TERMINAL FORECAST PHILOSOPHY (Appendix C)

1. USE OF THE TERMINAL FORECAST
2. COMPOSING THE TERMINAL FORECAST
3. AMENDMENT CRITERIA
4. AMENDMENT PHILOSOPHY
5. PROBABILITY TERM USE RESTRICTION
6. LENGTH OF BECMG, TEMPO, AND PROB TIME PERIODS
7. LIABILITY CONCERNS

TERMINAL FORECAST ISSUANCE GUIDELINES (Appendix D)
CONTRACTIONS USED IN TERMINAL FORECASTS (Appendix E)
GENERIC INTERNATIONAL TAF CODE FORMAT (Appendix F)
INTERNATIONAL TERMINOLOGY (Appendix G)
MINIMUM AMENDMENT CRITERIA (Appendix H)
SIGNIFICANT WEATHER (Appendix I)
SIGNIFICANT WEATHER PHENOMENA MATRIX (Appendix J)
ACRONYMS (Appendix K)

APPENDIX C
TERMINAL FORECAST PHILOSOPHY

Terminal forecasting must be focused on operationally significant weather events and changes. Forecaster judgement, particularly with regard to amendments, plays a key role in the terminal forecast program.

1. USE OF THE TERMINAL FORECAST

The terminal forecast serves the pre-flight and in-flight meteorological service requirements of domestic and international aviation operations by providing a forecast of weather conditions at an airport. The terminal forecast is principally used for flight planning, but the product is not designed to contain all information pertaining to aviation weather at that location. For a pilot weather briefing, the terminal forecast is used with other important aviation weather products such as: SIGMETS, AIRMETS, Convective SIGMETS, Center Weather Advisories, Severe Weather Outlooks (AC), Area Forecasts (FA), pilot reports (PIREPs), and Surface and Radar Observations.

When pilots are briefed for a planned flight, they are given applicable hazardous weather products issued by the Aviation Weather Center (or Alaska Aviation Weather Unit) which may contain actual or anticipated convective activity or instrument flight rules (IFR) conditions. In addition to the terminal forecast, convective weather information may be obtained from the Severe Weather Outlook (AC), the Area Forecast (FA), and radar observations and imagery. IFR condition information, as well as convective information, may be obtained from PIREPs, surface observations, and visually interpreted satellite imagery.

If the aviation weather products contain information on possible convection or IFR conditions for an area and a terminal forecast for an airport within that area does not contain the lower probability of IFR conditions in convection, pilots can still be advised of the risk of those convection conditions near that airport using the other aviation products.

2. COMPOSING THE TERMINAL FORECAST

Forecasters should consider the following recommendations when composing a terminal forecast:

• The terminal forecast should be consistent with other forecasts prepared within the NWS office
  
• Forecast a concise description of the expected sequence of events. The forecasts are often obtained verbally and are manually transcribed.
  
• Do not provide a great level of detail on operationally insignificant expectations.
  
• Be aware of amendment criteria when formulating the forecast, but do not forecast just to satisfy criteria.
  

3. AMENDMENT CRITERIA

If an unexpected weather event occurs at a terminal site which is not expected to persist (i.e., brief shower or thunderstorm), and the forecast still represents the forecaster's best professional judgement on expected conditions, there is little or no need to amend. En route pilots will be using current weather information (i.e., observations and radar) to make short term in-flight decisions. They may use terminal forecasts for longer term decisions. Amended terminal forecasts, addressing only a temporary change in the forecast, occurring within the next hour, will not help en route pilots nor those making pre-flight arrangements.

The threshold values chosen as amendment criteria are those which impact aircraft and airport operations. When selecting forecast values, forecasters should remain aware of the operationally-oriented amendment criteria, but the terminal forecast values should not be selected solely to satisfy the criteria.

Discrete flight category value changes for visual flight rules (VFR), marginal visual flight rules (MVFR), IFR, and low IFR (LIFR) have significant operational impact (i.e., fuel requirements, alternates) and the terminal forecast must be responsive to those changes. Ceiling and visibility values of 600 feet and 2 miles, respectively, impact airport use as a flight planning alternate. Ceiling values of 2000 feet, 200 feet, and a visibility value of 1/2 mile impact arrivals, takeoffs, and landings particularly at hub and large airports. Significant weather and wind conditions have an operational impact at all airports.

