A Case Study of Severe
Winter Convection in
the Midwest
Paul Cody and Tim Gibbs
Lecture Outline
O Introduction
O Background
O Synoptic Analysis
O Mesoscale Analysis
O Lincoln and Davenport Sounding Analyses
O Radar Analysis
O Lightning Analysis
O Summary
Introduction
O On the evening of February 11th 2003, a line
of severe thunderstorms moved through
Southeast Iowa and Northwest and Central
Illinois.
O Winds exceeded 50 kts (27 𝑚𝑠 −1 ) and a
brief period of heavy snowfall created
whiteout conditions.
O The NWS in Lincoln, Illinois issued a severe
thunderstorm warning for several counties
due to strong winds and heavy snowfall.
Introduction
O The first severe
thunderstorm warning
was issued at 0002 UTC
on February 12th for
Marshall County, Illinois.
O Within the hour, the front
had passed, leaving wind
damage (gusts exceeding
50 knots), downed power
lines, and utility poles, as
well as a unique feature
called snowrollers that
formed in Peoria, Illinois
(PIA).
http://www.crh.noaa.gov/ilx/events/roller/roller.php
Introduction
O This case is unique as it did not fall into a
specific operational paradigm.
O How do we choose a forecast warning that bests
communicates the nature of the hazard?
O Severe Thunderstorm Warning(SVR)
O .75” diameter hail and wind gusts >58 mph
O High Wind Warning (HWW)
O Surface wind gusts of >58 mph due to synoptic
scale pressure gradients, terrain-forced winds or
mesoscale winds associated with a wake low
behind a squall line.
Background
O This event was uncommon for thundersnow
cases since it was not a case of elevated
convection due to the presence of an
extratropical cyclone.
O This event had maintained qualities related
to warm season/sector weather rooted in
the Planetary boundary layer, like squall
lines.
O A warm season event that occurred in the
cold season.
Synoptic Analysis
O Cold front was found on surface analysis at 1500 UTC over South
O
O
O
O
Dakota, progressing to southeast Northern Missouri by 0000 UTC.
During morning hours, majority of the precipitation was upstream of
the cold front
Transitioned to prefrontal during afternoon and at time of
thundersnow event
Significant pressure gradient ahead of the cold front in the warm
sector and in the cold air behind it
Cold front conditions:
O 1) Intense 33 min snowshower
O 2) Skies cleared post shower and dewpoints fell
O 3)Moderate veer in <1 hr tempered by blustery post frontal
conditions
O Huron, SD has severe wind criteria, but lacked precipitation or
lightning
1500 UTC Feb 11th
0000 UTC Feb 12th
Synoptic Analysis
O A 300-hPa polar jet
was present
stretching from
Central Canada to the
Ohio valley
O Winds exceeded 135
kts
O Iowa and Illinois were
near the core, but
more towards the left
entrance region:
O Not typical for severe
weather as it is a
region of upper level
convergence
Figure: 300-hPa wind field. Jet streak in
shaded region
Synoptic Analysis
O Low-amplitude
shortwave trough at
500-hPa stretched from
Hudson bay through midwest
O Significant slope into
cold air, consistent with
a cold front
O Circular absolute
vorticity maximum in the
base of the trough
suggests quasigeostrophic forcing on
polarward side of 300hPa jet
Shaded region shows absolute vorticity
shaded every 10 ∗ 10−5 𝑠 −1
Synoptic Analysis
O Q-Vector
convergence
occurred over
central Illinois
O Additional
supporting
evidence for
midtropospheric
ascent
Figure: Q-vectors and Q-vector Divergence
(700-400 hPa)
Mesoscale Analysis
O Observed low level frontogenesis, vertical motion, and CAPE
revealed an environment with necessary ingredients for
convection.
O Combination of substantially increasing frontogenesis and
collocation of CAPE (albeit decreasing) and vertical motion
produced whiteout conditions.
