A Case Study of Severe Winter Convection in the Midwest

1. A Case Study of Severe Winter Convection in the Midwest Paul Cody and Tim Gibbs

2. Lecture Outline • Introduction • Background • Synoptic Analysis • Mesoscale Analysis • Lincoln and Davenport Sounding Analyses • Radar Analysis • Lightning Analysis • Summary

3. Introduction • On the evening of February 11th 2003, a line of severe thunderstorms moved through Southeast Iowa and Northwest and Central Illinois. • Winds exceeded 50 kts (27 and a brief period of heavy snowfall created whiteout conditions. • The NWS in Lincoln, Illinois issued a severe thunderstorm warning for several counties due to strong winds and heavy snowfall.

4. Introduction • The first severe thunderstorm warning was issued at 0002 UTC on February 12th for Marshall County, Illinois. • 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

5. Introduction • This case is unique as it did not fall into a specific operational paradigm. • How do we choose a forecast warning that bests communicates the nature of the hazard? • Severe Thunderstorm Warning(SVR) • .75” diameter hail and wind gusts >58 mph • High Wind Warning (HWW) • 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.

6. Background • This event was uncommon for thundersnow cases since it was not a case of elevated convection due to the presence of an extratropical cyclone. • This event had maintained qualities related to warm season/sector weather rooted in the Planetary boundary layer, like squall lines. • A warm season event that occurred in the cold season.

7. Synoptic Analysis • Cold front was found on surface analysis at 1500 UTC over South 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: • 1) Intense 33 min snowshower • 2) Skies cleared post shower and dewpoints fell • 3)Moderate veer in <1 hr tempered by blustery post frontal conditions • Huron, SD has severe wind criteria, but lacked precipitation or lightning

8. 1500 UTC Feb 11th

9. 0000 UTC Feb 12th

10. Synoptic Analysis • A 300-hPa polar jet was present stretching from Central Canada to the Ohio valley • Winds exceeded 135 kts • Iowa and Illinois were near the core, but more towards the left entrance region: • Not typical for severe weather as it is a region of upper level convergence Figure: 300-hPa wind field. Jet streak in shaded region

11. Synoptic Analysis • Low-amplitude shortwave trough at 500-hPa stretched from Hudson bay through mid-west • Significant slope into cold air, consistent with a cold front • 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

12. Synoptic Analysis • Q-Vector convergence occurred over central Illinois • Additional supporting evidence for midtropospheric ascent Figure: Q-vectors and Q-vector Divergence (700-400 hPa)

13. Mesoscale Analysis • Observed low level frontogenesis, vertical motion, and CAPE revealed an environment with necessary ingredients for convection. • Combination of substantially increasing frontogenesis and collocation of CAPE (albeit decreasing) and vertical motion produced whiteout conditions. • Analyses depict: • A dynamically forced region along and ahead of the surface cold front • Strong frontogenetic forcing made significant direct thermal mixing likely • Relatively deep dry-adiabatic layer (~100 hPa) along and just ahead of cold front provided little inhibition for parcels to be lifted to LCL • The lower troposphere as a region conducive to vertical motion.

14. Mesoscale Analysis • Values (Areas along cold front): • Frontogenesis • 5.0 – 10 @ 925 hPa • Vertical Motion • -20 ≤ ω ≤ -10 • CAPE • Exceeded 50

15. Lincoln, IL Sounding1200 UTCWell ahead of cold front & mostly cloudy conditionsLowest 100 hPa- Very cold temps.- Significant veering -Weakly sheared unidirectional flow above the boundary layer0000 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

16. Davenport, IASounding1200 UTC:Also ahead of cold front, showed cold near-surface temps., and an unidirectional shear signature0000 UTC:-Also revealed significant 12-h warming in lowest 200 hPa and moistening in the lowest 100-hPa-One difference is the cooling and significant drying in mid-troposphere: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

17. Radar Analysis • REFLECTIVITY: • Maximum reflectivities: 40 - 45 dBZ • Storm tops never exceeded 3.7 km • Highest reflectivities limited to lower portions of strongest cells • Deviations from traditional severe weather producing squall lines (radar and velocity data): • No rear in-flow jet • No stratiform precipitation region • No front-to-rear flow • Did not evolve through usual stages of squall line evolution

18. Radar Analysis • Instead the was a convective line that more closely resembled a parallel stratiform convective system (MCS), however: • The area of stratiform precipitation on the left flank • No tendency for cell decay on left flank • No new cell generation on the right as should be expected • Severe winds were not the result of squall line convection but rather the mixing associated with the mesoscale and synoptic systems • 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

19. Lightning Analysis • Rare for winter event • Data clearly shows the temporal and spatial extent of thunderstorm activity • Convective line’s progression towards the southeast was depicted well using lightning data • Whereas most “winter” lightning is (+) in polarity, a majority of the strikes (97 of 101) were (-) in polarity • 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

20. Summary • RUC analysis supported the formation of convection along the cold front, through a dry adiabatic layer in the PBL • The thermodynamic profile was conducive to lightning production yet cold enough for snow • Large amount of negative lightning contrasts with prior studies • Severe thunderstorm warning was best response to conditions • Choice of SVR over HWW forecast: • Rapid movement of line • Rapid onset and decrease of severe winds • 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. • Main difference from warm season SVR (snow, blowing snow, could have easily been mentioned in warning text)