7-1-27. Thunderstorm Flying

1. Thunderstorm Avoidance. Never regard any thunderstorm lightly, even when radar echoes are of light intensity. Avoiding thunderstorms is the best policy. Following are some Do's and Don'ts of thunderstorm avoidance:
   1. Don't land or takeoff in the face of an approaching thunderstorm. A sudden gust front of low level turbulence could cause loss of control.
   2. Don't attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and wind shear under the storm could be hazardous.
   3. Don't attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme clear air turbulence.
   4. Don't fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated.
   5. Don't trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.
   6. Don't assume that ATC will offer radar navigation guidance or deviations around thunderstorms.
   7. Don't use data-linked weather next generation weather radar (NEXRAD) mosaic imagery as the sole means for negotiating a path through a thunderstorm area (tactical maneuvering).
   8. Do remember that the data-linked NEXRAD mosaic imagery shows where the weather was, not where the weather is. The weather conditions depicted may be 15 to 20 minutes older than indicated on the display.
   9. Do listen to chatter on the ATC frequency for Pilot Weather Reports (PIREP) and other aircraft requesting to deviate or divert.
   10. Do ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed.
   11. Do use data-linked weather NEXRAD mosaic imagery (for example, Flight Information Service-Broadcast (FIS-B)) for route selection to avoid thunderstorms entirely (strategic maneuvering).
   12. Do advise ATC, when switched to another controller, that you are deviating for thunderstorms before accepting to rejoin the original route.
   13. Do ensure that after an authorized weather deviation, before accepting to rejoin the original route, that the route of flight is clear of thunderstorms.
   14. Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This is especially true under the anvil of a large cumulonimbus.
   15. Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage.
   16. Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm.
   17. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar.
   18. Do give a PIREP for the flight conditions.
   19. Do divert and wait out the thunderstorms on the ground if unable to navigate around an area of thunderstorms.
   20. Do contact Flight Service for assistance in avoiding thunderstorms. Flight Service specialists have NEXRAD mosaic radar imagery and NEXRAD single site radar with unique features such as base and composite reflectivity, echo tops, and VAD wind profiles.
2. If you cannot avoid penetrating a thunderstorm, following are some Do's before entering the storm:
   1. Tighten your safety belt, put on your shoulder harness (if installed), if and secure all loose objects.
   2. Plan and hold the course to take the aircraft through the storm in a minimum time.
   3. To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15ºC.
   4. Verify that pitot heat is on and turn on carburetor heat or jet engine anti-ice. Icing can be rapid at any altitude and cause almost instantaneous power failure and/or loss of airspeed indication.
   5. Establish power settings for turbulence penetration airspeed recommended in the aircraft manual.
   6. Turn up cockpit lights to highest intensity to lessen temporary blindness from lightning.
   7. If using automatic pilot, disengage Altitude Hold Mode and Speed Hold Mode. The automatic altitude and speed controls will increase maneuvers of the aircraft thus increasing structural stress.
   8. If using airborne radar, tilt the antenna up and down occasionally. This will permit the detection of other thunderstorm activity at altitudes other than the one being flown.
3. Following are some Do's and Don'ts duringthe thunderstorm penetration:
   1. Do keep your eyes on your instruments. Looking outside the cockpit can increase danger of temporary blindness from lightning.
   2. Don't change power settings; maintain settings for the recommended turbulence penetration airspeed.
   3. Do maintain constant attitude. Allow the altitude and airspeed to fluctuate.
   4. Don't turn back once you are in the thunderstorm. A straight course through the storm most likely will get the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft.

7-1-28. Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR)

FIG 7-1-22
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Front)

FIG 7-1-23
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Back)

7-1-29. International Civil Aviation Organization (ICAO) Weather Formats

The U.S. uses the ICAO world standard for aviation weather reporting and forecasting. The World Meteorological Organization's (WMO) publication No. 782 “Aerodrome Reports and Forecasts” contains the base METAR and TAF code as adopted by the WMO member countries.

