Wednesday, February 18, 2009
Scattered supercell thunderstorms are currently present over the southeastern US. The most organized storm at 5pm Eastern time was tornado warned, had just crossed I-20, and is heading towards the Georgia state line. The storm has a fat "hook echo" on it indicating that it is a classic/HP storm. Radar returns to 70 dBZ suggest there is BIG hail in this thing... look out!
Tuesday, February 17, 2009
I went to the National Storm Chaser's Convention last weekend in Denver. It was a real nice time and I met lots of great people. I got to hang out with Reed Timmer and Chris Chittick from http://www.tornadovideos.net/ for a while. They were cool guys and I look forward to seeing them out there on the road this spring. Sorry if I talked your head off Reed! I've been isolated from storms for too long!
My buddy Sean Poling gave me a few pictures from last season so I thought I'd post them here given the lack of storms this time of year. The pictures were taken in Kansas on May 22, 2008. Maybe he will comment on them since I wasn't there (I was still at Snoqualmie Pass, WA at that point). Looks like he intercepted a nice supercell with an RFD dry-slot and a developing tornado. Sean, comment about your storm. (BTW I didn't add all the shots... it appears the storm went HP at some point as one of the shots has a really wet RFD).
Tuesday, February 10, 2009
Wow! is this really February? A line a tornadic supercells is barreling through central Oklahoma at the moment. There are 4 distinct supercells with well developed hook echoes on this radar image. The storms are fast movers... with speeds of 45 kt. This could be a dangerous situation as they move through the Oklahoma City area. Looks like the month of May out there!
Wednesday, February 4, 2009
I've had several people comment on "what the heck is an RFD and why do we care!". Well, I'm going to do my best at explaining what they are and why they're important to storm and tornado research. First lets look at each term. Rear - the rear side with respect to storm motion. Flank - side or lateral. Downdraft - downward flowing air. So an RFD is an area of downward flowing air found near the rear and side of a supercell thunderstorm. More specifically, it is found from the rear to right side of the storm's main updraft with respect to motion. The picture shows an RFD. The view is looking north and the lower gray cloud straight ahead on the road is a wall cloud. Wall clouds form on the interface between the updraft and RFD. To the left is clearing behind the storm and the RFD scours away clouds wrapping counter-clockwise around the rotating updraft forming a clear slot or notch (visual evidence of the RFD). RFD winds hit the ground and spread out along the surface. Storm chasers look for this signature, which manifests as a horse shoe shaped cloud, as a precursor to tornado formation in supercell thunderstorms. Once the RFD develops, it will do one of two things... 1). focus the broad storm scale rotation near the surface into a tight circulation (i. e. tornado) or 2). cause the storm scale circulation to become less focused near the surface and essentially kill off low level rotation (no tornado). This is why the RFD is so important to tornado formation (at least we think that is true at the moment). It takes a special RFD to form a tornado... and a REALLY special RFD to form a long-lived damaging tornado. The basics are this: an RFD that is neutral or positively buoyant (think of a balloon that either hovers in place or moves upwards on its own) will readily rotate inward and upward into the storms main rotating updraft. These downdrafts are special because most downdrafts fall out of the sky because the air in the downdraft is heavier than it's surroundings due to evaporative cooling. However, a warm RFD is forced downward by something else... most likely vertical pressure differences. So once this warm, buoyant air is forced down to the surface, it spirals in to a common point under the updraft and is recycled. It had angular momentum as it rotated downward (or developed it due to other reasons). As the RFD air spirals inward to the center of the updraft... angular momentum is conserved. You can test the physics of this at home by sitting in your computer chair and start spinning with your legs out. While spinning, bring your legs inward. You will quickly begin to rotate faster. This is due to the conservation of angular momentum. It is theorized that the same thing happens with the warm RFD air. However, more commonly RFD air is colder than its surroundings and the opposite effect will occur. Evaporatively cooled air (from rain and dry air) can contribute to this. Cold RFDs will hit the ground and be too heavy for the updraft to recycle much of the air. The RFD will surge away from the updraft and the conservation of angular momentum will cause the rotation to slow down (do the computer chair experiment starting with your legs in and then push them out... you will start fast and then rotate slower). So RFDs are very important to the development of tornadoes and the demise of rotation. In fact, it is likely that the RFD that causes tornadoes to develop in time will cause the tornado to rope out and die. Warm RFD's that initially fuel the tornado turn colder in time and snuff off the circulation. This is where lots of current tornado research is being done today. Why are some RFDs cold and others warm? Why do they transition over time. Why can one storm produce multiple RFDs over time that are sometimes warm and other times cold. There are lots of questions to be answered.
Tuesday, February 3, 2009
Since it's the middle of winter and I'm thinking a lot about storms, I thought I would post a few shots and some radar data from my tornado intercept last August. I'm glad to be back out on the Plains after being away for 4 years. .. Here's a view of the Alta Vista, CO tornado developing (view is from the west looking east). A nice RFD notch can be seen with a dust whirl developing underneath. The tornado was a mile to my east. The storm was a neat little LP supercell that had a very compact base as seen from Dann's shots on the other side of the storm. Note that the pictures were taken by an iphone camera. Not bad quality for a phone, huh. I wasn't prepared to chase on this day. Keri & Tommy were in Florida with our main camera and we had just moved back to CO from WA. All of our stuff was still in boxes. Anyways, it was a fun chase, got to see a tornado, and get some half way decent pictures.
Here's the Pueblo, CO radar image from 4:19 local time. The storm was very sheared out west to east and had no visible hook echoes. It was an LP storm with all of its precipitation well downshear from the main updraft. There were also lots of surface boundaries in the vicinity. Once of which was a large north to south boundary that was likely a combined dry line - outflow boundary. This played a role in convective initiation and possibly tornadogenesis.
Finally, here's a picture of the tornado when it was mature. The funnel was not visible from my perspective. Notice that swirl-like cloud above and left of the tornado... that was a long-lived feature on this storm and was likely the low level mesocyclone. Rapid cloud based rotation was present for about an hour prior to the tornado. This was a very interesting storm from many aspects.
Monday, February 2, 2009
Photo taken by Dann Cianca of the Alta Vista, CO LP supercell that later goes on to produce a tornado on 13 Aug. 2008. I've drawn in where I think a mini-RFD surge was occurring. I was located just left of where I labelled "RFD" and got blasted by a warm and dry RFD surge as the tornado developed.