With sunshine and cold temperatures forecast through Monday and beyond here in West Michigan, I’d say we’re in the process of becoming nicely ridged. The GFS and NAM agree on a little trough digging into the southern plains on Saturday, with a surface low just southeast of the trough axis fetching moisture up into Dixie Alley. Shear is ample, as you’d expect this time of year, so maybe the South will see some organized storms. But up here in the north, none of that matters. Chilly and uneventful are the words for us this next week or so.
So it’s nice to know that chasers got in a little early play Tuesday in Iowa. The Storm Prediction Center shows 18 tornado reports, including the photogenic Creston tornado: a beautiful, sunlit tube that a number of chasers captured on camera and video. That had to have been a sight to see, and I congratulate those of you who witnessed it.
The big surprise came a day later, though, yesterday in Greensburg. No, not Greensburg, Kansas; Greensburg, Pennsylvania, 25 miles southeast of Pittsburgh. Who would have expected a tornado out there in March in hill country? True, the SPC had issued a moderate risk for the east, but that was considerably farther south, and the affected area was teetering on the very edge of a light risk. The bulk shear certainly was capable of producing supercells, but I figured they’d just be hailers. I had written off the setup as a straight-line wind event.
Yet a supercell drifting across the jumbled terrain of southwest Pennsylvania exhibited a pronounced mesocyclone that culminated in the Greensburg tornado–this in an environment where temperature and moisture that seemed inadequate to begin with dropped rapidly over the course of just a few miles. Lapse rates had to have been steep in order to sustain the storm in those conditions.
As near as I can judge, the Pennsylvania supercell appeared to drift along some kind of warm frontal boundary, with just enough moisture and heat to sustain it and enough helicity to produce a tornado. VAD wind profiles at KPBZ were straight from the west at all levels; farther east-northeast, however, at KCXX, backing surface winds veered rapidly with height, providing decent low-level torque.
But that was, I believe, in colder air. It strikes me as a pretty delicate balance that sustained the Pennsylvania supercell. Greensburg is located in the Allegheny Plateau in the foothills of the Appalachian Mountains, so perhaps the terrain may have influenced local boundary-layer winds enough to produce the Greensburg tornado. I can only speculate. The Plateau overall is mountainous, but when I look at photos of Greensburg, the town appears to be situated in a fairly level area. What the case is five miles southwest in Hempfield Township, where tornado damage was reported along Route 136, may be a different matter. I don’t know the area. However, the highway map shows curvy roads consistent with a rugged landscape.
It’s fun to think about when there’s no other weather to contemplate, which right now there isn’t. But it’s on the way. It’s spring, the sun is higher, the Gulf is setting up shop, and Storm Season 2011 is moving in.
ADDENDUM: Having looked at a few videos of the tornado in Hempfield since I made this post, I’m inclined to think that terrain wasn’t a factor. Looks like the right ingredients just came together on the mesoscale level. Interesting storm!
I was personally out of town when this storm hit–won’t get to take a look at the damage for a few days. I don’t know the slightest thing about weather, but it’s been my experience growing up there that the region between Irwin, Greensburg, and New Stanton typically gets hit hardest when storms pass through the Pittsburgh area. This is the first full scale tornado that I’m aware of, but I’d say every 2 to 3 years we get what the media labels a microburst, taking down a lot of trees and occasionally collapsing a barn or two. I mean, I might be mislead since I live there, but I always felt our area was somehow geographically situated to bear the brunt of most storms.
Thanks for your comment, Nathan. You could well be right about the lay of the land affecting the weather; topography certainly can have an influence. Every storm is different, so you can’t make any rules (such as, you’re protected from tornadoes by a ridge), just be aware that terrain and weather can interact. I recall reading a paper a few years ago on the impact of valley wind flow on tornado formation out East; interesting to speculate about! Since you live in the Greensburg area, you can no doubt answer my question: What’s the topography like out there? Looking at the map, I thought it might be pretty hilly and rugged, but the pictures and videos I’ve seen all make it look level, and the hills not particularly tall–not much different from some areas of my own state of Michigan.
Hilly is kind of a relative term, and I don’t know what I’d really compare the area to. It’s pretty far from flat, but the hills aren’t nearly as severe as they get another half hour or so farther east. Below are two links to google maps street views to maybe put it into perspective. The areas directly south of the first and north of the second were both directly hit.
https://local.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=greensburg+pa&sll=37.0625,-95.677068&sspn=42.987658,107.138672&ie=UTF8&hq=&hnear=Greensburg,+Westmoreland,+Pennsylvania&ll=40.286575,-79.669574&spn=0.001277,0.00327&z=19&layer=c&cbll=40.286581,-79.66942&panoid=AjIKFxsOI_WjB_p1Yx79Zg&cbp=12,174.56,,0,11.07
https://local.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=greensburg+pa&sll=37.0625,-95.677068&sspn=42.987658,107.138672&ie=UTF8&hq=&hnear=Greensburg,+Westmoreland,+Pennsylvania&layer=c&cbll=40.280163,-79.648058&panoid=2A5ltO0ogew9SrvrD402DA&cbp=12,351.19,,0,4&ll=40.280045,-79.647773&spn=0.001269,0.00327&z=19
I just noticed your post from last March about a tornado in Hempfield Township near Greensburg. I grew up there. We never had any tornadoes there when I was a kid in the fities. But there was one in the late eighties or early nineties. It came right through the former farm where my grandfather’s stone house is located and snapped and toppled hundreds (or thousands) of big old trees.
At the time I thought this was a once-in-a-lifetime event. But then, as you noted on your site, another tornado came though in March 2011 a few miles to the south and hit Hempfield High (where I went to school) and knocked down the entrance to the football field.
This made me wonder if there was something about the topography around here that precipitates or otherwise channels tornadoes through the area. I never noticed that before but I was aware that summer rainstorms have caused huge floods in the area over the decades, including one that hit Jeannette (a mile or two to the north) perhaps 80 or 100 years ago, breaking a dam and killing many people (our own local version of the Johnstown Flood).
I notice that Nathan says he always thought this area was especially hard hit by storms. I never heard anyone say that before. Now I wonder.