What a squall line looks like from underneath
- View
A line of storms gives you a shelf cloud, a lot of wind, and almost none of the visual vocabulary the rest of this module just taught you.
In this lesson you will
- Identify the leading shelf cloud of a squall line and the bow echo concept.
- State that QLCS tornadoes are often rain-wrapped, brief, and concentrated near and north of a bowing segment apex.
- Explain why QLCS rotation is hard to see and often carries short warning lead time.
- Contrast QLCS visual spotting with discrete-supercell spotting.
What you actually get to see
A squall line arrives as a nearly solid wall with the gust front on its leading edge, marked by a shelf cloud that can run from horizon to horizon. That shelf is lesson 5, and it means the same thing it always means: wind, imminent, not rotation.
And then the line passes over you and you are in heavy rain, and your visual spotting for the next twenty minutes is finished.
Where the tornadoes are
Module 2 lesson 8 gave the mechanism and the diagram: mesovortices form along the leading edge, favoured just north of a bowing segment apex, and the strongest straight-line winds are at the apex and to its south. What this lesson adds is that the circulation "may be wrapped within precipitation", which is the normal case rather than the unlucky one.
The numbers are worth carrying because they set how seriously to take a line. Trapp and colleagues classified US tornadoes by parent storm and found "79% were produced by cells, 18% were produced by QLCSs, and the remaining 3% were produced by other storm types". Ashley and colleagues, over a longer radar-based record, found "Over 21% of tornadoes, 28% of severe winds, and 10% of severe hail reports are due to QLCSs across the central and eastern United States", with more than half of tornado and wind reports attributable to QLCSs in some locations during the overnight and cool season.
A 2012 severe storms conference paper puts the spotter reality plainly: with rain-embedded mesocyclones, "storm spotters will have little chance of seeing a tornado if it is preceded by heavy rain." This is a case where visual spotting is not the primary tool, and knowing that is itself a skill. Your value on a QLCS night is damage reports, measured wind, and being where the radar cannot see, not heroics with binoculars.
How the job changes
| Discrete supercell | QLCS | |
|---|---|---|
| What you watch | One updraft base, from the inflow side | The leading edge, and bowing segments |
| Warning you get | Wall cloud may precede by up to nearly an hour | Often a couple of minutes, sometimes none |
| Visibility | Often good from the right position | Usually poor, usually rain-wrapped, usually at night |
| What to report | Structure, rotation, the tornado | Wind, damage, any sustained lowering even if brief |
The instruction that follows from all of that: report a sustained lowering or rotation on a line even if it is brief and even if you are not certain, and say what the visibility was. A QLCS spin-up does not give you the luxury of watching it for two minutes to be sure, which is otherwise this module's standard advice.
Check yourself
A squall line is bearing down with a long shelf cloud on its leading edge, and you can see a pronounced bowing segment on radar to your north-west. Where is the tornado risk concentrated, and what should you expect visually?
- Near and just north of the bow apex, and expect the circulation to be rain-wrapped and brief
- In the shelf cloud along the leading edge, which is where the rotation is
- At the southern end of the bow, where the strongest winds are
- Behind the line, once the heavy rain has passed and visibility improves
Hint
Module 2 lesson 8 drew this. The winds and the tornadoes are not in the same place.
Answer
Near and just north of the bow apex, and expect the circulation to be rain-wrapped and brief
Mesovortices are favoured near and just north of the apex, and they are what produce QLCS tornadoes. The apex itself and the area south of it is where the strongest straight-line winds land. Expect the circulation to be inside the precipitation: seeing it is the exception, and a report of wind damage may be the only observation anybody gets.
Sources
- NWS Louisville, "Squall Line, Bow Echo and QLCS"
- NWS St. Louis, QLCS and mesovortex reference pages
- Trapp et al. 2005, Wea. Forecasting 20, 23–34: tornadoes from squall lines and bow echoes, part I
- Ashley, Haberlie & Strohm 2019, Wea. Forecasting 34(6), 1605–1631: a climatology of quasi-linear convective systems and their hazards
- 26th Conf. on Severe Local Storms (2012), leading-stratiform and trailing-mesocyclone spotter visibility