Summary

Numerous severe thunderstorms erupted across south central and southeastern Pennsylvania during the late afternoon on July 18th, 2006. These storms formed in a high buoyancy, weak sheared environment resulting in strong updrafts that produced a lot of wind damage and large hail. The graphical plot of severe weather reports and warnings show the extent of this event. In Central Pennsylvania there were numerous reports of hail and strong winds. Local public and spotters also reported severe weather.

The morning soundings showed the potential for strong updrafts. The KLWX 1200 UTC sounding showed large CAPE values of 3388 J/KG. A special 1700 UTC sounding was launched which showed the CAPE had decreased to 2476 J/KG but was still fairly high. Both soundings showed very weak vertical wind shear indicating the main severe weather threat would be hail and strong winds instead of tornados.

Early in the event 2 large storms formed over Huntingdon and Franklin counties at 20:00 UTC. These storms continued to move east and produce nickel size hail at 20:15 UTC (4:15 pm) at Mapleton in Huntingdon county. The Vertically Integrated Liquid radar product at 20:04 UTC (4:04 pm) shows the potential for large hail across Huntingdon and Franklin counties. A 2-panel reflectivity and storm relative velocity loop shows these 2 storms moving across Huntingdon and Franklin counties between 19:43 UTC (3:43 pm) and 20:13 UTC (4:13 pm)

Another strong storm developed in western Schuylkill county around 20:00 UTC and produced marble to nickel sized hail and strong winds as it moved east. A 2-panel radar image loop of reflectivity and storm relative velocity shows the storm tracking east across Schuylkill county between 20:00 UTC and 20:31 UTC.

In York county a storm produced 1.00 inch diameter hail 5 miles east of Codorus at 22:00 UTC (6:00 pm).

By 22:00 UTC (6:00 pm) there was a line of severe thunderstorms stretching from Lebanon county southwest to York county. At 22:15 (6:15 pm) radar showed a strong storm over western Lancaster county. At this time a local trained spotter estimated wind gusts of 50 to 60 MPH at Neffville in Lancaster county.

Also in Lancaster county at 6:30 pm a large tree blew down across the road in Ephrata and the power was out. An Emergency Manager reported that a barn was blown down and several homes were damaged by fallen trees at 6:45 pm in Churchtown in extreme eastern Lancaster county. Radar showed these strong storms moving across eastern Lancaster county at 22:30 UTC (6:30 pm) and 22:45 UTC (6:45 pm).

Images

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KLWX 1200 UTC upper air sounding showing high CAPE and weak shear.

Storms erupting across Huntingdon and Franklin counties at 2000 UTC (4:00 pm).

Vertically Integrated Liquid radar image at 20:04 UTC (4:04 pm). Purple colors indicate potential for large hail in Huntingdon and Franklin counties.

Storms stretching from Lebanon county southwest to York county at 22:02 UTC (6:02 pm).

Storm over Schuylkill county producing penny size hail and wind gusts of 60 mph at Spring Glen at 20:15 UTC (4:15 pm).

Vertically Integrated Liquid radar image at 20:59 UTC (4:59 pm). Purple and white colors indicate potential for large hail over eastern Schuylkill county.

York county storm producing 1.00 inch diameter hail 5 miles east of Codorus at 22:00 UTC (6:00 pm). A radar loop also shows the storms moving across York county between 21:45 UTC (6:45 pm) and 22:19 UTC (6:19 pm).

Vertically Integrated Liquid radar image at 22:35 UTC (6:35 pm). Purple and white colors indicate potential for large hail over northeastern Lancaster county. At 6:30 pm a large tree blew down across the road in Ephrata and the power was out.