Why Are Terminal Area Forecasts Amended So Frequently?

For example, the Greenville-Spartanburg WFO in Greer, South Carolina, issues a TAF for six terminal areas to include, Charlotte Douglas International Airport (KCLT), Hickory Regional Airport (KHKY), Anderson Regional Airport (KAND), Asheville Regional Airport (KAVL), Greenville-Spartanburg International Airport (KGSP), and Greenville Downtown Airport (KGMU).  

These TAFs are routinely issued every six hours. Between those issuance times, they may be amended for many conditions that were not accurately forecast. These include discrete flight category value changes for VFR, Marginal VFR, IFR, and low IFR, which have significant operational impact for IFR operations. If thunderstorms occur that weren’t forecast—or don’t occur when they were forecast—an amendment is required. The same holds true for freezing rain, freezing drizzle, and ice pellets.  

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 below, 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.  

Forecasters and pilots understand that wind can fluctuate continuously throughout the day. So, forecasters are careful not to amend the TAF due to a change in wind direction or wind speed that is short-lived. However, when the actual mean wind direction differs from the forecast by 30 degrees or more, with an accompanying mean wind speed of 13 knots or greater, the TAF will likely be amended. 

Similar to the wind direction, when the actual mean wind speed differs from the forecast by 11 knots or greater, and the original wind speed or newly expected wind speed is greater than 13 knots, the TAF will likely be amended. Essentially this means that when the winds are light, don’t expect the TAF to be amended even if they are 180 degrees in the wrong direction.  

However, sometimes what looks like an amendment isn’t one at all. For high-impact airspace such as the Chicago, Atlanta, and New York City terminal areas, TAFs are issued every two or three hours. The NWS began this as part of an enhanced aviation project for the FAA. It went over so well that it adopted it permanently. For Chicago O'Hare International Airport (KORD), you might even see updates every two hours at certain times during the day. In fact, in these high-impact terminal areas there typically is a forecaster dedicated to aviation. 

Here’s the ugly side of this improvement. The forecasts issued on a two- or three-hour cycle are marked as an amended forecast, not a newly constructed TAF. That’s why it seems there are more amendments than in the past. Unfortunately, there’s no way to tell if the forecast was changed because the amendment criteria was reached or because it was time for a new forecast. If you fly into or out of a busy airspace such as Chicago, Atlanta, or New York, just keep in mind that forecasts will be updated much more frequently even on days with benign weather. A terminal forecast tagged with “AMD” may not be because the previous forecast was misaligned with reality. It simply may be a new and improved forecast for you to ponder.