4. AMENDMENT PHILOSOPHY

Terminal forecast amendment criteria are based on two principles;

1. Specific thresholds which are significant to aviation operations and;
   
2. A forecaster's determination that the new conditions are likely to persist long enough to actually be significant.

The requirement for issuing amendments is driven by customers needing to know when significant weather conditions, liable to affect their operations, are expected; and when those expectations change. Forecasters have expressed concern that the terminal forecast amendment criteria are too restrictive and thus, too many amendments are required. Forecasters perceive that forcing amendments of essentially good forecasts seems unnecessary because it requires as much effort as amending unrepresentative forecasts. The increased number of amended terminal forecasts has directly impacted forecasters' workload. There is also a concern that the increased number of amendments has reduced customer confidence in the integrity of the forecast.

The frequency of issuing amendments can be effectively controlled by exercising the second terminal forecast amendment principle. The decision to amend the forecast relies on the forecaster's judgement to determine whether it is actually warranted by assessing the combination of existing conditions and forecaster expectations. However, terminal forecast amendments must be issued promptly whenever the forecaster becomes aware that conditions meeting the amendment criteria have occurred and judges that those conditions will persist.

Forecasters are encouraged to use application programs that flag a terminal forecast which does not satisfy the first terminal forecast amendment principle. Forecasters amending every terminal forecast alarmed by such programs may be just "chasing the observation" and not applying the second terminal forecast amendment principle. As AWOS and ASOS observations become more prevalent at terminal forecast sites, the frequency of observations are likely to increase as well as the probability that the observations may occur outside of the forecast amendment ranges. Increased emphasis on forecaster judgement should throttle the urge to "chase the observations".

Brief changes in observed weather, which are not expected to persist and are not within the forecast ranges, do not require amendments if the forecast still represents expected conditions for the period.

Example: A terminal forecast does not contain "PROB40 (times) TSRA" because the inclusion of the group is prohibited in the first 6 hours. An unexpected thunderstorm occurs at the terminal forecast site during this period and no amendment was issued prior to its occurrence. The forecaster expects the thunderstorm to last about 30 minutes and does not foresee any other thunderstorms at the terminal. If the other forecast parameters are expected to remain representative, then the original terminal forecast is still valid and no amendment should be issued. However, a terminal forecast for a site downwind from the thunderstorm may need to be amended. If the thunderstorm was detected and expected to affect the site, then the terminal forecast should have been amended to reflect the anticipated conditions.

Both the timing and duration of the actual weather event and the forecast of the event must be considered when deciding to amend. If the conditions change earlier or later than forecast and the terminal forecast shows the expected trend and will soon recover, an amendment may not be needed. In addition, small fluctuations in the observation should not result in a minor adjustment to the terminal forecast. Forecasters should use their best judgement on deciding when to amend.

With the above guidance in mind, forecasters are encouraged to amend terminal forecasts whenever necessary to keep them representative.

5. PROBABILITY TERM USE RESTRICTION

PROB30 and PROB40 are not permitted in the first 6 hours of each new terminal forecast's valid period, including amendments. These restrictions relieve forecasters of responsibility for describing low probability events in the first 6 hours of the forecast period and relieves customers from having to respond operationally to forecasts of events which, by definition, probably will not occur.

There is an understandable forecaster concern that forecasts of convective events, particularly possible hazardous conditions within severe weather watch areas, are not adequately portrayed in the first 6 hours of the terminal forecast. This concern is based on the questionable premise that the customer is better off if the potentially hazardous weather conditions are stated in the terminal forecast, regardless of the operational impact when it doesn't occur. The low Probability of Precipitation (PoP) event (less than 50 percent) forecast in a terminal forecast, regardless of the associated conditions, exerts a disproportionate influence on the operations associated with the terminal forecast and thus its use has been disallowed in the first 6 hours of the forecast.

