squall lines: meteorology, skywarnspotting, & a brief look
Post on 21-Apr-2022
4 Views
Preview:
TRANSCRIPT
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
Thunderstorm Ingredients
• Lifting Mechanism(s)
– Fronts
– Jet Streams
– “other” boundaries– “other” boundaries
– topography
Thunderstorm Ingredients
• 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
Squall Line Spotting
Squall Line Spotting
Squall Line Spotting
• 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
Squall Line Spotting
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?
Wall or Shelf?
Wall or Shelf?
Wall or Shelf?
Wall or Shelf?
Wall or Shelf?
Squall Line Spotting
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
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
Squall Line Tornadoes
• 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
Challenges With Mesovortices
• 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
Challenges With Mesovortices
• 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…
Questions?
© Walker Ashley
Gino.Izzi@noaa.gov
top related