1. The 4 August 2004 Central Pennsylvania Severe Weather Event – Environmental and Topographical Influences on Storm Structure Evolution Joe Villani NOAA/NWS, Weather Forecast Office, Albany, NY

2. Background  Tornadic thunderstorms moved through the Lower Susquehanna Valley during the evening of 4 August 2004.  Tornadoes touched down in several locations. –F0 and F1 tornadoes struck the Harrisburg metro area. –Resulted in numerous downed trees and structural damage to homes.

3. Motivation  Earlier, severe thunderstorms further to the west produced straight line wind damage, but no tornadoes.  Main objective: Determine what factors contributed to the evolution of straight line wind producing storms into tornado spawning storms.

4. Synoptic Overview - 1845 UTC Water Vapor, RUC 500 hPa Heights

5. Synoptic Overview - 1815 UTC Visible Satellite, Lightning Plot

6. Synoptic Overview - 1500 UTC Mean Sea Level Pressure, 3 Hr Pressure change, station plots

7. Synoptic Overview - 1800 UTC Mean Sea Level Pressure, 3 Hr Pressure change, station plots

8. PIT 1200 UTC Sounding

9. Convective Parameters - 1900 UTC Convective Available Potential Energy (CAPE), Lifted Index

10. Significant Radar Features - Bow Echo: Centre County, Middle Susquehanna Valley 1938 UTC: KCCX (State College) 0.5 o Reflectivity, Storm Relative Motion (SRM)

11. Significant Radar Features - Bow Echo: Centre County, Middle Susquehanna Valley 1943 UTC: KCCX 0.5 o Reflectivity, SRM - Well-defined Mesocyclone

12. Significant Radar Features - Bow Echo: Centre County, Middle Susquehanna Valley 1948 UTC: KCCX 0.5 o Reflectivity, SRM - Circulation weakens

13. Significant Radar Features - Bow Echo: Centre County, Middle Susquehanna Valley 2003 UTC: KCCX 0.5 o Reflectivity, Base Velocity - Large area of 45-50 KTS

14. Significant Radar Features - Bow Echo: Centre County, Middle Susquehanna Valley 2013 UTC: KCCX 0.5 o Reflectivity, Base Velocity

15. Significant Radar Features - Bow Echo: Centre County, Middle Susquehanna Valley 2038, 2043 UTC: KCCX 0.5 o Reflectivity - Pronounced Rear Inflow Notch (RIN), classic structure RIN

16. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2138, 2142 UTC: KCCX 0.5 o Reflectivity - Transitioned from Bows Echoes to Mesocyclones

17. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2151 UTC: KCCX 0.5 o Reflectivity, SRM - Mesocyclones develop quickly MESO

18. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2155 UTC: KCCX 0.5 o Reflectivity, SRM - Rotational convergence broadens

19. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2156 UTC: KLWX (Sterling) 4-Panel Reflectivity

20. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2156 UTC: KLWX (Sterling) 4-Panel SRM

21. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2159 UTC: KCCX 0.5 o Reflectivity, SRM - Strong gate-to-gate shear develops

22. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2201 UTC: KLWX 4-Panel Reflectivity

23. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2201 UTC: KLWX 4-Panel SRM

24. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2204 UTC: KCCX 0.5 o Reflectivity, SRM - Mesocyclone and gate-to-gate shear persists

25. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2206 UTC: KLWX 4-Panel Reflectivity

26. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2206 UTC: KLWX 4-Panel SRM

27. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2212 UTC: KCCX 0.5 o Reflectivity, SRM - Mesocyclone over Harrisburg metro

28. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2211 UTC: KLWX 4-Panel Reflectivity, 71 dBZ core at 1.5 o

29. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2216 UTC: KCCX 0.5 o Reflectivity, SRM - Mesocyclone persists east of Harrisburg

30. Significant Radar Features - Tornadic Mesocyclones: Lower Susquehanna Valley 2216 UTC: KLWX 4-Panel Reflectivity

31. Analysis/Conclusions  Main objective: Determine what factors contributed to the evolution of straight line wind producing storms into tornado spawning storms.  Two key factors were instrumental in generating sufficient low level wind shear for tornadogenesis to occur.

32. Analysis/Conclusions 1)Surface low pressure center moving eastward through south central PA just ahead of the convection induced a low level southerly wind component. 2)Topography of Lower Susquehanna Valley enhanced channeling of low level south to southeast flow up the valley.

33. Analysis - High Resolution Topographic Map Lower Susquehanna Valley

34. Analysis - 1800 UTC surface dewpoint image, wind barbs and streamlines

35. Analysis - 2100 UTC surface dewpoint image, wind barbs and streamlines

36. Synoptic Overview - 2000 UTC Mean Sea Level Pressure, 3 Hr Pressure change, station plots

37. Summary -A significant severe weather event with multiple F0 and F1 tornadoes touching down in the Lower Susquehanna Valley. -An F1 tornado in a northeast metropolitan area (Harrisburg). -Nearly $400K in property damage. -Fortunately no fatalities or injuries.

38. Summary -Mainly linear mode of convection earlier during the event, transitioned to tornadic in the Lower Susquehanna Valley. -Surface low pressure center moved across favorable topographic area at a crucial time, increasing the low level directional wind shear sufficiently for tornadogenesis to occur.

39. End