Storm cells

Base URL: https://api.wxalerts.org

Every storm cell NOAA’s ProbSevere is tracking over the CONUS, the rotations found inside them, where each has been, and where its fitted motion puts it next. This is the data behind the storm layer on the live map, which is just a client of these routes.

GET /storms/active/geojson
GET /storms/active/geojson?lead=45&profile=tight&west=-103&south=33&east=-94&north=40
GET /storms/{storm_id}/track?minutes=60
GET /storms/{storm_id}/forecast-track?lead=30
GET /storms/{storm_id}/cone?lead=30&profile=wide

No key needed for these four. They share the per address limit. Browsers block calls from other websites (CORS allows only wxalerts.org), so call them from a server, script or app.

GET /v1/storms/active?min_prob_tor=10
GET /v1/storms/{storm_id}
GET /v1/storms/couplets/{couplet_id}/cone?lead=15

Needs an API key (storms:read) for these three, and they are served only under /v1. Without the prefix they answer 401 api_key_required.

This page is the wire format. What the shapes mean, how far to trust a cone, and why the tornado and severe numbers are on different scales is in Storm analysis, starting with Reading the map.

Ids

storm_id is WxAlerts’ own id for a storm. ProbSevere renames a cell when it merges or splits, and the ingest stitches those back into one storm; every ProbSevere ID it has worn is in feed_ids. Use storm_id for anything you keep. couplet_id is the id of a tracked rotation, which is linked to a cell (storm_id, possibly null) but tracked on its own.

The per storm routes take any id ever issued, not only active ones, so a client holding an id from earlier still gets an answer.

Freshness

Every collection response carries feed_valid_time, the newest ProbSevere scan ingested, and feed_age_seconds. The feed publishes about every two minutes and lands two to three and a half minutes after its valid time, so an age of 100 to 220 seconds is normal. Check it: an empty response looks the same whether the sky is quiet or the feed stopped. A storm drops out of the active set once it has not been seen for 15 minutes. Responses are cached for 30 seconds (Cache-Control: public, max-age=30); polling faster gains nothing.

GET /storms/active/geojson

Everything a map layer needs in one FeatureCollection: cells, their tracks, forecast lines and cones, and the rotations with theirs. One request rather than several, so a cone never belongs to a cell that has since moved.

Cells are ordered by ProbTor, then ProbSevere, highest first, so limit keeps the ones that matter.