NOTE: When a PROB30 or PROB40 group is warranted after the first 6 hours of an amended or delayed terminal forecast, it is recommended that the PROB group become effective on the next whole hour following the end of the exact 6 hour period under consideration. The 6 hour period begins at the issuance (effective) time of the amended or delayed terminal forecast. The use of intermediate times as beginning times for forecast groups, just for the purpose of including a PROB group, is discouraged.

At times when forecasters do not have rapid access to real-time radar imagery, other methods of quickly detecting convective activity near terminal forecast sites must be used. Besides observations, forecasters should rely on satellite imagery and lightning display graphics. For determining short term convective potential, forecasters must keep abreast of hourly mesoscale analyses.

The use of PROB in the terminal forecast should reflect the fact that the terminal forecast area is limited to a 5-statute mile radius around the airport. This is usually a significantly smaller area than the corresponding public forecast zone. The PROB group in the terminal forecast should be consistent with the zone PoP, but not necessarily the same because the PROB group often includes the probability of lower ceilings and restricted visibilities with the precipitation instead of just the statistical PoP. The terminal forecast has the advantage of being able to accommodate the PoP for hourly intervals, whereas the public zone must use PoPs for 6- or 12-hour periods. The terminal forecast PoP can refine the zone PoP for a terminal forecast site by providing more specific timing and details on precipitation type.

Example: A 70 percent chance of showers is forecast for a zone for the afternoon. At a terminal forecast site in that zone, a 30 percent chance of showers is forecast for the early afternoon and a 70 percent chance for late in the afternoon. The corresponding terminal forecast would not include a probability group in the first 6 hours for a 30 percent PoP. The terminal forecast PROB group reflecting the 70 percent PoP may only be a 40 percent PoP when the possibility of low ceilings and visibilities in the showers are taken into account. Conversely, if the situation warrants, the 70 percent PoP forecast may be carried as the prevailing group (FM or BECMG) instead of in a PROB30 or PROB40.

6. LENGTH OF BECMG, TEMPO, AND PROB TIME PERIODS

BECMG: The duration of the change period covered a by BECMG group, indicated by GGGeGe, shall never exceed 2 hours in an NWS-prepared terminal forecast.

TEMPO: In general, the period of time covered by a TEMPO group should not exceed 4 hours. In addition, a TEMPO forecast that is in effect for more than 2 hours without an occurrence of the stated conditions should be reviewed for amending.

PROB30 and PROB40: The period of time covered by a PROB group should generally be 6 hours or less, excluding widespread or self-sustaining convective systems.

These recommended practices should help satisfy the customer requirement that the terminal forecast be as specific as possible and representative of conditions which are occurring and those which may be expected.

7. LIABILITY CONCERNS

Forecasters are concerned that they will be personally accountable for their terminal forecast if an aircraft accident occurs at or near the terminal forecast site, and it is followed by an investigation and subsequent litigation. If the forecaster has followed NWS operational policies and procedures, which include the use of professional judgement, then the agency, and not the forecaster, will assume the responsibility for any legal implications. Therefore, forecasters should keep current with the observed weather and consider the need for amendments using agency policies and guidelines in the decision process. If an unexpected condition occurs at an airport, which, due to procedures or judgement, is not specifically addressed in the terminal forecast, and a weather related accident occurs at that airport, then the agency policy will be accountable and not the forecaster applying the policy.

APPENDIX D
TERMINAL FORECAST ISSUANCE GUIDELINES

Issue scheduled/amended terminal forecast when:

There are two consecutive observations not less than 30 minutes nor more than about 1 hour apart. This is a daily requirement for sites with part-time manual or part-time augmented automated observations.

If an element(s) or observation is missing, forecasters shall use their judgement and the total observation concept (see Section 6.1) to continue terminal forecast service when possible.

Delay terminal forecast issuance when:

Observations considered critical in the judgement of the forecaster are missing or required elements which are critical in the judgement of the forecaster of an observation are missing at the time of the scheduled issuance.

Amend terminal forecast:

a. Whenever it becomes, in the forecaster's judgement, unrepresentative of existing or expected conditions, particularly regarding those elements and events detailed in Section 6.7 and/or Appendix H, or

b. If the forecaster determines that missing observations or element(s) are not critical to the current forecast situation. In such event, a forecast amendment shall be issued with the statement "NIL AMD" appended, on a separate line and indented 5 spaces, to the end of existing forecast groups as the last line of the forecast.

