Observed weather condition reports are often used in the creation of forecasts for the same area. A variety of different forecast products are produced and designed to be used in the preflight planning stage. The printed forecasts that pilots need to be familiar with are the terminal aerodrome forecast (TAF), aviation area forecast (FA), inflight weather advisories (SIGMET, AIRMET), and the winds and temperatures aloft forecast (FD).

12.5.1 Terminal Aerodrome Forecasts (TAF) #

A TAF is a report established for the five statute mile radius around an airport. TAF reports are usually given for larger airports. Each TAF is valid for a 30-hour time period, and is updated four times a day at 0000Z, 0600Z, 1200Z, and 1800Z. The TAF utilizes the same descriptors and abbreviations as used in the METAR report. The TAF includes the following information in sequential order: 1.

Type of report—a TAF can be either a routine forecast (TAF) or an amended forecast (TAF AMD). 2. ICAO station identifier—the same as used in a METAR. 3. Date and time of origin—a six-number code (date, then time in UTC, followed by Z). 4. Valid period date and time—a six-digit group (date, two-digit beginning time, two-digit ending time). 5. Forecast wind—a five-digit group; first three digits give direction in reference to true north, last two give speed in knots (“KT”). 6. Forecast visibility—in statute miles, whole numbers or fractions; greater than six miles is coded “P6SM.” 7. Forecast significant weather—coded as in the METAR; “NSW” (no significant weather) appears in becoming/temporary groups when none is expected. 8. Forecast sky condition—as in the METAR; only cumulonimbus (CB) clouds are forecast. 9. Forecast change group—”FM” (from, rapid significant change usually within an hour), “BECMG” (gradual change over no more than 2 hours), “TEMPO” (temporary fluctuations expected to last less than one hour). 10. Probability forecast—a percentage describing the probability of thunderstorms and precipitation; not used for the first 6 hours.

Example: KPIR 111130Z 111212 15012KT P6SM BKN090 TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 1408KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT=.

Explanation: Routine TAF for Pierre, South Dakota, on the 11th day of the month at 1130Z, valid for 24 hours from 1200Z on the 11th to 1200Z on the 12th; wind from 150° at 12 knots; visibility greater than 6 sm; broken clouds at 9,000 feet; temporarily between 1200Z and 1400Z visibility 5 sm in mist; from 1500Z winds from 160° at 15 knots gusting to 25, visibility greater than 6 sm, clouds scattered at 4,000 feet and broken at 25,000 feet; from 0000Z wind from 140° at 12 knots, visibility greater than 6 sm, clouds broken at 8,000 feet, overcast at 15,000 feet; between 0000Z and 0400Z 40 percent probability of visibility 3 sm, thunderstorm with moderate rain showers, clouds broken at 3,000 feet with cumulonimbus; from 0400Z winds from 140° at 8 knots, visibility greater than 6 miles, clouds scattered at 4,000 and overcast at 8,000; temporarily between 0400Z and 0800Z visibility 3 miles, thunderstorms with moderate rain showers, clouds overcast at 3,000 feet with cumulonimbus; becoming between 0800Z and 1000Z wind from 320° at 7 knots; end of report (=).

12.5.2 Area Forecasts (FA) #

The FA gives a picture of clouds, general weather conditions, and visual meteorological conditions (VMC) expected over a large area encompassing several states. There are six areas for which area forecasts are published in the contiguous 48 states. Area forecasts are issued three times a day and are valid for 18 hours. This type of forecast gives information vital to en route operations, as well as forecast information for smaller airports that do not have terminal forecasts.

Area forecasts are typically disseminated in four sections: 1.

Header—gives the location identifier of the source of the FA, the date and time of issuance, the valid forecast time, and the area of coverage. 2.

Precautionary statements—IFR conditions, mountain obscurations, and thunderstorm hazards; heights are MSL unless noted AGL or ceiling (CIG). 3. Synopsis—a brief summary identifying the location and movement of pressure systems, fronts, and circulation patterns. 4. VFR Clouds and Weather—expected sky conditions, visibility, and weather for the next 12 hours, plus an outlook for the following 6 hours. When information is given, locations may be given by states, regions, or specific geological features such as mountain ranges. [Figure 12-12]

12.5.3 Inflight Weather Advisories #

Inflight weather advisories, which are provided to en route aircraft, are forecasts that detail potentially hazardous weather. These advisories are also available to pilots prior to departure for flight planning purposes. An inflight weather advisory is issued in the form of either an AIRMET, SIGMET, or convective SIGMET.

12.5.4 AIRMET #

AIRMETs (WAs) are examples of inflight weather advisories that are issued every 6 hours with intermediate updates issued as needed for a particular area forecast region. The information contained in an AIRMET is of operational interest to all aircraft, but the weather section concerns phenomena considered potentially hazardous to light aircraft and aircraft with limited operational capabilities.

An AIRMET includes forecast of moderate icing, moderate turbulence, sustained surface winds of 30 knots or greater, widespread areas of ceilings less than 1,000 feet and/or visibilities less than three miles, and extensive mountain obscurement.

Each AIRMET bulletin has a fixed alphanumeric designator, numbered sequentially. Sierra is the AIRMET code used to denote IFR and mountain obscuration; Tango denotes turbulence, strong surface winds, and low-level wind shear; and Zulu denotes icing and freezing levels.

