Reading the variables
Two kinds of field here. Simulated-storm fields (UH, reflectivity) come from storms the model itself produces — treat them as "the HRRR made a rotating storm here", not a literal tornado forecast; position can be off by tens of miles. Environment fields (SRH, CAPE) describe the ingredients available to any storm that forms. The classic tornado setup is high UH tracks overlapping large 0–1 km SRH and adequate CAPE. Click a row to display that field.
| Field | What it is | Why it matters for tornadoes | Values to watch |
|---|---|---|---|
| UH 2–5 km | Updraft helicity: updraft speed × spin, integrated 2–5 km up. The past-hour maximum, so storm motion leaves "tracks". | The model's mesocyclone detector — a supercell's rotating mid-level updraft lights up here. Long, intense tracks = persistent supercells. | >25 rotating storm · >75 strong supercell · >150 intense, tornado-capable |
| UH 0–2 km | Same quantity but for the lowest 2 km of the storm. | Rotation near the ground is the more direct tornado signal — mid-level mesocyclones are common, low-level ones much less so. | >10 notable · >30 strong low-level rotation · >75 high-end |
| Reflectivity | Simulated radar: the strongest echo in each column, as a radar would see it. | Shows storm mode — discrete cells vs. squall lines. Discrete supercells (isolated strong cores) are the most tornado-prone mode. | >35 dBZ thunderstorms · >50 strong cores · hook/kidney shapes = supercell structure |
| SRH 0–1 km | Storm-relative helicity: how much spin the wind profile in the lowest 1 km feeds into a moving storm. | The key environmental ingredient for tornadoes specifically — large low-level SRH is what lets a supercell tighten rotation to the surface. | >100 supportive · >200 favorable for significant tornadoes · >300 high-end |
| SRH 0–3 km | Same, over the lowest 3 km. | Governs whether storms rotate at all — supercell potential. Necessary but not sufficient for tornadoes. | >150 supercells possible · >300 strongly favorable |
| SBCAPE | Surface-based CAPE: buoyant energy for air rising from the ground — the fuel. | Storms need instability to form and sustain updrafts; tornado setups need surface-based (not "elevated") instability. Modest CAPE is enough if SRH is large. | >500 enough with strong shear · >1500 moderate · >3000 very unstable |