Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
• Turbulence: Moderate or greater turbulence. For the coastal waters portion of the FAGX, users will be referred to the appropriate CONUS AIRMET.
7.1.7.1 Area Forecast – Gulf of Mexico (FAGX) Example
FAGX20 KKCI 091812 (ICAO product header) OFAGX (NWS AWIPS Communication header) SYNOPSIS VALID TIL 101900Z FCST...091900Z-100700Z OTLK...100700Z-101900Z
INTERNATIONAL OPERATIONS BRANCH
AVIATION WEATHER CENTER KANSAS CITY MISSOURI .
CSTL WATERS FROM COASTLINE OUT TO HOUSTON OCEANIC FIR AND GLFMEX MIAMI OCEANIC FIR AND W OF 85W. HOUSTON OCEANIC FIR AND GLFMEX MIAMI OCEANIC FIR.
.
TS IMPLY SEV OR GTR TURB SEV ICE LLWS AND IFR CONDS. HGTS MSL.
.
01 SYNOPSIS...HIGH PRES OVR NRN GLFMEX.
.
02 SIGNIFICANT CLD/WX...
.
CSTL WATERS...
SCT020. OTLK...VFR.
.
HOUSTON OCEANIC FIR… SCT020. OTLK...VFR.
.
GLFMEX MIAMI OCEANIC FIR...
SCT020. OTLK...VFR.
.
03 ICE AND FRZLVL...
CSTL WATERS…SEE AIRMETS ZULU WAUS44 KKCI AND WAUS42 KKCI.
HOUSTON OCEANIC FIR… NO SGFNT ICE EXP OUTSIDE CNVTV ACT.
GLFMEX MIAMI OCEANIC FIR…NO SGFNT ICE EXP OUTSIDE CNVTV ACT.
FRZLVL…140 THRUT.
.
04 TURB...
CSTL WATERS…SEE AIRMETS TANGO WAUS44 KKCI AND WAUS42 KKCI.
HOUSTON OCEANIC FIR… NO SGFNT TURB EXP OUTSIDE CNVTV ACT.
GLFMEX MIAMI OCEANIC FIR…NO SGFNT TURB EXP OUTSIDE CNVTV ACT.
7.1.8 Area Forecast Format – Caribbean (FACA) Area forecasts issued for the Caribbean (FACA) cover the airspace between the surface and 24,000 feet AMSL and include the following elements. Each geographical section will have an entry even if it is negative.
• Synopsis: brief discussion of the synoptic weather affecting the FACA area during the 24-hour valid period.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-13
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
• Significant Clouds and Weather: description of the significant clouds and weather for the first 12 hours including the following elements.
o Cloud amount (SCT, BKN or OVC) for cloud bases below FL180 o Cloud bases and tops associated with the above bullet o Precipitation and thunderstorms o Visibility below 7 SM and obstruction(s) to visibility o Sustained surface winds greater than or equal to 20 knots o 12- to 24-hour categorical outlook (IFR, MVFR or VFR)
• Icing and Freezing Level: moderate or greater icing and freezing level
• Turbulence: moderate or greater turbulence
7.1.8.1 Area Forecast – Caribbean (FACA) Example
FACA20 KKCI 121530 (ICAO product header) OFAMKC (NWS AWIPS Communication header)
INTERNATIONAL OPERATIONS BRANCH
AVIATION WEATHER CENTER KANSAS CITY MISSOURI VALID 121600-130400 OUTLOOK...130400-131600 .
ATLANTIC S OF 32N W OF 57W...CARIBBEAN...GULF OF MEXICO BTN 22N AND 24N.
.
TS IMPLY SEV OR GTR TURB SEV ICE LLWS AND IFR CONDS. SFC TO 400 MB.
.
SYNOPSIS...WK CDFNT EXTDS FM NR 28N60W TO 23N63W TO THE MONA PASSAGE. CDFNT WL MOV EWD AND WKN TODAY. EXP NARROW BAND OF CLDS WITH ISOL SHRA INVOF CDFNT.
