Step 1 — Observe everything, everywhere
A forecast starts as a snapshot of the atmosphere right now: surface stations, weather balloons (launched twice daily from ~100 US sites), aircraft sensors, ocean buoys, radar, and satellites measuring temperature, humidity, pressure and wind through the depth of the sky. Gaps matter — the atmosphere over the Pacific is watched mostly by satellite, which is one reason West Coast forecasts for day 5 wobble more than East Coast ones.
Step 2 — Run the physics on a supercomputer
Numerical weather prediction divides the atmosphere into millions of grid boxes and solves the fluid-dynamics equations inside each, stepping forward minutes at a time. NOAA's GFS (Global Forecast System) and the European Centre's IFS (“the Euro model”) are the two giants US forecasters watch; regional models like the HRRR zoom into the US at 3 km resolution for tomorrow's thunderstorms. A single global run is tens of trillions of calculations — this is genuinely some of the largest computing humanity does daily.
Step 3 — Ensembles: asking the model 30 questions at once
Because the initial snapshot is never perfect, forecasters run the model dozens of times with slightly different starting conditions (an ensemble). If all runs agree on rain Thursday, confidence is high. If they split — half rain, half sun — Thursday is honestly uncertain, and the forecast's 50% precipitation chance is that disagreement made visible. A probability is not hedging; it is measurement.
Why accuracy decays with time
The atmosphere is chaotic: small errors grow roughly exponentially. Modern skill is remarkable — today's 5-day forecast is about as accurate as a 3-day forecast was a generation ago — but beyond 7–10 days a daily forecast is a trend statement, not a schedule. That is why this site shows 7 days and why any app promising your exact temperature on the 14th day is selling fiction.
The human layer
National Weather Service forecasters in 122 local offices blend the models with local knowledge (sea breezes, mountain effects, urban heat) and issue the watches and warnings that matter most. The apps — this one included — are windows onto that system; for dangerous weather, weather.gov is the source of record.