Written by Brian P Pettegrew Patrick S Market Raymond A Wolf Ronald L Holle and Nicholas WS Demetriades Presentation by Marcello Andiloro Introduction and Background On the evening of 11 Feb 2003 a line a severe thunderstorms moved through southeast Iowa and northwest and central ID: 679408
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Slide1
A Case study of Severe Winter Convection in the Midwest
Written by;
Brian P. Pettegrew, Patrick S. Market, Raymond A. Wolf, Ronald L. Holle, and Nicholas W.S. Demetriades
Presentation by;
Marcello AndiloroSlide2
Introduction and Background
On the evening of 11 Feb. 2003 a line a severe thunderstorms moved through southeast Iowa and northwest and central Illinois with winds in excess of 50
kt
.
Storms were associated with a passing cold front and resembled a small squall line.
Along with the wind gusts, they produced brief heavy snow, causing blinding conditions.
First severe thunderstorm warning was issued in Marshall County, Illinois at 0002 UTC on Feb. 12.Slide3
Introduction and Background
Why is this case important?
It resembled a warm season convective squall line, but was in the cold season.
There is little in the way of literature explaining events like this that occur in the cold season.
This specific case was atypical to
thundersnow
cases because it was not a case of elevated convection.
Resembled warm season severe weather rooted in the planetary boundary later.Slide4
Synoptic-scale Analysis
Bulk of precipitation started out behind the cold front, then as the storms started to grow the storms pushed ahead of the cold front.
There was a high pressure gradient both behind the cold front as well as in the warm sector.
Indications of cold front passage:
Several stations showed the passage of cold front with temperatures dropping along with dew points.
Thirty-three minute intense snow shower and clearing of skies under and hour after the shower at Davenport, Iowa.Slide5Slide6
Synoptic-scale Analysis
Polar jet analysis:
A 300
hPa
polar jet flowed southward out of central Canada with a centered core over the Ohio Valley and wind speeds of up to 135
kt
.
Southern Iowa and central Illinois were toward the left entrance region, which is atypical for the ideal occurrence of severe weather.
Where strong mass convergence below and strong mass divergence aloft are preferredSlide7
Synoptic –scale Analysis
A low amplitude shortwave trough axis was present at 500
hPa
from just west of Hudson Bay, southwest through Wisconsin revealed a significant slope into the cold air, consistent with a strong cold front.
Circular absolute
vorticity
maximum in base of shortwave trough suggested
quasigeostrophic
forcing for ascent downstream across eastern Iowa and northern Illinois.
Q-vector convergence over central Illinois at 0000 UTC supported evidence of
midtropospheric
forcing for ascent.Slide8
Synoptic-scale Analysis
Trough at 850
hPa
running parallel to surface cold front extending from Great Lakes to Iowa and Missouri. Along trough axis, dew point depressions were low, indicating a moist environment.
Temps behind trough dropped drastically, indicating a front at this level.
Evidence of prefrontal moisture and a frontal lifting mechanism were present at this level.Slide9
Mesoscale Analysis
At 2100 UTC Feb. 11 925
hPa
frontogenesis
was found along and behind the location of the surface cold front (using the 20-km RUC initial fields).
Along with significant region of
ω
along the frontal zone of values less than -15
μ
bs
-1
As well as CAPE values blossoming and maximized in area and magnitude, exceeding 50 Jkg
-1
over a large area coincident with the area where
ω
were less than -10
μ
bs
-1
This arrangement coincided with the area of the bulk of the precipitation at this time.Slide10
Mesoscale Analysis
Hourly analyses showed a gradual decrease in CAPE.
However, vertical motion values of -20
μ
bs
-1
were collocated with the small area of CAPE and the region ahead of the front where
precip
was occurring along with whiteout conditions.
These analyses depicted a dynamically forced region of ascent immediately along and ahead of the surface cold front capable of raising near surface parcels to their LCL.Slide11Slide12
Sounding Analysis
Lincoln, Illinois
Davenport, Iowa
Feb. 11 1200 UTC flew under mostly cloudy skies. Very cold temperatures existed in the lowest 100
hPa
.
The wind profile showed significant veering in lowest 100
hPa
.
Weakly sheared unidirectional flow above boundary layer.
Feb 12 0000 UTC revealed much warmer, moister environment in lowest 200
hPa
Noteworthy that the atmospheric profile was nearly dry adiabatic from 860~625.
Feb. 11 1200 UTC similar to Lincoln, very cold air near surface.
Similar unidirectional shear signature
Feb. 12 0000 UTC also in parallel with Lincoln
Significant 12-h warming in the lowest 200
hPa
, and 12-h moistening in the lowest 100
hPa
.
In contrast, cooling and drying in the
midtroposphere
which encouraged the creation of potential instability.Slide13
Radar Analysis
The strongest reflectivities were located along a line parallel to the fast-moving surface cold front.
Max
dBZ
values never exceeded 40-45.
Storm tops also never exceeded 3.7 km and the highest reflectivities were restricted to the lower portions of the strongest cells.
Showed no traits of traditional severe weather producing squall lines
No rear-inflow jet, no
stratiform
precip
region, or front-to-rear flow
Nor did it evolve through the stages of squall-line evolution.Slide14
Radar Analysis
Instead we see a convective line along the cold frontal zone that most closely resembled the parallel
stratiform
mesoscale
convective system.
But no
stratiform
precip
developed. There was no tendency even for cell decay on the left flank or new cell generation on the right as would be expected with the parallel
stratiform
MCS.
Furthermore, the convective line was found to tilt slightly down shear that was likely due to the lack of CAPE.
Therefore, the severe winds were a result of vertical mixing associated with the
mesoscale
and synoptic systems.Slide15
Lightning Analysis
A total of 101 cloud-to-ground flashes were detected by
Vaisala’s
National Lightning Detection Network.
Four of which were positive strikes.
The 26 flashes that were associated with the severe wind reports were all negative cloud-to-ground strikes.
This fact contrasted the larger body of work on winter lightning where most winter lightning documented was of positive polarity.
These results actually validated the work of Taniguchi et al. (1982) and Brooke et al. (1982) that shear in the cloud layer may have some control over the polarity of a given lightning flash. Slide16
Summary
This was a unique winter storm event:
Strong background pressure gradient and winds
Significant low-level
frontogenesis
Presence of modest near-surface CAPE
Dry-adiabatic near-surface layer .
This all allowed for shallow convection and the subsequent mixing of winds to the surface in excess of the severe criterion.
Therefore, a severe thunderstorm warning was the best response to this event as opposed to a high wind warning.