June 2, 1999, Severe Weather

Three main lines of convection formed on Wednesday, June 2, 1999. The atmosphere across Pennsylvania was slightly unstable... with CAPE values generally between 200-600 J/kg, and LI's mostly between -2C and -3C. Surface temperatures were in the 80's... and dew points (measure of humidity) were in the 60's. So, the air was fairly moist and muggy.

The first and second band of convection formed well ahead of a cold front... which was positioned in Ohio during the afternoon. The third line was near (just ahead of) the cold front. You can see the NWS Local Storm Reports (LSR) here.

The first produced isolated, marginal, severe weather across a few areas of south-central PA and the lower Susquehanna River Valley. The first five reports in the NWS State College LSR issued (linked above) show the damage from this first line of storms.

The second line of thunderstorms was by far the most severe... producing numerous occurrences of damaging thunderstorm winds from southwest and west-central PA to the Susquehanna Valley. Most of the damage was west/southwest of State College. East of State College, or east of the main central PA mountain range, into the Susquehanna Valley... the thunderstorms weakened into mostly showers. Damage was more widely scattered... with wind speeds marginally severe... and damage occurring along the leading edge of the line of convection.

As said above, the worst damage from these storms was west/southwest of State College. In southeastern Armstrong county, violent thunderstorm winds, deemed as a microburst, caused significant damage in a section of Plum Creek Township... near Elderton. 1 house was completely destroyed... and 4 others damaged quite a bit, which includes roofs ripped off and structural damage. 10 other homes had more minor damage, and 2 vehicles were damaged. Unfortunately, there was an injury. Numerous trees were snapped and uprooted along the 3 mile long damage path. Thunderstorm winds were likely 75-100 mph, per NWS survey.

Numerous other reports of severe weather were received across the Laurel Highlands. Cambria county appears to have the most widespread damage... but damage also occurred in surrounding counties. See LSR's for details.

The third line of showers and thunderstorms was the weakest of the three lines. One report of severe weather was received from north-central PA... in McKean county (this is shown as the last report on the NWS State College LSR).

The following radar images are available from this event:

2:40 pm reflectivity data shows the first line of storms crossing the Lower Susquehanna Valley (with some showers stretching into the Mid Susquehanna Valley). Also take note of the second line of storms forming in southwest PA.

3:03 pm reflectivity data show the line of storms in the Lower Susquehanna Valley weakening further... while the storms over southwest PA continue to strengthen and take shape. You'll notice the bowing section over southern Westmoreland county to near the Somerset county border.

3:03 pm echo tops data shows thunderstorm tops around the bow echo area in SW PA growing... while the tops with the convection in the Lower Susquehanna Valley lower with the weakening convection.

3:27 pm reflectivity data shows the storms continue with the embedded bow echo causing most of the damage.

3:44 pm reflectivity data.

3:56 pm reflectivity data.

4:38 pm reflectivity data.

4:38 pm velocity data.

5:00 pm reflectivity data.

5:05 pm reflectivity data.

5:20 pm reflectivity data.

5:20 pm VIL data.

5:25 pm reflectivity data.

5:45 pm reflectivity data shows the line weakening with time as it moves east.

5:45 pm velocity data shows that very gusty winds, still causing some damage, are present along the leading edge of these storms. The radar scan by the Union/Snyder/Centre/Clinton county borders, where the strong winds are shown, is roughly 3,000 feet up. Thus, radar shows that wind is occurring at that height.. not at ground level. But as is fairly frequently the case with storms like these, a lot of that wind is mixed to the ground.

6:40 pm reflectivity data shows that the convection has weakened significantly more... below severe limits... although gusty wind was still occurring along the leading edge of the line.

Regional radar loop from 3-8 pm shows the three main lines.

Go here for a complete listing of all images/text available for this event. Included are additional radar images not linked above and warnings issued.