Jersey funnel cloud and waterspout watch: an experimental guide to when the air around the Channel Islands is right for funnel clouds and waterspouts, with member sightings.
EXPERIMENTAL
A new idea for Jersey. SPOUT WATCH blends two published waterspout methods — Szilagyi's waterspout chart (Great Lakes, tested in the Mediterranean) and KNMI's North Sea index — rebuilt from their published ingredients into one score, with a cloud check on top. It can tell you when the air is right for funnel clouds and waterspouts; it can't tell you where or exactly when one will form. Not an official forecast: Jersey Met is the authority.
LOADING LIVE MODEL DATA FOR JERSEY…
SPOUT WATCH
EXPERIMENTAL
HOW THE SCORE WORKS ?
SPOUT SCORE · How the Blend Works
Jersey Weather Chat's own experimental blend of two published methods, the waterspout chart and the North Sea index. Each one rates the hour from 0 to 100%, and the spout score is the average of the two, scaled by a cloud check: a funnel needs a cumulus or shower cloud above it, so if the model shows no low cloud or showers and no cumulus can grow, the score drops to zero. FAVOURABLE needs a score of 60 or more with both methods at 45% or better, so neither can carry it alone, and a ground wind below Force 5 (17 knots) — spouts can form in a stronger breeze, but they are less likely and shorter-lived. WATCH THE SKY needs 35 or more, or one method very keen — that catches fair-weather cumulus days that only one method spots. If the wind at 1.5 km reaches 40 knots, the whole thing is switched off: strong winds tear funnels apart.
LOADING…
FETCHING LIVE MODEL DATA FOR JERSEY
WATERSPOUT CHART · SZILAGYI ?--
Cold air over warm sea, and how tall the shower clouds can grow.
WATERSPOUT CHART · Szilagyi Waterspout Index
Built by forecaster Wade Szilagyi from 172 waterspouts over the Great Lakes, and since tested in the Mediterranean and Adriatic. It looks at two things. First, how much colder the air is at 1.5 km than the sea surface: the bigger the gap, the more readily air warmed by the sea keeps rising. Second, how tall the shower clouds can grow, from cloud base to cloud top. Plotted against each other, waterspouts cluster in a few zones (see The Waterspout Chart further down). One rule overrides the rest: no waterspouts if the wind at 1.5 km is 40 knots or more. The original was only published as a drawing, so here we rate how far into its favourable ranges the hour sits, from 0 to 100%. In a 2011 Adriatic test it caught 14 of 19 waterspout days.
NORTH SEA INDEX · KNMI ?--
Light winds, a sharp chill in the lowest 500 m and moist air near the ground.
NORTH SEA INDEX · KHS (KNMI)
Created by Kuiper and Van der Haven at KNMI, the Dutch weather service, for waterspouts off the Dutch coast, and run every day in their forecast model. It checks four things: a light wind at the surface; little change in wind speed from the ground up to 3 km, because changing winds tear a young spin apart; a sharp drop in temperature through the lowest 500 m, which makes the air bubbly and buoyant; and moist air in the lowest kilometre. It's good at the fair-weather funnel days with shallow cumulus that the waterspout chart can miss. KNMI never published its exact weights, so here each ingredient gets its own rating and all four need to be in place for a high score. In the same 2011 Adriatic test it caught 13 of 19 waterspout days.
NEXT 3 DAYS — HOUR BY HOUR
TAP AN HOUR
LOWWATCH THE SKYFAVOURABLENIGHT
THE INGREDIENTS
WHAT MAKES A SPOUT DAY
THE WATERSPOUT CHART
INTERACTIVE
WHERE THIS HOUR SITS
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Each dot is one hour of the day you've picked, joined in order; the bright dot is the hour you've selected. Tap a dot to jump to it. The coloured areas are where waterspouts have been seen before, in Szilagyi's study of 172 Great Lakes events.
Zones are approximate, redrawn from the published ranges (the original chart was only published as a drawing). If the wind at 1.5 km is 40 knots or more, the chart is switched off: strong winds tear funnels apart.
MEMBER SIGHTINGS
FROM THE GROUP
Seen a funnel cloud or waterspout? Report it on the Storm Reports page as Funnel Cloud / Waterspout. Each report shows here with what SPOUT WATCH said for that hour and that day, and a 🌪️ marks it on the timeline above, so over time Jersey builds its own record of how well this works. The hour is when the report was sent, so a report made later on may not match the exact moment of the sighting.
A funnel cloud is a spinning column of air hanging from a cumulus or shower cloud that hasn't reached the ground.
They form when a fast-rising cloud stretches a patch of gently spinning air near the surface — often where two breezes meet, like along a sea breeze. That's why you can get them with only a few fair-weather cumulus about. Most last a few minutes and never touch down.
WATERSPOUTS
A waterspout is the same thing reaching the sea: look for a ring of spray where it touches the water.
Most around Jersey are fair-weather waterspouts — much weaker than tornadoes, short-lived and slow-moving, and they usually fade if they drift ashore. The busy season is late summer and autumn, when the sea is still warm and colder air arrives above it.
STAYING SAFE
Enjoy them from a distance. On the water, move away at right angles to its path and never try to get close. If one comes ashore, treat it like a small tornado: get indoors and away from windows.
If a boat or anyone is in difficulty, call 999 and ask for the Coastguard. Share your photos with the group and log them above.