Suspend terminal forecast issuance when:

Observations or one or more essential elements of the observation become unavailable for two consecutive hours; and

a. The forecaster judges the lack of observations or elements to be critical to maintaining the weather watch and the essential elements cannot be obtained using the total observation concept (see Section 6.1). In such event, a forecast amendment shall be issued containing only the statement, "NIL";

or

b. After an amendment continuing a terminal forecast in spite of missing observations, it becomes apparent that changing weather conditions make the forecast questionable or unrepresentative, an amendment shall be issued containing only the statement "NIL".

If, in the forecaster's judgement, adequate observations, including supplementary and complementary data sources (the total observation concept), do not resume, the next and subsequent regularly scheduled terminal forecast issuances shall simply state, "NIL." This shall continue until adequate observations resume or terminal forecast service is discontinued.

 

APPENDIX E 
CONTRACTIONS USED IN TERMINAL FORECASTS

NOTE: Some of the expressions -- short words, in common English, for which there are no ICAO contractions -- are completely spelled out, e.g., "AND" and "WIND". "TO" and "NIL" are both listed in the ICAO contraction manual. Both are common words in English.

AAx      Code used in the WMO abbreviated heading to indicate an amended
         forecast, where x is the letter A through X (see Section 7.1).
         NOTE: AAx is not used in the forecast text. 

AFT      After 

AGL      Above ground level 

AMD      Amended terminal forecast. Used in the forecast text only. AMD is
         not used in the WMO abbreviated heading. 

BC       Patches 

BECMG    Becoming. Indicator of a forecast change to prevailing
         meteorological conditions, occurring at either a regular or
         irregular rate at an unspecified time within the indicated period
         of time. The change occurs during the indicated period of time and
         the indicated conditions persist until the next forecast change
         indicator. The duration of the change period covered by BECMG,
         indicated by GGGeGe, shall never exceed 2 hours in NWS-prepared
         terminal forecasts. Refer to Section 7.2.9.b. 

BKN      Broken cloud layer (5 to 7 oktas cloud amount). Lowest broken layer
         is implied to be the ceiling. 

BL       Blowing 

BR       Mist 

CAVOK    Not used in NWS-prepared terminal forecasts. Contraction for
         "Ceiling and Visibility OK" and pronounced KAV-OH-KAY. Replaces
         visibility, present weather and cloud data under specified 
         conditions. See Appendix G, Section 1.1, for more specific
         information. 

CB       Cumulonimbus cloud 

CCCC     Generic WMO format code group for a four-letter location 
         identifier. Four-letter location identifiers for specific
         airports are listed in ICAO document 7910, "Location Indicators". 

CCx      Code used in the WMO abbreviated heading to indicate a corrected 
         forecast, where x is the letter A through X (see Section 7.1).
         CCx is not used in the forecast text. 

CLD      Cloud 

CLR      Not used in the terminal forecast. In the METAR code, CLR 
         indicates "clear below 12,000 feet above ground level" and applies
         to automated observations only. 

DR       Low drifting 

DS       Dust storm 

DU       Dust 

DZ       Drizzle 

FC       Funnel cloud 

FEW      Few clouds ( > 0 oktas to 2 oktas cloud amount) 

FG       Fog 

FMGGgg   From the time (UTC) indicated by GGgg. Generic WMO format code 
         group, indicating a significant and rapid (in less than one hour)
         change to a new set of prevailing conditions. Refer to Section
         7.2.9.a. 

FT       Feet 

FU       Smoke 

FZ       Freezing 

G        Gust. Defined as rapid fluctuations in wind speed with a variation
         of 10 knots or more between peaks and lulls. 

GR       Hail (diameter of largest hailstone > 1/4 inch) 

GS       Small hail and/or snow pellets (diameter of hailstones < 1/4 inch) 

HZ       Haze 

IC       Ice crystals 

KT       Knots 

LTD      Limited 

MI       Shallow 

NIL      No or None or I have nothing to send you 

NSC      Contraction for "no significant cloud", which replaces cloud data
         under specified conditions. See Appendix G, Section 1.2, for more
         specific information. Not used in NWS-prepared terminal forecasts. 

