What SRH measures
Storm-Relative Helicity (SRH) measures the streamwise vorticity available to a storm's updraft within a given layer, expressed in m²/s². In plain terms, SRH measures how much of the wind shear in your environment is oriented to make an updraft rotate when a storm moves through it.
It's "storm-relative" because the wind a storm experiences depends on how it is moving — not just on the ground-relative wind profile. A storm moving northeast at 30 kt feels a different wind field than a storm moving east at 15 kt, even in the same atmosphere.
The two layers you'll see most
- 0–3 km SRH — historically the standard tornado parameter. Useful but a little dated.
- 0–1 km SRH — modern preferred parameter for tornadoes; correlates strongly with low-level mesocyclone strength.
- 0–500 m SRH — newer and emerging, captures the very low-level shear most relevant to tornadogenesis.
Most chasers focus on 0–1 km SRH as the headline number, with 0–500 m as a tiebreaker on borderline setups.
Rough buckets for 0–1 km SRH
| 0–1 km SRH (m²/s²) | What it tells you |
|---|---|
| 0–100 | Weak; tornado threat low |
| 100–150 | Marginal; possible brief tornadoes |
| 150–250 | Favorable; supercells likely to be tornadic |
| 250–400 | Strong; significant tornadoes increasingly likely |
| 400+ | Extreme; outbreak-level low-level shear |
Like all severe weather parameters, these numbers are rules of thumb, not laws. SRH only matters when paired with CAPE, moisture, and a way to actually form storms.
Why "streamwise" matters
The technical reason SRH is the parameter, not just "shear," is that streamwise vorticity — vorticity oriented along the storm-relative wind — is what gets tilted into the updraft and amplified into rotation. Crosswise vorticity doesn't do that as efficiently.
This is why veering wind profiles (winds turning clockwise with height — south at the surface, southwest at 850 mb, west at 500 mb) generate the most SRH and the most tornadoes. Backed surface winds (more east-southeast than south) supercharge this further by increasing the low-level curvature of the hodograph.
The hodograph view
If you've ever looked at a sounding and seen a curly hodograph in the lower atmosphere, you've seen SRH visualized. A long, sickle-shaped low-level hodograph means lots of streamwise vorticity available — a tornado factory. A straight or short hodograph means little SRH, and even big CAPE will mostly produce hail.
The hodograph is also why boundaries matter so much. North of a warm front, surface winds back from south to southeast, which dramatically lengthens the low-level hodograph and pumps up SRH. The triple point — where a dryline meets a warm front — often has the most extreme local SRH on the whole map.
SRH plus CAPE — the real recipe
A common tornado-day signature on SPC mesoanalysis:
- 0–1 km SRH ≥ 200 m²/s²
- MLCAPE ≥ 1500 J/kg
- Surface dewpoints ≥ 60°F
- Effective bulk shear ≥ 40 kt
- LCL heights ≤ 1000 m
When all five line up, the Significant Tornado Parameter (STP) lights up, and SPC usually starts hatching the tornado probability map.
How chasers actually use SRH
- Check the 0–1 km SRH field on SPC mesoanalysis before locking a target.
- Look for localized SRH maxima along boundaries — that's where storms become tornadic.
- Compare model-forecast SRH with observed surface winds. If winds aren't as backed as modeled, SRH is overdone.
- Watch for SRH increase through the afternoon as the low-level jet ramps up — a classic late-day tornado signal.
Common mistakes
- Reading 0–3 km SRH only. The 0–1 km value is more discriminating for tornadoes.
- Ignoring CAPE. SRH without CAPE gives you a pretty hodograph and no storms.
- Treating SRH as a single number. Where on the map it's maximized — and whether your target storm will actually get there — matters more than the peak value.
- Forgetting it evolves. SRH can double in two hours when the nocturnal low-level jet kicks in.
Sources to know
- SPC Mesoanalysis page — the canonical real-time SRH product
- NWS Glossary entry on SRH
- The classic Thompson, Edwards, Hart (2003) paper on effective-layer SRH
How Meso reads SRH
Meso pulls 0–1 km and 0–500 m SRH from the latest HRRR and SPC mesoanalysis as part of the storm-evaluation stack behind "Is this storm going to produce?" — one of the eight core chaser questions Meso is built around. When SRH spikes locally — say, on the cool side of a freshly-laid outflow boundary — Meso flags the change under "Did anything important just change?" so you can re-evaluate which storm to be on before the hodograph wraps up.
SRH is one of the few parameters where the change through the day is often more informative than any single snapshot. Meso watches the change so you can stay focused on driving and visual.