Squall Lines & Tornadoes

[Mike Krzywonski]

How often do squall lines (not individual cells associated w/ squall lines, but the squall line itself) produce tornadoes?

 

[Paul Knightley]

A good question! It really depends on a number of things, but mainly it's to do with the nature of the squall line.

A summertime squall line, which has generated from extensive cold pool generation may not produce any tornadoes, as the storms are strongly outflow dominant and there's little inflow, and no real mechanism to generate rotation.

A strong winter-type squall line (by this I mean a very dynamic one, driven by a strong upper system, and not necessarily confined to winter!) may produce tornadoes via misocyclones. These can develop when a strong low-level jet ahead of the squall line causes portions of it to "roll up" into small circulations, or misocyclones. These can locally enhance convergence, causing stronger cores to develop, and at the same time can enhance SREH. If the circulation can be stretched by a strong updraught, a tornado can develop.

In the UK, this happens several times a year, although the tornadoes are generally weak and short-lived. In addition, the misocyclones themselves can bring strong non-tornadic winds in a similar fashion to a supercell's RFD, although dynamically different.

However, in answer to your actual question, I'm not entirely sure, but I would say more squall lines do not produce tornadoes than do!

 

[Darrin Rasberry]

How much truth is there to my weatherman's description of a tornado that happened in southern Iowa last year: "this tornado was spawned by an embedded supercell in this 'thunderstorm line'," and, if so, how often do squall lines have "embedded supercells?"

 

[Mike Krzywonski]

How much truth is there to my weatherman's description of a tornado that happened in southern Iowa last year: "this tornado was spawned by an embedded supercell in this 'thunderstorm line'," and, if so, how often do squall lines have "embedded supercells?"

I just read this from SPC's Convective Outlook for 2/12/08: "CURRENT SQUALL LINE WAS CHARACTERIZED BY PREDOMINATELY MULTICELLS
WITH OCCASIONAL SUPERCELL STRUCTURES."
http://www.spc.noaa.gov/products/outlook/day1otlk.html

 

[Karen Politte]

A large part of how squall lines manage to produce tornadoes is in the type of cell contained within the squall line.

If you have a continuous squall line that has propagated itself and consists of not individual cells, but of one long strip of thunderstorms - then the chances of getting a tornado out of that mess are non-existant.

However, if you have a squall line that is comprised of many cells all interacting with each other and crashing into each other, in a regime where it is possible to produce tornadoes, then it is quite possible to have a cell undergo the appropriate processes to produce tornadogenesis.

The most intense cells in a squall line usually occur where there is a break in the line of convection. The strongest cell will be a storm that has relatively unimpeded inflow and that is not being constantly seeded by a close neighbor - and these circumtances are most likely to exist in a cell that is nearest a break/clearing in the squall line. This could actually be read in this way: squall lines do NOT produce tornadoes, but if a squall line is broken enough to where it can contain embedded supercells - then these embedded supercells can and do produce tornadoes.

KL

 

[Jeff Miller]

This could actually be read in this way: squall lines do NOT produce tornadoes, but if a squall line is broken enough to where it can contain embedded supercells - then these embedded supercells can and do produce tornadoes.

I'm going to disagree in a way here.

There are other ways and processes to produce tornado circulations in a squall line besides broken discrete cellular processes. Line Echo Wave Patterns and Derecho events have produced tornadoes as well. Linear squall lines with a portion of the squall bowing out in front of the remaining squall in an otherwise linear event can produce tornadoes due to convergence and complex inflow interactions. And of course, there are times that gustando tornadoes can become a true tornado due to vortex stretching - though rare.

A good article to read is this one:

http://www.nssl.noaa.gov/users/brooks/public_html/papers/trappetal.pdf

This is a NSSL/NOAA study on tornadoes that come from squall lines (quasi-linear convective systems). Of note as one of many examples:

However this risk of strong and even violent
tornadoes is not limited to supercells. Indeed, consider
the tornadic squall line that occurred on 11 November
1995 near Flora, Mississippi (Fig. 1). At 0910 UTC
(0310 LST), this
300 km long squall line produced an
F3 (Fujita 1981) tornado that persisted on the ground
for 15 mi. Another F3 tornado formed at 1055 UTC
(0455 LST), and had a 7-mi pathlength. Neither tornado
was preceded by a user-defined tornadic vortex
signature

The primary objective of this study was to estimate the percentage of U.S. tornadoes that are spawned
annually by squall lines and bow echoes, or quasi-linear convective systems (QLCSs). This was achieved by
examining radar reflectivity images for every tornado event recorded during 1998–2000 in the contiguous
United States. Based on these images, the type of storm associated with each tornado was classified as cell,
QLCS, or other.
Of the 3828 tornadoes in the database, 79% were produced by cells, 18% were produced by QLCSs, and
the remaining 3% were produced by other storm types, primarily rainbands of landfallen tropical cyclones.
Geographically, these percentages as well as those based on tornado days exhibited wide variations. For
example, 50% of the tornado days in Indiana were associated with QLCSs.
In an examination of other tornado attributes, statistically more weak (F1) and fewer strong (F2–F3)
tornadoes were associated with QLCSs than with cells. QLCS tornadoes were more probable during the
winter months than were cells. And finally, QLCS tornadoes displayed a comparatively higher and statistically
significant tendency to occur during the late night/early morning hours. Further analysis revealed a
disproportional decrease in F0–F1 events during this time of day, which led the authors to propose that
many (perhaps as many as 12% of the total) weak QLCSs tornadoes were not reported.