Pennsylvania Tornadoes

Wednesday’s severe weather outbreak was remarkable given the time of year. 22 tornadoes touched down between Florida and Pennsylvania with 4 of those tornadoes being significant.

The Pennsylvania tornadoes were QLCS tornadoes. The Lancaster County tornado was an EF-2 and on the ground for 5 miles while the one in Bradford County was an EF-1 and on the ground for only a mile. The Lancaster County one was quite impressive on radar and very well warned by the National Weather Service in State College, PA.

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You can see a well defined QLCS with a developing bowing section over southern Lancaster County here at 0017 UTC. The couplet has a rotational velocity of around 24 knots (delta V of 48 knots). Within 11 minutes the couplet becomes substantially stronger. In fact, rotational velocity increases to nearly 60 knots! For a QLCS this is exceptionally powerful and is supportive of a significant tornado.

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By the time tornadogenesis occurs at 0038 UTC the velocity signature is “lost in the haze” so to speak on KDIX (the closest radar site) due to range folding.

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It’s somewhat surprising the tornado didn’t touch down earlier given the strength of the tornado vortex signature that was sampled. However, it’s important to realize that this signature was being sampled nearly 7,000 feet above radar level and the environment was not particularly favorable for tornadoes. Here’s the 1-hour 00z 4km NAM forecast for Lancaster, PA showing anomalously high instability for February (MUCAPE approaching 1,000 j/kg) but a relatively stable boundary layer and certainly not much surface based CAPE.

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The tornado in northeastern Pennsylvania was much less impressive on radar.

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With only about 20 knots of rotational velocity this tornado signature isn’t going to appear in a textbook anytime soon. What I found most interesting about this storm was how poorly the models were even initializing the nearby environment. Take the 00z 4km NAM for Scranton/Wilkes-Barre (KAVP).

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You can see almost no instability (surface based or otherwise) but exceptionally strong shear. Given this model sounding you’d expect the chance for severe weather to be quite low. However, take a look at the 0043 UTC METAR from KAVP.

KAVP 250043Z 16015G27KT 10SM VCTS -RA FEW024 BKN070 OVC110 17/13 A2931 RMK AO2 PK WND 15031/0027 LTG DSNT S-NW P0002 T01670133

The temperature was nearly 6C higher than modeled! What was a model sounding with virtually no instability is really now one with some instability – probably surface based too! This is a good reminder to not get lost in the models during events like this one – observations are important!

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