Severe Weather Discussion for April 16-17, 2013

Severe Weather Discussion for April 16-17, 2013.
Issued:  04/13/2013
Highlights:
–  Severe thunderstorms are possible across western and northern Oklahoma on Tuesday.
–  An additional threat of severe thunderstorms is possible on Wednesday.
Discussion:
12z NAM/EC/GFS all agree that a western U.S. medium wavelength trof will evolve by Tuesday.  Gulf moisture did not get pushed very far south with the prior cold front.  As such, moisture is lurking nearby and will rapidly return by Tuesday.  There are several differences from the prior system.  First, the surface low will be weaker, although still present.  Next, the system will not dig as far south, and finally the incoming cold front will be oriented east/west not north/south.
On Monday, a cold front is expected to move into the state and stall in southeast sections.  This will occur as a northern stream system translates eastward across the northern U.S.  By Tuesday the surface low should form in the Texas panhandle and Monday’s front return slowly northward.  Models all move this front in the general area of the KS/OK border by Tuesday afternoon.  Increasing flow aloft should provide favorable directional and speed shear for organized storms.  Not sure if Tuesday will rise to a medium-end event but SPC has appropriately risked it for the past two issuance of the Day4-8 product.  CAPE values look to be in the 2500 J/Kg range and CIN is seemingly reduced during afternoon heating.  NAM does not develop convection, even though CIN values decrease to near 0 J/Kg during peak heating.  There is some deep layer convergence and lift along the boundaries, so seemingly storms should develop.  GFS does create two areas of convection and this would fit with the pattern.  There doesn’t appear to be much help from the 500mb level, other than winds.  However, we’re still a ways out, so we’ll see what changes.  Model soundings are a little different with the NAM showing a long curved hodograph and the GFS S-shaped (veer-back-veer).  However, in this situation the warm front may compensate as 0-3km helicity should increase due to this boundary.  The boundary is expected to be in a favorable configuration for storms to move along it.  Even though SPC has D4 highlighted, not sure if they’ll do a moderate on the upcoming D3.
12z GFS would argue that another setup occurs on Wednesday but the 12z EC blasts the front into Texas.  This is a unkind swap from prior runs of both models where the EC kept the front north and the GFS blasts it into Texas.  Tough call on what will happen, but ongoing negative phase of the NAO would argue for a southward push.  Wednesday is beyond the forecast period.
Probabilities:
Risk area for any part of Oklahoma (valid:  04/14 Day 3):
Slight:  100%
Moderate:  25%
High:  N/A
Chase Status:
Level 4 – Monitoring Tuesday
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