.
SIGNIFICANT CLD/WX...
ERN MONTERREY FIR...NRN MERIDA FIR SCT025 SCT060. OTLK...VFR.
.
ATLC SWRN NEW YORK FIR...SAN JUAN FIR NW OF CDFNT...SCT025 SCT060. LYR OCNL BKN. TOP 120. ISOL SHRA.
OTLK...VFR.
VCNTY CDFNT...SCT025 BKN060. OCNL BKN025. TOP 120. WDLY SCT SHRA. ISOL TSRA TIL 20Z. OTLK...VFR SHRA.
SE OF CDFNT...SCT025 SCT060. ISOL SHRA. OTLK...VFR.
.
ATLC MIAMI FIR SCT025 SCT060. LYR OCNL BKN. TOP 120. ISOL SHRA. OTLK...VFR.
.
WRN PIARCO FIR...NRN MAIQUETIA FIR...CURACAO FIR BTN 61W-63W...SCT025 BKN060. OCNL BKN025. TOP 120. WDLY SCT SHRA. OTLK...VFR SHRA.
RMNDR...SCT025 SCT060. ISOL SHRA. OTLK...VFR.
.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-14
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) SANTO DOMINGO FIR...PORT-AU-PRINCE FIR SCT025 SCT060. LYR OCNL BKN. TOP 120. ISOL SHRA. OTLK...VFR.
.
NRN BARRANQUILLA FIR...NRN PANAMA FIR SCT025 SCT060. ISOL SHRA. SFC WND NE 20-25KT. OTLK...VFR.
.
KINGSTON FIR...NERN CNTRL AMERICAN FIR...HABANA FIR SCT025 SCT060. ISOL SHRA. OTLK...VFR.
.
ICE AND FRZLVL...
NO SGFNT ICE EXP OUTSIDE CNVTV ACT.
FRZLVL... 145-170.
.
TURB...
NO SGFNT TURB EXP OUTSIDE CNVTV ACT.
7.1.9 Area Forecast Format - Hawaii Area forecasts issued for Hawaii cover the airspace between the surface and 45,000 feet AMSL and include the following elements.
• Synopsis: brief discussion of the significant synoptic weather affecting the FA area during the 18-hour valid period.
• Clouds and Weather: description of the clouds and weather for the first 12-hour period including the following elements.
o Cloud amount (SCT, BKN or OVC) with bases and tops o Visibilities of 6 SM or less with obstruction(s) to visibility o Precipitation and thunderstorms o Sustained surface winds 20 knots or greater
• 12- to 18-hour categorical outlook: IFR, marginal MVFR, or VFR, including expected precipitation and/or obstructions to visibility
7.1.9.1 Area Forecast – Hawaii Example
FAHW31 PHFO 080940 (ICAO product header) FA0HI (NWS AWIPS Communication header) .
HNLC FA 080940 (Area forecast region, product type, issuance date/time)
SYNOPSIS AND VFR CLD/WX SYNOPSIS VALID UNTIL 090400 CLD/WX VALID UNTIL 082200...OUTLOOK VALID 082200-090400 .
SEE AIRMET SIERRA FOR IFR CLD AND MT OBSC.
TS IMPLY SEV OR GREATER TURB SEV ICE LOW LEVEL WS AND IFR COND.
NON MSL HGT DENOTED BY AGL OR CIG.
.
SYNOPSIS...SFC HIGH FAR N PHNL NEARLY STNR.
.
BIG ISLAND ABOVE 060.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-15
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) SKC. 20Z SCT090. OUTLOOK...VFR.
.
BIG ISLAND LOWER SLOPES...COAST AND ADJ WATERS FROM UPOLU POINT TO CAPE KUMUKAHI TO APUA POINT.
SCT030 BKN050 TOPS 080 ISOL BKN030 VIS 3-5SM -SHRA BR. 21Z SCT030 SCT- BKN050 TOPS 080 ISOL BKN030 5SM -SHRA. OUTLOOK...VFR.
