Severe Weather Discussion for May 18-21, 2013

Severe Weather Discussion for May 18-21, 2013.

Issued:  05/16/2013.

 Highlights:

 –  A few severe thunderstorms are expected across western and central Oklahoma on Saturday.  Primary threats are hail to baseball size and damaging winds to 70mph.

 –  Severe thunderstorms are likely Sunday and Monday across much of Oklahoma.  Potential remains for a significant severe weather episode one or both days.

 Discussion:

 Operational model suite over the past 24 hours continues prior trends of develop a western U.S. trof by late Friday.  Said trof is then progged to translate eastward into the central U.S. by late Saturday and wrap up into a closed low by Monday.  Models are in reasonably good agreement and will follow consensus for this forecast.  I’m very tempted to use enhanced wording to describe the threat Sunday and Monday, but forecast rules prohibit such this far out.  Also, Monday’s threat will be strongly governed by Sunday’s convection and subsequent surface feature location. 

 00z NAM remains consistent with prior runs and matches up decently with the 18z GFS/NAM along with 12z ECMWF for Saturday.  As the upper trof approaches on Saturday, a surface low will develop across the KS/TX area.  Robust moisture return is ongoing and is expected to continue through the next several days.  NAM dewpoints in Oklahoma show upper 60s common with some low 70s.  This appears reasonable given upper 60s just south of the Red River and Mesonet showing common low/mid 60s across much of the state.  For the past several runs the NAM/GFS have both had very low 850mb dewpoints across much of Texas Saturday morning.  Previously, this has resulted in a lot of mixing in western OK.  The 00z NAM doesn’t show this as much and has 850mb dewpoints near 12.5 C.  Even the 18z GFS backed off some.  As such, CAPE values are near 3000 J/Kg along the dryline and potentially near 4000 J/Kg along I-35.  Upper level winds are not all that favorable, but low/mid level winds are decent.  500mb jet max will remain west but winds will still be 20-25kts by late Saturday afternoon.  I finally get that the models punch the dryline east into western Oklahoma Saturday afternoon and then retreat it by early evening.  00z NAM develops convection from just south of I-40 northward Saturday afternoon.  Degree of instability and modest low-level shear should support supercells.  NAM supercell composite map indicates much of the area near HWY 183 to be marginally favorable.  Either way, the going forecast for some severe thunderstorms on Saturday appears good.  The tornado threat is seemingly low at this point but one or two can be expected.

 Upper system will continue to evolve across the central U.S. on Sunday.  A dryline is expected to punch east well into Oklahoma Sunday morning and afternoon.  The models are disagreeing a lot regarding the expected evolution of surface features on Sunday with the NAM trying to drive a cold front through NW OK into central sections late Sunday.  Other models do not agree, but will need to trend this development a few more runs.  It doesn’t seem to mitigate the overall threat as the front arrives near 7pm and after convection has already developed.  As with Saturday, instability will be impressive as dewpoints and temperatures remain robust.  CAPE values are progged @3000 J/Kg across central/eastern Oklahoma.  Winds increase across the state with impressive flow at all levels.  Depending on the actual location of the dryline, storms are expected to develop by mid/late afternoon.  Supercells will be favored and the NAM supercell composite map confirms such. 

 Yet another potential severe weather event is possible on Monday.  Luckily that is beyond the scope of this forecast.  I should note that the 00z NAM at 84 hours (7am Monday) shows this boundary lifting back north through Oklahoma City.  Monday will be heavily dependent on what happens Sunday.  However, winds remain strong throughout the atmosphere.   Warm and moist conditions are expected yet again during the day.  This would seemingly favor the redevelopment of thunderstorms across parts of the state.

 I expect SPC to do a slight risk on the Day 2 and probably the same on the Day 3.  They may go higher probabilities on Day 3.  I think a moderate is in the offering for Sunday at some point, but not sure they’ll do it on the Day 3.  The Day 2 will seemingly have a 15% area with 10% hatched in western Oklahoma.  The bigger threat will be north in KS/NE.

 Probabilities:

 Risk area for any part of Oklahoma (valid:  05/17 Day 2, Day 3):

 –  Slight:  100%, 100%

–  Moderate:  0%, 75%

–  High:  0%, N/A

Chase Status:

 Level 3 – Still interested in Saturday.  Initial spot is Snyder.  Also, interested in Sunday, initial spot is Weatherford to Watonga.

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