16 September 2005 Northern Pennsylvania Supercell Thunderstorm by Richard H. Grumm National Weather Service Office State College, PA 16803

Transcription

1 16 September 2005 Northern Pennsylvania Supercell Thunderstorm by Richard H. Grumm National Weather Service Office State College, PA INTRODUCTION During the afternoon hours of 16 September 2005, with tropical storm Ophelia moving northeastward along the East Coast, a few severe thunderstorms developed over the eastern United States. One large and persistent supercell developed in north-central Pennsylvania. This storm produced large hail and damaging winds in the few locations were spotter reports were available. The overall situation at 2045 UTC is shown in Figure 1. Tropical storm Ophelia dominates the cloud pattern to the southeast while the lightning strikes show the regions of convection, to include the supercell over northern Pennsylvania. This note will document the supercell thunderstorm of 16 September Three severe thunderstorm warnings, based on radar, were issued during the event. The storm verified 2 of the 3 warnings. This storm had minimal impact due in part to its path over a sparsely populated region. 2. METHODS very All data were collected an archived in real-time at the National Weather Service office in State college. All data shown were derived from AWIPS and images were made after the fact using the 7-day archive to display the data. 3. RESULTS Figure 2 shows the Storm Prediction Centers (SPC) storm reports for 16 September The only report in Pennsylvania was 2 inch diameter hail near Wellsboro in northern Pennsylvania. Due to the lack of reports and the difficult chore of acquiring information in the sparsely populated areas affected by the supercell storm, wind damage found 2 days later did not make the SPC database. Figure 3 shows the KCCX 0.5 degree reflectivity and storm relative velocity data valid at 1818 UTC. The emergent supercell, with a pendant in the reflectivity data and weak rotation was clearly visible at this time. The storm was in Elk County, just east of St. Mary s over Elk State Forest. Figure 2 SPC storm reports on 16 September The 2 inch diameter hail in Wellsboro was the only report to make the product. Wind was found 2 days later and is not in this product. By 1904 UTC (Fig ure 4) the storm had passed north of Emporium in Cameron County. A strong rotating

2 center was located near the Potter County border. Outbound velocities were on the order of 50KT on the eastern flank of the mesocyclone. Warnings had been issued for both counties. However, the storm avoided the only populated town of Emporium and tracked over State Forecast land. No verification was ever received from Cameron County, despite a hook echo, a strong mesocyclone and gate-to-gate shear values at the 0.5 and 1.5 degree slices in the -30/+30 range. By 1910 ( Figure 5) the storm was south of Austin in Potter County. Composite reflectivity showed elevated areas of 70 dbz cores and velocities over 50 KTS. The storm had an pendant and at times, a hook echo in the region of interest. In real-time no damage reports were observed from this storm as it traversed Potter County. Wind damage would be obtained nearly 48-hours after the event. By 2014 UTC (Figure 6) the storm was headed into Tioga County. In real-time there were not reports of any damage with the storm. Some three body scattering was observed, mainly on KBGM radar (not shown). By 2053UTC (Figure 7) the storm had a large area of 65-70dBZ cores in the composite reflectivity. Hail reports, initially around penny size, came in from Wellsboro, beneath the storm at this time. Shortly afterwards, reports of 2 inch hail were called into the office by Emergency management personnel. Note the weak rotating storm back over Cameron County at this time. By 2113 UTC (Figure 8) the storm began to weaken. 4. CONCLUSIONS One long-lived and persistent thunderstorm traversed across 3 counties during the late afternoon and evening hours of Friday, 16 September In real-time, only large hail verified one of the 3 severe thunderstorm warnings. This was probably due to the fact that the hail fell in the population center of Wellsboro and was observed by trained personnel. This storm developed over eastern Elk County and once it was fully developed, a warning was issued for Cameron County. There was some concern that the storm would pass over forested regions and be difficult to verify. However, the strong winds, pendant, and high reflectivity on radar warranted a warning. The same rational was used for the Potter County warning. No verification was available for Potter County to strengthen the arguments to warn for Tioga County. This was the second significant supercell to move over State Forests that was difficult to verify. The 12 August 2005 storm produced similar problems. Clearly, supercells are good signatures to warn on, and those with hooks and pendants should be capable of producing damage about 80% of the time. In this event, it was 66% and this number was likely skewed due to the lack of anyone to verify the hail and/or high winds in Cameron County. Thus verification remains a problematic issue.

3 5. Figure 1 GOES visual image over the eastern United States valid at 2045UTC 16 September with lightning and surface observations overlayed. Tropical storm Ophelia was off the coast at this time. Return to text.

4 6. Figure 3 KCCX 0.5 degree reflectivity and storm relative velocity data valid at 1818 UTC Friday September White arrow shows the initial storm with location shown by the white arrow. Return to text. Figure 4 As in Figure 4 except valid at 1904 UTC. Return to text. Alternate Figure 4 with composite reflectivity.

5 Figure 5 As in Figure 3 except valid at 1910 UTC. Return to text. Figure 6. As in Figure 5 except for composite reflectivity and SRM data valid at 2014 UTC. Return to text

6 Figure 7 As in Figure 6 except valid at 2058 UTC. Return to text. Figure 8 As in Figure 7 except valid at 2113 UTC. Return to text.

7 Figure 9 As in Figure 4 except composite reflectivity valid at 1904 UTC. Return to figure 4.