squall lines: meteorology, skywarnspotting, & a brief look

Squall Lines:

Meteorology,

Skywarn Spotting,

& A Brief Look At The

18 June 2010 Derecho

Gino Izzi

National Weather Service, Chicago IL

Outline

• Meteorology 301: Squall lines

– Brief review of thunderstorm basics

– Squall lines

– Squall line tornadoes

– Mesovorticies– Mesovorticies

• Storm spotting for squall lines

• Brief Case Study of 18 June 2010 Event

Thunderstorm Ingredients

• Moisture

– Gulf of Mexico

most common most common

source locally


• Lifting Mechanism(s)

– Fronts

– Jet Streams

– “other” boundaries– “other” boundaries

– topography


• Instability

– Measure of potential

for air to accelerate

upwardupward

– CAPE: common variable

used to quantify magnitude

of instability

< 1000: weak 1000-2000: moderate

2000-4000: strong 4000+: extreme

Thunderstorms

Thunderstorms

• Moisture + Instability + Lift = Thunderstorms

• What kind of thunderstorms?

– Single Cell– Single Cell

– Multicell/Squall Line

– Supercells

Thunderstorm Types

• What determines T-storm Type?

– Short/simplistic answer: CAPE vs Shear

Thunderstorm Types

• What determines T-storm Type?

(Longer/more complex answer)

– Lot we don’t know, other factors (besides

CAPE/shear) includeCAPE/shear) include

• Strength of forcing

• Strength of CAP

• Shear WRT to boundary

• Other stuff

Thunderstorm Types

• Multi-cell squall lines most common type of

severe thunderstorm type locally

• Most common type of severe weather is • Most common type of severe weather is

damaging winds

• Hail and brief tornadoes can occur with most

the intense squall lines

Squall Lines

&

Spotting

Squall Line Terminology

• Squall Line: a relatively narrow line of thunderstorms, often fast moving

• Bow Echo: line of thunderstorms shaped like a • Bow Echo: line of thunderstorms shaped like a “bow”, frequently produces damaging winds

• Derecho: a fast moving line of thunderstorms that produces widespread, significant wind damage (long track, duration

Squall Lines

• Cold Frontal:

– can extend across

great distances

(many states)(many states)

– most common spring

and fall

Squall Lines

• Cold pool driven

(MCS)

– most common late

spring into summerspring into summer

– tends to occur in

weaker forcing,

thus often harder

to forecast

Squall Lines

Shelf CloudShelf Cloud

Squall Line Spotting

• Big visual clue:

Shelf Cloud

• Shelf Clouds:• Shelf Clouds:

– extend across entire

sky

– likely location of

damaging winds


• Important to distinguish between shelf cloud

and wall clouds

• Difficult to remember the difference, very • Difficult to remember the difference, very

common mistake spotters make reporting

shelf clouds as wall clouds

• Shelf clouds Stretch across the horizon


Shelf Clouds

• Stretches across the

entire horizon

• Associated with

Wall Clouds

• Localized lowering

• Associated with

supercells• Associated with

squall lines

• Primary hazard is

damaging winds

• Could rotate

horizontally

supercells

• If tornado forms, this

is where

• Could rotate

vertically

Quiz Time!

Wall or Shelf?


So you have shelf cloud now what…

• Expect the potential for damaging winds

• Be sure you are in a safe location, your safety

is importantis important

• If you can do so safely, measure winds

• If not, do your best to estimate winds

• Keep in mind tornado threat isn’t zero…

Squall Line Tornadoes


• Tend to have shorter live spans than supercell

counterparts

• Very rarely can reach EF-2/EF-3 intensity, but • Very rarely can reach EF-2/EF-3 intensity, but

majority are EF-0/EF-1


• Very difficult to spot for multiple reasons:

– Form quickly

– Short lived

– Move rapidly– Move rapidly

– Typically poor contrast and/or rain-wrapped

• Not only difficult to spot, but can also put

spotter potentially in harms way

Mesovortices

• Mesovortex: a circulation, usually along

leading edge of bow echoes and squall lines,

that often causes enhanced damaging winds

and sometimes tornadoes and sometimes tornadoes

• Relatively newly discovered phenomena

• Our region has a high regional frequency

Mesovortices

• Most typically form along boundaries

intersecting bows or in strong low level shear

environment

• Requires a “balanced” bow echo/squall line,

not too cold pool dominant:

– Fairly upright updrafts

– Outflow not out-running leading edge

Mesovortex vs. Mesovortex vs. MesocycloneMesocyclone

•• In Supercells In Supercells ––the rotating the rotating updraft is the updraft is the mesocyclonemesocycloneand it’s located and it’s located at back edge at back edge at back edge at back edge (SW) portion of (SW) portion of stormstorm

–– Tornadoes in Tornadoes in the the back!back!