NSW      A contraction for "no significant weather". An indication that
         significant weather conditions, as expressed by Appendix I 
         (WMO Code Table 4678), are forecast to end. Refer to Section 7.2.6. 

OVC      Overcast cloud layer (8 oktas cloud amount) 

P        Greater than (as in P6SM) 

P6SM     Visibility forecast to be greater than 6 statute miles 

PE       Ice pellets 

PO       Well-developed dust/sand whirls 

PR       Partial 

PROBC2C2 Forecaster's assessment of the probability of occurrence of a
         thunderstorm (and associated precipitation) or precipitation
         event, along with associated weather elements (wind, visibility,
         and/or sky condition) whose occurrences are directly related to,
         and contemporaneous with, the thunderstorm or precipitation event.
         Only PROB30 or PROB40 are allowed. PROB30 and PROB40 are not
         permitted in the first six hours of each new terminal forecast's
         valid period, including amendments. The period of time covered by
         a PROB group should generally be six hours or less, excluding 
         widespread or self-sustaining convective systems. Refer to Section
         7.2.9.d. 

PY       Spray 

RA       Rain 

RRx      Code used in the WMO abbreviated heading to indicate a delayed 
         forecast, where x is the letter A through X (see Section 7.1).
         RRx is not used in the terminal forecast text. 

SA       Sand 

SCT      Scattered cloud layer (3 to 4 oktas cloud amount) 

SG       Snow grains 

SH       Shower 

SKC      Sky clear. No clouds; zero oktas cloud amount. The contraction 
         CLR is not used in the terminal forecast. 

SKED     Scheduled 

SM       Statute miles 

SN       Snow 

SQ       Squall 

SS       Sandstorm 

TAF      Aerodrome forecast code format. The international standard for the
         TAF code, FM 51-X Ext. TAF, is included in WMO Manual on Codes,
         WMO No. 306, Volume I.1, Part A. 

TEMPO    Temporarily. Indicator of temporary fluctuations to forecast
         meteorological conditions which are expected to last less than 
         1 hour in each instance and, in the aggregate, to cover less than
         half of the indicated period. In general, the period of time
         covered by a TEMPO group should not exceed 4 hours. Refer to
         Section 7.2.9.c. 

TS       Thunderstorm 

UP       Not used in terminal forecasts. Unknown precipitation (used in 
         METAR code only) 

VA       Volcanic ash 

VC       Vicinity - two definitions:

         NWS:
         An area encompassed between circles with radii of 5 and 10 statute
         miles, respectively, from the center of the runway complex of an 
         airport. This definition applies to NWS-prepared terminal
         forecasts. In NWS terminal forecasts, VC shall only be used in 
         the initial time period, FM and BECMG groups, all of which 
         forecast prevailing conditions, and shall only be used in 
         combination with fog (FG), shower(s) (SH), and thunderstorm(s)(TS).

         WMO:
         (An area encompassed) within 8 kilometers (5 statute miles) of the
         aerodrome but not at the aerodrome. (Words in parentheses inferred.
         See Note 1 under WMO Regulation 15.8.10). VC is not used in 
         international terminal forecasts. 

VIS      Visibility 

VRB      Variable wind direction. Wind direction is considered variable 
         when it is impossible to forecast a mean wind direction due to its
         expected variability, e.g., for very light winds (3 knots or less)
         or during convective activity. 

VV       Vertical Visibility 

Z        Indicator letter (an abbreviated symbol for Coordinated Universal 
         Time - UTC) appended to the date-time of forecast origin group.

 APPENDIX F
GENERIC INTERNATIONAL TAF CODE FORMAT

The format of the forecast text in the standard international TAF code is made up of code groups, represented generically, as shown below. The optional groups (enclosed in parentheses) are to be used as called for by ICAO RAN agreement(s). NOTE: Groups shown in parentheses shall not be used in NWS terminal forecasts. The generic terms and groups used in NWS terminal forecasts are described in Sections 7.2.1 through 7.2.9 of this chapter. The generic terms and groups shown in parentheses, CAVOK, NSC, optional groups 6Ic (icing), 5B (turbulence), T (temperature), and QNH (lowest altimeter setting), are described in Appendix G.

