Why TAFs Get Amended

Scott:

“TAFs are routinely issued by the National Weather Service every six hours. Between those issue 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.

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.

Sometimes what looks like an amendment isn’t one at all. For the Chicago, Atlanta, and New York City terminal areas, TAFs are now issued every two or three hours. The forecast is issued as an amended forecast, not a newly constructed TAF. That’s why it seems there are more amendments than in the past. 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. “