Severe Weather Discussion for April 7-9, 2013

Severe Weather Discussion for April 7-9, 2013.
Issued:  04/06/2013.
Highlights:
–  A few severe thunderstorms are expected on Sunday across northwest Oklahoma.  Primary threat is large hail to golfball size.
–  Severe thunderstorms are possible on Monday in western Oklahoma.  Primary threats are large hail to baseball size and a few tornadoes.
–  Active severe weather day is possible on Tuesday for much of Oklahoma.
Discussion:
Finally in the forecast time period for the upcoming severe weather threats.  Model resolution and agreement is no better than it was a few days ago and the finer details are still quite illusive.  At this point, I’d typically be trying to nail down a few important things about Monday (cap strength, shear, CAPE, dryline location, etc).  Not completely impossible at this point, but certainly not easy.
18z GFS and 12z ECMWF are primarily used for this forecast, the 18z NAM is somewhat ignored.  The latter is used for comparison of some surface features and upper system.  Overall I have elected this course of action re the NAM, due to its characteristic moisture issues and lack of convection Mon/Tue.  The NAM (as I have seen numerous times) lowers 850mb dewpoints below zero in the warm sector.  This creates moisture profile issues and seemingly impacts the model’s decision to activate the convective parameters (generate QPF) or not.  CIN goes to zero Monday and Tuesday with no signs of convection, somewhat difficult to believe.  GFS and EC both generate convection.  Okay, enough of that, on to the forecast.
No changes to Sunday and SPC Day 2 is the best thing going.  Models are actually consistent and all generate QPF.  Many of the ingredients for severe thunderstorms will be in-place but they won’t be that robust.  As such, only a few severe thunderstorms are expected with large hail to golfball size the primary threat.  Storms should decrease after sunset, per the models.
Monday continues to look interesting as the ingredients increase some across western/central Oklahoma.  GFS/EC both take CAPE values to at least 2000 J/Kg as low to maybe mid 60s dewpoints move into the state.  Additionally, deep layer moisture will be present as 850mb dewpoints of 12.5C move into the state.  Dryline reaches just inside the western border of Oklahoma by early afternoon, providing surface convergence.  Cap will be strong and heating, then maybe a rogue shortwave trof will be needed to breach it.  A well-timed jet max may also assist in the endeavor.  GFS develops convection @5pm and the EC a tad earlier.  As noted previously, the NAM is quiet.  Wind shear is an issue and I’m still trying to wrap myself around the possibilities.  NAM creates S-curved hodographs in northwest Oklahoma with more a long-curved hodograph I-40 and south.  The GFS is more of a long curved hodograph in western sections.  I believe the models are still struggling to represent the pre-convective environment, so more changes are likely.  Despite the ongoing model issues and resulting disagreement, I’m looking to chase on Monday.  Clinton is my pick for initial location as I can head north if necessary.  I’m not sure about the need to go a little farther west, we’ll see.  Not convinced I can pin the dryline location down to a few miles this far out.
Backing up a little, the prior frontal passage into the Gulf has been real.y noted by many forecasters.  Models are likely too robust with moisture return on Sunday and maybe much of Monday, however by Tuesday I’d think moisture quality will be much improved across Oklahoma.  The NAM moves 65F dewpoints to the Gulf Coast and leaves them there for the better part of 36 hours as pressures fall across the region.  This may be right, but I think it suspect with the GFS much different and the EC somewhat.  Still not convinced that the expected moisture is that big of a deal either way and is seasonable.
Ingredients continue to increase late Monday into Tuesday along with the upper system approaching the region.  This is where the EC and GFS/NAM diverge the most.  The EC is slower than the GFS on ejecting the system but the GFS has the NAM on its side.  The EC was consistent two runs re being slower.  The GFS and NAM both eject a big part of the trof on Tuesday, holding back some of it in the SW US.  We’ve seen this a few times this year and so the solution is believable.  However, without better model agreement, somewhat tough to determine what will happen on Tuesday.  I’ve elected to go a little higher with the wording since the NAM/GFS are in decent agreement and if they verify, a medium-end event is in the offering.  GFS and EC both generate QPF on Tuesday, along the dryline.  As such, severe thunderstorms are expected.
Later Tuesday night into Wednesday morning, another round of severe thunderstorms are likely as the cold front blasts thru Oklahoma.  More beneficial rain is expected across the state.
That’s all for now, this is already way longer than I intended.
Probabilities:
Risk area for any part of Oklahoma (valid 04/07 Day 1, Day 2, Day 3):
Slight:  100%, 100%, 100%
Moderate:  0%, 40%, 75%
High:  0%, 0%, N/A
Chase Status:
Level 3 – Monitoring Monday.
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