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                   Friday, March 25, 2011 - 8:48PM CST

March 26 STL snowstorm; 3/23 southwestern Pennsylvania tornado event

By DAN ROBINSON
Editor/Photographer
25 Years of Storm Observing
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A strong early-spring snowstorm will impact the St. Louis region on Saturday. Models show strong upward vertical motion under an intense mid-and upper-level jet during the day on Saturday, with strong baroclinicity just to the south (temperature gradient between warm and cold) and heavy precipitation overspreading the area by mid-afternoon. These conditions are ingredients indicative of thundersnow, and subsequently I will be out 'observing' and streaming live. Stay tuned to the Facebook or Twitter feeds if you would like a notification when the stream is 'on the air'.

March 23 Greensburg/Hempfield, PA tornado

NWS DAMAGE SURVEY: EF2 tornado in Hempfield Township

In yet another example of how chaseable tornadoes can and do happen outside of traditional "Tornado Alley", supercells and at least one tornado struck the Pittsburgh area on Wednesday as part of a widespread, multi-state severe weather outbreak. A rather impressive high-contrast tornado struck the Greensburg-Hempfield area just southeast of Pittsburgh, heavily damaging a high school and several homes. The tornado was rated EF2. (See footage links below)

The Pittsburgh area tornado occured ahead of a surface low along a warm/stationary front draped across northern Ohio and southern Pennsylvania. Supercells fired in central/northern Ohio and moved toward this boundary, with at least one other storm developing strong low-level rotation (and subsequently tornado warned). The Hempfield storm initiated just southwest of Pittsburgh and produced the tornado as it interacted with the frontal boundary.

Due to the long distance (over 9 hours one-way) and high gas prices, this setup was never in the running for an expedition day for me. However, I did do some forecasting excercises in the morning, chose a 'virtual' target (which I posted on the Facebook page in the morning) and kept tabs on the event throughout the day as it unfolded. The RUC model showed the surface low tracking into southwestern PA along the frontal boundary, with 50s dewpoints under very cold temperatures moving in aloft. This indicated strong ambient instability over the region. While mid and upper level winds were very strong (maxing out at 100 knots at 500mb), they were all out of the southwest (unidirectional) as were the low levels. The meant there was virtually no directional shear across the entire risk area, except up at the northern end of the warm sector along and north of the I-70 corridor, where winds were backed along and north of the frontal boundary. Due to models indicating very early storm initiation and fast storm motions, an expeditioner would have wanted to position themselves along the frontal boundary well ahead of the initial activity. For this reason, I chose Pittsburgh as my 'virtual' target, that is, one I would have traveled to if I had been able to chase.

Here are some Youtube links showing the tornado captured by residents. Be forewarned, these contain strong language, so you might want to mute your speakers.

VIDEO LINK 1: Tornado approaches high school
VIDEO LINK 2: Hempfield tornado
VIDEO LINK 3: Close-up view with debris

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