{TAF
{or
{TAF AMD
CCCC           YYGGggZ         Y1Y1G1G1G2G2        dddffGfmfmKT 

[location      [date-time      [valid              [wind forecast]
 identifier]    of forecast      period]
                origin]

 

                                   {NsNsNshshshs
{VVVV          {w'w'               {or
{or            {or                 {VVhshshs
{(CAVOK)       {NSW                {or
                                    {SKC (or NSC) 

[visibility     [significant       [cloud and
 forecast]       weather            obscuration
                 forecast]          forecast]

 

(6IchihihitL)   (5BhBhBhBtL)   (TTFTF/GFGFZ)    (QNHPIPIPIPIINS) 

[icing           [turbulence    [temperature     [lowest altimeter
forecast]         forecast]      forecast]        setting]

 

{TTGGgg
{or               PROBC2C2 GGGeGe
{TTTTT GGGeGe 

[forecast change  [probability
 indicators]       forecast]

 APPENDIX G
INTERNATIONAL TERMINOLOGY AND FORECAST GROUPS NOT USED IN NWS TERMINAL FORECASTS

This appendix describes terminology and code groups which are used in the international TAF code but not used in NWS-prepared terminal forecasts. This Appendix is included to assist in the understanding of terminal forecasts prepared by other agencies and/or countries.

1. INTERNATIONAL TAF TERMS NOT USED IN NWS-PREPARED TERMINAL FORECASTS

1.1 CAVOK. The WMO Manual on Codes states that the contraction CAVOK (pronounced KAV-OH-KAY) shall be included in place of the visibility, significant weather and cloud/obscuration groups when the following conditions are expected to exist simultaneously.

a. Visibility: 10 km or more (more than 6 statute miles);

b. No cloud below 1,500 meters (5,000 feet) or below the highest minimum sector altitude, whichever is greater, and no cumulonimbus;

(Highest minimum sector altitude is defined by ICAO as the lowest altitude which may be used under emergency conditions which will provide a minimum clearance of 1,000 feet [300 meters] above all objects located in an area contained within a sector of a circle of 25 nautical miles [46 km] radius centered on a radio aid to navigation. In the U.S., minimum sector altitudes have been established for each airport for which instrument approach procedures have been established; the altitudes are shown on instrument approach procedure charts.)

c. No significant weather phenomena (see Appendix I)

1.2 NSC. The contraction "NSC" (meaning "no significant clouds") indicates no clouds are forecast below 5,000 feet (1,500 meters) or below the highest minimum sector altitude, whichever is greater, and no CB (any height) is expected. In applying this limitation, and when the use of neither CAVOK (see above) nor SKC would be appropriate, the term NSC could be used in non-NWS-prepared terminal forecasts.

The use of the contraction NSC is appropriate only if called for by an ICAO RAN agreement. No such RAN agreement is in effect for the United States or any of its territories or possessions. Thus, "NSC" is not used in NWS terminal forecasts.

1.3 PROBC2C2 GGGeGe in combination with BECMG and/or TEMPO. WMO TAF regulations allow the use of PROB30 or PROB40 in combination with the TEMPO group. The WMO regulations state that, when used, the PROB group is to be placed immediately before the TEMPO group and that the group GGGeGe is to be placed after TEMPO, for example, PROB30 TEMPO 1214 or PROB40 TEMPO 1214.

The PROBC2C2 group shall not be used by NWS offices as a direct modifier of the BECMG or TEMPO group(s) or with the FMGGgg group. Similarly, none of these groups may be used by NWS offices as a direct modifier of the PROB group. The reason for these prohibitions is to ensure the terminal forecast is as easy as possible to understand and correctly interpret.

1.4 Optional Groups. The optional groups (enclosed in parentheses in Appendix F) are to be used as called for by ICAO RAN agreement(s). At present, there is no requirement for the inclusion in NWS-prepared terminal forecasts of any of the optional groups. Arrangements for the issuance of optional group data as/when required will be made by NWS headquarters. (See ICAO Air Navigation Plans, documents 8733/14 and 8755/13, Part IV, Meteorology, Section 5, Forecast.) Explanations concerning the use of these optional groups follow.

