UPDATE: The NWS State College say that it was the outflow from the eastern storm that killed this one saying in am email that it "not only decreased the fuel available to the cell and increased the boundary layer stability, but also changed the storm relative flow inside the low and mid levels of the storm." So the cool, dry air not only reduced the energy available to the storm, it disrupted the updraft which also powers thunderstorms.

UPDATE: Thanks for all the Comments. The NWS in State College has put up a great summary page on the storm with lots of radar images which I have borrowed below, but they don't say on that page why the storm died (nor did they in their Warning cancellation), and unfortunately I was on vacation so I don't have any data that would tell us (I was driving north towards town when I heard about the storm). But I think the Comments covered the possible explanations (I have an email in to NWS-CTP and will post here if they respond).

The first thought from myself and Meteo Madness Man (PREMIUM | PRO) - who I was on the phone with at the time - was that the storm had lost its punch as it descended the mountains to our Northwest. Although this happens a lot east of the Appalachians (descending air dries out a storm) as we are here in State College (see map below), I'm not convinced that alone was responsible for the death of this storm. While the storm did roll into several valleys, it was still strengthening in northeast Clearfield county.

The air was certainly juicy enough over Centre County to support strong storms - you can see (rare) dew points in the lower to mid 70s on the map below.

A more likely scenario than the one above (I'm guessing here without data) is that the outflow boundary from the first line of storms swept in front of this new storm, reducing the energy in front of it (you can see the outflow boundary as a weak light blue line expanding away from the storm line on the NWS radar image - I have identified it below).

Outflow boundaries happen when winds descend from a storm, hit the ground, and spread out in front of it (more on WikiPedia). Although ouflow boundaries can also cause new storms to form on the boundary, they spoil storms that move into the air behind the boundary, which is now cooler and drier.

ORIGINAL BLOG ENTRY: A possibly tornadic severe thunderstorm was bound for State College, Pennsylvania, home of AccuWeather HQ [Google Map] yesterday afternoon. What killed it? If you have a guess, leave me a Comment. The answer coming later today.

Comments (7): Jeff Wilson:

My guess would be the the convection that existed NE of State College produced a cool, stabilizing outflow boundary ahead of the severe thunderstorm.

Posted by Jeff Wilson | August 10, 2009 1:01 PM Chris Hayes:

Jesse,

Were there showers that developed ahead of the main thunderstorm? That could have stabilized the atmosphere enough causing the thunderstorm to dissipate. Just my two cents. Sorry first post had wrong wording (destabilized should have been stabilized)

Posted by Chris Hayes | August 10, 2009 12:40 PM sharky:

Bill Gates?...

But seriously, I think maybe the topography caused it to run out of energy

Posted by sharky | August 10, 2009 12:23 PM Wade:

The terrain?

Posted by Wade | August 10, 2009 12:21 PM Matthew:

I think because State College had more of a stable atmosphere from either rain that they got from earlier or from more clouds than sun.

Posted by Matthew | August 10, 2009 12:19 PM Chris:

My guess is that since State College is known as "Happy Valley" the topography of the mountains around State College caused the storm to dissipate. Probably the effects of downsloping caused it to dry up and run out of steam. If thats not right then State College is a bubble, just like where i live.

Posted by Chris | August 10, 2009 12:09 PM Dayton Spy:

Without looking for further info I will guess either it was riding a frontal boundry that lifted north or it ran into drier air.

Posted by Dayton Spy | August 10, 2009 11:57 AM