Chapter 27
Forecasts
In this example the visibility is not forecast in the TEMPO group. Therefore, the visibility is expected to remain the same (P6SM) as forecast in the prevailing conditions group. Also, note that in the TEMPO 0912/0915 group, all three cloud layers are included, although the lowest layer is not forecast to change from the initial time period.
27.4.2.10.3 Probability (PROB) Group (PROB30 YYGG/YeYeGeGe)
The probability group (PROB30 YYGG/Y Y G G ) is only used by NWS forecasters to forecast a low-probability occurrence (30 percent chance) of a thunderstorm or precipitation event and its associated weather and obscuration elements (e.g., wind, visibility, and/or sky condition) at an airport.
The PROB30 group is the forecaster’s assessment of probability of occurrence of the weather event that follows it. The first two digits (YY) are the day of the month for the start of the PROB30. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (Y Y ) are the ending day of the month, while the last two digits (G G ) are the ending hour (UTC) of the PROB30 period. PROB30 is the only PROB group used in NWS’ TAFs.
The PROB30 group is located within the same line of the prevailing condition group, continuing on the line below if necessary.
Only one PROB30 group may be used in the initial forecast period and in any subsequent FM groups. Note that the U.S. military and international TAFs do not have these restrictions.
PROB30 groups do not include forecasts of significant weather in the vicinity (VC) or non-convective LLWS.
Example:
FM012100 18015KT P6SM SCT050 PROB30 0123/0201 2SM TSRA OVC020CB
In this example, the PROB30 group is valid on the 1st day of the month at 2300 UTC to the second day of the month at 0100 UTC.
27.4.2.10.4 TAFs for Joint-Use (Joint Civilian/Military) Airports
The TAF format at some joint-use airports is different from the NWS TAF format as follows:
- Visibility is in meters instead of statute miles.
- Example: BECMG 0504/0505 21006KT 9000 BR SKC QNH3005INS.
- Includes a forecast of the lowest barometric altimeter setting (QNH) during the forecast period in inches of mercury.
- Example: BECMG 0504/0505 21006KT 9000 BR SKC QNH3005INS.
- Includes a forecast of the maximum temperature (in whole degrees Celsius) and expected time of occurrence.
- Example: BECMG 0511/0512 16005KT 9999 NSW SKC QNH3006INS TX28/0420Z TN22/0410Z.
- Includes a forecast of the minimum temperature (in whole degrees Celsius) and expected time of occurrence.
- Example: TN22/0410Z BECMG 0511/0512 16005KT 9999 NSW SKC QNH3006INS TX28/0420Z TN22/0410Z.
27.4.3 TAF Examples
TAF KPIR 111140Z 1112/1212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1112/1114 5SM BR FM111500 16015G25KT P6SM SCT040 BKN250 FM120000 14012KT P6SM BKN080 OVC150 PROB30 1200/1204 3SM TSRA BKN030CB FM120400 14008KT P6SM SCT040 OVC080 TEMPO 1204/1208 3SM TSRA OVC030CB
TAF Terminal Aerodrome Forecast.
KPIR Pierre, South Dakota.
111140 Prepared on the 11th day of the month at 1140 UTC.
1112/1212 Valid from the 11th day of the month at 1200 UTC until the 12th day of the month at 1200 UTC.