1.4.1 Icing Group (6IchihihitL). The icing group (6IchihihitL) provides the means to include forecasts of layers and types of icing at or in the vicinity of an aerodrome in the terminal forecast. The group may be repeated as often as necessary to indicate more than one layer or type of icing. If the forecast thickness of the layer for any one type of forecast icing is greater than 9,000 feet (2,700 meters), the group should be repeated, and the base indicated in the second group is to coincide with the height of the top of the layer as given in the preceding group.

To explain the code: 6 is an indicator identifying the icing group; Ic is the type of forecast ice accretion on the external parts of aircraft (refer to WMO Code Table 1733, below); hihihi (refer to WMO Code Table 1690, below) is the height of the lowest level of icing, expressed in hundreds of feet/units of 30 meters above the aerodrome elevation (001 = 100 feet/30 meters); and tL is the thickness of the layer of icing (refer to WMO Code Table 4013, at the end of this Appendix) expressed in thousands of feet/units of 300 meters.

Examples of icing forecasts in the TAF code:

WMO CODE TABLE 1733 (ICING)

WMO CODE TABLE 1690 (ICING AND TURBULENCE)  

NOTES:
(1)   The code figures (hBhBhB and hihihi) are direct readings in
      units of 100 feet(300 meters).
(2)   The Code Table shall be considered as a coding device in which
      certain code figures are assigned values. These are discrete
      values, not ranges. Any observation or forecast of values to be
      coded in the Code Table shall be made without regard to the code
      table. The forecast is coded according to the following rule: 
      if the observed or forecast value is between two of the heights
      as given in the table, the code figure for the lower height
      shall be given. 

 

WMO CODE TABLE 4013 (ICING AND TURBULENCE)

NOTE: The code figure t_L is a direct reading in 1,000s of feet/units of 300 meters.

 

1.4.2 Turbulence Group (5Bh_Bh_Bh_Bt_L). The turbulence group (5Bh_Bh_Bh_Bt_L) provides the means to include forecasts of layers and severity/frequency of turbulence at or in the vicinity of an aerodrome in the terminal forecast. The group may be repeated as often as necessary to indicate more than one layer and/or severity/frequency of turbulence. If the thickness of the layer for any one frequency or severity of turbulence is greater than 9,000 feet (2,700 meters), the group should be repeated and the base indicated in the second group should coincide with the top of the layer as given in the preceding group.

To explain the code: 5 is an indicator identifying the turbulence group; B indicates the severity and frequency of the forecast turbulence (refer to WMO Code Table 0300, below); h_Bh_Bh_B (refer to WMO Code Table 1690, shown above in Section 1.4.1, Icing) is the height of the lowest level of turbulence, expressed in hundreds of feet/units of 30 meters above aerodrome elevation; and t_L is the thickness of the layer of turbulence (refer to WMO Code Table 4013, shown above in Section 1.4.1, Icing).

Examples of turbulence forecasts in the TAF code:

530907 - Forecast moderate turbulence in clear air, frequent, from
         a height of 9,000 feet/2,700 meters above aerodrome elevation
         up to 16,000 feet/4,800 meters above aerodrome elevation. 

563004 - Forecast severe turbulence in clear air, occasional, from a
         height of 30,000 feet/9,000 meters above aerodrome elevation
         up to 34,000 feet/10,200 meters above aerodrome elevation. 

 

WMO CODE TABLE 0300 (TURBULENCE)

* Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.

 

1.4.3 Surface Temperature Group (TT_FT_F/G_FG_FZ). The surface temperature group (TT_FT_F/G_FG_FZ) provides the mechanism for including forecast temperature (T_FT_F, whole degrees Celsius) in the TAF code for specific times. One or more such groups may be used to provide, for example, forecast temperatures at certain times or to indicate expected maximum temperatures and the forecast time of occurrence.

To explain the code: T is an indicator, meaning "temperature"; T_FT_F is symbolic for the forecast temperature in whole degrees Celsius (C); G_FG_F is symbolic for the valid time to the nearest whole hour UTC of the temperature forecast; and Z is an abbreviated symbol meaning Universal Coordinated Time (UTC). Temperatures between +9^o C and -9^o C are to be preceded by 0; temperatures below 0^o C are to be preceded by the letter M (for minus).

