A Case study of Severe Winter Convection in the Midwest

1. 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 Andiloro

2. 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.

3. 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.

4. 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.

6. 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 preferred

7. 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.

8. 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.

9. Mesoscale Analysis • At 2100 UTC Feb. 11 925 hPafrontogenesis 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 μbs-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 μbs-1 • This arrangement coincided with the area of the bulk of the precipitation at this time.

10. Mesoscale Analysis • Hourly analyses showed a gradual decrease in CAPE. • However, vertical motion values of -20 μbs-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.

12. 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.

13. 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 stratiformprecip region, or front-to-rear flow • Nor did it evolve through the stages of squall-line evolution.

14. Radar Analysis • Instead we see a convective line along the cold frontal zone that most closely resembled the parallel stratiformmesoscale convective system. • But no stratiformprecip 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.

15. 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.

16. 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.