A TTF or Trend Type Forecast also known simply as a Trend, is an aviation weather forecast written by a real person on location at a major airport or military base. A TTF is a professionally considered forecast for weather over a three hour period and is based on an actual weather report, such as a METAR

METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....

 or SPECI.

A TTF is similar to or sometimes in addition to a TAF

Terminal Aerodrome Forecast

In meteorology and aviation, TAF is a format for reporting weather forecast information, particularly as it relates to aviation. "TAF" is an acronym of Terminal Aerodrome Forecast or, in some countries, Terminal Area Forecast....

, a Terminal Aerodrome Forecast, but during the TTF's validity period is considered superior to a TAF.