Severe Weather Alerts, Explained — Warning vs Watch vs Advisory

At 3 p.m. the phone buzzes: Tornado Watch until 9 PM. At 5:40 it buzzes again: Tornado Warning — take shelter now. The two messages sound nearly identical, and every year emergency managers repeat the same plea, because confusing them can cost lives. This guide is the plain-language version of the US alert system — what each level means, who issues it, what the colours on the Severe Alerts layer mean, and what actually happens between a forecaster's desk and a shaded shape on the live map.

Watch, warning, advisory — the ladder

The National Weather Service (NWS), the US government's forecasting agency, issues every official weather alert in the United States from its network of over a hundred local forecast offices. Its alerts come in three working levels:

A watch means the ingredients are there. Conditions are favourable for the hazard over a broad area in the coming hours — the classic example is a Tornado Watch covering parts of several states for an afternoon. Nothing is confirmed yet; you're being told to stay alert, keep an eye on the sky and know what you'd do. The NWS shorthand: be prepared.

A warning means it's happening, or about to. The hazard has been detected — a radar-indicated rotation, a spotter report, a flash-flood gauge spiking — and the threat to life or property is imminent. Warnings are smaller and shorter than watches, and they mean act now: take action. A Tornado Warning means take shelter immediately, not "monitor the situation."

An advisory sits below a warning: the event is happening or likely, but it's expected to be a hassle rather than a disaster if people use caution — Heat Advisories, Wind Advisories, Dense Fog Advisories, the mariner's Small Craft Advisory. Don't dismiss the tier: heat is among the deadliest weather hazards in the US, and it usually arrives wearing an advisory first.

The same event name climbs the ladder as confidence and danger rise: Winter Storm Watch → Winter Storm Warning; Flood Watch → Flood Warning. Learn the ladder once and every alert type reads instantly.

The severity colours

Beyond the watch/warning ladder, every alert in the NWS data feed carries a machine-readable severity field — Extreme, Severe, Moderate or Minor — and that is what shades the map. Extreme paints red. Severe paints amber. Moderate paints blue. Minor paints grey. These are the map's semantic alert colours, deliberately kept apart from its everyday brand palette, so that red on this layer only ever means one thing. On a big severe-weather day you can read the country's danger at a glance: a red-cored amber mass over the Plains, blue heat advisories draped across the South, grey marine advisories tracing the coasts.

Tap any shaded region and the full alert opens: the event name, the severity, the exact area, the official instructions, the expiry time and the forecast office that issued it — the same text your phone's emergency alert quotes, unabridged.

From a forecaster's desk to a shape on the map

Here is the honest engineering story, because it explains something you'd otherwise notice and wonder about.

When an NWS forecaster issues a short-fuse warning — tornado, severe thunderstorm, flash flood — they draw a storm-based polygon around the actual threat, and that shape travels with the alert. But most alerts aren't like that. Heat advisories, winter storm watches, coastal and marine warnings are issued for predefined zones — forecast zones, counties, marine areas — and the raw public feed carries only the zone's identifier, not its geometry. On a typical day the zone-coded alerts are the majority: a live check of the feed on July 4, 2026 found 426 active alerts, of which 256 were zone-coded and only 170 carried their own polygon.

A map that painted only the ready-made polygons would silently drop most of the country's active alerts — every heat advisory, most winter weather, nearly all marine warnings. So the Severe Alerts layer resolves each zone ID to its real official boundary, published by the NWS itself, and keeps those boundaries cached (zones are stable — they change rarely). The result is that the whole alert set paints, warnings and watches and advisories alike. When an alert can't yet be resolved to a shape, it is counted honestly on the layer's readout rather than invented — the map will tell you "mapped 279 of 282" rather than quietly pretend.

Everything on the layer comes from NWS / weather.gov, is attributed as such on the map, and refreshes on a minutes cadence — alerts are short-fuse by nature.

The US-only scope, stated plainly

The layer currently covers the United States and its territories, because that is what the NWS feed covers — and the map says so on the glass rather than leaving you to guess. Official alert feeds exist for much of the world (Europe's MeteoAlarm aggregates the EU national services), but redistribution rights differ country by country, and each source's licence is being reviewed properly before it ships. The rule here is simple: nothing gets painted from a feed the site isn't clearly licensed to show. More regions are planned.

Using it

Open the live weather map and flip SEVERE ALERTS in the rail — or start from the Severe Alerts layer page, which boots the map with the layer already on. Stack it with the rain radar to watch warnings track with the storms that triggered them, or with Storm Energy to see the fuel behind a Tornado Watch. If you use My Local Forecast or pin a place in My Weather Station, any active alert for that exact point appears as a strip right on the forecast — with the expiry time shown in the place's own clock, not yours.

One ladder, four colours, one honest map. The next time the phone buzzes, you'll know exactly which kind of buzz it is.