Severe Weather Discussion for October 12 – 13, 2012

Severe Weather Discussion for October 12 – 13, 2012.
Issued:  10/11/2012.
Highlights:
–  Severe thunderstorms are expected Friday afternoon and evening, mainly across northwest Oklahoma and the panhandle.  Primary threats are damaging hail and wind.
–  Severe thunderstorms are possible on Saturday across much of Oklahoma.
Discussion:
For the first time since mid-April, see the need to issue this product.  The last tornado in Oklahoma was May 29th and we’ll see if that holds past Saturday.
18z NAM and general model outputs agree that a southern stream closed low will translate eastward the next few days and emerge into the Plains as a shortwave trof on Saturday.  This afternoon, moisture return is occurring across much of Oklahoma with unexpected CAPE values AOA 1500 J/Kg across the state.  Wind fields aloft are decent as Oklahoma is still influenced by the northern jet.  As such, any storms that can develop will likely be organized and pose a large hail threat.
On Friday the closed low meanders into the southwest U.S.  In response to this system, a surface low will develop in eastern Colorado, although this may take some time as the cold surface high shifts east.  Moisture return is expected to remain the same as it is now, even with the 1020mb line near Houston.  Then again, this is fall and not spring, so not attempting to recover a wiped out Gulf.  Current think is for storms to initiate in the western Texas/Oklahoma panhandles and move northeast in 50kt 500mb/35kt 700mb flow.  Assuming the NAM isn’t underestimating CAPE values again, the progged numbers in the threat are a respectable 1500 J/Kg in the southern Texas panhandle but much lower in northern sections.  Either way, at least a few severe thunderstorms appear likely on Friday.
The closed low is progged to open up and eject eastward on Saturday.  At what point this occurs and how it happens will be the driving factor on how significant Saturday becomes.  The models are in decent agreement aside for some differences, nothing unusual this far out.  The setup in the panhandles on Friday will translate eastward on Saturday such that atmospheric winds will increase at all levels during the day.  Moisture values should remain similar, although the NAM is playing with dewpoints some.  Can’t rule the lower dewpoints out as the surface ridge to the east tries to nudge a little bit into the eastern Gulf.  The main issue really is early morning convection.  The NAM wants to develop convection along the dryline @10am and shift it unusually slowly eastward during the day.  CAPE values do get impressive at 1500 J/Kg but there is the overriding concern of cloud cover.  Since many of the players are present for Saturday, the issue flatly remains how much we can warm up.  October ground is much easier to warm than April, as seen today.
For now Saturday sits at a slight risk but SPC may consider nudging a moderate southward if surface heating appears more likely.  For chasing purposes, I’m looking at the Watonga area.  Storm motion will be a bear, so need an area with good roads and certainly not too close to the dryline.
Probabilities:
Risk area for any part of OK/TX (Day 1, Day 2):
Slight:  100%, 100%
Moderate:  0%, 20%
High:  0%, 0%
Chase Status:
Level 3 – monitoring Saturday.
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