{
  "type": "FeatureCollection",
  "server_time": "2026-11-09T19:07:28.027599Z",
  "feed_valid_time": "2026-11-09T19:04:34Z",
  "feed_age_seconds": 172.44656,
  "lead_minutes": 45,
  "profile": "tight",
  "features": [
    {
      "type": "Feature",
      "geometry": { "type": "Polygon", "coordinates": [[[-96.88402, 25.28124], "...", [-96.88402, 25.28124]]] },
      "properties": { "kind": "cone", "storm_id": 297397, "lead_minutes": 45, "profile": "tight",
                      "prob_tor": 1, "prob_severe": 3, "alerted": false }
    },
    {
      "type": "Feature",
      "geometry": { "type": "Polygon", "coordinates": [[[-96.98, 25.87], [-96.92, 25.87], "...", [-96.98, 25.87]]] },
      "properties": {
        "kind": "cell", "storm_id": 297397, "tier": null,
        "prob_tor": 1, "prob_severe": 3, "prob_hail": 0, "prob_wind": 1,
        "peak_prob_tor": 1, "peak_prob_severe": 6,
        "speed_kmh": 20.2, "cone_speed_kmh": 24.4, "bearing_deg": 49,
        "alerted": false, "alerted_at": null,
        "valid_time": "2026-11-09T19:04:34Z", "first_seen": "2026-11-09T17:32:36Z",
        "scan_count": 47, "feed_ids": [81531], "lon": -96.922616, "lat": 25.585429,
        "max_llaz": 0.008, "p98_llaz": 0.006,
        "couplet_lon": -96.98750000000001, "couplet_lat": 25.8425, "couplet_shear": 0.00800000037997961
      }
    },
    {
      "type": "Feature",
      "geometry": { "type": "LineString", "coordinates": [[-96.9125, 25.7825], [-96.84111, 25.76027]] },
      "properties": {
        "kind": "couplet-forecast", "couplet_id": 96819, "storm_id": 297397, "state": "confirmed",
        "lead_minutes": 15, "speed_kmh": 30.3, "bearing_deg": 109,
        "ticks": [{ "minutes": 15, "lon": -96.84111, "lat": 25.76027, "at": "2026-11-09T19:19:34Z" }],
        "shear": 0.006000000052154064, "alerted": false
      }
    },
    {
      "type": "Feature",
      "geometry": { "type": "Point", "coordinates": [-95.1525, 29.7375] },
      "properties": {
        "kind": "couplet", "couplet_id": 96829, "tier": null, "storm_id": 297366, "state": "emerging",
        "shear": 0.00800000037997961, "peak_shear": 0.00800000037997961, "mid_shear": 0.003000000026077032,
        "rqi": 1.0, "prob_tor": 0, "diameter_km": 0.82, "vrot_ms": 3.3, "stacked": false,
        "debris_confirmed": null, "rho_hv": null, "confirmed_at": null,
        "speed_kmh": null, "bearing_deg": null, "alerted": false, "alerted_at": null, "scan_count": 2,
        "first_seen": "2026-11-09T19:02:37Z", "last_seen": "2026-11-09T19:04:34Z",
        "lon": -95.1525, "lat": 29.7375
      }
    },
    {
      "type": "Feature",
      "geometry": { "type": "LineString",
                    "coordinates": [[-96.92262, 25.58543], [-96.87691, 25.62156], [-96.83118, 25.65769], [-96.78543, 25.69379]] },
      "properties": {
        "kind": "forecast-track", "storm_id": 297397, "lead_minutes": 45,
        "speed_kmh": 20.2, "cone_speed_kmh": 24.4, "bearing_deg": 49,
        "ticks": [
          { "minutes": 15, "lon": -96.87691, "lat": 25.62156, "at": "2026-11-09T19:19:34Z" },
          { "minutes": 30, "lon": -96.83118, "lat": 25.65769, "at": "2026-11-09T19:34:34Z" },
          { "minutes": 45, "lon": -96.78543, "lat": 25.69379, "at": "2026-11-09T19:49:34Z" }
        ],
        "prob_tor": 1, "prob_severe": 3, "alerted": false
      }
    },
    {
      "type": "Feature",
      "geometry": { "type": "LineString", "coordinates": [[-97.12435, 25.80059], [-97.06814, 25.68414], "...", [-96.92262, 25.58543]] },
      "properties": {
        "kind": "track", "storm_id": 297397, "points": 29, "minutes": 60, "prob_tor": 1, "alerted": false,
        "jumps": [0, 1, 27, 28],
        "times": ["2026-11-09T18:08:37Z", "2026-11-09T18:10:33Z", "...", "2026-11-09T19:04:34Z"]
      }
    }
  ]
}

That example was requested with detail_min_prob_tor=0&emerging_couplets=true on a quiet afternoon, so the low numbers are real.

Feature kinds

Every feature carries properties.kind. Features come in draw order: cones first, then cells, then the couplets with their cones, lines and tracks, then the cell forecast lines and tracks. Render in sequence and the lines land on top.

“Detailed” means at or above detail_min_prob_tor. A cell or rotation with no motion fit yet (its first scans) gets no cone or forecast line rather than a circle that would claim it is standing still.

A turn-cone is part of the same cone, reaching along the HRRR storm motion heading for a storm that may turn. Draw it like the cone, a shade quieter, never as a second forecast. It is a separate feature so a client that does not know about it still draws the main cone. What the shapes mean on the map: The four shapes and The second, solid cone.

Cell properties

Couplet properties

A couplet is a rotation found in the MRMS azimuthal shear grid and tracked scan to scan, independent of whichever cell holds it. How they are found and gated: Rotation detection.