.
BIG ISLAND LOWER SLOPES…COAST AND ADJ WATERS FROM APUA POINT TO SOUTH CAPE TO UPOLU POINT. SKC. 21Z SCT-BKN060 TOPS 080. 23Z SCT030 SCT- BKN060 TOPS 080 ISOL BKN030 -SHRA. OUTLOOK...VFR.
.
BIG ISLAND LOWER SLOPES…COAST AND ADJ WATERS FROM SOUTH CAPE TO PHKO TO UPOLU POINT.
SCT050 ISOL BKN050 TOPS 080. 18Z FEW050. 23Z SCT-BKN050 TOPS 080.
OUTLOOK...VFR.
.
N AND E FACING SLOPES...COAST AND ADJ WATERS OF THE REMAINING ISLANDS.
SCT020 BKN045 TOPS 070 TEMPO BKN020 VIS 3-5SM -SHRA...FM OAHU EASTWARD ISOL CIG BLW 010 AND VIS BLW 3SM SHRA BR WITH TOPS 120. 22Z SCT025 SCT-BKN050 TOPS 070 ISOL BKN025 3-5SM -SHRA. OUTLOOK...VFR.
.
REST OF AREA.
SCT035 SCT-BKN050 TOPS 070 ISOL BKN030 -SHRA. 20Z SCT050 ISOL SCT030 BKN045 TOPS 070 -SHRA. OUTLOOK...VFR.
7.1.10 Area Forecast Format - Alaska Area forecasts issued for Alaska cover the airspace between the surface and 45,000 feet AMSL and include the following elements. Clouds and weather, turbulence and icing information is included in each geographical zone.
• Synopsis: a brief description of the significant synoptic weather affecting the FA area during the first 18 hours of the forecast period.
• Clouds and Weather: a description of the clouds and weather for each geographical zone during the first 12 hours of the forecast period including the following elements:
o SIGMET information for thunderstorms and volcanic ash o AIRMET information for IFR ceiling and visibility, mountain obscuration, and strong surface winds o Cloud amount (SCT, BKN or OVC) with bases and tops o Visibilities of 6 SM or less with obstruction(s) to visibility o Precipitation and thunderstorms o Surface wind greater than 20 knots o Mountain pass conditions using categorical terms (for selected zones only) o 12- to 30-hour categorical outlook (VFR, MVFR, and IFR)
• Turbulence: a description of expected turbulence conditions including the following elements.
o SIGMET information for turbulence o AIRMET information for turbulence or low level wind shear AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-16
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) o Turbulence not meeting SIGMET or AIRMET criteria during the 6- to 12-hour period o If no significant turbulence is forecast, NIL SIG will be entered.
• Icing and freezing level: a description of expected icing conditions including the following elements.
o SIGMET information for icing o AIRMET information for icing and freezing precipitation o Icing not meeting SIGMET or AIRMET criteria during the 6- to 12-hour period o Freezing level o If no significant icing is forecast, NIL SIG will be entered followed by the freezing level.
7.1.10.1 Area Forecast – Alaska Example
FAAK48 PAWU 251345 (ICAO product header) FA8H (NWS AWIPS Communication header) ANCH FA 251345 (Area forecast region, product type, issuance date/time) AK SRN HLF EXC SE AK...
.
AIRMETS VALID UNTIL 252000 TS IMPLY POSSIBLE SEV OR GREATER TURB SEV ICE LLWS AND IFR CONDS.
NON MSL HEIGHTS NOTED BY AGL OR CIG.
.
SYNOPSIS VALID UNTIL 260800 972 MB BRISTOL BAY LOW WL MOV N TO 50 S PAOM AT 987 MB BY END OF PD.
ASSOCIATED OCCLUDED FRONT FM PALJ..KENNEDY ENTRANCE..SE WL MOV NE TO PAMH..PACV..SE BY 08Z.
.
COOK INLET AND SUSITNA VALLEY AB...VALID UNTIL 260200 ...CLOUDS/WX...
