Every 10 minutes we read every alert the National Weather Service has in force (api.weather.gov). We count the warnings: alerts named "… Warning" or "… Emergency", such as a Tornado Warning, Flash Flood Emergency or Winter Storm Warning. Watches and advisories are left out. So are marine warnings, which cover water, and civil messages that emergency managers send through the same system, such as a Child Abduction Emergency. A warning counts once it has begun; one issued for later counts when it starts.
We use the 2020 Census's 83,848 census tracts in the 50 states and DC, and the place where each tract's people live on average, which the Census Bureau publishes as its centre of population.
A person under two warnings counts once in the totals and once under each kind of warning. River flood warnings are often drawn as a thin outline along the river with no tract's centre inside it. Those are listed with no one counted.
It is an estimate of how many people live where a warning is in force, not of how many were affected or received it. A tract is all in or all out, so at the edge of an outline the count can be high or low by a tract, about 4,000 people. The population is the 2020 Census count, not today's.
The monthly counts come from the Iowa Environmental Mesonet's archive of National Weather Service warnings (Iowa State University; public domain, with thanks). One warning is one event the NWS numbers (a VTEC event), counted in every state it touches and in the month it was first issued, by Coordinated Universal Time. The national figure counts each warning once. We compare months from 2008 on, when the archive is complete for every office.
The numbers and maps are free to quote and republish under CC BY 4.0: credit "InstaMaps" with a link. If we find a mistake we correct the page and say so at the bottom of the report it was in. Questions: gbade@get-instamaps.com (InstaMaps data team).