The 4 August 2004 Central Pennsylvania Severe Weather Event – Environmental and Topographical...

The 4 August 2004 Central The 4 August 2004 Central Pennsylvania Severe Weather Event Pennsylvania Severe Weather Event – Environmental and Topographical – Environmental and Topographical

Influences on Storm Structure Influences on Storm Structure EvolutionEvolution

Joe VillaniJoe Villani

NOAA/NWS, NOAA/NWS, Weather Forecast Weather Forecast Office, Albany, NYOffice, Albany, NY

BackgroundBackground

Tornadic thunderstorms moved Tornadic thunderstorms moved through the Lower Susquehanna Valley through the Lower Susquehanna Valley during the evening of 4 August 2004.during the evening of 4 August 2004.

Tornadoes touched down in several Tornadoes touched down in several locations.locations.– F0 and F1 tornadoes struck the F0 and F1 tornadoes struck the

Harrisburg metro area.Harrisburg metro area.– Resulted in numerous downed trees Resulted in numerous downed trees

and structural damage to homes.and structural damage to homes.

MotivationMotivation

Earlier, severe thunderstorms further Earlier, severe thunderstorms further to the west produced straight line wind to the west produced straight line wind damage, but no tornadoes. damage, but no tornadoes.

Main objective: Determine what Main objective: Determine what factors contributed to the evolution of factors contributed to the evolution of straight line wind producing storms straight line wind producing storms into tornado spawning storms.into tornado spawning storms.

Synoptic OverviewSynoptic Overview- 1845 UTC Water Vapor, RUC 500 hPa - 1845 UTC Water Vapor, RUC 500 hPa

HeightsHeights

Synoptic OverviewSynoptic Overview- 1815 UTC Visible Satellite, Lightning Plot- 1815 UTC Visible Satellite, Lightning Plot

Synoptic OverviewSynoptic Overview- 1500 UTC Mean Sea Level Pressure, 3 Hr - 1500 UTC Mean Sea Level Pressure, 3 Hr

Pressure change, station plotsPressure change, station plots

PIT 1200 UTC SoundingPIT 1200 UTC Sounding

Convective ParametersConvective Parameters- 1900 UTC Convective Available Potential - 1900 UTC Convective Available Potential

Energy (CAPE), Lifted IndexEnergy (CAPE), Lifted Index

Significant Radar FeaturesSignificant Radar Features- - Bow Echo: Centre County, Middle Susquehanna Bow Echo: Centre County, Middle Susquehanna

ValleyValley1938 UTC: KCCX (State College) 0.51938 UTC: KCCX (State College) 0.5o o Reflectivity, Reflectivity,

Storm Relative Motion (SRM)Storm Relative Motion (SRM)


ValleyValley1943 UTC: KCCX 0.51943 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM

- Well-defined Mesocyclone- Well-defined Mesocyclone


ValleyValley1948 UTC: KCCX 0.51948 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM

- Circulation weakens- Circulation weakens


ValleyValley2003 UTC: KCCX 0.52003 UTC: KCCX 0.5o o Reflectivity, Base VelocityReflectivity, Base Velocity

- Large area of 45-50 KTS- Large area of 45-50 KTS


ValleyValley2013 UTC: KCCX 0.52013 UTC: KCCX 0.5o o Reflectivity, Base VelocityReflectivity, Base Velocity


ValleyValley2038, 2043 UTC: KCCX 0.52038, 2043 UTC: KCCX 0.5o o ReflectivityReflectivity

- Pronounced Rear Inflow Notch (RIN), classic - Pronounced Rear Inflow Notch (RIN), classic structurestructure

RIN RIN

Significant Radar FeaturesSignificant Radar Features- - Tornadic Mesocyclones: Lower Susquehanna ValleyTornadic Mesocyclones: Lower Susquehanna Valley

2138, 2142 UTC: KCCX 0.52138, 2142 UTC: KCCX 0.5o o ReflectivityReflectivity- Transitioned from Bows Echoes to Mesocyclones- Transitioned from Bows Echoes to Mesocyclones


2151 UTC: KCCX 0.52151 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM- Mesocyclones develop quickly- Mesocyclones develop quickly