***AIRMET IFR/MT OBSC***AK RANGE/W SIDE COOK INLET..OCNL CIGS BLW 10 VIS BLW 3SM -RA BR. NC...
OTHERWISE..AK RANGE/W SIDE INLET..SCT005 OVC020 VIS 3-5SM -RA BR.
ELSEWHERE..SCT025 BKN045 OVC080 LYR ABV TO FL250. OCNL BKN025 OVC045 - RA.
COOK INLET..SFC WND NE 20G30 KTS. THRU TERRAIN GAPS..ERN MTS/AK RANGE..SFC WND E 30G60 KTS.
OTLK VALID 260200-262000...MVFR CIG RA WND.
PASSES...LAKE CLARK..MERRILL..RAINY..IFR CIG RA WND. WINDY..MVFR CIG RA. PORTAGE..IFR CIG RA WND.
...TURB...
***SIGMET***KILO 1 VALID 251607/252000 PANC- OCNL SEV TURB FCST BLW 080 WI AN AREA FM TKA-JOH-MDO-AKN-SQA-TKA.
THIS IS THE AREA S OF A PAHZ-PATK LN.
***AIRMET TURB/LLWS***OCNL MOD TURB BLW 120. LLWS. NC...
...ICE AND FZLVL...
***AIRMET ICE***OCNL MOD RIME/MX ICEIC 050-160. FZLVL 050. NC...
.
COPPER RIVER BASIN AC...VALID UNTIL 260200 AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-17
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) ...CLOUDS/WX...
FEW045 SCT090 BKN-OVC180 TOP FL250.
SFC WND SE G 25 KTS.
WRN MTS..ISOL BKN025 OVC045 4SM -SHRA.
OTLK VALID 260200-262000...VFR.
PASS...TAHNETA..MVFR CIG.
...TURB...
NIL SIG.
...ICE AND FZLVL...
NIL SIG. FZLVL 050.
.
CNTRL GLF CST AD...VALID UNTIL 260200 ...CLOUDS/WX...
***AIRMET MT OBSC***MTS OBSCD IN CLDS/PRECIPITATION. NC...
SCT020 OVC040 LYRD ABV TO FL250 -RA.
OCNL SCT005 OVC020 VIS 3-5SM -RA BR.
SFC WND E 20G35 KTS. THRU TRRN GAPS WND E-NE 25G50 KTS.
ALONG KENAI PENINSULA..ISOL CIGS BLW 10 VIS BLW 3SM RA BR.
OTLK VALID 260200-260200..MVFR CIG RA WND.
...TURB...
***SIGMET***KILO 1 VALID 251607/252000 PANC- OCNL SEV TURB FCST BLW 080 WI AN AREA FM TKA-JOH-MDO-AKN-SQA-TKA.
THIS IS THE AREA E OF A JOH-PAMD LN.
***AIRMET TURB/LLWS***OCNL MOD TURB BLW 120. LLWS NR TRRN. NC...
...ICE AND FZLVL...
***AIRMET ICE***OCNL MOD RIME ICEIC 050-160. FZLVL 050. NC...
.
KODIAK ISLAND AE...VALID UNTIL 260200 ...CLOUDS/WX...
***AIRMET MT OBSC***MTS OBSCD IN CLDS/PRECIPITATION. NC...
SCT020 OVC040 LYRD ABV TO FL250 -RA.
OCNL SCT005 OVC020 VIS 3-5SM -RA BR.
E SIDE..ISOL CIGS BLW 10 VIS BLW 3SM RA BR.
SFC WND SE G 25 KT.
OTLK VALID 260200-262000...MVFR CIG SHRA WND. AFT 06Z..VFR.
...TURB...
NIL SIG.
...ICE AND FZ LVL...
