Chase Log: October 17th, 2007
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Total Distance: 200 miles
Target Area: Chickasha to Norman, OK
Chase Area: Oklahoma City to Cushing, OK
Maximum SPC Risk category: Moderate
Watches:
Mesoscale Discussions (MCDs):
Similar story to most folks who chased in Oklahoma... I left OUN with Mike French, Jana Houser, and Howie B. upon seeing initiation down near Chickasha. We got in good view of that storm as we passed through Moore (driving nearly alongside Roger E and company), and we ended up chasing it to Cushing. It was apparent to us that things were not going to go well upon feeling cold, westerly outflow after stopping between Tryon and Perkins miles southeast of the primary updraft base. We meandered southward towards sunset to see if anything coming up from just east of OKC would do anything. The storm(s) appeared to have weak rotation on radar as they passed S and E of Stroud, but it was then getting dark, so we called it off.
I did get a call from my wife at our house in northwestern Arcadia as we were driving N of Wellston on 177 to tell me that quarter size hail had fallen ~5-10 minutes earlier (that's the LSR that should be 2 mi E of I35 on Waterloo, though it's on the LSR as having occurred at I35 and Waterloo). She took a few pictures of some of the hailstones -- some were very spikey, and many were quite irregularly-shaped, something I don't remember seeing a lot with hail only the size of quarters. I supsect that some of those oddly-shaped stones were likely larger at one time, but had melted (either falling through the storm or on the ground in rainwater).
Interestingly, many of the storms that developed in C and NE OK seemed to produce killer outflow that spread well outward from the updraft bases. This was readily apparent as a fineline on radar, and it was equally apparent in the field. Here are a couple of my thoughts, developed after some post-chase discussion on the ride back to OUN:
1. Although it appeared that deep-layer shear was quite strong, I'm not convinced that the shear "experience" by the bulk of the depth of the updrafts was... If we assume that the OUN sounding from 1km and above is likely at least 0th-order representative of the environment immediately ahead of the dryline in central OK (i.e. the environment of the storms from OKC to Cushing, and the later storms that developed E of OKC near Stroud), then I contend that the magnitude of the 0-6km shear is quite misleading in terms of the effect of that shear on the storms. The 00z OUN sounding shows 45kts from the SW just off the surface, with 55-65kts at 6km. Sure, the winds at the surface ahead of the dryline were southerly or SSWrly at ~10-15kts, but it's likely prudent to average the sfc wind to formulate a mean lower-layer averaged wind (similar to using 75 mb ML CAPE as opposed to purely SB CAPE). Doing so, a rough estimate of the near-surface wind could be 30kts from the SSW or SW. This means that, though the sfc-6km shear may have been 55-65 kts, the shear from 500m-6km or 1km-6km was only 20-30kts. Though we had pretty strong shear in the low-levels, the shear from just above cloud base to 6km was actually quite weak, much much suggestive of multi-cell mode than supercell mode. In addition, the EL was likely quite low (not surprising -- most were expecting relatively low-topped convection), so the 800m-6km layer encompassed the vast majority of the updrafts. Visually, we saw something similar to a more typical multicell mode, with new updrafts that would develop upshear / to the south-southwest of ongoing convection... These new updrafts would intensify quickly, but then a new "puff" of convection would develop to its south or southwest while the previous updraft weakened. Intermittent supercell structures were observed, but these features were much more transient than, certainly, I had expected.
2. The veered flow off the surface also reduced the depth of the moisture along and a little ways ahead of the dryline. I haven't chased too many supercells in an environment of low-level moisture characterized by a quickly decreasing mixing ratio. Farther east, the 850mb flow hadn't yet ushered in the drier air from western OK/TX, and the 850mb flow a little less veered, so moisture depth better and deep-layer shear was stronger. In addition, with very strong near-surface shear, shallow moisture depth may have allowed for a considerably lower MEAN "updraft parcel" RH (i.e. a 75mb mixed-layer mixing ratio or specific humidity considerably lower than the surface observations suggested). My first observation of the cell on the SW side of OKC was "Hmm, that cloud base looks higher than I thought it would given surface dewpoint deficits of ~10-13 F... In addition, drier air just off the sfc may have allowed for stronger cold pool generation, which may have lead to outflow containment problems I saw through the afternoon.Just a couple of things to ponder, I suppose.
