Severe Weather Discussion for April 17, 2013

Severe Weather Discussion for April 17, 2013.

 Issued:  04/16/2013.

 Highlights:

 –  Active severe weather day remains possible on Wednesday.  Potential exists for a significant severe weather episode.

 –  Winter precipitation is possible early Thursday morning.

 Discussion:

 Models remain very divergent, rather odd for 36 hours out.  They also remain intra-model consistent.  As such, the forecast confidence remains very low.  A cold front moved through much of Oklahoma overnight and is now located from Walters to Ada Stillwell.  This boundary has slowed significantly during the day and is essentially stalled.  This boundary may move some more during the late afternoon/early evening hours but significant movement is not expected.

 12z NAM continues prior trends of slowly lifting the front northward on Wednesday and reaches I-44 by the afternoon hours.  The GFS slams the front into Kansas, while the EC is OK/KS border.  This is very frustrating since these models have held to their respective solutions for several days/runs.  As such, picking one or the other remains problematic.  The best guess is a NAM/EC compromise.  I think the NAM is over-convecting as there is not an obvious source for widespread showers and thunderstorms in Texas.  Additionally, the NAM develops very little convection all the cold front until late Wednesday.  The NAM fed models (SPC WRF) do similar things.  The NAM appears to generate a small shortwave trof late tonight and hence convection.  It then intensifies the trof and develops additional convection in the warm sector of southeast OK and much of north Texas.  Again, this seems suspect but not beyond reason.  GFS is largely ignored for its too far north warm front position.

 The forecast assumption going forward is that warm sector convection will be limited through noon Wednesday and as such decent surface heating will occur.  CAPE values increase dramatically by 1pm, with EC values > 3000 J/Kg (actually by 10am).  Storms should rapidly develop along the dryline in far western Oklahoma and move rapidly east-northeast at 40-45mph.  NAM storm motion map shows 30-35 kts, so I may be a little high, but still these things will move.  NAM supercell/sig tornado maps have decrease some in value but likely due to decrease in instability and previously mentioned warm sector convection. 

 18z NAM has arrived and now suppresses overnight and early morning convection in the warm sector.  The warm front location is still farther south than the other models.  Additionally, it rapidly develops convection along I-35 between 2-4pm.  I’m still not convinced this model is correct, but at least it seems to understand the evening stuff better.  EC moves the warm front north and develops impressive CAPE values by 1pm.  It then develops develops convection along the dryline in western Oklahoma by 3pm. 

 Long story short, still have a big difference in model solutions.  These differences have fundamental impacts on the forecast and where the greatest severe weather threat might evolve.

 SPC has a moderate risk up and modified it some for the 17:30z Day 2 issuance.  I don’t see them making any big changes to it until 11:30am tomorrow unless it is obvious the threat is modulated downward.  Another discussion is likely this evening.

 Probabilities:

 Risk area for any part of Oklahoma (valid:  Day 1):

 Slight:  100%

 Moderate:  100%

 High:  10%

 Chase Status:

 Level 2 – certainly interested, depending on things line up.

 

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