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In-Flight Weather Advisories

Introduction:

  • Inflight Aviation Weather Advisories are forecasts to advise en route aircraft of development of potentially hazardous weather
  • Advisories can be found based on geographic location:
    • In the conterminous U.S., In-Flight Weather Advisories are issued by the Aviation Weather Center (AWC) in Kansas City, Missouri and can be found on their web page at http://aviationweather.gov/adds/airmets//http://aviationweather.gov/adds/airmets/java/
    • For the Hawaiian Islands, In-flight Weather Advisories are issued by the Weather Forecast Office (WFO) in Honolulu
    • In Alaska, In-flight Weather Advisories are issued by the Alaska Aviation Weather Unit (AAWU)
    • All heights are referenced MSL, except in the case of ceilings (listed as "CIG") which indicates AGL
  • There are three types of inflight aviation weather advisories:
  • All of these advisories use the same location identifiers (either VORs, airports, or well−known geographic areas) to describe the hazardous weather areas. See FIG 7−1−3 and FIG 7−1−4. Graphics with improved clarity can be found in the latest version of Advisory Circular AC 00−45 series, Aviation Weather Services, which is available on the following Web site: http://www.faa.gov
  • Inflight Aviation Weather Advisory Supplements:

  • All of these advisories use the same location identifiers (either VORs, airports, or well-known geographic areas) to describe the hazardous weather areas Figure 1 & 2]
  • Graphics with improved clarity can be found in the Advisory Circular AC 00−45 series, Aviation Weather Services, which is available at http://www.faa.gov
In-flight Advisory Plotting Chart
Figure 1: In-flight Advisory Plotting Chart

Validity:

  • AIRMET valid for 6 hours
  • SIGMET valid for 4 hours
  • Convective SIGMET valid for 2 hours
  • The G-AIRMET is issued over the CONUS every 6 hours, valid at 3-hour increments through 12 hours with optional forecasts possible during the first 6 hours
    • The first 6 hours of the G-AIRMET correspond to the 6-hour period of the AIRMET

Convective SIGMET
Figure 2: AIRMET (Tango)

Airman Meteorological Information:

  • AIRman's METeorological products are issued corresponding to the Area Forecasts (FA) areas shown in Figure 3, 4 & 5]
  • AIRMETs (WAs) are advisories of significant weather phenomena but describe conditions at intensities lower than those which require the issuance of SIGMETs
    • AIRMETs are intended for dissemination to all pilots in the preflight and en route phase of flight to enhance safety
  • AIRMET information are issued in two formats:
    • A textual bulletin (WA), or
    • Graphical report (G-AIRMET)
  • text and graphical bulletins are issued 6 times daily (beginning at 0245 UTC) or as required:
    • The maximum forecast period is 6 hours for AIRMETs

  • Text AIRMET (WA):

    • AIRMET text (WA) products are issued corresponding to the Area Forecast (FA) areas described in Figure 3, 4 & 5]
    • Considered to be widespread because they must be affecting or be forecast to affect an area of at least 3,000 square miles at any one time
      • However, 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
    • Amended as necessary due to changing weather or issuance/cancellation of a SIGMET
    • AIRMETs contain details about IFR, extensive mountain obscuration, turbulence, strong surface winds, icing, and freezing levels
    • AIRMET Sierra
      Figure 3: AIRMET (Sierra)
    • There are three types AIRMETs:

        AIRMET Zulu
        Figure 4: AIRMET (Zulu)
      1. AIRMET Sierra:
        • AIRMET Sierra describes IFR conditions and/or extensive mountain obscuration
        • For operations under Instrument Flight Rules (IFR)
          • Ceilings less than 1000' and/or visibility less than 3 miles affecting over 50% of an area at one time
      2. AIRMET Tango:
        • AIRMET Tango describes moderate turbulence, sustained surface winds of 30 knots or greater, and/or non-convective low-level wind shear
      3. AIRMET Zulu:
        • AIRMET Zulu describes moderate icing and provides freezing level heights
      • After the first issuance each day, scheduled or unscheduled bulletins are numbered sequentially for easier identification
  • Graphical AIRMETs (G-AIRMETs):

    • Graphical AIRMETs (G−AIRMETs) are graphical forecasts of en-route weather hazards valid at discrete times no more than 3 hours apart for a period of up to 12 hours into the future
      • Example: 00, 03, 06, 09, and 12 hours
    • Additional forecasts may be inserted during the first 6 hours
      • 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
    • Forecasts valid at 06 through 12 hours correspond to the text bulletin outlook
    • Considered to be widespread because they must be affecting or be forecast to affect an area of at least 3,000 square miles at any one time
      • However, 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
    • G−AIRMET depicts the following en route aviation weather hazards:
      • Instrument flight rule conditions (ceiling < 1000’ and/or surface visibility <3 miles)
      • Mountain obscuration
      • Icing
      • Freezing level
      • Turbulence
      • Low level wind shear (LLWS)
      • Strong surface winds G−AIRMETs are snap shots at discrete time intervals as defined above
    • The text AIRMET is the result of the production of the G-AIRMET, but provided in a time smear for a 6 hr valid period
    • G-AIRMETs provide a higher forecast resolution than text AIRMET products
    • Since G-AIRMETs and text AIRMETs are created from the same forecast "production" process, there exists perfect consistency between the two
    • Using the two together will provide clarity of the area impacted by the weather hazard and improve situational awareness and decision making
    • Interpolation of time periods between G-AIRMET valid times:
      • Users must keep in mind when using the G-AIRMET, 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
        • 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
    • The G−AIRMET is issued over the CONUS every 6 hours, valid at 3−hour increments through 12 hours with optional forecasts possible during the first 6 hours. The first 6 hours of the G−AIRMET correspond to the 6−hour period of the AIRMET

