TAF amendment criteria & format

TAF amended when it stops matching current or expected conditions.

Triggers:

• METAR, SPECI, or PIREP showing significant change from forecast
• Forecaster's judgment: TAF no longer representative

Amendment criteria (threshold changes in):

• Ceiling
• Visibility
• Present weather
• Wind speed and direction
• Existence of low-level wind shear

Also amended:

• Unforecast freezing precipitation begins or is expected (TC AIM MET 3.1 Table 3.1)
• Both ways: weather better than forecast too
• Typos or omissions making meaning unclear, even with no weather change

Small METAR AUTO vs TAF difference does not by itself mean TAF needs amending. Forecaster uses more data than automated sensor alone (TC AIM MET 7.4).

Exception: no amendment needed for a ceiling or visibility change when both values are below minima:

• Normal VFR minima here: ceiling < 1,000 ft and/or ground visibility < 3 SM, not the general controlled/uncontrolled airspace VFR minima.

Format of an amended TAF (TC AIM MET 8.2):

• TAF AMD replaces TAF before the aerodrome identifier.
• Covers only the remaining period of the original forecast. No fresh 24 to 30 h window.
• Issue time in the body always shows the latest version.

AWOS-based TAF, sensor missing, inoperative, or below standard (TC AIM MET 7.4):

1. Forecaster tries to infer missing element from other data.
2. May include a remark in the TAF.
3. If not inferable: may cancel TAF outright, pending a fix.

Trap: only deterioration triggers amendment (improvement counts too); only observed or only forecast below minima skips amendment (both must be below); TAF AMD opens a fresh full validity period; cancelling before trying to infer the missing element.

Practice