Definition
Hazardous weather conditions encountered in flight that were not predicted by the weather forecast the pilot used during preflight planning. Typical examples include unexpected icing, thunderstorms, low ceilings, reduced visibility, or strong winds that the forecast did not warn about.
Plain English
Bad weather you run into in flight that the forecast did not say would be there.
Context Anchor
Used in instrument flying and emergency operations when the actual weather becomes worse than the forecast or preflight briefing led the pilot to expect.
Derivation
Unforecast' simply means 'not forecast' -- the prefix 'un-' (Old English, meaning 'not') attached to 'forecast.' 'Adverse' comes from the Latin 'adversus,' meaning 'turned against' or 'opposing.' Together the phrase describes weather that works against the flight and was not predicted.
Why Pilots Care
It can force an immediate diversion, altitude change, or emergency declaration because the pilot prepared for different conditions.
Grounding Statement
A pilot may depart with a legal and reasonable plan, then find that the weather ahead is much lower, rougher, or more dangerous than anyone forecast.
Intuition Check
Do not assume “unforecast” means impossible to prepare for; it means the forecast did not predict it. Do not assume “adverse” means merely inconvenient; in this context it means weather that can affect safety or the ability to continue the flight as planned.
Example Sentence 1
The pilot encountered unforecast adverse weather over the mountains and requested a lower altitude to stay clear of building clouds.
Example Sentence 2
Unforecast adverse weather along the route prompted the crew to declare an emergency and request a lower altitude.