4/26-27/14 severe weather

   Flow associated with a developing surface low in the High Plains in advance of an upper trough ejecting from the Southwest will result in rapid low-level moisture advection into the Southern and Central Plains on Saturday.  Dewpoints in the low to mid 60s will result in considerable instability (2000-3000 J/kg CAPE), but a stout cap will remain in place through most of the day as the upper-level forcing to weaken it will lag to the west.  Convergence and convective mixing along the dryline will be nearly enough to overcome the cap as stronger flow aloft/differential vorticity advection finally arrives in the evening.  Models differ on timing and location of convective initiation, but the most likely scenario is a storm or two in southwestern Oklahoma or western North Texas just around sunset.  Large low-level hodographs and increasing deep-layer shear will support supercells capable of producing very large hail.  LCLs will initially be too high for an appreciable tornado threat.  However, as the boundary layer cools and moistens with loss of diurnal heating, cloud bases may lower.  Model consensus is that a ~50 kt low-level jet will emerge over northwestern Texas and western/central Oklahoma in this time frame.  Extreme low-level vertical shear combined with any cloud cover that results from distant anvils/synoptic lift/moist advection in the LLJ should prevent rapid stabilization of the boundary layer in the 00-04z period.  Remaining instability up to 2000 J/kg CAPE and large values of SRH owing to the LLJ would mean a risk of a few tornadoes, possibly significant, if mature surface-based convection continues beyond sunset.  The temporal window for tornadoes will be short.  The most likely area for this tornado threat, if it materializes, would be southwestern to west-central Oklahoma.  Given modest storm motion, storms do not appear likely to reach the OKC metro before boundary layer decoupling ends tornado chances for the night.

   Sunday's primary severe weather setup will be to the east of I-35 after the dryline surges through in the morning hours, with southeastern Oklahoma into the Arklatex seeing the highest risk.  Current trends in the NAM suggest against ruling out morning convection in the I-35 corridor, though, and will need to be monitored.