Low ceilings pin Louisville and Charleston. The rest of the network flies clear.

The cooperative weather. Visibility's good, ceilings are high, and controllers can run a normal arrival rate. This is each airport's baseline for lag-matched delay pressure.

The sneaky one. Still legal for plenty of operations, but ceilings or visibility have dropped enough that throughput starts to dip, sometimes a lot, depending on the airport.

Instrument Flight Rules. Low or very low ceilings/visibility. Spacing widens, arrival rates fall, and at constrained airports the queue starts piling up fast.

The half-hourly airport observation. Wind, visibility, clouds, temperature, pressure, present weather. Boring until it isn't.

The airport forecast for the coming hours. We treat it as a leading indicator, not a guarantee. A TAF that calls for IFR doesn't always deliver one.

A pilot reporting what they actually flew through: turbulence, icing, cloud tops. FL means flight level, roughly hundreds of feet. PIREPs are gold; they're real conditions, not modelled ones.

The official heads-up that something hazardous is happening over a region: thunderstorms, severe icing, big turbulence. We let these inflate scores in the affected boxes.

Forecast models. NBM is a blended ensemble; GFS is the global standard. We use them as guidance. They explain a piece of risk, never the whole thing.