SIGMET:

  • SIGnificant METeorological information
  • SIGMETs/AIRMET text (WA) products are issued corresponding to the Area Forecasts (FA) areas shown in Figure 3, 4 & 5]
  • Issued as required to advise of non-convective weather that is potentially hazardous to all aircraft
  • Considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time
    • However, 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
  • SIGMETs are unscheduled products that are valid for 4 hours
    • Conditions that are associated with hurricanes are valid for 6 hours
    • Unscheduled updates and corrections are issued as necessary
    • The maximum forecast period is 4 hours for SIGMETs
  • SIGMETS can be identified by an alphabetic designator from November through Yankee excluding 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 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
    • No two different phenomena across the country can have the same alphabetic designator at the same time
    • 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
  • Contents:
    • In the conterminous U.S., SIGMETs are issued when the following phenomena occur or are expected to occur:
      • Severe icing not associated with thunderstorms
      • Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms
      • Dust storms or sandstorms lowering surface or in-flight visibilities to below 3 miles
      • Volcanic ash
    • In Alaska and Hawaii, SIGMETs are also issued for:

      • Tornadoes
      • Lines of thunderstorms
      • Embedded thunderstorms
      • Hail greater than or equal to 3/4 inch in diameter
Convective SIGMET
Figure 5: Convective SIGMET

Convective SIGMET:

  • Convective SIGMETS (also known internationally as SIGMET for Convection) are text/graphical reports are issued for the contiguous U.S
  • Any convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear(different from above) issued for any convective situation that the forecaster feels is hazardous to all categories of aircraft
  • Convective SIGMET bulletins are issued for the western (W), center (C), and eastern (E) United States
    • Convective SIGMETs are not issued for Alaska or Hawaii
  • The areas are separated at 87° and 107° west longitude with sufficient overlap to cover most cases when the phenomenon crosses the boundaries
  • Bulletins are issued hourly at H+55
    • If conditions warrant, 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 at H+55 for each area
  • 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
  • Contents:
    • Convective SIGMETs are issued in the conterminous U.S. for any of the following:
      • Severe thunderstorms 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
      • Embedded Thunderstorms
      • A line of thunderstorms
      • Thunderstorms that produce precipitation levels greater than or equal to heavy-intensity precipitation, affecting 40% or more of an area at least 3,000 square miles

International SIGMET:

  • Some NWS offices have been designated by the ICAO as Meteorological Watch Offices (MWOs)
    • These offices are responsible for issuing International SIGMETs for designated areas that include Alaska, Hawaii, portions of the Atlantic and Pacific Oceans, and the Gulf of Mexico
  • The offices which issue international SIGMETs are:
    • The AWC in Kansas City, Missouri
    • The AAWU in Anchorage, Alaska
    • The WFO in Honolulu, Hawaii
  • Considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time
  • The International SIGMET is issued for 6 hours for volcanic ash events, 6 hours for hurricanes and tropical storms, and 4 hours for all other events
  • Like the domestic SIGMETs, international SIGMETs are also identified by an alphabetic designator from Alpha through Mike and are numbered sequentially until that weather phenomenon ends
  • Found on the Aviation Weather Center web page at http://www.aviationweather.gov/products/sigmets/intl
  • Issued for 12 hours for volcanic ash events, 6 hours for hurricanes and tropical storms, and 4 hours for all other events
  • Numbered like SIGMETs
  • Issued for:
    • Thunderstorms occurring in lines, embedded in clouds, or in large areas producing tornadoes or large hail
    • Tropical cyclones
    • Severe icing
    • Severe or extreme turbulence
    • Dust storms and sandstorms lowering visibilities to less than 3 miles
    • Volcanic ash
  • Example of an International SIGMET:
    • WSNT06 KKCI 022014
    • SIGA0F
    • KZMA KZNY TJZS SIGMET FOXTROT 3 VALID 022015/030015 KKCI− MIAMI OCEANIC FIR NEW YORK OCEANIC FIR SAN JUAN FIR FRQ TS WI AREA BOUNDED BY 2711N6807W 2156N6654W 2220N7040W 2602N7208W 2711N6807W. TOPS TO FL470. MOV NE 15KT. WKN. BASED ON SAT AND LTG OBS
    • MOSHER

Severe Weather Watch Bulletins (WW) and Alert Messages (AWW):