Examples of temperature forecasts in the TAF code:

T17/20Z - forecast temperature of 17^o C at 2000 UTC
T08/21Z - forecast temperature of 8^o C at 2100 UTC
T00/18Z - forecast temperature of 0^o C at 1800 UTC
TM10/07Z - forecast temperature of minus 10^o C at 0700 UTC

1.4.4 Lowest Altimeter Setting Group (QNHP_IP_IP_IP_IINS). The lowest altimeter setting group (QNHP_IP_IP_IP_IINS) is used to forecast the lowest altimeter setting (P_IP_IP_IP_I, in inches of mercury) during the initial forecast period and in each FM and BECMG group. QNH shall not be encoded in a TEMPO group.

2. TERMS OR GROUPS USED DIFFERENTLY IN NWS-PREPARED TERMINAL FORECASTS THAN IN INTERNATIONAL TERMINAL FORECASTS

2.1 Cloud Height (hshshs). WMO procedures allow for the height of clouds or vertical visibility (hshshs)to be forecast in hundreds of feet (units of 30 meters) above ground level, at all levels.

In NWS-prepared terminal forecasts, the resolution for forecast cloud heights is relaxed above 3,000 feet. Refer to the table in Section 7.2.7.

2.2 BECMG GGGeGe. The guidelines for using BECMG GGGeGe in the international TAF code state that the duration of the change period covered by BECMG, indicated by GGGeGe, should not normally exceed 2 hours and in any case should not exceed 4 hours.

In NWS-prepared terminal forecasts, the duration of the change period covered by BECMG shall never exceed 2 hours. Refer to Sections 6.7 and 7.2.9.b.

APPENDIX H
U.S. TERMINAL FORECAST MINIMUM AMENDMENT CRITERIA

IF CEILING FORECAST IS:                  AMEND IF: 

No ceiling or ceiling above              in the forecaster's judgement the forecast is
3000 feet                                non representative or the ceiling falls to
                                         3000 feet or below 

3000 feet to 2000 feet                   increases to greater than 3000 feet or falls
                                         below 2000 feet 

Less than 2000 feet to                   increases to equal or exceed 2000 feet, or
1000 feet                                falls below 1000 feet 

Less than 1000 feet to                   increases to equal or exceed 1000 feet, or 
600 feet                                 falls below 600 feet 

Less than 600 feet to                    increases to equal or exceed 600 feet, or
200 feet                                 below 200 feet 

Less than 200 feet                       increases to 200 feet or more 

IF VISIBILITY FORECAST IS:               AMEND IF: 

Greater than 5 miles                     decreases to 5 miles or less 

3 to 5 miles                             increases to 7 miles or more, or falls to below 3 miles 

2 miles                                  increases to 3 miles or more, or falls to below 2 miles 

1 mile or 1 1/2 miles                    increases to 3 miles or more, or falls below 1 mile 

1/2 to 3/4 mile                          increases to 1 mile or more, or falls below 1/2 mile 

Less than 1/2 mile                       increases to 1/2 mile or more 

IF FORECAST IS:                                 AMEND IF: 

thunderstorms, freezing precipitation,        phenomenon is no longer expected
or ice pellets at the airport                 expected at the airport 

no thunderstorm, freezing precipitation,      occurs or is expected at the
or ice pellets at the airport                 the airport  

NOTE: Vicinity (VC) events, by definition, are not at the airport 

NOTE: "forecast mean" refers to the mean wind direction or speed expected
for the specified forecast group time period.  

For any forecast mean wind direction: Amend if the mean wind direction for the
remainder of the forecast period is expected to differ by 30 degrees or more, 
with an accompanying mean wind speed of 12 knots or more. 

For forecast mean wind speed: Amend if the actual mean wind speed will differ
differ from forecast group mean speed by 10 knots or more, and 

 For any forecast peak gust (or forecast of no gust), Amend if: 

IF FORECAST:                          AMEND IF: 

Wind shear                            No wind shear 

No wind shear                         Wind shear