Example: AIRMET TANGO UPDT 3 FOR TURBC… STG SFC WINDS AND LLWS VALID UNTIL 242000 … OK TX … FROM OKC TO DFW TO SAT TO MAF TO CDS TO OKC OCNL MDT TURBC BLO 60 DUE TO STG AND GUSTY LOW LVL WINDS. CONDS CONTG BYD 2000Z. Explanation: Issued by Dallas–Fort Worth on the 24th at 1650Z. On this third update, AIRMET Tango is issued for turbulence, strong surface winds, and low-level wind shear until 2000Z. The area from Oklahoma City to Dallas to San Antonio to Midland to Childress to Oklahoma City will experience occasional moderate turbulence below 6,000 feet due to strong and gusty low-level winds, continuing beyond 2000Z.

12.5.5 SIGMET #

SIGMETs (WSs) are inflight advisories concerning non-convective weather that is potentially hazardous to all aircraft. They report weather forecasts that include severe icing not associated with thunderstorms, severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms, dust storms or sandstorms that lower surface or inflight visibilities to below three miles, and volcanic ash. SIGMETs are unscheduled forecasts that are valid for 4 hours, but if the SIGMET relates to hurricanes, it is valid for 6 hours.

A SIGMET is issued under an alphabetic identifier, from November through Yankee, excluding Sierra and Tango. The first issuance of a SIGMET is designated as an Urgent Weather SIGMET (UWS).

Example: SFOR WS 100130 SIGMET ROME02 VALID UNTIL 100530 OR WA FROM SEA TO PDT TO EUG TO SEA OCNL MOGR CAT BTN 280 AND 350 EXPCD DUE TO JTSTR. CONDS BGNG AFT 0200Z CONTG BYD 0530Z.

Explanation: This is SIGMET Romeo 2, valid until the 10th at 0530Z, for Oregon and Washington, for a defined area from Seattle to Portland to Eugene to Seattle. It calls for occasional moderate or greater clear air turbulence between 28,000 and 35,000 feet due to the jet stream, beginning after 0200Z and continuing beyond 0530Z.

12.5.6 Convective Significant Meteorological Information (WST) #

A convective SIGMET (WST) is an inflight weather advisory issued for hazardous convective weather.

Convective SIGMETs are issued for severe thunderstorms with surface winds greater than 50 knots, hail at the surface greater than or equal to ¾ inch in diameter, or tornadoes. They are also issued to advise pilots of embedded thunderstorms, lines of thunderstorms, or thunderstorms with heavy or greater precipitation that affect 40 percent or more of a 3,000 square mile or greater region. Convective SIGMETs are issued for the eastern (E), western (W), and central (C) United States (not Alaska or Hawaii). Each report is issued at 55 minutes past the hour, valid for 2 hours, numbered sequentially each day from 1–99 beginning at 00Z. If no hazardous weather exists, the convective SIGMET still states “CONVECTIVE SIGMET…NONE.”

Example: MKCC WST 221855 CONVECTIVE SIGMET 21C VALID UNTIL 2055 KS OK TX VCNTY GLD-CDS LINE NO SGFNT TSTMS RPRTD LINE TSTMS DVLPG BY 1955Z WILL MOV EWD 30-35 KT THRU 2055Z HAIL TO 2 IN PSBL. Explanation: Convective SIGMET number 21C, the 21st consecutive report for the central United States, issued the 22nd at 1855Z, valid until 2055Z. For an area from Kansas to Oklahoma to Texas in the vicinity of a line from Goodland, Kansas, to Childress, Texas. No significant thunderstorms are being reported, but a line of thunderstorms will develop by 1955Z and move eastward at 30–35 knots through 2055Z, with hail up to 2 inches possible.

12.5.7 Winds and Temperature Aloft Forecast (FD) #

Winds and temperatures aloft forecasts (FD) provide wind and temperature forecasts for specific locations in the contiguous United States, including network locations in Hawaii and Alaska. The forecasts are made twice a day based on the radiosonde upper air observations taken at 0000Z and 1200Z. Through 12,000 feet are true altitudes and above 18,000 feet are pressure altitudes. Wind direction is always in reference to true north and wind speed is given in knots; the temperature is given in degrees Celsius. No winds are forecast when a given level is within 1,500 feet of the station elevation; temperatures are not forecast for any station within 2,500 feet of the station elevation.

If the wind speed is forecast to be greater than 100 but less than 199 knots, the computer adds 50 to the direction and subtracts 100 from the speed (to decode, reverse it): “731960” means 230° at 119 knots, temperature –60 °C. If the wind speed is forecast to be 200 knots or greater, the wind group is coded as 99 knots: “7799” means 270° at 199 knots or greater. When the forecast wind is calm or less than 5 knots, the data group is coded “9900” (light and variable). [Figure 12-13]

Explanation of Figure 12-13: This FD was transmitted on the 15th at 1640Z and is based on the 1200Z radiosonde. The valid time is 1800Z and should be used for the period between 1700Z and 2100Z. Temperatures above 24,000 feet MSL are negative (the minus sign is omitted). At Amarillo, Texas (AMA, elevation 3,605 feet), the lowest reportable altitude is 6,000 feet; “2714” means wind from 270° at 14 knots. At Denver (DEN, elevation 5,431 feet), the lowest reportable altitude is 9,000 feet; “2321-04” means wind from 230° at 21 knots with a temperature of –4 °C.