10/16/06 NOW: Se TX, LA

Bill Tabor

Bill Tabor

EF5
Joined
Apr 16, 2004
Messages
1,613
Location
Austin, Tx
On SPC's Mesoscale Analysis products @ 16:00 in southern LA I'm seeing 8 for STP, 11 for 3 km EHI, 9 for 1 km EHI, and 32 for Supercell Composite. This seems a bit extreme of a value to me considering the scenario, and I'd like opinions. Do the rest of you feel this is accurate, or are these indicators somehow perhaps by feedback or some other mechanism over reacting to the environment?
 
The 12z LCH sounding has an extremely impressive shear profile / hodograph, one of the best I've seen for an area in the warm sector and in tropical moisture. The lapse rates are nearly moist adiabatic, though I assume that sounding went through the updraft of deep convection. Regardless, area VWPs continue to indicate extremely strong shear profiles (just look at the length of the 0-1km section of the hodographs!) over the area. (un)fortunately, deep convergence is also very strong, leading to widespread precipitation that is limiting instability greatly. Some discrete activity has been able to develop on the south edge of the precip shield and take on decent rotation, though the limited CAPE is certainly limiting the extent of the tornado risk.

The 12z LCH sounding is a good example of the importance of the virtual temperature correction when computing CAPE and the parcel trace. The very deep, high-quality moisture decreases the density of a surface parcel to a value similar to a parcel that has a higher temperature. So, the environmental T and Td profile doesn't look like it'd support much CAPE, but the Tv correction indicates that there actually is some marginal CAPE.
 
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Yeah, that is a stunning hodograph on the 12z LCH sounding -- I haven't seen low-level shear of that magnitude juxtapose with that degree of total CAPE / low-level CAPE in monthes. In fact, I don't think I've seen this type of environment anywhere since the early spring setups (i.e. 3/13). The hodograph indicates 664m2/s2 SRH accruing in the lowest 0-1km AGL and RUC/SPC mesoanalysis shows very similar amounts across much of the entire warm sector -- in addition to >1000 MLCAPE across much of the same area. That's pretty impressive...
 
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Pretty wild to see tornado producing rain showers going on. Given the amt. of 0-1km 389m2/s2 & 0-3km SRH 561 m2/s2 on the 12z JAN sounding...it is not surprising to see that anything convective was spinning like a top. It would be pretty scary if some 50k topped supercell
gets going in that environment. Just now getting to peak heating time so may be time to watch the Jackson radar for some interesting stuff in next hour or two.

12zJAN.JPG
 
Ok, I agree there is some good shear, and hodographs, etc. Given these readings with the SPC Composite Indices should we have expected to see extremely severe (large violent) damaging tornadoes, that are long tracked? That's what I'm getting at. No doubt they are picking up some stuff and there are tornadoes being generated - perhaps even from just rain showers, however should these composite indices be able to distinguish a bit better? Looking right now at SPC storm reports I don't see that much in the area of LA I mentioned, and probably not long tracked violent.

Is it the lack of instability due to widespread convection limiting a harsher tornado scenario today? I wonder what modifications these indices could use to be a bit more accurate? Opinions?
 
Bill,

I think the marginal CAPE was the limiting factor in today's events. The low-level shear is ridiculous right now, with an ~85kt LLJ (at 850mb) yielding >1800 0-3km SRH over parts of TN and KY. The RUC shows 850mb flow increasing to over 90kts in a few hours! I don't know the last time I saw 90-100kts at 850mb associated with an extratropical cyclone in the US. I'm sure it's not entirely uncommon, but it's been a while since I seen that strong of low-level flow. Heck, 0-1km shear is over 70kts in that area...

The 00z JAN, LIX, and BMX
soundings have hodographs beyond insane. That BMX sounding shows >1050m2/s2 0-1km SRH and nearly 1300 m2/s2 0-3km SRH. Of course, it's also extremely stable for a surface-based parcel. The LIX sounding, with ~400 m2/s2 0-1km SRH, does have a narrow, long CAPE profile. Being a narrow profile, the absolute updraft velocities probably are relatively weak, particularly given water-loading effects inherent with mid-70 Tds and 2.14" precipitable water.
 
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