Chase Log: April 13th, 2007
Click on any of the SPC products below (Convective Outlooks, Watches, or MCDs) to see the SPC's "Severe Thunderstorm Events" page pertaining to this event.
Total Distance: 415 miles
Target Area: Decatur, TX
Chase Area: Decatur, TX, to Weatherford and Ft. Worth
Maximum SPC Risk category: High
Watches:
PDS Tornado Watches 135 and 136, Tornado Watches 133 and 138, Severe Watch 134
Mesoscale Discussions (MCDs): 479-485
A strong upper-level low was progged to move out of New Mexico, spreading strong southwesterly flow aloft over much of the southern plains. Cyclogenesis began the previous day, with mid-60 dewpoints streaming into central Texas by Friday morning. A surface low was forecast to deepen and move just south of the Red River, with a warm front stretching east-southeastward from the low. Original concerns revolved around the forecast widespread convection that would prohibit destabilization across most are the area north of I20. Despite questions about instability, there was very high confidence that the warm sector would be very strongly sheared. In fact, southeasterly sfc winds beneath 50kt southerly 850mb flow would create low-level shear profiles supportive of strong and violent tornadoes. When I awoke Friday and noticed that there was no convection across northern Texas, I had an awful good feeling that there was the potential for a very active tornado day.
I chased with my semi-regular group of folks (Dan Dawson, Robin Tanamachi, Jana Houser, and Mike French) west of the DFW metroplex. Our original goal was to target the area around Decatur, in hopes that it would keep all options open (e.g. we could intersept any northeastward-moving storms that developed near or sw of DFW, while also giving us space to head west or east if necessary). I was pleased to see the lack of convection in the warm sector during the morning (the models had forecast widespread convection across northern TX during the morning), and I was pleased to see some insolation occurring across parts o western north Texas. By the time we made it into Gainesville, we were well aware of the cell near Seymour, but we though it would quickly cross into more stable air. So, we waited for new initiation near or SW of I20, thinking it would move northeastward at us. Well, we waited in Decatur for what seemed like an hour (it probably was, actually) before opting to head towards Weatherford in hopes that the developing convection to our west would be more discrete in that area. Well, that didn't happen. The squall line (with embedded mesos and leading-edge mesovorticies that were very apparent on the Fort Worth 88D from time-to-time) passed over us near Springtown. Just south of Springtown, we came upon a pretty serious head-on collision, and we were just the 2nd car to come across the accident (the first car was just ahead of us). I didn't know where the driver of one of the vehicles was (the driver of the truck), but the driver of the car was pinned. I worked with a few others to pull the door open enough for someone to pry the guy's leg out. Regardless to say, he will probably have a very sore leg for some time (I can't imagine it isn't badly broken in some places, especially considering the stearing column was pressed all the way to the driver's seat). When it appeared as though he was free of serious injury, with emergency services 'en route, and a dozen others at the scene, we figured we couldn't provide any more help, so we continued onward.
By the time we made it to Weatherford, we heard about the tornado warning for Tarrant county. We briefly thought about (and tried) to catch up to it, but 4 accidents in a 2-mile stretch of I20 east of Weatherford made that impossible. Instead, we met up with Amos M., Eric N., Tony L. and his crew at a mexican food joint in northern Fort Worth. It was great to chat with Amos and Eric again, and it was great to finally meet Tony.
I'm not not entirely sure what that fineline was that was ahead of much of the convection west of FTW. First guess was an OFB, but it was continuous with a very fast-moving front that was sweeping eastward across the area. In addition, it was readily apparent before much convection developed along and behind it. With such intense low-level shear, I also can't imagine the outflow and gust front would have been able to surge that far ahead of the convection. So, given all this, I think it was more of a pacific front / cold front that moved so quickly that it undercut the convection and provided for an intense, linearly-oriented source of surface convergence. By early to mid-afternoon, I saw several obs near I20 in western TX that had westerly winds at 20-40kts. During the mid-afternoon, it looked as though the Canadian cold front was located E-W near Lubbock (northerly winds N of that), with a Pacific airmass located south and southeast of Lubbock (with very strong westerly winds). I thought briefly that this main boundary was a sort of dryline, but, IIRC, the air behind it was not "hot and dry" as we typically see with dryline (it was mild and relatively dry, not atypical of Pacific cold fronts). By the time 23z approached, it appeared as though the Canadian cold front had joined up with this Pacific cold front across much of the northern TX, as winds had turned to the NW and NNW behind the fineline and convection.
