Ever wonder how an aviation forecast, officially known as a Terminal Aerodrome Forecast (TAF), is put together and what the constraints are?  Sometimes it’s easier to understand the strengths and weaknesses of a forecast product if you’re aware of the process used to prepare it. This article will focus on rules a forecaster must follow when writing a TAF concerning BECMG (Becoming), TEMPO (Temporary) and PROB (Probably) groups. 

The BECMG group was deleted from the National Weather Service TAF in 2004. The main reason was uncertainty and confusion from the users interpreting this group. Pilots expressed a need for better timing on forecasted conditions, versus a two or three hour window. The result of being more exact can lead to more amended TAFs and additional lines. For example, old rules allowed the following TAF:

KLCH 221138Z 221212 VRB03KT 1/2SM FG OVC002
       BECMG 1315 VRB03KT P6SM NSW SCT030
      FM1800 23005KT P6SM SKC
 

In this example the TAF is issued at 1138Z and goes into effect at 1200Z. The second line is a BECMG group covering the period from 13 to 15Z. So the question is, when should the second line go into effect?

The forecaster expects weather to improve between 13 and 15Z, but the forecaster is unsure. It’s a best guess. As a user, you’re expected to assume the first line remains in effect until 15Z. So even though the weather is expected to improve sometime between 13 and 15Z, you still must plan on using the forecasted 200 feet and one-half mile until 15z. After 15Z, it’s VFR.

Under the new rules, the above TAF would look something like this: 

KLCH 221138Z 221212 VRB03KT 1/2SM FG OVC002
     FM1300 VRB03KT 1SM BR OVC006
     FM1400 VRB03KT 3SM BR BKN015
     FM1500 VRB03KT P6SM SCT030
     FM1800 23005KT P6SM SKC

 The TEMPO group is another area where rule changes have drastically affected the appearances of TAFs. Prior to a few years ago, forecasters could have TEMPO groups for as long as 12 or even 24 hours. Now forecasters must keep the TEMPO groups to 4 hours or less. With new technology and better tools, forecasters are working to decrease false alarms by keeping TEMPO groups as short as possible.

While reducing false alarms is moving in the right direction, pilots may be put off by TAFs that seem wordy and repetitive. Certain weather conditions lead to a forecast that is tough to nail down and provide specifics for. When this is the case, TAFs can become quite long.

An example along the Gulf Coast is when sea fog moves ashore. When this is the case, visibilities and ceilings can vary over a short period throughout a 24 hour or longer period.  So what does the forecaster do? Several years ago the TAF may have looked something like this:

KLCH 251135Z 251212 18006KT 5SM BR 0VC008
       TEMPO 1212 1/4SM FG OVC001 

This forecast may be meteorologically accurate, but lacks detail. Here’s how the TAF might look when limiting the TEMPO group to four hours. 

KLCH 251135Z 251212 18006KT 1SM BR OVC004
      TEMPO 1216 1/4SM FG OVC001
     FM1600 18006KT 3SM BR OVC006
      TEMPO 1620 1/2SM FG OVC001
     FM2000 18006KT 5SM BR OVC008
      TEMPO 2024 2SM BR OVC004
     FM0200 18006KT 3SM BR OVC006
      TEMPO 0206 1/2SM FG OVC002
     FM0600 1SM BR OVC004
      TEMPO 0812 1/4SM FG OVC001

The user will see a long TAF, but it is much more detailed and useful for planning purposes.
 

The PROB group has also changed over the last few years. The PROB40 has been eliminated; only PROB30 is allowed now and is restricted when it can be used. PROB30 groups are no longer allowed in the first 9 hours of the TAF. So basically, forecasters cannot mention certain aspects of the weather in the TAF if the probability for that weather to occur is less than 50 percent in the first nine hours of the TAF.
 

For example, if the probability for thunderstorms is less than 50 percent in the first nine hours of the TAF, they are not to be mentioned. What forecasters can do is use the VCTS remark. VCTS literally means thunderstorms are forecasted to be within a 5 to 10 mile radius of the terminal, but it is often used to state that thunderstorms could be at the terminal during a certain period. It’s just that the chance will be less than 50 percent.  If the chance is 50 percent or greater, the thunderstorms will be mentioned in a TEMPO or FM group.

This can present obvious problems if a pilot happens to look at a zone forecast that has a 40 percent chance of thunderstorms and then a TAF that has no mention of thunderstorms. But a zone forecast is designed to cover a larger area as compared to a TAF, which is more of a point forecast, and is designed to be more fine-tuned than a zone forecast.