1. Although the METAR code is adopted worldwide, each country is allowed to make modifications or exceptions to the code for use in their particular country, e.g., the U.S. will continue to use statute miles for visibility, feet for RVR values, knots for wind speed, and inches of mercury for altimetry. However, temperature and dew point will be reported in degrees Celsius. The U.S reports prevailing visibility rather than lowest sector visibility. The elements in the body of a METAR report are separated with a space. The only exceptions are RVR, temperature, and dew point which are separated with a solidus (/). When an element does not occur, or cannot be observed, the preceding space and that element are omitted from that particular report. A METAR report contains the following sequence of elements in the following order:
   1. Type of report.
   2. ICAO Station Identifier.
   3. Date and time of report.
   4. Modifier (as required).
   5. Wind.
   6. Visibility.
   7. Runway Visual Range (RVR).
   8. Weather phenomena.
   9. Sky conditions.
   10. Temperature/dew point group.
   11. Altimeter.
   12. Remarks (RMK).
2. The following paragraphs describe the elements in a METAR report.
   1. Type of report. There are two types of report:
      1. Aviation Routine Weather Report (METAR); and
      2. Nonroutine (Special) Aviation Weather Report (SPECI).

         The type of report (METAR or SPECI) will always appear as the lead element of the report.

   2. ICAO Station Identifier. The METAR code uses ICAO 4-letter station identifiers. In the contiguous 48 States, the 3-letter domestic station identifier is prefixed with a “K;” i.e., the domestic identifier for Seattle is SEA while the ICAO identifier is KSEA. Elsewhere, the first two letters of the ICAO identifier indicate what region of the world and country (or state) the station is in. For Alaska, all station identifiers start with “PA;” for Hawaii, all station identifiers start with “PH.” Canadian station identifiers start with “CU,” “CW,” “CY,” and “CZ.” Mexican station identifiers start with “MM.” The identifier for the western Caribbean is “M” followed by the individual country's letter; i.e., Cuba is “MU;” Dominican Republic “MD;” the Bahamas “MY.” The identifier for the eastern Caribbean is “T” followed by the individual country's letter; i.e., Puerto Rico is “TJ.” For a complete worldwide listing see ICAO Document 7910, Location Indicators.
   3. Date and Time of Report. The date and time the observation is taken are transmitted as a six-digit date/time group appended with Z to denote Coordinated Universal Time (UTC). The first two digits are the date followed with two digits for hour and two digits for minutes.
   4. Modifier (As Required). “AUTO” identifies a METAR/SPECI report as an automated weather report with no human intervention. If “AUTO” is shown in the body of the report, the type of sensor equipment used at the station will be encoded in the remarks section of the report. The absence of “AUTO” indicates that a report was made manually by an observer or that an automated report had human augmentation/backup. The modifier “COR” indicates a corrected report that is sent out to replace an earlier report with an error.
   5. Wind. The wind is reported as a five digit group (six digits if speed is over 99 knots). The first three digits are the direction the wind is blowing from, in tens of degrees referenced to true north, or “VRB” if the direction is variable. The next two digits is the wind speed in knots, or if over 99 knots, the next three digits. If the wind is gusty, it is reported as a “G” after the speed followed by the highest gust reported. The abbreviation “KT” is appended to denote the use of knots for wind speed.
      1. Peak Wind. Whenever the peak wind exceeds 25 knots “PK WND” will be included in Remarks, e.g., PK WND 28045/1955 “Peak wind two eight zero at four five occurred at one niner five five.” If the hour can be inferred from the report time, only the minutes will be appended, e.g., PK WND 34050/38 “Peak wind three four zero at five zero occurred at three eight past the hour.”
      2. Wind shift. Whenever a wind shift occurs, “WSHFT” will be included in remarks followed by the time the wind shift began, e.g., WSHFT 30 FROPA “Wind shift at three zero due to frontal passage.”
   6. Visibility. Prevailing visibility is reported in statute miles with “SM” appended to it.
      1. Tower/surface visibility. If either visibility (tower or surface) is below four statute miles, the lesser of the two will be reported in the body of the report; the greater will be reported in remarks.
      2. Automated visibility. ASOS/AWOSvisibility stations will show visibility 10 or greater than 10 miles as “10SM.” AWOSvisibility stations will show visibility less than ¹/₄ statute mile as “M¹/₄SM” and visibility 10 or greater than 10 miles as “10SM.”
      3. Variable visibility. Variable visibility is shown in remarks (when rapid increase or decrease by ¹/₂ statute mile or more and the average prevailing visibility is less than three miles) e.g., VIS 1V2 “visibility variable between one and two.”
      4. Sector visibility. Sector visibility is shown in remarks when it differs from the prevailing visibility, and either the prevailing or sector visibility is less than three miles.
   7. Runway Visual Range (When Reported). “R” identifies the group followed by the runway heading (and parallel runway designator, if needed) “/” and the visual range in feet (meters in other countries) followed with “FT” (feet is not spoken).
      1. Variability Values. When RVR varies (by more than on reportable value), the lowest and highest values are shown with “V” between them.
      2. Maximum/Minimum Range. “P” indicates an observed RVR is above the maximum value for this system (spoken as “more than”). “M” indicates an observed RVR is below the minimum value which can be determined by the system (spoken as “less than”).
   8. Weather Phenomena. The weather as reported in the METAR code represents a significant change in the way weather is currently reported. In METAR, weather is reported in the format:

