FORECASTDBZ.COM · HOW IT WORKS HOMERADARCOLOURS ABOUT FORECAST EAGLE

What this is

Forecast dBZ shows MRMS — NOAA's Multi-Radar/Multi-Sensor system, which merges every NEXRAD radar in the country, plus Canadian radars, satellite, lightning and surface observations, into one national grid at about one kilometre. It is not one radar's picture stitched to its neighbour's; it is a single mosaic in which the seams between radars have already been reconciled.

It is a companion to Forecast Eagle, which covers the forecast models, and weather.city, which covers places. This site covers what is happening right now.

The nine fields

AND WHAT THE NUMBERS ACTUALLY MEAN
FieldUnitUpdatesWhat it tells you
Composite ReflectivitydBZ~2 min The strongest echo anywhere in the column. Roughly: 20 is light rain, 40 is heavy, 50+ is a downpour or hail, 60+ is almost always hail.
Precipitation Ratemm/h~12 min How hard it is falling now. Derived with a relationship that accounts for hail and snow, so it does not read a hailstorm as a cloudburst.
QPE, 1 hourmm~12 min How much actually fell in the last hour. 25 mm is an inch.
QPE, 24 hoursmm~30 min A day of accumulation — the flooding view.
Hail (MESH)mm~12 min Maximum estimated hail size the column can support. 25 mm (one inch) is severe criteria, and the colour scale breaks there deliberately.
VILkg/m²~12 min Total water mass in the column — the storm's weight. A fast climb is the classic hail signature.
Echo Top (18 dBZ)kft~12 min How tall the storm is, in thousands of feet. Tops climbing through 50 kft means an updraft worth watching.
Rotation (120-min)×10⁻³ s⁻¹~15 min Where rotation has passed in the last two hours — a swath, not an instant. A strong mesocyclone reads 5–15.
Lightningfl/km²/min~12 min Cloud-to-ground strike density over the last five minutes.

How it is built

The frames store numbers, not pictures

Most radar sites store their imagery already coloured in. That is fine until you want two things this site wants. A coloured picture cannot tell you what the value under your finger is — you can only match a colour against a legend by eye, and guesswork about dBZ is the one thing worth refusing to ship. And a baked-in palette makes the colour scale permanent: changing it would mean repainting the whole archive.

So every frame here is stored as 255 levels of a physical quantity, and the colour is applied at the moment a tile is drawn. That is why you can switch between four reflectivity scales instantly, and why tapping the map returns a measured number. It also happens to be smaller — a sparse field of numbers compresses better than a picture of one.

Nothing is thrown away between scans

MRMS publishes a new reflectivity scan roughly every two minutes, at no fixed second. Any system that simply polls on a timer will drift against that and quietly lose scans — a sister site measured 11–16% of its frames disappearing this way before the problem was found. Here every scan is written under its own permanent name the moment it arrives, so a slow moment delays a frame instead of destroying it, and the loop you scrub has no invisible holes in it.

The map underneath is ours

The basemap is OpenStreetMap, built into vector tiles and served from our own hardware down to street level — no third-party tile service, no API key, nothing about your location leaving this site. Place names are drawn over the weather rather than under it, with a heavy halo, because a town buried beneath a 55 dBZ core has stopped being a town.

How many names appear is a control rather than a consequence of how far you have zoomed: watching a squall line cross a rural county wants more labels than the default, and a national screenshot wants far fewer.

Using it

What this is not

Forecast dBZ is an independent project. It is not affiliated with or endorsed by NOAA, the National Weather Service, or any other government agency, and it is not an official source of weather warnings.

Radar shows what is happening, not what has been warned. If there is severe weather near you, get warnings from weather.gov, NOAA Weather Radio, or your phone's emergency alerts — and act on those, not on this map.

Credits