A Case Study of Severe Winter Convection in

the MidwestPaul Cody and Tim Gibbs

Lecture Outline

O IntroductionO BackgroundO Synoptic AnalysisO Mesoscale AnalysisO Lincoln and Davenport Sounding

Analyses O Radar AnalysisO Lightning AnalysisO 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 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.

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

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.

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.

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

1500 UTC Feb 11th

0000 UTC Feb 12th

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

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

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)

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.

Mesoscale AnalysisO Values (Areas along cold front):

O FrontogenesisO 5.0 – 10 @ 925 hPa

O Vertical MotionO -20 ≤ ω ≤ -10

O CAPEO Exceeded 50

Lin

coln

, IL S

ou

nd

ing

12

00

UT

CW

ell a

head

of co

ld fro

nt &

mostly clo

ud

y con

ditio

ns

Low

est 1

00

hP

a- V

ery co

ld te

mp

s.- S

ign

ifica

nt ve

erin

g

-Weakly sh

eare

d u

nid

irectio

nal fl

ow

ab

ove

the b

ou

nd

ary la

yer

00

00

UT

C-R

eve

ale

d a

warm

er, m

oiste

r en

viron

men

t in lo

west 2

00 h

Pa

-CA

PE

of m

ost u

nsta

ble

parce

l lifted

from

862

hP

a – m

eag

er 3

J/kg

, bu

t atm

osp

heric p

rofi

le w

as

nearly d

ry ad

iab

atic (8

62

– 62

5 h

Pa)

1200 UTC: solid lines0000 UTC: dashed lines

Dave

nport

, IA

Sou

ndin

g1200 U

TC

:A

lso a

head o

f co

ld f

ron

t, s

how

ed c

old

near-

surf

ace

tem

ps.

, an

d a

n

un

idir

ect

ion

al sh

ear

sign

atu

re0000 U

TC

:-A

lso r

eve

ale

d s

ign

ifica

nt

12-h

warm

ing in

low

est

200 h

Pa a

nd m

ois

ten

ing

in t

he low

est

100-h

Pa

-On

e d

iffere

nce

is

the c

oolin

g a

nd s

ign

ifica

nt

dry

ing in

mid

-tro

posp

here

: en

cou

ragin

g d

eve

lopm

en

t of

pote

nti

al in

stabilit

y-L

au

nch

ed a

s th

e p

reci

pit

ati

on

ban

d a

ppro

ach

ed

- S

urf

ace

tem

ps

were

ch

an

gin

g r

apid

ly a

s a r

esu

lt-

Su

ggest

ed a

su

per-

adia

bati

c su

rface

laye

r

1200 UTC- Solid lines0000 UTC- Dashed lines

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

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

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

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)