Lines, cones and tracks

• Forecast lines (forecast-track, couplet-forecast) start at the object and carry ticks, evenly spaced about every 15 minutes and ending exactly on the lead (20 gives one tick at 20; 45 gives 15, 30, 45). Each tick has minutes from the scan, its lon/lat, and at, the absolute time it stands for. Use at for anything that quotes an arrival. The line is the middle of the cone, not a promise. The forecast track.
• Cones (cone, turn-cone, couplet-cone) carry lead_minutes, profile, and the owner’s prob_tor, prob_severe (cells) or state, shear (couplets) so you can color them without a lookup.
• Tracks (track) run oldest first. times gives each coordinate’s scan time, and jumps lists the indexes where the centroid moved more than 10 km in one scan because the polygon absorbed a neighbour: break the line there rather than draw a storm teleporting. couplet-track carries points, shear and alerted, with no times.

The tier object

tier on a cell or couplet is null or the WXA/SA tier, served so that every client renders the same label and the same sentence:

Render claim rather than writing your own. Where the tiers come from: The ladder.

Cone profiles

profile picks how much area a cone spends for how much certainty. Both are measured against where storms actually went.

Area is the real cost: a wide cell cone grows about five times between 10 and 60 minutes of lead, and everyone inside it is being told something. A cell cone has no rear margin; a rotation’s cone does, because rotations die and re-form upstream. Rotation cones stop at 20 minutes because past that they grow larger than the cell cone they are meant to narrow. The reasoning, with numbers, is in Accuracy and The rotation’s own cone.

GET /storms/{storm_id}/track

Where one storm has been: every scan in the last minutes (1 to 180, default 60), oldest first, across every ProbSevere ID it has worn. The window is counted back from now, so a storm that ended long ago returns an empty list. An unknown id is also an empty list, not a 404.

{
  "storm_id": 297397,
  "points": [
    { "valid_time": "2026-11-09T19:02:37Z", "lon": -96.927201, "lat": 25.479222,
      "prob_tor": 0, "prob_severe": 2, "prob_hail": 0, "prob_wind": 0,
      "speed_kmh": 20.236125946044922, "bearing_deg": 48.74721145629883,
      "feed_id": 81531, "jumped": true },
    "..."
  ],
  "geometry": { "type": "LineString", "coordinates": [[-96.91801, 25.75726], "...", [-96.92262, 25.58543]] }
}

geometry is null until there are two points. jumped marks a scan whose centroid moved more than 10 km: a real observation, left out of the motion fit.

GET /storms/{storm_id}/forecast-track

The forecast line on its own, for any storm, including the ones under detail_min_prob_tor that the bulk feed draws no line for. lead is 1 to 60, default 30. There is no profile: both profiles share this line.

{
  "type": "Feature",
  "geometry": { "type": "LineString", "coordinates": [[-96.92262, 25.58543], [-96.86167, 25.63361]] },
  "properties": {
    "kind": "forecast-track", "storm_id": 297397, "lead_minutes": 20,
    "speed_kmh": 20.2, "cone_speed_kmh": 24.4, "bearing_deg": 49, "valid_time": "2026-11-09T19:04:34Z",
    "ticks": [{ "minutes": 20, "lon": -96.86167, "lat": 25.63361, "at": "2026-11-09T19:24:34Z" }]
  }
}

The line is drawn at cone_speed_kmh (the track speed blended with the HRRR storm motion, or the track speed alone where there is none); speed_kmh is the raw track speed. Read positions and times from ticks rather than multiplying either.

GET /storms/{storm_id}/cone

One storm’s cone. lead 1 to 60 (default 30), profile wide or tight.

{
  "type": "Feature",
  "geometry": { "type": "Polygon", "coordinates": [["...", "..."]] },
  "properties": {
    "kind": "cone", "storm_id": 297397, "lead_minutes": 30, "profile": "wide",
    "steered": true, "speed_kmh": 20.2, "cone_speed_kmh": 24.4, "bearing_deg": 49,
    "area_km2": 4407, "turn_cone": null
  }
}

steered is true when the cone is drawn at the HRRR blended speed with the steered profile. area_km2 is the main cone’s area. turn_cone is the turn lobe’s Polygon geometry when the storm has one, else null; it overlaps the main cone, so the two areas do not add.

