W

• warm (air mass)

  Weather

• west or western longitude

  ICAO

• White

  ICAO

W has 3 meanings; the right one depends on context. A weather report, a NOTAM or an ATC clearance. Got a NOTAM full of contractions? Paste it into the decoder.

Source: FAA Order JO 7340.2 Contractions. Not for operational use.