Hard cases: QLCS, rain-wrapped and nocturnal
Rotation you cannot see, in a line, inside the rain, or in the dark, and how to be useful anyway.
Rotation inside a line
Module 2 covered Quasi-Linear Convective SystemA long line of storms rather than a discrete cell, often bowing forward. Its tornadoes tend to spin up fast, stay brief and give very little warning. structure and module 3 covered what a A segment of a line of storms that has surged forward into a bow shape on radar. The apex of the bow is where the strongest straight-line winds are. looks like. What matters here is the spotting problem: a QLCS A small, shallow circulation along the leading edge of a line of storms, and the source of most tornadoes that lines produce. They spin up in a minute or two, often between one radar scan and the next. spins up fast, lives briefly, and sits inside a wall of precipitation with no clean The smooth, flat, dark cloud base on the rear flank of a storm, where warm air is flowing in and rising. It is named for the air going up, not for being dry: hail and large drops may still fall from it. to watch. There is often no A localized, persistent lowering beneath the updraft base, on the rear inflow side of a storm, marking where air is being drawn up. Not all of them rotate, and a rotating one is what you report., no A brightening slot of sinking, drying air cutting into the cloud base around the back of a wall cloud. It marks the rear-flank downdraft wrapping in, and it often appears shortly before a tornado. and no warning sky. National Severe Storms LaboratoryA research laboratory, not an operational office. It develops the techniques forecasters use, but issues no products and takes no reports. puts nearly 20% of all tornadoes with QLCSs, and notes they frequently occur late at night and early in the morning and are a particular problem in the Southeast.
Every cue in lessons 2 through 4 assumes a discrete storm with a visible updraft base. In a line, you may have none of them.
Hidden inside the precipitation around it. A rain-wrapped tornado breaks none of the identification rules; it just denies you the observation those rules rely on.
In an High Precipitation supercellA supercell whose precipitation wraps around and can completely hide the updraft. Rain-wrapped tornadoes and flash flooding, and the common mode on the Gulf Coast. the precipitation wraps around the circulation itself, so the tornado can be inside the rain curtain with no visible funnel from any safe vantage. The dangerous version of this is a spotter concluding, correctly, that they cannot see a tornado, and then concluding, incorrectly, that there is not one.
The general term for a tropical depression, tropical storm or hurricane. Their outer rainbands produce small, shallow supercells that are a recognised tornado threat well away from the centre. tornadoes
Landfalling tropical cyclones produce tornadoes in their outer rainbands, from A supercell with a shallower, smaller and weaker rotating updraft than a classic one, typical of tropical cyclone rainbands. Produces tornadoes in an environment with a fraction of the instability a Plains supercell needs.: shallower, smaller and weaker mesocyclones than a Plains storm has. The environment is different in kind, with average Convective Available Potential EnergyHow much energy is available to an updraft, in joules per kilogram. Bigger numbers mean a stronger updraft is possible, and a number on its own says nothing about what kind of storm you get. around 253 J/kg in US tropical cyclone tornado environments, a fraction of what a classic A thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds. day carries.
At night
Ashley, Krmenec and Schwantes quantified what everyone suspected, and the numbers are worse than the intuition.
The headline figure from the same paper: a tornado between local midnight and sunrise is 2.5 times as likely to kill as one in the daytime.
The blue-green glow of electrical equipment arcing as something destroys it, often the only evidence available at night. Supporting evidence for damage at a point, never confirmation of a tornado.
A power flash is a transformer or line arcing as something destroys it, and at night it may be the only evidence available. Storm Prediction CenterThe national office in Norman, Oklahoma that issues severe weather outlooks, mesoscale discussions and watches. It does not issue warnings. describes them as blue-green or turquoise, tending to linger from a single point on the ground, and unaccompanied by thunder, which is what separates them from lightning.
How to be useful when you cannot see
- Say what you cannot see. "Rain-wrapped, no visual on the circulation, heavy rain and hail at my location" is a real observation and offices use it.
- Report the things that do not need light. Wind damage, hail size, water over roads, a roar with no train nearby.
- Trust radar more, and say you are. In these conditions the The radar product showing how fast the rain, hail and debris in the beam are moving toward or away from the radar. Only motion along the beam is measured, and rotation shows up here as inbound and outbound speeds side by side. data is doing the work. Module 9 covers reading it.
- Move your standard for safety, not your standard for reporting. Being unable to confirm is not a reason to guess.
At 2am you see two or three blue-green flashes lingering low on the horizon in the direction of an approaching line, with no thunder. What do you report?
Ask what else could destroy electrical infrastructure in a squall line at 2am.Power flashes are supportive, never confirmatory. Straight-line winds and falling trees produce them too, and at night you have no way to tell which. Reporting exactly what you observed, with a time and a direction, gives the office something to combine with velocity data. Reporting a tornado you did not see does not.