GET /v1/storms/active

The same cells as the GeoJSON route, without geometry, as plain JSON. Takes west, south, east, north, min_prob_tor, min_prob_severe, alerted_only and limit exactly as above. Needs an API key (storms:read).

{
  "server_time": "2026-11-09T19:07:20.745325Z",
  "feed_valid_time": "2026-11-09T19:04:34Z",
  "feed_age_seconds": 166.645814,
  "storms": [
    {
      "id": 297397,
      "first_seen": "2026-11-09T17:32:36Z", "last_seen": "2026-11-09T19:04:34Z",
      "valid_time": "2026-11-09T19:04:34Z",
      "prob_severe": 3, "prob_hail": 0, "prob_wind": 1, "prob_tor": 1,
      "peak_prob_tor": 1, "peak_prob_severe": 6,
      "speed_kmh": 20.236125946044922, "bearing_deg": 48.74721145629883,
      "cone_speed_kmh": 24.379592895507812,
      "steer_u_kmh": 6.4435505867004395, "steer_v_kmh": 27.785707473754883,
      "cross_track_km": 55.9759635925293, "along_track_km": 38.771297454833984,
      "scan_count": 47, "feed_ids": [81531],
      "alerted": false, "alerted_at": null,
      "lon": -96.922616, "lat": 25.585429
    },
    "..."
  ]
}

Beyond the cell properties above: steer_u_kmh and steer_v_kmh are the HRRR storm motion sampled at the storm (east and north components, km/h), and cross_track_km and along_track_km are the storm’s measured width across and along its motion, which set the cone’s starting width. Values here are unrounded; the GeoJSON route rounds motions to 0.1 km/h and whole degrees.

GET /v1/storms/{storm_id}

One storm in full: everything in /v1/storms/active plus the polygon and the diagnostics behind the score. 404 for an unknown id. Needs an API key (storms:read).

{
  "id": 297397,
  "...": "every field of /v1/storms/active",
  "geometry": { "type": "Polygon", "coordinates": [["...", "..."]] },
  "latest_feed_id": 81531,
  "active": true,
  "missing_scans": 0,
  "motion_fit_points": 0,
  "properties": {
    "ID": "81531", "ProbTor": "1", "ProbSevere": "3", "ProbHail": "0", "ProbWind": "1",
    "MLCAPE": "728", "MUCAPE": "1262", "EBSHEAR": "30", "SRH01KM": "28", "MESH": "0.06",
    "MAXLLAZ": "0.008", "P98LLAZ": "0.006", "FLASH_RATE": "0", "COMPREF": "56.0",
    "MOTION_EAST": "2.8", "MOTION_SOUTH": "-3",
    "...": "..."
  }
}

• active is whether the storm is still in the tracked set; missing_scans is how many scans in a row it has not been found.
• motion_fit_points is how many centroids (of a ten point window) back the motion fit on this scan. 0 means a merge just cut its history and it is coasting on its previous velocity, which is still the best estimate.
• properties is ProbSevere’s own property set for the newest scan, verbatim, every value a string. Its MOTION_EAST and MOTION_SOUTH are in no identifiable unit; use speed_kmh and bearing_deg.

GET /v1/storms/couplets/{couplet_id}/cone

A rotation’s own cone, for any couplet id, whatever its state or alert status (the bulk feed draws couplet cones only for alerting rotations). lead 1 to 25, default 15, drawn at no more than 20. profile as above. Needs an API key (storms:read).

{
  "type": "Feature",
  "geometry": { "type": "Polygon", "coordinates": [["...", "..."]] },
  "properties": {
    "kind": "couplet-cone", "couplet_id": 96829, "storm_id": 297366,
    "state": "confirmed", "alerted": false, "lead_minutes": 15, "profile": "wide",
    "shear": 0.00800000037997961, "vrot_ms": 3.3, "diameter_km": 0.82, "prob_tor": 0,
    "speed_kmh": 8.3, "bearing_deg": 180, "area_km2": 301
  }
}

lead_minutes is the lead actually drawn, so lead=25 comes back as 20.

Errors

The shared error shapes are on the REST overview.