Central Pennsylvania supercells and spearhead echo

of

15 June 2000

by

Richard H. Grumm

National Weather Service

State College PA 16801

1. Introduction

During the late morning hours of Thursday, 15 June, a lone rotating thunderstorm developed over Potter County in northern Pennsylvania. This storm moved slowly to the northeast. A short time later, similar storms developed to the west of Clearfield and Cambria Counties. These storms slowly moved eastward across these Counties, merged and formed a line of storm, which moved across Centre and Huntingdon Counties.

The largest storm of the day developed over Bedford County and moved across Fulton County and western Franklin County. This storm dissipated as it moved across Franklin County.

The storms on this day were confined to central Pennsylvania. Storms moving into eastern Pennsylvania died as they moved into the cool, stable air to the east. This cool stable air had dominated most of central and eastern Pennsylvania on Tuesday and Wednesday (13-14 June). For several days, temperatures remained in the 60s and 70s over these sections, while to the west, in the clear air; daytime temperatures were mainly in the 80s.

The few severe storms on 15 June appear to demonstrate the role of instability, shear, and orography on thunderstorm development in central Pennsylvania. The use of observational data, such as satellite, surface observations and objectively analyzed data showed the stable air to the east and probably provided clues that the severe weather would not move too far to the east.

2. Method

All radar data were obtained in real-time using the RIDDS WSR-88D tape device at the National Weather Service Office in State College. These data were played back using the archive II and displayed in WATADS. A list of all radar imagery is available here. (bed is Bedford Storm, sc is State College, po is Potter; ref is reflectivity, cr composite, vel velocity, srm storm relative velocity; finally an x signifies a cross section).

Other imagery, such as satellite and surface data is no longer available.

3. Results

a. Tioga storm

This storm had only a weak low-level rotation in it and was never very large. The circulation was only up to about 14 kft in the storm. It appeared to generate enough wind to down two trees in Potter County. But it was truly a low-end storm that probably did not produce severe storm winds at the surface. Unfortunately 35-50 kts of wind is enough to damage weak trees.

Reflectivity Images:

1609 || 1619 || 1629 || 1639 || |1639x ||

Reflectivity Images:

1609 || 1619 || 1629 || 1639 || |1609x SRM Cross section ||

b. State College Storm

The State College storm had its origins over Clearfield County and formed due to the merging of several large thunderstorms. These storms formed a line, which could be seen in the base reflectivity image at 1759 UTC. The 1804 UTC image looked quite similar. Just south of Philipsburg, in both reflectivity images, was a subtle weak inflow notch suggesting and acceleration from rear to front in the storm as it moved off the Allegheny front (zoomed image).

By 1823 UTC, the effects of the interaction with Mount Nittany can be seen in the reflectivity image. The southern part of the storm as accelerated relative to the northern portion. The storm then took a slight left turn as it moved up Happy Valley toward the east-northeast as seen in the 1833 UTC image. To the south, a similar spearhead echo was entering northern Huntingdon County.

The velocity data were of little use early in this storms development due to the method the Doppler velocities are computed. The storm may also have been rapidly developing at this time too. It is likely that the WSR-88D grossly underestimated the wind speeds due to the nearly orthogonal relationship of the radar beam and the winds. However they did reveal the leading edge of the gust front at 1758 UTC extending from near Philipsburg southward toward Tyrone and then southwestward to a point north west of Altoona. This feature was useful in tracking the timing the rain and strong gusty winds. Under these conditions, reflectivity data needs to be better utilize or if another radar is close enough and can see down the radial, these data need to be examined (unfortunately KPBZ was too far to the west of this storm).

By 1804 UTC, the zero isodop showed that the gust front had past Tyrone and the radar detected circulations along it. By 1809 UTC, the radar began to detect some outbound velocity behind he gust front. These outbounds became more pronounced as the angle the radar viewed the system became less orthogonal as seen in the 1814 UTC image. These outbounds became stronger as the storm entered State College around 1818 UTC. The storm had taken a left turn as it moved up Happy Valley striking Boalsburg around 1823 UTC. The radar had a clear view of the spearhead echo as it moved up the valley between Tussey Mountain and Mount Nittany as seen in the 1828 and 1833 UTC images.

Storm relative velocity did slightly better showing the outbound velocity data associated with the echo as it moved through the State College region between 1814 and 1818

UTC.

Series of reflectivity data:

1758 || 1804 || 1809 || 1814 || 1818 || 1823 || 1828 || 1833 ||

Series of velocity data:

1758 || 1804 || 1809 || 1814 || 1818 || 1823 || 1828 || 1833 ||

c. Bedford-Fulton Supercell

The base reflectivity image at 1749 UTC (149 Local) shows the storm over eastern Bedford County. A reflectivity cross section through this storm revealed greater then 65 dBZ’s to 28kft, the composite reflectivity also revealed how strong this storm was. This storm exceeded the Lemon criteria for pulse storms capable of producing damaging downdrafts. The VIL in this storm was quite high at 1749, VIL with a value of about 69 indicated in eastern Bedford County.

Storm relative velocity (SRM) revealed a mesocyclone over the storm at 1749 UTC over eastern Bedford County. An SRM cross-section taken at 1754 UTC revealed the circulation in the storm. This mesocyclone weakened as it moved over central Fulton County as seen in the 1818 UTC SRM.

At 1749 the reflectivity cross-section revealed that this high reflectivity cores rapidly descended over the mountains of eastern Bedford County. This is a likely location and time of downburst damage.

A series of images shows the storm evolution in the reflectivity data:

|| 1749 || 1754 || 1759 || 1804 || 1814 || 1823 || 1833 ||

A series of SRM images:

|| 1749 || 1759 || 1804 || 1804 || 1818 || 1833 ||

4. Conclusions

The severe thunderstorm, which moved through downtown State College and up Happy Valley was clearly the result of thunderstorm outflows interacting with terrain features. It appears that the downdraft accelerated off the Allegheny front, providing a spearhead or mini-bow echo shape to the storm. Upon cross Bald Eagle ridge, the northern flank of the storm struck Mount Nittany, this may have allowed the southern flank to accelerate and move up Happy Valley. The result was downed trees and limbs south of Mount Nittany. Damage was more sporadic farther eastward up the Valley.

The supercell thunderstorm over Bedford, Fulton, and Franklin Counties was the strongest storm of the day and only about the fourth of the year to produce a tornado vortex on the WSR-88D in central Pennsylvania. This storm produced no known damage in Bedford County where it reached its peak vertical extent. The only known damage was in Fulton County where the storm blew 5 vehicles off I-70. This storm weakened in Franklin County where it encountered cooler, stable, cloudy air.

Other storms over Huntingdon and Juniata Counties produced little damage. As these storms moved eastward, they weakened upon encountering the cooler cloudy air to the east.

5. Acknowledgements