Rotation you cannot see, in a line, inside the rain, or in the dark, and how to be useful anyway.

In this lesson you will

  • Explain why QLCS and mesovortex tornadoes are visually difficult and give short lead times.
  • Describe how HP supercells conceal a tornado in precipitation.
  • State the nocturnal tornado fatality multiplier and the limits of power flashes as evidence.
  • Adjust reporting and safety behavior for low-contrast conditions.

Rotation inside a line

Module 2 covered QLCS structure and module 3 covered what a bow echo looks like. What matters here is the spotting problem: a QLCS mesovortex spins up fast, lives briefly, and sits inside a wall of precipitation with no clean updraft base to watch. There is often no wall cloud, no clear slot and no warning sky. NSSL 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.

Rain-wrapped

Field Guide, on HP supercells, verbatim
Potentially obscured by heavy precipitation (rain-wrapped).

In an HP supercell 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.

Tropical cyclone tornadoes

Landfalling tropical cyclones produce tornadoes in their outer rainbands, from mini-supercells: shallower, smaller and weaker mesocyclones than a Plains storm has. The environment is different in kind, with average CAPE around 253 J/kg in US tropical cyclone tornado environments, a fraction of what a classic supercell day carries.

Why this matters here
Low instability, shallow rotation, embedded in rainbands, often at night, frequently while the region is already being hit by a hurricane. This is the tropical cyclone tornado threat, which reaches far inland and up the Atlantic seaboard, and lesson 9 walks a real one.

At night

Ashley, Krmenec and Schwantes quantified what everyone suspected, and the numbers are worse than the intuition.

Nocturnal tornadoes, 1950 to 2005Share
Of all tornadoes27.3%
Of all tornado fatalities39.3%
Of killer tornado events42.1%

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.

Field Guide, verbatim
Nighttime storm spotting in a vehicle can be dangerous and is not recommended. It is difficult to observe key storm features at night.

Power flashes

A power flash is a transformer or line arcing as something destroys it, and at night it may be the only evidence available. SPC 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.

What a power flash is worth
Supporting evidence, and a last resort. It says something is destroying infrastructure at a point; it does not say a tornado is. Straight-line winds, falling trees and failing equipment all make the same light. Report it as a power flash, with a direction and a time, and let the office weigh it. Do not report a tornado on the strength of one.

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 velocity 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.

Check yourself

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?

  1. Power flashes, with time and direction, and let the office weigh them against radar
  2. A tornado, since lingering blue-green flashes confirm one at night
  3. Nothing, since power flashes are not a recognized report type
  4. Lightning, since any flash without visible cause is lightning
Hint

Ask what else could destroy electrical infrastructure in a squall line at 2am.

Answer

Power flashes, with time and direction, and let the office weigh them against radar

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.

Sources

  1. Ashley, Krmenec & Schwantes 2008, Wea. Forecasting 23(5), 795–807: vulnerability due to nocturnal tornadoes
  2. Edwards 2012, Electronic J. Severe Storms Meteor. 7(6), 1–61: tropical cyclone tornadoes, a review of knowledge in research and prediction
  3. McCaul 1991, Mon. Wea. Rev. 119, 1954–1978: buoyancy and shear characteristics of hurricane-tornado environments
  4. NSSL Severe Weather 101, tornado basics and tornado types
  5. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
  6. NWS online glossary