December 1999 3-17 ALB 1200 RADAT 84M019045051 The Albany, New York, RADAT indicates three crossings of the freezing level. The three crossings of the zero-degree Celsius isotherm occurred at 1,900 feet MSL, 4,500 feet MSL, and at 5,100 feet MSL.
The relative humidity was 84% at the middle crossing which was 4,500 feet MSL.
Figure 3-6. ALB RADAT.
DNR 1200 RADAT ZERO The Denver, Colorado, RADAT indicates that the entire RADAT information was below zero degrees Celsius.
Figure 3-7. DEN RADAT.
0°C Surface of ground 0°C Surface of ground 5,100 4,500 1,900 Mean Sea Level Mean Sea Level
December 1999 3-18 ABR 0000 RADAT MISG The Aberdeen, South Dakota, RADAT was terminated before the first crossing of the zero-degree Celsius isotherm. All temperatures were above freezing.
Surface of ground 0°C Figure 3-8. ABR RADAT.
Mean Sea Level
December 1999 4-1 Section 4
AVIATION WEATHER FORECASTS Good flight planning involves considering all available weather information, including weather forecasts.
This section explains the following aviation forecasts:
1. Aviation Terminal Forecast (TAF)
2. Aviation Area Forecast (FA)
3. Inflight Aviation Weather Advisories
4. Alaska, Gulf of Mexico, and International Area Forecasts (FAs)
5. Transcribed Weather Broadcasts (TWEB) Text Products
6. Winds and Temperatures Aloft Forecast (FD)
7. Center Weather Service Unit (CWSU) Products Also discussed are the following general forecasts that may aid in flight planning:
1. Hurricane Advisory (WH)
2. Convective Outlook (AC)
3. Severe Weather Watch Bulletins (WW) and Alert Messages (AWW)
AVIATION TERMINAL FORECAST (TAF) An Aviation Terminal Forecast (TAF) is a concise statement of the expected meteorological conditions within a 5-statute-mile radius from the center of an airport’s runway complex during a 24-hour time period.
The TAFs use the same weather code found in METAR weather reports. Detailed explanations of the code are found only in Section 2.
The National Weather Service (NWS) requires an airport to have two consecutive METAR observations, not less than 30 minutes apart nor more than 1 hour apart, before a TAF will be issued. After the TAF has been issued, the forecaster will use all available weather data sources to maintain the TAF. If during this time a METAR is missing or part of the METAR is missing, the forecaster can use other weather sources to obtain the necessary data to maintain the TAF. However, if the forecaster feels that the other weather sources cannot provide the necessary information, the forecaster will discontinue the TAF.
A TAF contains the following elements in the order listed:
1. Type of report
2. ICAO station identifier
3. Date and time of origin
4. Valid period date and time
5. Wind forecast
6. Visibility forecast
7. Significant weather forecast
8. Sky condition forecast
9. Nonconvective low-level wind shear forecast (optional data)
10. Forecast change indicators
11. Probability forecast
December 1999 4-2 International and U.S. military TAFs also contain forecasts of maximum and minimum temperature, icing, and turbulence. These three elements are not included in NWS-prepared TAFs. For forecast icing and turbulence, see page 4-23, Inflight Aviation Weather Advisories.
The following paragraphs describe the elements in a TAF report. A sample report will accompany each element with the subject element in bold letters.
TYPE OF REPORT TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The report type header will always appear as the first element in the TAF. There are two types of TAF reports: a routine forecast, TAF; and an amended forecast, TAF AMD. An amended TAF is issued when the forecaster feels the TAF is not representative of the current or expected weather conditions. An equal sign at the end of the TAF signifies the end of the report.
ICAO STATION IDENTIFIER TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The TAF code uses ICAO four-letter location identifiers as described in Section 2. TAF locations are in Figures 4-1, 4-2, 4-3, and 4-4 located on pages 4-13 through 4-16.
DATE AND TIME OF ORIGIN TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= This element is the date and universal coordinated time (UTC) the forecast is actually prepared. The format is a two-digit date and four-digit time followed without a space by the letter Z. Routine TAFs are prepared and filed approximately one-half hour prior to scheduled issuance times.
Examples:
111140Z Forecast prepared on the eleventh day of the month at 1140Z.
050530Z Forecast prepared on the fifth day of the month at 0530Z.
December 1999 4-3
VALID PERIOD DATE AND TIME TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The valid period of the forecast is a two-digit date followed by the two-digit beginning and two-digit ending hours in UTC. Routine TAFs are valid for 24 hours and are issued four times daily at 0000Z, 0600Z, 1200Z, and 1800Z. All ending times throughout the TAF of 00Z are indicated by the number 24.