  • WWs define areas of possible severe thunderstorms or tornado activity
  • Issued by the Storm Prediction Center (SPC) in Norman, OK
  • They are unscheduled and therefore issued as required
  • Severe thunderstorm watches describe areas of expected severe thunderstorms (Severe thunderstorm criteria are 3/4−inch hail or larger and/or wind gusts of 50 knots [58 mph] or greater)
  • A tornado watch describes areas where the threat of tornadoes exists
  • In order to alert the WFOs, CWSUs, FSSs, and other users, a preliminary notification of a watch called the Alert Severe Weather Watch bulletin (AWW) is sent before the WW (WFOs know this product as a SAW)
  • Soon after the AWW goes out, the actual watch bulletin itself is issued
  • A WW is in the following format:

    • Type of severe weather watch, watch area, valid time period, type of severe weather possible, watch axis, meaning of a watch, and a statement that persons should be on the lookout for severe weather
    • Other watch information; i.e., references to previous watches
    • Phenomena, intensities, hail size, wind speed (knots), maximum cumulonimbus (CB) tops, and estimated cell movement (mean wind vector)
    • Cause of severe weather
    • Information on updating Convective Outlook (AC) products
  • 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
  • When tornadoes or severe thunderstorms have developed, the local WFO office will issue the warnings covering those areas
  • Example of an AWW:

    • MKC AWW 011734
    • WW 75 TORNADO TX OK AR 011800Z−020000Z
    • AXIS..80 STATUTE MILES EAST AND WEST OF A LINE..60ESE DAL/DALLAS TX/ - 30 NW ARG/ WALNUT RIDGE AR/
    • ..AVIATION COORDS.. 70NM E/W /58W GGG - 25NW ARG/ HAIL SURFACE AND ALOFT..1 3/4 INCHES. WIND GUSTS..70 KNOTS. MAX TOPS TO 450. MEAN WIND VECTOR 24045
  • Example of a WW:

    • BULLETIN - IMMEDIATE BROADCAST REQUESTED
    • TORNADO WATCH NUMBER 381
    • STORM PREDICTION CENTER NORMAN OK
    • 556 PM CDT MON JUN 2 1997
    • THE STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF NORTHEAST NEW MEXICO TEXAS PANHANDLE
    • EFFECTIVE THIS MONDAY NIGHT AND TUESDAY MORNING FROM 630 PM UNTIL MIDNIGHT CDT
    • TORNADOES...HAIL TO 2 3/4 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 80 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS
    • THE TORNADO WATCH AREA IS ALONG AND 60 STATUTE MILES NORTH AND SOUTH OF A LINE FROM 50 MILES SOUTHWEST OF RATON NEW MEXICO TO 50 MILES EAST OF AMARILLO TEXAS
    • REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE 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...CONTINUE... WW 378...WW 379...WW 380
    • DISCUSSION...THUNDERSTORMS ARE INCREASING OVER NE NM IN MOIST SOUTHEASTERLY UPSLOPE FLOW. OUTFLOW BOUNDARY EXTENDS EASTWARD INTO THE TEXAS PANHANDLE AND EXPECT STORMS TO MOVE ESE ALONG AND NORTH OF THE BOUNDARY ON THE N EDGE OF THE CAP. VEERING WINDS WITH HEIGHT ALONG WITH INCREASING MID LVL FLOW INDICATE A THREAT FOR SUPERCELLS
    • AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2 3/4 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM MOTION VECTOR 28025

Center Weather Advisories (CWAs):

  • A Center Weather Advisory is an aviation warning for use by air crews to anticipate and avoid adverse weather conditions in the en-route and terminal environments [Figure 1]
  • By nature of its short lead time, the CWA is not a flight planning product
  • It is generally a nowcast for conditions beginning within the next two hours
  • CWAs will be issued:
    • As a supplement to an existing SIGMET, Convective SIGMET or AIRMET
    • 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
    • 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
  • The following example is a CWA issued from the Kansas City, Missouri, ARTCC:
    • ZKC3 CWA 032140 ZKC CWA 301 VALID UNTIL 032340 ISOLD SVR TSTM over KCOU MOVG SWWD 10 KTS ETC
    • 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
  • Found at http://www.aviationweather.gov/products/cwsu/
Center Weather Service Unit Products.jpg
Figure 6: Center Weather Service Unit Products
  • Validity:
    • Issued as required
    • Valid for 2 hours
    • Not for use in flight planning
    • Reflects actual or short range forecasts expected to begin within 2 hours

  • Translation:
    • Location: ZMA (Miami) ARTCC
    • Issuance occurrence: 204, phenomenon number (2) with the (04)th issue
    • Validity: valid until the 21st day at 0005Z
      • Some advisories may include an issuance time

    Miami (ZMA) Center Weather Advisory
    Figure 7: Miami (ZMA) Center Weather Advisory

  • Area: based off of defined points and distances from them to form a geographic area
  • Description: varies by report but the example talks about 40 NM wide area of thunder storms moving from the south west at 15 knots. Tops are estimated to be at 46,000' with little expected change in coverage through 0005Z

  • Conclusion:

    References: