1. FACTORS CONTRIBUTING TO A HEAVY SNOWFALL EVENT ACROSS NORTHERN ILLINOIS DECEMBER 2009 Nathan Marsili/Stephen Rodriguez 2010 GLOM Workshop Toronto, Ontario

2. Introduction  Synoptic Background and Forcing Mechanisms  Mesoscale Features/Forcing Mechanisms  Snow/Liquid Ratios

4. OBSERVED SNOWFALL TOTALS

5. FORECAST SNOWFALL TOTALS 2 to 4 INCHES 1 to 3 INCHES AROUND AN INCH

6. Radar Overview (06Z-00Z)

7. Synoptic Overview 300 MB 12/26/09 12Z

8. Synoptic Overview 700 MB 12/26/09 12Z

9. Synoptic Overview 850 MB 12/26/09 12Z

10. Synoptic Overview SFC 12/26/09 12Z

11. Synoptic Overview 12/26/09 12Z ROAB KDVN DGZ

12. Radar Evolution (KLOT) 06Z-12Z

13.  Equally spaced west-east oriented snow bands (max reflectivity 35-40 dBZ).  Bands short-lived but multiple bands affect same areas to produce 2-4 inches of snow accumulation between midnight and 6am.  Occurred in differential negative vorticity advection region, seem to feed off shallow low level convective instability. Phase 1: 06Z-12Z December 26th

14. Horizontal Convective Rolls (HCRs) Schultz (2003) - Develop from thermal instability. - Enhanced by wind shear, but shear minimized in cross-roll direction. - Noted with large surface/air heat fluxes (warm ground/cold air).

15. Horizontal Convective Rolls (HCRs) A B A B KLOT 0.5 base reflectivity 0841Z 0-2.5 km shear vectors Reflectivity cross section

16. Phase 2: 12Z-00Z  Radar Storm Evolution 12Z-00Z

17. Phase 2: 12Z-00Z Synoptic Forcing  Persistent warm advection associated with slowly retreating cold dome 12Z NAM 280K

18. Phase 2: 12Z-00Z Mesoscale Factors  Moderate mid level frontogenesis 12Z NAM 6-hr forecast 700 mb frontogenesis 12Z NAM 6-hr forecast 700 mb Fvec dvg (color-filled) KLOT 0.5 deg base reflectivity ~1800Z

19. Phase 2: 12Z-00Z Mesoscale Factors  Bands become more enhanced when entering mid level frontogenesis region  In addition to area of apparent enhancement due to frontogenesis, smaller scale linear elements also noted with locally more intense snowfall rates.  Complex banding structure shows up early in the event during the overnight hours on KDVN radar.

20. Radar Imagery (KDVN) KDVN 0.5 base reflectivity (~ 0900Z-1130Z)

21. Phase 2: 12Z-00Z Mesoscale Factors  West-east banding appears to be convectively driven – horizontal convective rolls  North-south oriented bands do not necessarily line up nicely with DPVA, frontogenesis, or mid level warm advection?  Enhanced reflectivities where bands intersect (and deeper reflectivity signatures)  CSI evaluated and found not to be playing a role in this case  What else could be forcing these bands?

22. Phase 2: 12Z-00Z Mesoscale Factors  North-south bands may have been enhanced by gravity waves?  Enhancement of updrafts due to HCR/gravity wave interaction. Christian 87

23. Phase 2: 12Z-00Z Mesoscale Factors Stable ducting layer Convectively unstable capping layer

24. Phase 2: 12Z-00Z Mesoscale Factors ~1820Z

25. Snow to Liquid Ratios Images from SLU Snow:Liquid Research webpage (Marty Baxter)

26. Snow to Liquid Ratios

28. Conclusions  Up to 1 foot of snow due to several synoptic/mesoscale factors.  Upper level low and associated isentropic lift with departing cold dome provided extended duration snowfall.  Thermal instability led to HCR development early morning producing storm total forecasted amounts by 12Z over localized areas.

29. Conclusions  Mid level frontogenetical forcing providing greatest snowfall rates midday.  Frontogenetical forcing co-located with deep DGZ leading to highly anomalous SLR.  Prolonged forcing combined with rare SLR’s led to very challenging forecast.

30. Thank you  Any questions?