A Case Study of Severe Winter Convection in the Midwest
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A Case Study of Severe Winter Convection in
the MidwestPaul Cody and Tim Gibbs
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Lecture OutlineO IntroductionO BackgroundO Synoptic AnalysisO Mesoscale AnalysisO Lincoln and Davenport Sounding
Analyses O Radar AnalysisO Lightning AnalysisO Summary
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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 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.
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IntroductionO 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
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IntroductionO 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 mphO 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.
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BackgroundO 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.
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Synoptic AnalysisO Cold front was found on surface analysis at 1500 UTC over South
Dakota, progressing to southeast Northern Missouri by 0000 UTC.O During morning hours, majority of the precipitation was upstream
of the cold frontO Transitioned to prefrontal during afternoon and at time of
thundersnow eventO Significant pressure gradient ahead of the cold front in the warm
sector and in the cold air behind itO Cold front conditions:
O 1) Intense 33 min snowshowerO 2) Skies cleared post shower and dewpoints fellO 3)Moderate veer in <1 hr tempered by blustery post frontal
conditionsO Huron, SD has severe wind criteria, but lacked precipitation or
lightning
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1500 UTC Feb 11th
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0000 UTC Feb 12th
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Synoptic AnalysisO A 300-hPa polar jet
was present stretching from Central Canada to the Ohio valleyO Winds exceeded 135
ktsO 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
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Synoptic AnalysisO Low-amplitude
shortwave trough at 500-hPa stretched from Hudson bay through mid-west
O Significant slope into cold air, consistent with a cold front
O Circular absolute vorticity maximum in the base of the trough suggests quasi-geostrophic forcing on polarward side of 300-hPa jet Shaded region shows absolute
vorticity shaded every
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Synoptic AnalysisO Q-Vector
convergence occurred over central Illinois
O Additional supporting evidence for midtropospheric ascent
Figure: Q-vectors and Q-vector Divergence (700-400 hPa)
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Mesoscale AnalysisO 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:O A dynamically forced region along and ahead of the surface
cold frontO Strong frontogenetic forcing made significant direct thermal
mixing likelyO 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.
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Mesoscale AnalysisO Values (Areas along cold front):
O FrontogenesisO 5.0 – 10 @ 925 hPa
O Vertical MotionO -20 ≤ ω ≤ -10
O CAPEO Exceeded 50
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Lincoln, IL Sounding1200 U
TCW
ell ahead of cold front & m
ostly cloudy conditionsLow
est 100 hPa- Very cold tem
ps.- Significant veering
-Weakly sheared unidirectional flow
above the boundary layer
0000 UTC
-Revealed a w
armer, m
oister environment in low
est 200 hPa-C
APE
of most unstable parcel lifted from
862 hPa – meager 3 J/kg, but atm
ospheric profile was nearly dry
adiabatic (862 – 625 hPa)
1200 UTC: solid lines0000 UTC: dashed lines
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Dav
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1200 UTC- Solid lines0000 UTC- Dashed lines
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Radar AnalysisO REFLECTIVITY:
O Maximum reflectivities: 40 - 45 dBZO Storm tops never exceeded 3.7 kmO 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 jetO No stratiform precipitation regionO No front-to-rear flowO Did not evolve through usual stages of squall line
evolution
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Radar AnalysisO 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 flankO No tendency for cell decay on left flankO 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 systemsO 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
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Lightning AnalysisO Rare for winter eventO Data clearly shows the temporal and spatial
extent of thunderstorm activityO Convective line’s progression towards the
southeast was depicted well using lightning dataO 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
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SummaryO 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 snowO Large amount of negative lightning contrasts with prior
studiesO Severe thunderstorm warning was best response to
conditionsO Choice of SVR over HWW forecast:
O Rapid movement of lineO Rapid onset and decrease of severe windsO 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)