      Intensity/Proximity/Descriptor/Precipitation/Obstruction to visibility/Other

      1. Intensity applies only to the first type of precipitation reported. A “-” denotes light, no symbol denotes moderate, and a “+” denotes heavy.
      2. Proximity applies to and reported only for weather occurring in the vicinity of the airport (between 5 and 10 miles of the point(s) of observation). It is denoted by the letters “VC.” (Intensity and “VC” will not appear together in the weather group).
      3. Descriptor. These eight descriptors apply to the precipitation or obstructions to visibility:
         TS thunderstorm
         DR low drifting
         SH showers
         MI shallow
         FZ freezing
         BC patches
         BL blowing
         PR partial
      4. Precipitation. There are nine types of precipitation in the METAR code:
         RA rain
         DZ drizzle
         SN snow
         GR hail (¹/₄” or greater)
         GS small hail/snow pellets
         PL ice pellets
         SG snow grains
         IC ice crystals (diamond dust)
         UP unknown precipitation (automated stations only)
      5. Obstructions to visibility. There are eight types of obscuration phenomena in the METAR code (obscurations are any phenomena in the atmosphere, other than precipitation, that reduce horizontal visibility):
         FG fog (vsby less than ⁵/₈ mile)
         HZ haze
         FU smoke
         PY spray
         BR mist (vsby ⁵/₈ - 6 miles)
         SA sand
         DU dust
         VA volcanic ash
      6. Other. There are five categories of other weather phenomena which are reported when they occur:
         SQ squall
         SS sandstorm
         DS duststorm
         PO dust/sand whirls
         FC funnel cloud
         +FC tornado/waterspout

         Examples:

         TSRA thunderstorm with moderate rain
         +SN heavy snow
         -RA FG light rain and fog
         BRHZ mist and haze (visibility ⁵/₈ mile or greater)
         FZDZ freezing drizzle
         VCSH rain shower in the vicinity
         +SHRASNPL heavy rain showers, snow, ice pellets (intensity indicator refers to the predominant rain)

   9. Sky Condition. The sky condition as reported in METAR represents a significant change from the way sky condition is currently reported. In METAR, sky condition is reported in the format:
      
