Definition
A forecast weather chart issued by the National Weather Service that depicts expected significant weather conditions from the surface up to the 400 millibar pressure level (approximately 24,000 feet MSL). The chart shows forecast areas of IFR and marginal VFR conditions, turbulence, freezing levels, and the expected position of fronts and pressure systems for specified valid times, typically 12 and 24 hours into the future.
Plain English
A weather map that predicts what the weather will look like at lower altitudes — the part of the sky most general aviation flying happens in — for a few hours ahead. It highlights the bad-weather areas a pilot needs to plan around.
Context Anchor
Seen during preflight weather planning and in FAA weather chart discussions, especially when a pilot is checking forecast weather along a route below 24,000 feet.
Derivation
‘Prognostic’ comes from the Greek prognōstikos, meaning ‘knowing in advance’ or ‘foretelling.’ A prog chart is literally a forecast chart — a picture of what is expected to happen, not what is happening now. Knowing this distinguishes it from a current-conditions chart like a surface analysis.
Why Pilots Care
Allows pilots to anticipate and route around weather that could compromise safety or comfort on low-altitude flights.
Grounding Statement
Before flying, a pilot can use this chart to picture where the main weather trouble spots are expected to be in the lower part of the atmosphere.
Intuition Check
“Low-level” does not mean unimportant or only near the ground; here it means the lower aviation weather layer, from the surface up to about 24,000 feet. “Prognostic” means forecast, not current conditions.
Example Sentence 1
Before departing on the cross-country, she pulled up the low-level significant weather prognostic chart to see whether the forecast IFR area along her route would clear by her planned arrival time.
Example Sentence 2
Moderate turbulence appeared on the low-level significant weather prognostic chart over the planned route after midday.