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How this is built. Every two minutes NOAA's Multi-Radar/Multi-Sensor system (MRMS) merges the nation's weather radars into one 1 km grid. We fold every frame into a 2-minute bin (rain, heavy-rain, snow and ≥40 dBZ minutes; peak reflectivity, rain rate and MESH hail size; rain totals), and those into 5-minute bins together with NLDN cloud-to-ground lightning density, then into hourly bins, which use MRMS's gauge-corrected MultiSensor QPE for rainfall once it posts (about 2 hours later), and then into weather days that start at 12 UTC. Now, and 1 to 3 hours, move every two minutes, as each frame lands; 6 to 72 hours every fifteen minutes and 5 to 30 days hourly, because they do not change in two. The 5- and 15-minute windows step on the 5-minute beat instead, so that they are exactly five and fifteen minutes rather than whichever side of it a 2-minute edge fell on. Each window carries the moment it runs through — hover a window button — and every one is assembled exactly: to the frame from 1 to 3 hours, to the 5 minutes out to 72, to the hour for 5 to 30 days. Every 1 km cell carries a share of its census tract's people, homes and owner-occupied home value (ACS 2020–2024 5-year, spread evenly over the tract's cells), so "people under rain" counts the people whose own square kilometre had radar-indicated precipitation. MRMS posts a new scan every two minutes, so Now — the latest scan — is the shortest window there is to have; lightning is a 5-minute product that posts about ten minutes late, so it moves on the 5-minute beat rather than the 2-minute one. "Partial" marks a window whose time the archive does not fully cover yet. Radar sees precipitation, not what reached the ground: mountains block beams, far ranges overshoot shallow rain, and MESH is an estimate of the largest hailstone, not a report. Wind is not radar. Peak gust, Sustained wind and Gusty hours (time under gusts of 40 mph or more) come from the Weather Service's 2.5 km surface analyses, one per hour, each 1 km cell taking the analysis point it falls in. The record is URMA, which waits about seven hours for late station reports; the hours it has not reached yet are RTMA, and each one is replaced by URMA when it posts, as the rainfall is by the gauge-corrected estimate. Over 18 hours measured side by side the two differ by about half a mile an hour on average, but the people counted under ≥ 40 mph gusts can differ by a factor of two in a single hour, because the few cells that change can sit where people live. Now is the 15-minute RTMA-RU analysis. Wind windows are the last N hourly analyses and run through the newest one, so they end about an hour behind the radar, and there are no 5- or 15-minute wind windows. A wind field is never zero, so "had it" means gusts of 40 mph or more, or sustained winds of 31 mph (a Wind Advisory's threshold); 58 mph gusts are the severe-thunderstorm criterion and 74 mph is hurricane force. Temperature comes from the same analyses: High and Low temp are the warmest and coldest of the hourly analyses (a peak between two of them reads a degree or so short of a thermometer's), Heat index uses the Weather Service's formula wherever it is 80°F or more, Wind chill wherever it is 50°F or colder with a 3 mph wind, and Heat hours and Freezing hours count the hours with a heat index of 100°F or more, or a temperature at or below 32°F. "Had it" is a 90°F high, a freeze, a 100°F heat index or a wind chill of zero. Stations puts what was measured over what was estimated: every METAR and special report from 2,270 Lower-48 stations (the Station Interrogator's roster, kept live from the Aviation Weather Center every two minutes), over the same hours as the map — the radar's window for rainfall, the analyses' for wind and temperature — with the analysis at each station's cell beside its reading. A station's peak gust is the larger of each report's gust group and its PK WND remark, the peak since the previous report, so it catches the bursts an hourly analysis cannot; its rainfall is shown only where the gauge covered 80% of the window's hours. Gauges and thermometers the Interrogator's QC condemned are left out. What happened steps out of the live windows and onto a finished weather day, 12 UTC to 12 UTC, for any of the last thirty: the day's radar hour by hour, the URMA rainfall analysis over it, the nearest station's own record of it, every severe and flood warning the Weather Service issued, and the people, homes and dollars inside five miles of wherever you put the pin.

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