Examples:
111212 Forecast valid from the eleventh at 12Z to the twelfth at 12Z.
300024 Forecast valid from the thirtieth at 00Z to the first at 00Z.
Amended, canceled, or delayed forecasts may have valid periods less than 24 hours.
Examples:
231512 Forecast valid from the twenty-third at 15Z to the twenty-fourth at 12Z.
091006 Forecast valid from the ninth at 10Z to the tenth at 06Z.
For airports with less than 24-hour observational coverage for which part-time terminal forecasts are provided, the TAF will be valid until the end of the scheduled forecast even if the observations have ceased before that time. AMD NOT SKED (amendment not scheduled) or NIL AMD (no amendment) will be issued after the forecast information. AMD NOT SKED AFT (closing time)Z (amendment not scheduled after [closing time]Z) will be used if the times of the observations are known and judged reliable. During the time the station is closed and a TAF is issued, there will be no forecast as indicated by NIL (no TAF) after the valid date and time group. Only after two METARs observations have been disseminated will a TAF be issued. AMD LTD TO CLD VIS AND WIND (amendment limited to clouds, visibility, and wind) is used at observation sites that have part-time manual augmentation. This remark means that there will be amendments only for clouds, visibility, and wind. There will be no amendments for thunderstorms or freezing/frozen precipitation.
WIND FORECAST TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The surface wind forecast is the wind direction in degrees from true north (first three digits) and mean speed in knots (last two or three digits if 100 knots or greater). The contraction, KT, denotes the units of wind speed in knots. Wind gusts are noted by the letter G appended to the mean wind speed followed by the highest expected gust (two or three digits if 100 knots or greater). Calm winds are encoded as 00000KT. A variable wind is encoded as VRB when wind direction fluctuates due to convective activity or low wind speeds (3 knots or less).
December 1999 4-4 Examples:
13012KT, 18010KT, 35012G26KT, or VRB16G28KT
VISIBILITY FORECAST TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The prevailing visibility is forecasted in whole and fractions of statute miles followed by SM to note the units of measurement. Statute miles followed by fractions of statute miles are separated with a space; for example, 1 1/2SM. Forecasted visibility greater than 6 statute miles is indicated by coding P6SM. If prevailing visibility is 6 statute miles or less, one or more weather phenomena must be included in the significant weather forecast. If volcanic ash is forecasted, the visibility must also be forecasted even if the visibility is greater than 6 statute miles. Sector or variable visibility is not forecasted.
Examples:
1/2SM, 2 1/4SM, 5SM, or P6SM
SIGNIFICANT WEATHER FORECAST TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The expected weather phenomenon or phenomena are coded in TAF reports using the same format, qualifiers, and phenomena contractions as METAR reports (except UP). (See Section 2.)
Obscurations to vision will be forecasted whenever the prevailing visibility is forecasted to be 6 statute miles or less. Precipitation and volcanic ash will always be included in the TAF regardless of the visibility forecasted.
Examples:
FM2200 18005KT 1SM BR SKC FM0100 12010KT P6SM -RA BKN020 FM1500 22015KT P6SM VA SCT100 If no significant weather is expected to occur during a specific time period in the forecast, the weather group is omitted for that time period. However, if after a time period in which significant weather has been forecasted, a change to a forecast of “no significant weather” occurs, the contraction NSW (no significant weather) will appear as the weather included in BECMG or TEMPO groups. NSW will not be used in the initial time period of a TAF or in FM groups.
December 1999 4-5 Example:
FM0600 16010KT 3SM RA BKN030 BECMG 0810 P6SM NSW If the forecaster determines that in the vicinity of the airport there could be weather that impacts aviation, the forecaster will include those conditions after the weather group. The letters VC describe conditions that will occur within the vicinity of an airport (5-10 SM) and will be used only with fog, showers, or thunderstorms (FG, SH, or TS).
Examples:
P6SM VCFG - fog in the vicinity.
5SM BR VCSH - showers in the vicinity .
P6SM VCTS - thunderstorms in the vicinity.
SKY CONDITION FORECAST TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= TAF sky condition forecasts use the METAR format described in Section 2. Cumulonimbus clouds (CB) are the only cloud type forecasted in TAFs.