MESO


2155 UTC: KCCX 0.52155 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM- Rotational convergence broadens- Rotational convergence broadens


2156 UTC: KLWX (Sterling) 4-Panel2156 UTC: KLWX (Sterling) 4-Panel ReflectivityReflectivity


2156 UTC: KLWX (Sterling) 4-Panel2156 UTC: KLWX (Sterling) 4-Panel SRMSRM


2159 UTC: KCCX 0.52159 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM- Strong gate-to-gate shear develops- Strong gate-to-gate shear develops


2201 UTC: KLWX 4-Panel2201 UTC: KLWX 4-Panel ReflectivityReflectivity


2201 UTC: KLWX 4-Panel2201 UTC: KLWX 4-Panel SRMSRM


2204 UTC: KCCX 0.52204 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM- Mesocyclone and gate-to-gate shear persists- Mesocyclone and gate-to-gate shear persists


2206 UTC: KLWX 4-Panel2206 UTC: KLWX 4-Panel SRMSRM


2212 UTC: KCCX 0.52212 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM- Mesocyclone over Harrisburg metro- Mesocyclone over Harrisburg metro

Significant Radar FeaturesSignificant Radar Features- - Tornadic Mesocyclones: Lower Susquehanna ValleyTornadic Mesocyclones: Lower Susquehanna Valley2211 UTC: KLWX 4-Panel2211 UTC: KLWX 4-Panel Reflectivity, Reflectivity, 71 dBZ core at 71 dBZ core at

1.51.5oo


2216 UTC: KCCX 0.52216 UTC: KCCX 0.5o o Reflectivity, SRMReflectivity, SRM- Mesocyclone persists east of Harrisburg- Mesocyclone persists east of Harrisburg

Analysis/ConclusionsAnalysis/Conclusions

Main objective: Determine what Main objective: Determine what factors contributed to the evolution factors contributed to the evolution of straight line wind producing storms of straight line wind producing storms into tornado spawning storms.into tornado spawning storms.

Two key factors were instrumental in Two key factors were instrumental in generating sufficient low level wind generating sufficient low level wind shear for tornadogenesis to occur.shear for tornadogenesis to occur.

Analysis/ConclusionsAnalysis/Conclusions

1)1) Surface low pressure center moving Surface low pressure center moving eastward through south central PA eastward through south central PA just ahead of the convection induced just ahead of the convection induced a low level southerly wind a low level southerly wind component.component.

2)2) Topography of Lower Susquehanna Topography of Lower Susquehanna Valley enhanced channeling of low Valley enhanced channeling of low level south to southeast flow up the level south to southeast flow up the valley.valley.

AnalysisAnalysis- High Resolution Topographic Map- High Resolution Topographic Map

Lower Susquehanna Valley

AnalysisAnalysis- 1800 UTC surface dewpoint image, wind - 1800 UTC surface dewpoint image, wind

barbs and streamlinesbarbs and streamlines

AnalysisAnalysis- 2100 UTC surface dewpoint image, wind - 2100 UTC surface dewpoint image, wind

barbs and streamlinesbarbs and streamlines

SummarySummary

- A significant severe weather event A significant severe weather event with multiple F0 and F1 tornadoes with multiple F0 and F1 tornadoes touching down in the Lower touching down in the Lower Susquehanna Valley.Susquehanna Valley.

- An F1 tornado in a northeast An F1 tornado in a northeast metropolitan area (Harrisburg).metropolitan area (Harrisburg).

- Nearly $400K in property damage.Nearly $400K in property damage.- Fortunately no fatalities or injuries.Fortunately no fatalities or injuries.

SummarySummary

- Mainly linear mode of convection Mainly linear mode of convection earlier during the event, earlier during the event, transitioned to tornadic in the transitioned to tornadic in the Lower Susquehanna Valley.Lower Susquehanna Valley.

- Surface low pressure center moved Surface low pressure center moved across favorable topographic area across favorable topographic area at a crucial time, increasing the low at a crucial time, increasing the low level directional wind shear level directional wind shear sufficiently for tornadogenesis to sufficiently for tornadogenesis to occur.occur.

EndEnd

The 4 August 2004 Central Pennsylvania Severe Weather Event – Environmental and Topographical...

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