      Amount/Height/(Type) or Indefinite Ceiling/Height
      1. Amount. The amount of sky cover is reported in eighths of sky cover, using the contractions:
         SKC clear (no clouds)
         FEW >0 to ²/₈
         SCT scattered (³/_8s to ⁴/_8s of clouds)
         BKN broken (⁵/_8s to ⁷/_8s of clouds)
         OVC overcast (⁸/_8s clouds)
         CB Cumulonimbus when present
         TCU Towering cumulus when present
      2. Height. Cloud bases are reported with three digits in hundreds of feet above ground level (AGL). (Clouds above 12,000 feet cannot be reported by an automated station).
      3. (Type). If Towering Cumulus Clouds (TCU) or Cumulonimbus Clouds (CB) are present, they are reported after the height which represents their base.
      4. Vertical Visibility (indefinite ceiling height). The height into an indefinite ceiling is preceded by “VV” and followed by three digits indicating the vertical visibility in hundreds of feet. This layer indicates total obscuration.
      5. Obscurations are reported when the sky is partially obscured by a ground-based phenomena by indicating the amount of obscuration as FEW, SCT, BKN followed by three zeros (000). In remarks, the obscuring phenomenon precedes the amount of obscuration and three zeros.
      6. When sky conditions include a layer aloft, other than clouds, such as smoke or haze the type of phenomena, sky cover and height are shown in remarks.
      7. Variable ceiling. When a ceiling is below three thousand and is variable, the remark “CIG” will be shown followed with the lowest and highest ceiling heights separated by a “V.”
      8. Second site sensor. When an automated station uses meteorological discontinuity sensors, remarks will be shown to identify site specific sky conditions which differ and are lower than conditions reported in the body.
      9. Variable cloud layer. When a layer is varying in sky cover, remarks will show the variability range. If there is more than one cloud layer, the variable layer will be identified by including the layer height.
      10. Significant clouds. When significant clouds are observed, they are shown in remarks, along with the specified information as shown below:
          1. Cumulonimbus (CB), or Cumulonimbus Mammatus (CBMAM), distance (if known), direction from the station, and direction of movement, if known. If the clouds are beyond 10 miles from the airport, DSNT will indicate distance.
          2. Towering Cumulus (TCU), location, (if known), or direction from the station.
          3. Altocumulus Castellanus (ACC), Stratocumulus Standing Lenticular (SCSL), Altocumulus Standing Lenticular (ACSL), Cirrocumulus Standing Lenticular (CCSL) or rotor clouds, describing the clouds (if needed) and the direction from the station.
   10. Temperature/Dew Point. Temperature and dew point are reported in two, two‐digit groups in degrees Celsius, separated by a solidus (“/”). Temperatures below zero are prefixed with an “M.” If the temperature is available but the dew point is missing, the temperature is shown followed by a solidus. If the temperature is missing, the group is omitted from the report.
   11. Altimeter. Altimeter settings are reported in a four‐digit format in inches of mercury prefixed with an “A” to denote the units of pressure.
   12. Remarks. Remarks will be included in all observations, when appropriate. The contraction “RMK” denotes the start of the remarks section of a METAR report.

       Except for precipitation, phenomena located within 5 statute miles of the point of observation will be reported as at the station. Phenomena between 5 and 10 statute miles will be reported in the vicinity, “VC.” Precipitation not occurring at the point of observation but within 10 statute miles is also reported as in the vicinity, “VC.” Phenomena beyond 10 statute miles will be shown as distant, “DSNT.” Distances are in statute miles except for automated lightning remarks which are in nautical miles. Movement of clouds or weather will be indicated by the direction toward which the phenomena is moving.

       1. There are two categories of remarks:
          1. Automated, manual, and plain language.
          2. Additive and automated maintenance data.
       2. Automated, Manual, and Plain Language. This group of remarks may be generated from either manual or automated weather reporting stations and generally elaborate on parameters reported in the body of the report. (Plain language remarks are only provided by manual stations).
          1. Volcanic eruptions.
          2. Tornado, Funnel Cloud, Waterspout.
          3. Station Type (AO1 or AO2).
          4. PK WND.
          5. WSHFT (FROPA).
          6. TWR VIS or SFC VIS.
          7. VRB VIS.
          8. Sector VIS.
          9. VIS @ 2^nd Site.
          10. Lightning. When lightning is observed at a manual location, the frequency and location is reported.

              When cloud-to-ground lightning is detected by an automated lightning detection system, such as ALDARS:

