Severe Weather Discussion for April 12 – 15, 2012

Severe Weather Discussion for April 12 – 15, 2012.
Issued:  04/12/2012.
Highlights:
–  Severe thunderstorms remain possible this afternoon in western Oklahoma, with large hail and a few tornadoes the primary threats.
–  On Friday, severe thunderstorms are likely in western Oklahoma with the primary threats tornadoes and destructive hail.
–  An active severe weather day is still possible on Saturday across much of Oklahoma.  A significant severe weather event may be in the offering if parameters come together.
–  Severe thunderstorms and excessive rainfall are expected on Sunday, primary threats are large hail and damaging winds.
Discussion:
Little to no change from prior forecasts, upper system is evolving as models have progged for several days now.  There are finer details that must be reviewed and considered for the overall forecast and impacts on the same.  Forecast rules still dictate not using enhanced wording this far out for Saturday even though SPC has maxed out the probabilities for Saturday (Day 3).  We’ll see what happens on the Day 2 tomorrow.
SPC has an MD out for parts of western OK and the Day 1 has this well covered, no need to revisit it here.
A slight risk is posted tomorrow for western and parts of central Oklahoma.  The severe weather impacts are greater but storm coverage is in-doubt as models are lower on QPF than prior progs.  However, SPC may go to a moderate risk if it looks like storm coverage will meet the threshold.  18z NAM has lowered CAPE values a little but this is really from 4000 J/Kg to 3000 J/Kg which is probably much more realistic for mid-April.  Either way, favorable instability with expected speed/directional shear will be more than sufficient for supercells.  Any sustained supercell will be capable of destructive hail and a few tornadoes, especially as they move east of HWY 283 and LCL heights lower.  NAM develops some QPF, mainly near I-40 late Friday and this is in keeping with the expected evolution of this event.
The storm coverage on Friday will have major impacts on what happens Saturday and if the full extent of the event can be realized.  While May 24th last year was a significant day and well-known days in advance, that is late May and much greater heating.  This allows the atmosphere to readily reset after prior convection.  So, the main thing to remember, with all the hype about Saturday, is that lots of convection overnight and into Saturday will possibly modulate the event downward.  For now, the continued thinking for me is to forecast the potential and acknowledge the prior caveat.  18z NAM has CAPE values @3000J/Kg Saturday afternoon in the threat area with dewpoints in the mid 60s.  CIN is low and LCL values are near 1000 meters, the latter favorable for tornadoes.  LI values are -8 to -9 across central Oklahoma.  A streak of 850mb dewpoints greater than 15C nose into central Oklahoma by 4pm, which is flat ugly…for lack of better words.  No need to discuss winds, they are beyond favorable.  18z NAM is awfully quiet at 7pm Saturday but it did that for tomorrow and has now trended to convection.  I’d expect to see convection given impressive winds and incoming upper system.  By 10pm, NAM gets one little supercell going near Lawton.  Once again, not unusual for this model and I’d expect that to change.  Concern is that CAPE values remain high even at 10pm, indicating the storms may go well into the night time hours.  This would indicate potential for night time tornadoes and associated hazards.
Still not a clear picture on Sunday and the models aren’t in the best of agreement.  However, there is potential for low-CAPE, high shear thunderstorms, which translates to a severe weather threat.  For now, we’ll have to monitor but it is there.
Probabilities:
Risk area for any part of OK/TX (valid:  04/13 Day 1, Day 2, Day 3):
Slight:  100%, 100%, 100%
Moderate:  50%, 100%, 10%
High:  0%, 20%, N/A
Chase Status:
Level 2 – looking to chase on Friday, target area is Hobart.
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