Examples:
BKN100, SCT040 BKN030CB, or FEW008 BKN015 When the sky is obscured due to a surface-based phenomenon, vertical visibility (VV) into the obscuration is forecasted. The format for vertical visibility is VV followed by a three-digit height in hundreds of feet. Partial obscurations are not forecasted. Remember a ceiling is the lowest broken or overcast layer or vertical visibility.
Example:
VV008
NONCONVECTIVE LOW-LEVEL WIND SHEAR FORECAST (OPTIONAL DATA) TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= A forecast of nonconvective low-level wind shear is included immediately after the cloud and obscuration group when wind shear criteria have been or will be met. The forecast includes the height of the wind shear followed by the wind direction and wind speed at the indicated height. Height is given in hundreds of feet above ground level (AGL) up to and including 2,000 feet. Wind shear is encoded with the contraction WS, followed by a three-digit height, solidus (/), and winds at the height indicated in the same format as surface winds. The wind shear element is omitted if not expected to occur.
December 1999 4-6 Example:
WS020/36035KT
FORECAST CHANGE INDICATORS TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= If a significant change in any of the elements is expected during the valid period, a new time period with the changes is included. The following change indicators are used when either a rapid, gradual, or temporary change is expected in some or all of the forecasted meteorological conditions.
From (FM) Group The FM group is used when a rapid and significant change, usually occurring in less than 1 hour, in prevailing conditions is expected. Appended to the FM indicator is the four-digit hour and minute the change is expected to begin. The forecast is valid until the next change group or until the end of the current forecast.
The FM group will mark the beginning of a new line in a TAF report. Each FM group shall contain a forecast of wind, visibility, weather (if significant), sky condition, and wind shear (if warranted). FM groups will not include the contraction NSW.
Examples:
FM1500 16015G25KT P6SM SCT040 BKN250 FM0200 32010KT 3SM TSRA FEW010 BKN030CB Becoming (BECMG) Group The BECMG group is used when a gradual change in conditions is expected over a period not to exceed 2 hours. The time period when the change is expected to occur is a four-digit group containing the beginning and ending hours of the change that follows the BECMG indicator. The gradual change will occur at an unspecified time within the time period. Only the changing forecasted meteorological conditions are included in BECMG groups. Omitted conditions are carried over from the previous time group.
Example:
FM2000 18020KT P6SM BKN030 BECMG 0103 OVC015 This BECMG group describes a gradual change in sky condition from BKN030 to OVC015. The change in sky conditions occurs between 01Z and 03Z. Refer back to the FM2000 group for the wind and visibility conditions. The forecast after 03Z will be: 18020KT P6SM OVC015.
December 1999 4-7 Example:
FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= This BECMG group describes a gradual change in wind direction only beginning between 08Z and 10Z.
Refer back to the previous forecast group, in this case the FM0400 group, for the prevailing visibility, weather, and sky conditions. The forecast after 10Z will be: 32007KT P6SM SCT040 OVC080.
Temporary (TEMPO) Group The TEMPO group is used for temporary fluctuations of wind, visibility, weather, or sky condition that are expected to last for generally less than an hour at a time (occasional), and expected to occur during less than half the time period. The TEMPO indicator is followed by a four-digit group giving the beginning and ending hours of the time period during which the temporary conditions are expected. Only the changing forecasted meteorological conditions are included in TEMPO groups. The omitted conditions are carried over from the previous time group.
Example:
FM1000 27005KT P6SM SKC TEMPO 1216 3SM BR This temporary group describes visibility and weather between 12Z and 16Z. The winds and sky condition have been omitted. Go back to the previous forecast group, FM1000, to obtain the wind and sky condition forecast. The forecast between 12Z and 16Z is: 27005KT 3SM BR SKC.
Example:
FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= This temporary group describes visibility, weather, and sky condition between 04Z and 08Z. The winds have been omitted. Go back to the previous forecast group, FM0400, to obtain the wind forecast.
The forecast between 04Z and 08Z is: 14008KT 3SM TSRA OVC030CB.
PROBABILITY (PROB30 or PROB40) FORECAST TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= The probability forecast describes the probability or chance of thunderstorms or other precipitation events occurring, along with associated weather conditions (wind, visibility, and sky conditions). The probability forecast will not be used in the first 6 hours of the TAF.
The PROB30 or PROB40 group is used when the occurrence of thunderstorms or precipitation is in the 30% to less than 40% or 40% to less than 50% range, respectively. If the thunderstorms or precipitation chance is greater than 50%, it is considered a prevailing weather condition and is included in the significant weather section or the TEMPO change indicator group. PROB30 or PROB40 is followed by a four-digit time group giving the beginning and ending hours of the time period during which the thunderstorms or precipitation is expected.