              1. Within 5 nautical miles (NM) of the Airport Reference Point (ARP), it will be reported as “TS" in the body of the report with no remark;
              2. Between 5 and 10 NM of the ARP, it will be reported as “VCTS" in the body of the report with no remark;
              3. Beyond 10 but less than 30 NM of the ARP, it will be reported in remarks as “DSNT" followed by the direction from the ARP.
          11. Beginning/Ending of Precipitation/TSTMS.
          12. TSTM Location MVMT.
          13. Hailstone Size (GR).
          14. Virga.
          15. VRB CIG (height).
          16. Obscuration.
          17. VRB Sky Condition.
          18. Significant Cloud Types.
          19. Ceiling Height 2nd Location.
          20. PRESFR PRESRR.
          21. Sea-Level Pressure.
          22. ACFT Mishap (not transmitted).
          23. NOSPECI.
          24. SNINCR.
          25. Other SIG Info.
       3. Additive and Automated Maintenance Data.
          1. Hourly Precipitation.
          2. 3- and 6-Hour Precipitation Amount.
          3. 24-Hour Precipitation.
          4. Snow Depth on Ground.
          5. Water Equivalent of Snow.
          6. Cloud Type.
          7. Duration of Sunshine.
          8. Hourly Temperature/Dew Point (Tenths).
          9. 6-Hour Maximum Temperature.
          10. 6-Hour Minimum Temperature.
          11. 24-Hour Maximum/Minimum Temperature.
          12. Pressure Tendency.
          13. Sensor Status.
              PWINO
              FZRANO
              TSNO
              RVRNO
              PNO
              VISNO

              Examples of METAR reports and explanation:

              METAR KBNA 281250Z 33018KT 290V360 1/2SM R31/2700FT SN BLSN FG VV008 00/M03 A2991 RMK RAE42SNB42
              
              METAR aviation routine weather report
              KBNA Nashville, TN
              281250Z date 28^th, time 1250 UTC
              (no modifier) This is a manually generated report, due to the absence of “AUTO” and “AO1 or AO2” in remarks
              33018KT wind three three zero at one eight
              290V360 wind variable between two nine zero and three six zero
              1/2SM visibility one half
              R31/2700FT Runway three one RVR two thousand seven hundred
              SN moderate snow
              BLSN FG visibility obscured by blowing snow and fog
              VV008 indefinite ceiling eight hundred
              00/M03 temperature zero, dew point minus three
              A2991 altimeter two niner niner one
              RMK remarks
              RAE42 rain ended at four two
              SNB42 snow began at four two

              METAR KSFO 041453Z AUTO VRB02KT 3SM BR CLR 15/12 A3012 RMK AO2
              
              METAR aviation routine weather report
              KSFO San Francisco, CA
              041453Z date 4^th, time 1453 UTC
              AUTO fully automated; no human intervention
              VRB02KT wind variable at two
              3SM visibility three
              BR visibility obscured by mist
              CLR no clouds below one two thousand
              15/12 temperature one five, dew point one two
              A3012 altimeter three zero one two
              RMK remarks
              AO2 this automated station has a weather discriminator (for precipitation)

              SPECI KCVG 152224Z 28024G36KT 3/4SM +TSRA BKN008 OVC020CB 28/23 A3000 RMK TSRAB24 TS W MOV E
              
              SPECI (nonroutine) aviation special weather report
              KCVG Cincinnati, OH
              152228Z date 15^th, time 2228 UTC
              (no modifier) This is a manually generated report due to the absence of “AUTO” and “AO1 or AO2” in remarks
              28024G36KT wind two eight zero at two four gusts three six
              3/4SM visibility three fourths
              +TSRA thunderstorms, heavy rain
              BKN008 ceiling eight hundred broken
              OVC020CB two thousand overcast cumulonimbus clouds
              28/23 temperature two eight, dew point two three
              A3000 altimeter three zero zero zero
              RMK remarks
              TSRAB24 thunderstorm and rain began at two four
              TS W MOV E thunderstorm west moving east

3. Aerodrome Forecast (TAF). A concise statement of the expected meteorological conditions at an airport during a specified period. At most locations, TAFs have a 24 hour forecast period. However, TAFs for some locations have a 30 hour forecast period. These forecast periods may be shorter in the case of an amended TAF. TAFs use the same codes as METAR weather reports. They are scheduled four times daily for 24-hour periods beginning at 0000Z, 0600Z, 1200Z, and 1800Z.

   Forecast times in the TAF are depicted in two ways. The first is a 6-digit number to indicate a specific point in time, consisting of a two-digit date, two-digit hour, and two-digit minute (such as issuance time or FM). The second is a pair of four-digit numbers separated by a “/” to indicatea beginning and end for a period of time. In thiscase,each four-digit pair consists of a two-digit date and a two-digit hour.

