The US TAF package includes the information needed to model the filed differences maintained by the US for the TAF product. Two US documents govern the production of TAFs.
The National Weather Service maintains National Weather Service Instruction 10-813, Aviation Weather Services, NWSPD 10-8, Terminal Aerodrome Forecasts. The revision used for this package was published on 5 April 2012. A copy was accessed online in June 2013 at http://www.nws.noaa.gov/directives/sym/pd01008013curr.pdf.
The Secretary of the US Air Force maintains Air Force Manual 15-124, Meteorological Codes. The revision used for this package was published on 28 February 2013. A copy was accessed online in June 2013 at http://www.e-publishing.af.mil/.
A Record that is intended to contain non-Annex 3 meteorological forecasting conditions at an aerodrome as needed by US OPMET services.
Lowest Altimeter group. Lowest altimeter setting expected during the forecast period.
AFMAN 15-124, Section 1.3.4.10 describes this group as QNHP1P1P1P1INS for traditional alphanumeric TAFs issued by AFWA. It also specifies "Do not include QNH in Temporary (TEMPO) altimeter setting groups."
Volcanic Ash forecast. Indicates the forecast base and height of the volcanic ash cloud above an aerodrome.
AFMAN 15-124, Section 1.3.4.6 describes the Volcanic Ash Group (VAbbbttt) this way:
Include a VA group in the TAF, following the cloud and obscuration group, when US assets fall within the boundaries of a VAAC ash-plume forecast. Encode all VA plume forecasts provided by the VAAC in TAF coded products. The unit’s VA plume forecast must be horizontally consistent with the official VAAC forecast. NOTE: If the responsible VAAC cannot product the volcanic ash products, then the 2 WS will be the primary source.
Direction of the forecast peak wind gust.
Section 1.3.4.2.1.3 of AFMAN 15-124 says the following:
When it is possible to forecast the peak gust direction, but not the prevailing direction, encode the wind group as VRBffGfmfmKT and append the probable peak gust direction to remarks (e.g., GST DRCTN 250).
This code list contains the list of weather parameters that will be monitored and supported with amendments.
http://nws.weather.gov/codes/NWSI10-811/2011/AmendableTAFParameter
none
http://nws.weather.gov/codes/NWSI10-813/2012/IcingType/
none
http://nws.weather.gov/codes/NWSI10-813/2012/TurbulenceType/
none
Icing forecast for layer above aerodrome. AFMAN 15-124, Section 1.3.4.8, describes the Icing Group (6IchihihihitL) this way:
Forecast icing group, used to forecast icing not associated with thunderstorms (thunderstorm forecasts imply moderate or greater icing). Repeat as necessary to indicate multiple icing layers. Omit when no icing is forecast.
The type of icing forecast.
Describes the upper and lower limits of a layer above the aerodrome. This is needed for a number of weather elements, including non-convective low level wind shear, icing, turbulence, and volcanic ash.
The choice of CloudBaseHeight as the data type may seem odd. Strictly speaking, the top of the LLWS layer may or may not have a cloud layer associated with it. The top of the LLWS is a measured surface above the ground level of the aerodrome.
This class models the low level wind shear groups that can appear in US TAFs.
Forecasts of LLWS in the TAF will refer only to non-convective LLWS from the surface up to and including 2,000 feet AGL (LLWS is always assumed to be present in convective activity). It will be included in TAFs on an as-needed basis to focus the aircrew’s attention on LLWS problems which currently exist or are expected. Non-convective LLWS may be associated with the following: frontal passage, inversion, low-level jet, lee side mountain effect, sea breeze front, Santa Ana winds, etc. Mentioning LLWS whenever conditions are present or possible to occur is highly encouraged as it provides the TAF user with valuable information.
LLWS should be included in the TAF if a value of 30kts or greater is determined.
The wind direction at the top of the layer where low level wind shear is forecast
The wind speed at the top of the layer where low level wind shear is forecast.
This class models the various amendment limitations described in section 1.3 of NWSI 10-813 and in section 1.3.5.3 of AFMAN 15-124.
If observations are not available for a station 24 hours each day, or if human augmentation of observations is not available for a station 24 hour each day, then TAFs issued for that station will be governed by a special set of rules. These rules govern when and how amendments will be issued for the TAF.
In traditional alphanumeric TAFs issued by the NWS these limitations take on two forms. First, AMD NOT SKED, and second, AMD LTD TO CLDS VIS AND WND.
In traditional alphanumeric TAFs issued by the AFWA these limitations take on two forms. First LAST NO AMDS AFT YYGG NEXT YYGG (AmendableTAFParameter = NONE), and second, LIMITED METWATCH YYGG TIL YYGG (AmendableTAFParameter = LIMITED).
The time period when amendments are restricted.
The list of weather parameters that can be amended.
Turbulence forecast for layer above aerodrome. AFMAN 15-124, Section 1.3.4.9, describes the Turbulence group (5BhBhBhBtL) this way:
Forecast turbulence group, used only to forecast turbulence not associated with a thunderstorm (thunderstorms already imply severe or extreme turbulence). Turbulence forecasts apply to category II (CAT II) aircraft. Omit when no turbulence is forecasted.