Mesovortex vs. Mesovortex vs. MesocycloneMesocyclone

•• QLCS’sQLCS’s –– rotation (a rotation (a

mesovortexmesovortex) is ) is

located along the located along the

leading edge (E) leading edge (E)

portion of stormportion of stormportion of stormportion of storm

–– Tornadoes in the Tornadoes in the

front!front!

Gust Front – Shelf Cloud

Inflow NotchesMesovortices

Case studies ofCase studies of Squall Lines Squall Lines

tornadoes & examples tornadoes & examples of of

mesovorticesmesovortices ((MVsMVs)…)…

July 4, 2004 July 4, 2004 –– Southwest MissouriSouthwest Missouri

• Two leading

line MVs…

• This was the

first case that

got me got me

interested in

MVs

• 1 tornado, but

extreme wind

damage in a

“thin” swath

February 6, 2008 February 6, 2008 –– Central KentuckyCentral Kentucky

• Another

example of a

leading-line MV

• This squall line

produced 16 produced 16

tornadoes

across central

Kentucky

(overnight in

the winter!)

• 12 tornadoes

in 1 hour!

October 24, 2001 October 24, 2001 –– Northern IndianaNorthern Indiana

• Normally, MVs are

leading-line, low-level

circulations

• But a few MVs during

this event were well this event were well

behind leading edge,

wrapped in heavy rain

• No visual “heads up” =

much more dangerous

• The 2nd largest TOR

outbreak in IWX CWA!Base reflectivity – rear

inflow identified

Base storm-relative

velocity – MVs identified

Challenges With Mesovortices

• Still new to us, so meteorologists have a lot more to learn:

– How do they form?

– Why do some last so long, others brief?– Why do some last so long, others brief?

– Why do some produce tornadoes while most don’t?

• Tend to form (and dissipate) very quickly

– Can go from non-detectable on WSR-88D one volume scan to tornado producing the next


• Often smaller/shallower than supercell

mesocyclones� hard to see at > distances

• Not all mesovortices produce tornadoes • Not all mesovortices produce tornadoes

and/or wind damage

• Hard to tell on radar which will produce

damaging winds, tornadoes, or neither


• Pose big challenges, still much debate on how

to warn for them:

– Big tornado warning for entire squall line or bow

echo?echo?

– Severe thunderstorm warning w/hard hitting

wording?

– Multiple small/short tornado warnings?

QLCS DamageQLCS Damage

The 18 June 2010 Derecho

What Happened

• Bow echo produced widespread wind damage from MS River across all N IL/N IN/S Lower MI

– Numerous trees (1000s in

Chicago alone), powerlines

downed, w/some structural downed, w/some structural

damage

– Hundreds of thousands households lost power

– Many reports 70+ mph winds, including M77 mph @ Chicago Crib (lakefront)

– Multiple injuries, 1 fatality in Chicago area

Meteorological Set-up

• Strong heating of moist airmass led to strong

potential instability in advance of MCV

• Somewhat modest deep layer shear likely • Somewhat modest deep layer shear likely

augmented by stronger flow w/MCV

• Somewhat marginal shear (esp. LL) � weaker,

more transient MV’s, few tornadoes

RUC MLCAPE 20z 0 Hour Forecast

>2500 J/Kg

Mesovorticies

• Relatively few/weak

MV’s with bow

– 0-3km shear was weak

– No well defined

TMDW VWP

– No well defined

boundaries intersecting

bow

– Appeared to be more RIJ

dominant

Summary

• RIJ dominant bow produced widespread wind

damage, locally significant, despite lack of

MVs

• Timing & antecedent Wx led to potential

danger of bow for Chicago Metro

Summary

• Synoptically evident event, led to accurate

short term forecasts

• Challenge leading up to event was conveying • Challenge leading up to event was conveying

potential danger to decision makers/public…

squall lines: meteorology, skywarnspotting, & a brief look

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