   TAFs are issued in the following format:

   TYPE OF REPORT/ICAO STATION IDENTIFIER/DATE AND TIME OF ORIGIN/VALID PERIOD DATE AND TIME/FORECAST METEOROLOGICAL CONDITIONS

   TAF KORD 051130Z 0512/0618 14008KT 5SM BR BKN030
   TEMPO 0513/0516 1 1/2SM BR
   FM051600 16010KT P6SM SKC
   FM052300 20013G20KT 4SM SHRA OVC020
   PROB40 0600/0606 2SM TSRA OVC008CB
   BECMG 0606/0608 21015KT P6SM NSW SCT040

   TAF format observed in the above example:

   TAF = type of report

   KORD = ICAO station identifier

   051130Z = date and time of origin (issuance time)

   0512/0618 = valid period date and times

   14008KT 5SM BR BKN030 = forecast meteorological conditions

   Explanation of TAF elements:

   1. Type of Report. There are two types of TAF issuances, a routine forecast issuance (TAF) and an amended forecast (TAF AMD). An amended TAF is issued when the current TAF no longer adequately describes the on‐going weather or the forecaster feels the TAF is not representative of the current or expected weather. Corrected (COR) or delayed (RTD) TAFs are identified only in the communications header which precedes the actual forecasts.
   2. ICAO Station Identifier. The TAF code uses ICAO 4-letter location identifiers as described in the METAR section.
   3. Date and Time of Origin. This element is the date and time the forecast is actually prepared. The format is a two-digit date and four-digit time followed, without a space, by the letter “Z.”
   4. Valid Period Date and Time. The UTC valid period of the forecast consists of two four-digit sets, separated by a “/”. The first four-digit set is a two-digit date followed by the two-digit beginning hour, and the second four-digit set is a two-digit date followed by the two-digit ending hour. Although most airports have a 24-hour TAF, a select number of airports have a 30-hour TAF. In the case of an amended forecast, or a forecast which is corrected or delayed, the valid period may be for less than 24 hours. Where an airport or terminal operates on a part-time basis (less than 24 hours/day), the TAFs issued for those locations will have the abbreviated statement “AMD NOT SKED” added to the end of the forecasts. The time observations are scheduled to end and/or resume will be indicated by expanding the AMD NOT SKED statement. Expanded statements will include:
      1. Observation ending time (AFT DDHHmm; for example, AFT 120200)
      2. Scheduled observations resumption time (TIL DDHHmm; for example, TIL 171200Z) or
      3. Period of observation unavailability (DDHH/DDHH); for example, 2502/2512).
   5. Forecast Meteorological Conditions. This is the body of the TAF. The basic format is:

      WIND/VISIBILITY/WEATHER/SKY CONDITION/OPTIONAL DATA (WIND SHEAR)

      The wind, visibility, and sky condition elements are always included in the initial time group of the forecast. Weather is included only if significant to aviation. If a significant, lasting change in any of the elements is expected during the valid period, a new time period with the changes is included. It should be noted that with the exception of a “FM” group the new time period will include only those elements which are expected to change, i.e., if a lowering of the visibility is expected but the wind is expected to remain the same, the new time period reflecting the lower visibility would not include a forecast wind. The forecast wind would remain the same as in the previous time period. Any temporary conditions expected during a specific time period are included with that time period. The following describes the elements in the above format.

