It’s not always too difficult to tell if the weather is good enough to fly – in the moment.  Looking up to a clear blue windless sky is a pretty good sign.  The big question with weather is often “what’s going to happen next?”  On a recent flight to Nashville, Tennessee, my initial plan was to fly up in the morning on a Saturday and return the same evening. As the day approached, I made the call to fly up Friday night and return midday Saturday. Why? Because of the weather forecast. Several tools are used to maintain a good big picture of the weather, but a Terminal Area Forecast (TAF) is a pilot’s crystal ball for helping to see into the future.  One is issued every 6 hours (four times per day). If you look at How To Read A METAR, you’ll know that weather code often has both a “raw” and “decoded” version, and that the decoded version isn’t always around to save the day.  Learning to read the raw code is essential.  Each TAF will list the airport identifier it is being reported from. When looking at certain resources like Foreflight, you’ll notice that looking at one airport’s TAF is really pulling a nearby airport’s TAF. For instance, my home airport of Thomson McDuffie (KQHU) uses the Augusta Daniel Field (KDNL) TAF.  Like Dave Matthews said, it’s the best of what’s around.  Let’s look at a basic example of a raw TAF and break it down.

072334Z 0800/0824 03010KT P6SM VCSH BKN 009 OVC012
FM080900 02010G20KT P6SM SCT009 BKN020

Does that look like a bunch of nothing? That’s ok.  We’ll break down each piece, but I recommend being familiar with reading a METAR first, as a TAF builds upon that knowledge.

 

Date & Time of Forecast
072334Z 0800/0824 03010KT P6SM VCSH BKN 009 OVC012
FM080900 02010G20KT P6SM SCT009 BKN020

The first two digits are the day of the month that the report was generated. In this case, the 7th. The last four digits are the time the report was generated.  In this case, 23:34, which is 11:34pm in Zulu time.

Caption

Some TAFs, like this one issued at 11:20 Zulu on the 12th, are just one line. This simplicity indicates that not much weather change is in the forecast.

 

Valid Time of Forecast
072334Z 0800/0824 03010KT P6SM VCSH BKN 009 OVC012
FM080900 02010G20KT P6SM SCT009 BKN020

This is the range of times that the forecast is covering.  In this case, the forecast is for the 8th day of the month at 00:00 Zulu until the 8th day of the month at 24:00 Zulu. Basically, a 24 hour period on the 8th day of the month. Just don’t forget to convert the Zulu time to your local time.

Caption

You can see from the first line that this TAF is valid from 22:00 Zulu on the 11th (1122) until 18:00 Zulu on the 12th (1218).

 

Initial forecast for the period

072334Z 0800/0824 03010KT P6SM VCSH BKN 009 OVC012
FM080900 02010G20KT P6SM SCT009 BKN020

The wind, visibility and sky condition are displayed in a METAR format for the forecast period. Wind is from 030 at 10KT and the visibility is greater than 6 miles. VCSH is the code for “showers in the vicinity.”  Clouds are broken at 900ft, overcast at 1200ft.  Bring an umbrella.

Caption

The first line shows the initial observation. This TAF begins with light winds, 4 statute miles of visibility and scattered clouds at both 100 and 25000 feet.

 

Change in weather 
072334Z 0800/0824 03010KT P6SM VCSH BKN 009 OVC012
FM080900 02010G20KT P6SM SCT009 BKN020

When the weather is expected to change, another line will begin with the term FM (from) for a rapid change in weather, BECMG (becoming) for a more gradual change in weather, and TEMPO (temporary) for a temporary change in the weather. In this case, the FM indicates a raid change in weather starting at  09:00 Zulu on the 8th day (080900).

Caption

This TAF has a lot of weather change in the forecast.  Note the temporary forecast (TEMPO) in the second line that predicts light rain showers (-SHRA) somewhere between 12:00 Zulu to 14:00 Zulu as well as the scattered layer at 8000 filling in to a broken layer.

 

Forecast wind, visibility and sky condition 
072334Z 0800/0824 03010KT P6SM VCSH BKN 009 OVC012
FM080900 02010G20KT P6SM SCT009 BKN020

After FM, BECMG or TEMPO, the same coding as a METAR format is used to forecast the wind, visibility and clouds for the forecast period. Note that with the exception of the use of FM (from), any forecast changes in weather will only show what is expected to change. For instance, if the visibility is changing, but the wind and clouds will remain the same, only the new visibility will be listed. For today’s flight, starting at 09:00 Zulu, the wind is forecast to be from 020 at 10 knots gusting to 20 knots. The visibility is forecast to remain above 6 statute miles (P6SM).  Clouds are forecast to be scattered at 900ft and broken at 2000ft.

Caption

This TAF was for the approach of Hurricane Matthew.  Note the initial observations (first line) were winds at 14 knots gusting to 24 knots. But by 08:00 Zulu, winds were forecasted to be at 80 knots gusting to 100 knots (the line in gray)!

 

Predicting the future 
This is certainly not an exhaustive guide to the TAF. This overview is intended to demystify the code so that you have some familiarity with reading one. When you come across a component you don’t know, dig a little deeper in a resource like this one to look up some of the codes. Perhaps the most important thing to know about a forecast is this: expect it to change. Forecasts are a guess. Perhaps it’s a super high tech, data driven, educated guess, but it cannot be relied on with certainty. The best thing you can do to put this knowledge into practice is to build safe margins and always have a plan B.  If you are counting on the clouds being exactly as forecast to maintain a safe altitude over those hills on your flight, there is probably not enough margin for forecast error. And a plan B might be an elaborate backup, or it might just be a 180 degree turn back to good weather if you fly into weather that is worse than the forecast. You can go back for another hundred dollar hamburger and a second helping of forecast – and maybe fries.

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1 Comment

  1. Randall Kelley

    Thanks

    Reply

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  1. Weather Or Not - The Decision Making Process | Clayviation - […] close to the flight, a Terminal Aerodrome Forecast (TAF) can start to get us real numbers to work with.…
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