Severe Weather Discussion for April 7-9, 2013

Severe Weather Discussion for April 7-9, 2013.
Issued:  04/04/2013.
Highlights:
–  A few severe thunderstorms are possible Sunday late afternoon across western/central Oklahoma.
–  Severe thunderstorms are possible Monday afternoon and evening across western/central Oklahoma.
–  Severe thunderstorms are possible Tuesday across a large part of Oklahoma.
Discussion:
Skies have cleared across a large part of Oklahoma today, allowing temperatures to recover into the mid/upper 50s except for western sections which still have some clouds.  Despite the intense rainfall this week, temperatures have rebounded nicely.  Moisture has been pushed offshore and this push is expected to continue through early Friday morning.  By Saturday morning, lee trofing develops north of Oklahoma and winds shift southerly across the Gulf.  This is in best agreement with the NAM, which is a little slower to turn winds southerly than the GFS and hence slows initial moisture return.  If the GFS verifies, moisture would return a little quicker.  Initial moisture return will be characterized by modified Canadian air as low/mid 50s dewpoints move into Oklahoma later Saturday.  Moisture return will be somewhat shallow so not sure we’ll see the surge indicated by the models.  Although the widespread 2-4 inches of rain this week should put some moisture back into the air.
By Sunday afternoon NAM, GFS, EC all bring low 60s dewpoints into Oklahoma, ahead of a developing dryline across western sections.  850mb moisture also increases with common values @10C among the models.  Afternoon temperatures in the mid 70s yields CAPE values @1500 J/Kg with the GFS on the high side of 2400J/Kg and EC/NAM on the low side with @1400J/Kg.  Models are in decent agreement regarding the dryline position in western Oklahoma.  The EC’s position is a little more believable, to the east, given time of year, although even it is just east of Altus.  The main limiting factor for Sunday is limited forcing and a small pocket of 850mb winds in southwest Oklahoma.  The latter may change in future model runs but for now the best area seems to point to this area for storms.  Do not think storms will last much past dark as CIN increases.
Things get more interesting on Monday as the dryline retreats westward late Sunday.  By Monday afternoon the dryline will be just inside the western border of Oklahoma.  Winds increase across western/central sections during the day.  GFS is showing mid-60s dewpoints but I’m a little suspicious of it, probably too high.  Either way, good moisture return is likely as 850mb dewpoints approach 15C.  GFS is again higher on CAPE values at 3500J/Kg with the EC at 2200 J/Kg, the latter seems more believable right now, and both are good for this time of year.  Both models develop convection, although GFS is at 00z and EC 21z.  SPC’s Day5 doesn’t seem right and way too conservative.  Western Oklahoma did not get the rain central/east did, so can’t see how that is a factor here.  We’ll see what happens Friday morning, but I’d think more of western Oklahoma will be included in the new Day4.
GFS and EC continue in general agreement re Tuesday’s severe weather threat.  Upper system responsible for Monday’s setup will eject eastward towards Oklahoma and is seemingly well-timed.  Model dryline position near HWY 183 at 4pm is also believable.  Amazingly the GFS and EC are similar here.  CAPE values are in the 2500 J/Kg range and both models develop convection by mid-afternoon.
Monday and Tuesday are technically outside the normal range of my special forecast period, so things are likely to change.  However, given the potential for a medium-end event one or both days, important to pay attention.  Yes, we’re 4/5 days out, but we’re also 4/5 days out with decent model agreement.  Then again, the 18z GFS is running, so that can’t be good.
Probabilities:
Risk area for any part of Oklahoma (Valid:  04/05, Day 3):
–  Slight:  75%
–  Moderate:  0%
–  High:  N/A
Chase Status:
Level 4 – Normal
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