Chase Log: March 30th, 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.

MAP

Total Distance: 320 miles
Target Area: Breckenridge, TX
Chase Area: Near Decatur, TX
Maximum SPC Risk category: Moderate
Watches: Tornado Watches 92-96
Mesoscale Discussions (MCDs): 376-378, 380-388, and 390

The cut-off that set up over the central and southern Rockies was forecast to begin to move out this day. Ample moisture remained in place (62-68F Tds south of I40), though widespread convection was forecast to limit destabilization. Despite this, very strong shear was forecast east of the cold front that stretched from western Oklahoma into southwestern Texas. With ~20 SE sfc flow, 40-50kts SSE flow at 850mb, and 50kt SSW 500mb flow, shear profiles were moderate to strong, particularly by early evening. By the time we looked at things around noon, 0-3km SRH was already >400 m2/s2 near SJT. With a RUC forecast of widespread >400 m2/s2 0-3km SRH by 0z across much of Texas and Oklahoma, we were hoping that the strong shear and low LCLs were going to be enough to compensate for marginal instability (500-1000 j/kg CAPE).

Not much to report... I headed out (with Dan Dawson and another) towards an area west of Fort Worth around 1pm. Our thought was that the I20 corridor was going to be the southern line of the convection that had been occurring across northern Texas. Though there was tremendous low-level shear south of I20, we weren't sure we had enough time to target a place south or southeast of ABI. At any rate, by the time we made it to Gainesville, we realized that the ongoing convection was leading to poor lapse rates and relatively cool sfc conditions, two things which significantly stabilized the risk area. So, we putzed around Gainesville for a while, before heading southwest towards Decatur to intercept a bowing line segment approaching from Jacksboro. Waiting experienced very near the apex of the bow echo ~4-5 mi NW of Decatur, but we received no winds >20mph or even any lightning. We quickly realized there was absolutely no hope for significant discrete activity, and we called off the chase to head home.

I'm not sure why I'm surprised we saw the result we saw. Most models forecast pitiful instability, and the widespread convection certainly saw that it happened (terrible lapse rates and unimpressive sfc temps). I'm slightly surprised various forecasts remained bullish in light of marginal instability, but even my less-than-optimistic forecast was a complete bust. Oh well, it's only March, so I'm fine with a bust. LOL