ISOL MOD RIME ICEIC 030-120. FZLVL 030.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-18
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
7.2 Terminal Aerodrome Forecast (TAF) A Terminal Aerodrome Forecast (TAF) is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within five statute miles (SM) of the center of the airport’s runway complex (terminal). The TAFs use the same weather codes found in METAR weather reports (Section 2) and can be viewed on the National Weather Service (NWS) Aviation Digital Data Service (ADDS) web site at:
http://adds.aviationweather.noaa.gov/tafs/.
7.2.1 Responsibility TAFs are issued by NWS Weather Forecast Offices (WFOs). A map of WFO areas of responsibility can be found at:
http://www.srh.noaa.gov/
7.2.2 Generic Format of the Forecast Text of a NWS-Prepared TAF
Table 7-4. Generic Format of NWS TAFs TAF or
TAF AMD or
TAF COR
Type of report
CCCC YYGGggZ Y 1Y1G1G1/Y2Y2G2G2 dddffGf mfmKT Location identifier Date/time of forecast origin group Valid period Wind group
VVVV w'w' or NSW NsNsNshshshs or VVhshshs or SKC WShwshwshws/dddftKT Visibility group Significant weather group Cloud and vertical obscuration groups Non-convective low-level wind shear (LLWS) group
TTGGgg Forecast change indicator groups
FMY1Y1GGgg TEMPO Y 1Y1GG/YeYeGeGe PROB30 Y 1Y1GG/YeYeGeGe From group Temporary group Probability group
7.2.2.1 Type of Report (TAF or TAF AMD or TAF COR) The report-type header always appears as the first element in the TAF and is produced in three forms: a routine forecast, TAF, an amended forecast, TAF AMD, or a corrected forecast, TAF COR.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-19
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) TAFs are amended whenever they become, in the forecaster's judgment, unrepresentative of existing or expected conditions, particularly regarding those elements and events significant to aircraft and airports. An amended forecast is identified by TAF AMD (in place of TAF) on the first line of the forecast text.
7.2.2.2 Location Identifier (CCCC) After the line containing either TAF, TAF AMD, or TAF COR, each TAF begins with its fourletter International Civil Aviation Organization (ICAO) location identifier.
Examples:
KDFW – Dallas-Fort Worth PANC – Anchorage, Alaska PHNL – Honolulu, Hawaii
7.2.2.3 Date/Time of Forecast Origin Group (YYGGggZ) The date/time of forecast origin group (YYGGggZ) follows the terminal's location identifier. It contains the day of the month in two (2) digits (YY) and time in four (4) digits (GGgg in hours and minutes) the forecast is completed and ready for transmission, with a Z appended to denote UTC. This time is entered by the forecaster. A routine forecast, TAF, is issued 20 to 40 minutes before the beginning of its valid period.
Examples 061737Z The TAF was issued on the 6th day of the month at 1737 UTC.
121123Z The TAF was issued on the 12th day of the month at 1123 UTC.
7.2.2.4 Valid Period (Y1Y1G1G1/Y2Y2G2G2) The TAF valid period (Y1Y1G1G1/Y2Y2G2G2) follows the date/time of forecast origin group.
Scheduled 24- and 30-hour TAFs are issued four (4) times per day, at 0000, 0600, 1200, and 1800Z. The first two digits (Y1Y1) are the day of the month for the start of the TAF. The next two digits (G1G1) are the starting hour (UTC). Y2Y2 is the day of the month for the end of the TAF, and the last two digits (G2G2) are the ending hour (UTC) of the valid period. A forecast period that begins at midnight UTC is annotated as 00. If the end time of a valid period is at midnight UTC, it is annotated as 24. For example, a 00Z TAF issued on the 9th of the month and valid for 24 hours would have a valid period of 0900/0924.
Whenever an amended TAF (TAF AMD) is issued, it supersedes and cancels the previous TAF.
That is, users should not wait until the start of the valid period indicated within the TAF AMD to begin using it.
Examples:
1512/1612 The TAF is valid from the 15th day of the month at 1200 UTC until the 16th day of the month at 1200 UTC.
