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

2. 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

3. Introduction

4. 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

5. 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.

6. 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.

7. Synoptic Analysis O 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 front O Transitioned to prefrontal during afternoon and at time of thundersnow event O Significant pressure gradient ahead of the cold front in the warm sector and in the cold air behind it O 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

8. 1500 UTC Feb 11th

9. 0000 UTC Feb 12th

10. 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

11. Synoptic Analysis O 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

12. 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)

13. 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: O 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.

14. Mesoscale Analysis

15. 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

16. 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 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. Modified Sounding Davenport, Iowa 0000 UTC 12 February 2003 Changes include: -Surface ambient temperature -Dewpoint temp. -Well-mixed, dry- adiabatic env. (lowest 50 hPa) -Superadiabatic layers have been removed

18. Rationalization for and Results of Modified Sounding Davenport, Iowa O Modifications were made in order to approximate warm air immediately ahead of the surface frontal zone O Changes to surface and dewpoint temps. were determined by extrapolating the lapse rate in the 925-975 hPa layer to the surface. O Yielded a nearly-dry adiabatic surface layer beneath the frontal inversion and a conditionally unstable atmos. Above the inversion O Boundary layer adjusted to a well-mixed, dry adiabatic condition (lowest 50 hPa), temps. have been warmed by 1 deg. C above the inversion, & superadiabatic layers were removed O Resulted in CAPE values >100 J/Kg, LCL w/ temp. warmer than -10 deg. C, relatively low EL (592 hPa) – all necessary for lightning O Winds were free to mix into the lowest 50 hPa – layer w/ 50 kt+ winds

19. 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

20. 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

21. 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

22. 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)