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What's behind a Terminal Aerodrome Forecast (TAF) amendment?

Updated: Oct 24, 2022

Most of the official weather forecasts you will get on a standard briefing or via your favorite app are issued by aviation meteorologists located at the Aviation Weather Center (AWC) in Kansas City, Missouri. This includes G-AIRMETs, SIGMETs (WS) and convective SIGMETs (WST). Terminal Aerodrome Forecasts or TAFs, however, are not issued by the AWC nor are they issued by Flight Service; they are issued by your local NWS Weather Forecast Offices (WFO). The meteorologists at the local WFO are very familiar with any local weather effects and have the best opportunity to produce a quality forecast. Pictured to the right is the WFO in Greer, South Carolina (KGSP). Click here to see a map of the local WFOs.

Your local WFO typically has the responsibility for issuing the TAFs for 6 or 7 terminal areas. At the Greenville-Spartanburg WFO, for example, they issue the terminal forecasts for six airports to include KAND, KGSP, KGMU, KCLT, KHKY and KAVL as shown in the image below.

Scheduled TAFs are issued four times a day at 0000, 0600, 1200 and 1800 UTC. They are typically transmitted 20 to 40 minutes prior to these times. Once the TAFs hit the wire, the forecaster must continue to compare the forecast to the actual observations for the airport to be sure it accurately depicts the ceiling, visibility, wind and weather occurring at the airport. When there is a discrepancy or the forecaster feels that the TAF isn't representative of the weather that may occur in the terminal area within the TAF's valid period, they will issue what is called an unscheduled TAF, better known as an amendment.

Amendments are the absolute best way to provide the highest quality forecast. Just like instrument students are taught by their instructors not to chase the needles, forecasters are similarly encouraged not to chase the observations. For example, an unexpected, but brief rain shower may quickly develop and pass by the terminal area temporarily lowering visibility below the visibility in the TAF. The forecaster may be tempted to issue a quick amendment, but if the condition is expected to be brief, there's no value to issuing an amendment - especially if it doesn't change the flight category (i.e., VFR, MVFR, IFR, LIFR, VLIFR).

The forecaster doesn't have to literally watch the observations minute by minute per se. Instead, they have a software program appropriately called "weather watch" that monitors the observations. Based on programmed criteria, the software compares the terminal forecast to the latest observations for each TAF site issued by that forecaster and flags the forecast element as green when they match. When the program highlights a forecast element as yellow or red, this means the difference is near or has exceeded the amendment criteria. As can be seen above, the forecast for the most part matches the current observations for those airports in the Greenville-Spartanburg WFO (most elements are green). However, there are three terminal areas (KAVL, KHKY and KAND) that show yellow for wind implying that the forecast is not quite in line with the current observations.

This allows the forecaster to quickly scan the display to determine if there is an immediate need for an amendment to one or more of the TAFs. The forecaster that issues the TAFs is assigned to the "short term desk" and has other duties to include the area forecast discussions and the issuance of severe thunderstorm or tornado warnings for the WFO. The forecaster shown on the left is located at the Greenville-Spartanburg WFO and has recently transmitted the 1800 UTC scheduled TAFs and is now working on the gridded forecasts for the GSP region. Click here to see the national gridded forecasts. The weather watch software compares the current observations to the gridded forecasts looking for differences as well.

Most pilots are weatherwise, but some are otherwise

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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