2306/2412 This is a 30-hour TAF valid from the 23rd day of the month at 0600 UTC until the 24th day of the month at 1200 UTC.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-20
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
0121/0218 This is an amended TAF valid from the 1st day of the month at 2100 UTC until the 2nd day of the month at 1800 UTC.
0600/0624 This TAF is valid from the 6th day of the month at 0000 UTC until the 6th day of the month at 2400 UTC (or 7th day of the month at 0000 UTC).
7.2.2.5 Wind Group (dddffGfmfmKT) The initial time period and any subsequent FM groups begin with a mean surface wind forecast (dddffGfmfmKT) for that period. Wind forecasts are expressed as the mean three-digit direction (ddd - relative to true north) from which the wind is blowing, rounded to the nearest ten degrees and the mean wind speed in knots (ff) for the time period. If wind gusts are forecast (gusts are defined as rapid fluctuations in wind speeds with a variation of 10 knots or more between peaks and lulls), they are indicated immediately after the mean wind speed by the letter G, followed by the peak gust speed expected. KT is appended to the end of the wind forecast group. Any wind speed of 100 knots or more will be encoded in three digits. Calm winds are encoded as 00000KT.
The prevailing wind direction is forecast for any speed greater than or equal to seven (7) knots.
When the prevailing surface wind direction is variable (variations in wind direction of 30 degrees or more), the forecast wind direction is encoded as VRBffKT. Two conditions where this can occur are very light winds and convective activity. Variable wind direction for very light winds must have a wind speed of one (1) through six (6) knots inclusive. For convective activity, the wind group may be encoded as VRBffGfmfmKT, where Gfmfm is the maximum expected wind gusts. VRB is not used in the non-convective LLWS group.
Squalls are forecast in the wind group as gusts (G), but must be identified in the significant weather group with the code SQ.
Examples:
23010KT Wind from 230 degrees “true” (southwest) at 10 knots.
28020G35KT Wind from 280 degrees “true” (west) at 20 knots gusting to 35 knots.
VRB05KT Wind variable at 5 knots. This example depicts a forecast for light winds that are expected to variable in direction.
VRB15G30KT Wind variable at 15 knots gusting to 30 knots. This example depicts winds that are forecast to be variable with convective activity.
00000KT Wind calm
090105KT AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-21
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) Wind from 90 degrees at 105 knots
7.2.2.6 Visibility Group (VVVV) The initial time period and any subsequent FM groups include a visibility forecast (VVVV) in statute miles appended by the contraction SM.
When the prevailing visibility is forecast to be less than or equal to six (6) SM, one or more significant weather groups are included in the TAF. However, drifting dust (DRDU), drifting sand (DRSA), drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG) may be forecast with prevailing visibility greater than or equal to seven (7) statute miles.
When a whole number and a fraction are used to forecast visibility, a space is included between them (e.g., 1 1/2SM). Visibility greater than six (6) statute miles is encoded as P6SM.
If the visibility is not expected to be the same in different directions, prevailing visibility is used.
When volcanic ash (VA) is forecast in the significant weather group, visibility is included in the forecast, even if it is unrestricted (P6SM). For example, an expected reduction of visibility to 10 statute miles by volcanic ash is encoded in the forecast as P6SM VA.
Although not used by the National Weather Service in U.S. domestic TAFs, the contraction “CAVOK” (Ceiling and Visibility OK) may replace the visibility, weather, and sky condition groups if all of the following conditions are forecast: Visibility of 10 kilometer (6 statute miles) or more, no clouds below 1500 meters (5,000 feet) or below the highest minimum sector altitude (whichever is greater), no cumulonimbus, and no significant weather phenomena.
Examples
P6SM Visibility unrestricted
1 1/2SM Visibility 1 and ½ statute miles
4SM Visibility 4 statute miles
7.2.2.7 Significant Weather Group (w'w' or NSW) The significant weather group (w'w' or NSW) consists of the appropriate qualifier(s) and weather phenomenon contraction(s) or NSW (No significant weather).
If the initial forecast period and subsequent FM groups are not forecast to have explicit significant weather, the significant weather group is omitted. NSW is not used in the initial forecast time period or FM groups.