December 1999 4-8 Example:
FM0600 0915KT P6SM BKN020 PROB30 1014 1SM RA BKN015 This example depicts a 30% to less than 40% chance of 1statute mile, moderate rain, and a broken cloud layer (ceiling) at 1,500 feet between the hours of 10-14Z.
Example:
FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB In this example, there is a 40% to <50% chance of visibility 3 statute miles, thunderstorms with moderate rain showers, and a broken cloud layer (ceiling) at 3,000 feet with cumulonimbus between the hours of 00-04Z.
December 1999 4-9
EXAMPLES OF TAF REPORTS TAF KPIR 111140Z 111212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1214 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004 3SM TSRA BKN030CB FM0400 14008KT P6SM SCT040 OVC080 TEMPO 0408 3SM TSRA OVC030CB BECMG 0810 32007KT= TAF Aviation terminal forecast KPIR Pierre, South Dakota 111140Z prepared on the 11th at 1140Z 111212 valid period from the 11th at 1200Z until the 12th at 1200Z 13012KT wind 130 at 12 knots P6SM visibility greater than 6 statute miles BKN100 ceiling 10,000 broken WS020/35035KT wind shear at 2,000 feet, wind (at 2,000 feet) from 350 at 35 knots TEMPO 1214 temporary conditions between 1200Z and 1400Z 5SM visibility 5 statute miles BR mist FM1500 from 1500Z 16015G25KT wind 160 at 15 knots gusting to 25 knots P6SM visibility greater than 6 statute miles SCT040 BKN250 4,000 scattered, ceiling 25,000 broken FM0000 from 0000Z 14012KT wind 140 at 12 knots P6SM visibility greater than 6 statute miles BKN080 OVC150 ceiling 8,000 broken, 15,000 overcast PROB40 0004 40% probability between 0000Z and 0400Z 3SM visibility 3 statute miles TSRA thunderstorm with moderate rain showers BKN030CB ceiling 3,000 broken with cumulonimbus FM0400 from 0400Z 14008KT wind 140 at 8 knots P6SM visibility greater than 6 statute miles SCT040 OVC080 4,000 scattered, ceiling 8,000 overcast TEMPO 0408 temporary conditions between 0400Z and 0800Z 3SM visibility 3 statute miles TSRA thunderstorms with moderate rain showers OVC030CB ceiling 3,000 overcast with cumulonimbus BECMG 0810 becoming between 0800Z and 1000Z 32007KT= wind 320 at 7 knots; the equal sign signifies the end of the TAF
December 1999 4-10
TAF AMD KEYW 131555Z 131612 VRB03KT P6SM VCTS SCT025CB BKN250 TEMPO 1618 2SM TSRA BKN020CB FM1800 VRB03KT P6SM SCT025 BKN250 TEMPO 2024 1SM TSRA OVC010CB FM0000 VRB03KT P6SM VCTS SCT020CB BKN120 TEMPO 0812 BKN020CB= TAF AMD Amended aviation terminal forecast KEYW Key West, Florida 131555Z prepared on the 13th at 1555Z 131612 valid period from the 13th at 1600Z until the 14th at 1200Z VRB03KT wind variable at 3 knots P6SM visibility greater than 6 statute miles VCTS thunderstorms in the vicinity SCT025CB BKN250 2,500 scattered with cumulonimbus, ceiling 25,000 broken TEMPO 1618 temporary conditions between 1600Z and 1800Z 2SM visibility 2 statute miles TSRA thunderstorms with moderate rain showers BKN020CB ceiling 2,000 broken with cumulonimbus FM1800 from 1800Z VRB03KT wind variable at 3 knots P6SM visibility greater than 6 statute miles SCT025 BKN250 2,500 scattered, ceiling 25,000 broken TEMPO 2024 temporary conditions between 2000Z and 0000Z 1SM visibility 1 statute mile TSRA thunderstorms with moderate rain showers OVC010CB ceiling 1,000 overcast with cumulonimbus FM0000 from 0000Z VRB03KT variable wind at 3 knots P6SM visibility greater than 6 statute miles VCTS thunderstorms in the vicinity SCT020CB BKN120 2,000 scattered with cumulonimbus, ceiling 12,000 broken TEMPO 0812 temporary conditions between 0800Z and 1200Z BKN020CB= ceiling 2,000 broken with cumulonimbus; the equal sign signifies the end of the TAF