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

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Presentation transcript:

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

Background  Tornadic thunderstorms moved through the Lower Susquehanna Valley during the evening of 4 August  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.

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.

Synoptic Overview UTC Water Vapor, RUC 500 hPa Heights

Synoptic Overview UTC Visible Satellite, Lightning Plot

Synoptic Overview UTC Mean Sea Level Pressure, 3 Hr Pressure change, station plots

Synoptic Overview UTC Mean Sea Level Pressure, 3 Hr Pressure change, station plots

PIT 1200 UTC Sounding

Convective Parameters UTC Convective Available Potential Energy (CAPE), Lifted Index

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

Analysis - High Resolution Topographic Map Lower Susquehanna Valley

Analysis UTC surface dewpoint image, wind barbs and streamlines

Analysis UTC surface dewpoint image, wind barbs and streamlines

Synoptic Overview UTC Mean Sea Level Pressure, 3 Hr Pressure change, station plots

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.

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.

End