One or more significant weather group(s) is (are) required when the visibility is forecast to be 6SM or less. The exceptions are: volcanic ash (VA), low drifting dust (DRDU), low drifting sand (DRSA), low drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG). Obstructions to vision are only forecast when the prevailing visibility is less than 7 statute miles or, in the opinion of the forecaster, is considered operationally significant.
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-22
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) Volcanic ash (VA) is always forecast when expected. When VA is included in the significant weather group, visibility is included in the forecast as well, even if the visibility is unrestricted (P6SM).
NSW is used in place of significant weather only in a TEMPO group to indicate when significant weather (including in the vicinity (VC)) included in a previous sub-divided group is expected to end.
Multiple precipitation elements are encoded in a single group (e.g., -TSRASN). If more than one type of precipitation is forecast, up to three appropriate precipitation contractions can be combined in a single group (with no spaces) with the predominant type of precipitation being first. In this single group, the intensity refers to the total precipitation and can be used with either one or no intensity qualifier, as appropriate. In TAFs, the intensity qualifiers (light, moderate, and heavy) refer to the intensity of the precipitation and not to the intensity of any thunderstorms associated with the precipitation.
Intensity is coded with precipitation types, except ice crystals and hail, including those associated with thunderstorms and those of a showery nature (SH). No intensity is ascribed to blowing dust (BLDU), blowing sand (BLSA), or blowing snow (BLSN). Only moderate or heavy intensity is ascribed to sandstorm (SS) and duststorm (DS).
7.2.2.7.1 Exception for Encoding Multiple Precipitation Types When more than one type of precipitation is forecast in a time period, any precipitation type associated with a descriptor (e.g., FZRA) is encoded first in the precipitation group, regardless of the predominance or intensity of the other precipitation types. Descriptors are not encoded with the second or third precipitation type in the group. The intensity is associated with the first precipitation type of a multiple precipitation type group. For example, a forecast of moderate snow and light freezing rain is coded as -FZRASN although the intensity of the snow is greater than the freezing rain.
Examples:
Combinations of one precipitation and one non-precipitation weather phenomena:
-DZ FG Light drizzle and fog (obstruction which reduces visibility to less than 5/8 SM)
RA BR Moderate rain and mist
-SHRA FG Light rain showers and fog
+SN FG Heavy snow and fog
Combinations of more than one type of precipitation:
-RASN FG HZ Light rain and snow (light rain predominant), fog and haze
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-23
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) TSSNRA Thunderstorm with moderate snow and rain (moderate snow predominant)
FZRASNPL Moderate freezing rain, snow, and ice pellets (freezing rain mentioned first due to the descriptor, followed by other precipitation types in order of predominance)
SHSNPL Moderate snow showers and ice pellets
7.2.2.7.2 Thunderstorm Descriptor The TS descriptor is treated differently than other descriptors in the following cases:
• When non-precipitating thunderstorms are forecast, TS may be encoded as the sole significant weather phenomenon; and
• When forecasting thunderstorms with freezing precipitation ( FZRA or FZDZ), the TS descriptor is included first, followed by the intensity and weather phenomena.
Example:
TS -FZRA When a thunderstorm is included in the significant weather group (even using vicinity - VCTS), the cloud group (NsNsNshshshs) includes a forecast cloud type of CB. See the following example for encoding VCTS.
Example
-FZRA VCTS BKN010CB
7.2.2.7.3 Fog Forecast A visibility threshold must be met before a forecast for fog (FG) is included in the TAF. When forecasting a fog-restricted visibility from 5/8SM to 6SM, the phenomena is coded as BR (mist).
When a fog-restricted visibility is forecast to result in a visibility of less than 5/8SM, the code FG is used. The forecaster never encodes weather obstruction as mist (BR) when the forecast visibility is greater than 6 statute miles (P6SM).
The following fog-related terms are used as described below:
AC 00-45F, CHG 2 Section 7: Forecast Text Products Page 7-24