages, and Public Tornado Watch
Notification Messages.
It is important to note the difference between a Severe T hunderstorm (or Tornado)  Watch and a Severe
Thunderstorm (or Tornado) Warning. A watch means severe weather is possible during the next few hours, while
a warning means that severe weather has been observed, or is expected within the hour. Only the SPC issues
Severe Thunderstorm and Tornado Watches, while only NWS Weather Forecasts Offices issue Severe
Thunderstorm and Tornado Warnings.
1. The Aviation Watch Notification Message. The Aviation Watch Notification Message product is an
approximation of the area of the Public Severe Thunderstorm Watch or Public Tornado Watch. The area may be
defined as a rectangle or parallelogram using VOR navigational aides as coordinates.
The Aviation Watch Notification Message was formerly known as the Alert Severe Weather Watch Bulletin
(AWW). The NWS no longer uses that title or acronym for this product. The NWS uses the acronym SAW for
the Aviation Watch Notification Message, but retains AWW in the product header for processing by weather data
systems.
EXAMPLE-
Example of an Aviation Watch Notification Message:
WWUS30 KWNS 271559
SAW2
SPC AWW 271559
WW 568 TORNADO AR LA MS 271605Z - 280000Z
AXIS..65 STATUTE MILES EAST AND WEST OF LINE..
45ESE HEZ/NATCHEZ MS/ - 50N TUP/TUPELO MS/
..AVIATION COORDS.. 55NM E/W /18WNW MCB - 60E MEM/
HAIL SURF ACE AND ALOFT..3 INCHES. WIND GUSTS..70 KNOTS. MAX TOPS TO 550. MEAN STORM MOTION
VECTOR 26030.
LAT...LON 31369169 34998991 34998762 31368948
THIS IS AN APPROXIMATION TO THE WATCH AREA. FOR A COMPLETE DEPICTION OF THE WATCH SEE
WOUS64 KWNS FOR WOU2.
2. Public Severe Thunderstorm Watch Notification Messages describe areas of expected severe
thunderstorms. (Severe thunderstorm criteria are 1-inch hail or larger and/or wind gusts of 50 knots [58 mph]
or greater). A Public Severe Thunderstorm Watch Notification Message contains the area description and axis,
the watch expiration time, a description of hail size and thunderstorm wind gusts expected, the definition of the
watch, a call to action statement, a list of other valid watches, a brief discussion of meteorological reasoning and
technical information for the aviation community.
3. Public Tornado Watch Notification Messages describe areas where the threat of tornadoes exists. A
Public Tornado Watch Notification Message contains the area description and axis, watch expiration time, the
term "damaging tornadoes," a description of the largest hail size and strongest thunderstorm wind gusts expected,
the definition of the watch, a call to action statement, a list of other valid watches, a brief discussion of
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meteorological reasoning and technical information for the aviation community. SPC may enhance a Public
Tornado Watch Notification Message by using the words "THIS IS A PARTICULARLY DANGEROUS
SITUATION" when there is a likelihood of multiple strong (damage of EF2 or EF3) or violent (damage of EF4
or EF5) tornadoes.
4. Public severe thunderstorm and tornado watch notification messages were formerly known as the Severe
Weather Watch Bulletins (WW). The NWS no longer uses that title or acronym for this product but retains WW
in the product header for processing by weather data systems.
EXAMPLE-
Example of a Public Tornado Watch Notification Message:
WWUS20 KWNS 050550
SEL2
SPC WW 051750
URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 243
NWS STORM PREDICTION CENTER NORMAN OK
1250 AM CDT MON MAY 5 2011
THE NWS STORM PREDICTION CENTER HAS ISSUED A
*TORNADO WATCH FOR PORTIONS OF
WESTERN AND CENTRAL ARKANSAS
SOUTHERN MISSOURI
F AR EASTERN OKLAHOMA
*EFFECTIVE THIS MONDAY MORNING FROM 1250 AM UNTIL 600 AM CDT.
...THIS IS A P ARTICULARLY DANGEROUS SITUATION...
*PRIMARY THREATS INCLUDE
NUMEROUS INTENSE TORNADOES LIKELY
NUMEROUS SIGNIFICANT DAMAGING WIND GUSTS TO 80 MPH LIKELY
NUMEROUS VERY LARGE HAIL TO 4 INCHES IN DIAMETER LIKELY
THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE MILES EAST AND WEST OF A LINE
FROM 15 MILES WEST NORTHWEST OF FORT LEONARD WOOD MISSOURI TO 45 MILES SOUTHWEST OF HOT
SPRINGS ARKANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE
UPDATE (WOUS64 KWNS WOU2).
REMEMBER...A TORNADO WATCH M EANS CONDITIONS ARE F AVORABLE FOR TORNADOES AND SEVERE
THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE
LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE
WARNINGS.
OTHER WATCH INFORMATION...THIS TORNADO WATCH REPLACES TORNADO WATCH NUMBER 237. WATCH
NUMBER 237 WILL NOT BE IN EFFECT AFTER
1250 AM CDT. CONTINUE...WW 239...WW 240...WW 241...WW 242...
DISCUSSION...SRN MO SQUALL LINE EXPECTED TO CONTINUE EWD...WHERE LONG/HOOKED HODOGRAPHS
SUGGEST THREAT FOR EMBEDDED SUPERCELLS/PO SSIBLE TORNADOES. F ARTHER S...MORE WIDELY
SCATTERED
SUPERCELLS WITH A THREAT FOR TORNADOES WILL PERSIST IN VERY STRONGLY DEEP SHEARED/LCL
ENVIRONMENT IN AR.
AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURF ACE AND ALOFT TO 4
INCHES. EXTREME TURBULENCE AND SURF ACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH
MAXIMUM TOPS TO 500. MEAN STORM MOTION VECTOR 26045.
5. Status reports are issued as needed to show progress of storms and to delineate areas no longer under the
threat of severe storm activity. Cancellation bulletins are issued when it becomes evident that no severe weather
will develop or that storms have subsided and are no longer severe.
h. Center Weather Advisories (CWAs)
1. CWAs are unscheduled inflight, flow control, air traffic, and air crew advisory. By nature of its short lead
time, the CW A is not a flight planning product. It is generally a nowcast for conditions beginning within the next
two hours. CWAs will be issued:
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(a) As a supplement to an existing SIGMET, Convective SIGMET or AIRMET.
(b) When an Inflight Advisory has not been issued but observed or expected weather conditions meet
SIGMET/AIRMET criteria based on current pilot reports and reinforced by other sources of information about
existing meteorological conditions.
(c) When observed or developing weather conditions do not meet SIGMET, Convective SIGMET, or
AIRMET criteria; e.g., in terms of intensity or area  coverage, but current pilot reports or other weather
information sources indicate that existing or anticipated meteorological phenomena will adversely affect the safe
flow of air traffic within the ARTCC area of responsibility.
2. The following example is a CWA issued from the Kansas City, Missouri, ARTCC. The "3" after ZKC
in the first line denotes this CWA has been issued for the third weather phenomena to occur for the day. The "301"
in the second line denotes the phenomena number again (3) and the issuance number (01) for this phenomena.
The CWA was issued at 2140Z and is valid until 2340Z.
EXAMPLE-
ZKC3 CWA 032140
ZKC CWA 301 VALID UNTIL 032340
ISOLD SVR TSTM over KCOU MOVG SWWD 10 KTS ETC.