      1. Wind. This five (or six) digit group includes the expected wind direction (first 3 digits) and speed (last 2 digits or 3 digits if 100 knots or greater). The contraction “KT” follows to denote the units of wind speed. Wind gusts are noted by the letter “G” appended to the wind speed followed by the highest expected gust. A variable wind direction is noted by “VRB” where the three digit direction usually appears. A calm wind (3 knots or less) is forecast as “00000KT.”
      2. Visibility. The expected prevailing visibility up to and including 6 miles is forecast in statute miles, including fractions of miles, followed by “SM” to note the units of measure. Expected visibilities greater than 6 miles are forecast as P6SM (plus six statute miles).
      3. Weather Phenomena. The expected weather phenomena is coded in TAF reports using the same format, qualifiers, and phenomena contractions as METAR reports (except UP). Obscurations to vision will be forecast whenever the prevailing visibility is forecast to be 6 statute miles or less. If no significant weather is expected to occur during a specific time period in the forecast, the weather phenomena group is omitted for that time period. If, after a time period in which significant weather phenomena has been forecast, a change to a forecast of no significant weather phenomena occurs, the contraction NSW (No Significant Weather) will appear as the weather group in the new time period. (NSW is included only in TEMPO groups).
      4. Sky Condition. TAF sky condition forecasts use the METAR format described in the METAR section. Cumulonimbus clouds (CB) are the only cloud type forecast in TAFs. When clear skies are forecast, the contraction “SKC” will always be used. The contraction “CLR” is never used in the TAF. When the sky is obscured due to a surface-based phenomenon, vertical visibility (VV) into the obscuration is forecast. The format for vertical visibility is “VV” followed by a three-digit height in hundreds of feet.

         SKC “sky clear”
         SCT005 BKN025CB “five hundred scattered, ceiling two thousand five hundred broken cumulonimbus
         clouds”
         VV008 “indefinite ceiling eight hundred”

      5. Optional Data (Wind Shear). Wind shear is the forecast of nonconvective low level winds (up to 2,000 feet). The forecast includes the letters “WS” followed by the height of the wind shear, the wind direction and wind speed at the indicated height and the ending letters “KT” (knots). Height is given in hundreds of feet (AGL) up to and including 2,000 feet. Wind shear is encoded with the contraction “WS,” followed by a three-digit height, slant character “/,” and winds at the height indicated in the same format as surface winds. The wind shear element is omitted if not expected to occur.

         WS010/18040KT - “LOW LEVEL WIND SHEAR AT ONE THOUSAND, WIND ONE EIGHT ZERO AT FOUR ZERO”

4. Probability Forecast. The probability or chance of thunderstorms or other precipitation events occurring, along with associated weather conditions (wind, visibility, and sky conditions). The PROB30 group is used when the occurrence of thunderstorms or precipitation is 30-39% and the PROB40 group is used when the occurrence of thunderstorms or precipitation is 40-49%. This is followed by two four-digit groups separated by a “/”, giving the beginning date and hour, and the ending date and hour of the time period during which the thunderstorms or precipitation are expected.
5. Forecast Change Indicators. The following change indicators are used when either a rapid, gradual, or temporary change is expected in some or all of the forecast meteorological conditions. Each change indicator marks a time group within the TAF report.
   1. From (FM) group. The FM group is used when a rapid change, usually occurring in less than one hour, in prevailing conditions is expected. Typically, a rapid change of prevailing conditions to more or less a completely new set of prevailing conditions is associated with a synoptic feature passing through the terminal area (cold or warm frontal passage). Appended to the “FM” indicator is the six-digit date, hour, and minute the change is expected to begin and continues until the next change group or until the end of the current forecast. A “FM” group will mark the beginning of a new line in a TAF report (indented 5 spaces). Each “FM” group contains all the required elements-wind, visibility, weather, and sky condition. Weather will be omitted in “FM” groups when it is not significant to aviation. FM groups will not include the contraction NSW.
   2. Becoming (BECMG) group. The BECMG group is used when a gradual change in conditions is expected over a longer time period, usually two hours. The time period when the change is expected is two four-digit groups separated by a “/”, with the beginning date and hour, and ending date and hour of the change period which follows the BECMG indicator. The gradual change will occur at an unspecified time within this time period. Only the changing forecast meteorological conditions are included in BECMG groups. The omitted conditions are carried over from the previous time group.
   3. Temporary (TEMPO) group. The TEMPO group is used for any conditions in wind, visibility, weather, or sky condition which are expected to last for generally less than an hour at a time (occasional), and are expected to occur during less than half the time period. The TEMPO indicator is followed by two four-digit groups separated by a “/”. The first four digit group gives the beginning date and hour, and the second four digit group gives the ending date and hour of the time period during which the temporary conditions are expected. Only the changing forecast meteorological conditions are included in TEMPO groups. The omitted conditions are carried over from the previous time group.