O Analyses depict:
A dynamically forced region along and ahead of the surface cold
front
O Strong frontogenetic forcing made significant direct thermal mixing
likely
O Relatively deep dry-adiabatic layer (~100 hPa) along and just
ahead of cold front provided little inhibition for parcels to be lifted
to LCL
O The lower troposphere as a region conducive to vertical motion.
O
Mesoscale Analysis
O Values (Areas along cold front):
O Frontogenesis
O 5.0 – 10 𝐾 100𝑘𝑚
−1
3ℎ
−1
@ 925 hPa
O Vertical Motion
O -20 µ𝑏(𝑠)−1 ≤ ω ≤ -10 µ𝑏(𝑠)−1
O CAPE
O Exceeded 50 𝐽 𝑘𝑔
−1
Lincoln, IL
Sounding
1200 UTC
Well ahead of cold front &
mostly cloudy conditions
Lowest 100 hPa
- Very cold temps.
- Significant veering
-Weakly sheared unidirectional
flow above the boundary layer
0000 UTC
-Revealed a warmer, moister
environment in lowest 200 hPa
-CAPE of most unstable parcel
lifted from 862 hPa – meager 3
J/kg, but atmospheric profile
was nearly dry adiabatic (862 –
625 hPa)
1200 UTC: solid lines
0000 UTC: dashed lines
Davenport, IA
Sounding
1200 UTC:
Also ahead of cold front,
showed cold near-surface
temps., and an unidirectional
shear signature
0000 UTC:
-Also revealed significant 12-h
warming in lowest 200 hPa and
moistening in the lowest 100hPa
-One difference is the cooling
and significant drying in midtroposphere: encouraging
development of potential
instability
-Launched as the precipitation
band approached
- Surface temps were changing
rapidly as a result
- Suggested a super-adiabatic
surface layer
1200 UTC- Solid lines
0000 UTC- Dashed lines
Radar Analysis
O REFLECTIVITY:
O Maximum reflectivities: 40 - 45 dBZ
O Storm tops never exceeded 3.7 km
O Highest reflectivities limited to lower portions of strongest
cells
O Deviations from traditional severe weather producing
squall lines (radar and velocity data):
O No rear in-flow jet
O No stratiform precipitation region
O No front-to-rear flow
O Did not evolve through usual stages of squall line evolution
Radar Analysis
O Instead the was a convective line that more closely
resembled a parallel stratiform convective system
(MCS), however:
O The area of stratiform precipitation on the left flank
O No tendency for cell decay on left flank
O No new cell generation on the right as should be expected
O Severe winds were not the result of squall line
convection but rather the mixing associated with the
mesoscale and synoptic systems
O Convective line tilted slightly down shear, which was
likely due to a lack of CAPE ahead of the front and
convective line and the strong ambient vertical wind
shear
Lightning Analysis
O Rare for winter event
O Data clearly shows the temporal and spatial extent of
thunderstorm activity
O Convective line’s progression towards the southeast
was depicted well using lightning data
O Whereas most “winter” lightning is (+) in polarity, a
majority of the strikes (97 of 101) were (-) in polarity
O Taniguchi et al. (1982) and Brooke et al. (1982)
hypotheses that shear in the cloud layer controls the
polarity of a lightning strike were confirmed
Summary
O RUC analysis supported the formation of convection
along the cold front, through a dry adiabatic layer in the
PBL
O The thermodynamic profile was conducive to lightning
production yet cold enough for snow
O Large amount of negative lightning contrasts with prior
studies
O Severe thunderstorm warning was best response to
conditions
O Choice of SVR over HWW forecast:
O Rapid movement of line
O Rapid onset and decrease of severe winds
O Area affected closer to a warm season squall line than
either a high wind event due to synoptic-scale pressure
gradient or mesoscale wake low event.
O Main difference from warm season SVR (snow, blowing
snow, could have easily been mentioned in warning text)