Chaseit · wiki
A field guide to UK storms
Every entry has an At a glance line pitched at a layman and a Deep divewritten for working forecasters. If you've never heard of CAPE before, start with CAPE and follow the related links. If you're a meteorologist, the Chaseit features section tells you what feeds what.
Parameters & indices
The numeric ingredients forecasters read off a sounding or a mesoanalysis — energy, shear, moisture, lapse rates.
- Bulk shear (0–6 km)The vector difference between surface wind and the wind at 6 km — the single best predictor of whether a storm…
- CAPE — Convective Available Potential EnergyCAPE is how much fuel a rising parcel of air has in the tank. More CAPE means stronger updrafts and, all else …
- CIN — Convective InhibitionCIN is the lid on the pot. Before a storm can fire, the atmosphere has to spend energy breaking through a warm…
- EHI — Energy Helicity IndexA simple product of CAPE × helicity that rolls 'enough energy' and 'enough spin' into one number. Higher is mo…
- EL — Equilibrium LevelThe altitude where a rising storm parcel finally becomes neutral — roughly, where you see the anvil spread out…
- LCL — Lifting Condensation LevelThe height where cloud base forms if you lift a parcel from the surface. Lower LCLs mean wetter, cooler air ne…
- LFC — Level of Free ConvectionThe altitude at which a rising parcel becomes warmer than the air around it and can accelerate upward on its o…
- Lifted Index (LI)A one-number instability check: the temperature difference between a lifted parcel and the environment at 500 …
- PWAT — Precipitable WaterHow much rain would fall if you wrung every drop of water out of a vertical column of atmosphere. High PWAT = …
- SRH — Storm Relative HelicityHow much spin the low-level wind profile hands to a storm. High SRH is what lets a mesocyclone exist.
- STP — Significant Tornado ParameterA composite index that blends CAPE, low-level shear, helicity and LCL height into a single score calibrated to…
Storm types
From pulse cells through supercells to derechos — what's on the end of your radar.
- Bow echoA squall line bent into the shape of a bow — where the bow's apex is, the wind is strongest. The signature of …
- DerechoA long-lived, widespread windstorm produced by a fast-moving line of thunderstorms. Think of it as a bow echo …
- HP supercell (High Precipitation)A supercell whose mesocyclone is wrapped in heavy rain. You can't see what's inside — which makes them both th…
- MCS — Mesoscale Convective SystemA huge, organised cluster of thunderstorms that behaves as one entity. Responsible for most summer-night UK fl…
- MesocycloneThe rotating updraft inside a supercell. If it tightens and stretches to the ground, you get a tornado.
- MicroburstA concentrated blast of wind from a collapsing thunderstorm — small in area but capable of aviation-grade dama…
- Pulse storm (air-mass storm)A single-cell summer thunderstorm that blows up, drops a localised hail core or gust, and collapses — all with…
- Squall lineA long line of thunderstorms, all sharing a single gust front. Expect damaging wind and brief heavy rain as it…
- Storm splits — left and right moversWhen a thunderstorm splits in two, one half goes right of the mean wind and the other goes left. In most UK se…
- SupercellA long-lived, rotating thunderstorm. The mesocyclone at its core lets it persist for hours, produce giant hail…
- Tornado (and the EF scale)A violently rotating column of air in contact with both the ground and the cloud base. Rated after the fact by…
- WaterspoutA tornado over water. 'Fair weather' waterspouts are weak and come from growing cumulus; 'tornadic' ones are t…
Radar signatures
Hook echoes, BWERs, velocity couplets and dual-pol fingerprints — what to look for in a scan.
- BWER — Bounded Weak Echo RegionA pocket of *weak* radar return sitting inside an otherwise intense storm — the updraft is so strong it's hold…
- Hook echoA hook-shaped notch on the right-rear of a supercell's radar signature — the classic calling card of a rotatin…
- Rear-inflow jet (RIJ)A river of fast air blowing from behind a squall line into its back — what turns an orderly line into a bow ec…
- Reflectivity (dBZ) and dual-polThe main radar image — brighter colours mean more and bigger raindrops, hail, or whatever else is reflecting. …
- TVS — Tornadic Vortex SignatureA radar-detected circulation tight enough, low enough, and strong enough that the algorithm flags it as likely…
- Velocity coupletTwo neighbouring radar pixels showing opposite-direction winds — one coming at the radar, one going away. That…
Chasing techniques
Target selection, staging, escape routes, reading storm structure from the ground.
- FFD — Forward-Flank DowndraftThe heavy rain and hail region out in front of a supercell's updraft. You want to approach from the south/sout…
- Hodograph readingA polar plot of how wind changes with height. The shape tells you everything about what mode of storm you'll g…
- RFD — Rear-Flank DowndraftThe clear slot that wraps around the back of a supercell's updraft. Critical for tornado genesis — and dangero…
- Staging and escape routesGet close enough to see structure, but always have an exit — downwind from hail, perpendicular to storm motion…
- Target selectionPicking a spot to be by mid-afternoon so storms fire where you are, not over someone else. It's forecasting re…
UK context
Why UK convection doesn't look like the US Plains — Atlantic track, jet stream, maritime airmass.
- Jet streamA high-altitude river of fast air that separates cold and warm airmasses. The UK sits under or near it most of…
- Spanish PlumeA tongue of hot, unstable air pushed north from Iberia ahead of an Atlantic trough. Sets up most of the UK's b…
- Storm naming (MO / MÉ / KNMI)The Met Office, Met Éireann and KNMI jointly name extratropical storms that are expected to impact the UK / Ir…
- UK convective climatologyWhy UK storms are rarer, smaller, and wetter than their US Plains cousins — and when they do fire, why they're…
Chaseit features
How the pages on this site fit together — what feeds what, and what the numbers mean.
- SAR imagery (ICEYE)Synthetic Aperture Radar satellites that see through cloud. We use them for post-storm flood extent and ground…
- The Case ArchiveA curated record of notable UK storm days with reports, radar replays, soundings, and post-event verification.…
- The Outlook pageA multi-day risk digest of the UK. Combines CAPE, shear, helicity, moisture and timing into a daily convective…
- The Radar pageLive UK composite radar with cell tracking, velocity-couplet flags, and chase-target overlays. Built for nowca…
- The Skill page (forecast verification)How often we (and each forecaster) got it right. Rolling skill scores against observed outcomes — kept honest …
- The Soundings pageSkew-T plots — the full vertical profile of temperature, dew point, wind. The deepest look at atmospheric stru…
- The Stormfront modelOur in-house UK-localised convective intensity model. Not a full NWP — a regression layer that takes operation…
- The Targets pageSuggests specific chase locations based on the day's outlook — not just 'weather is bad over there' but 'here'…
- The Winds pageUpper-air wind profiles across the UK, derived from ADS-B aircraft measurements — higher resolution than radio…