Severe Weather Discussion for April 22 – 24, 2015

Severe Weather Discussion for April 22 – 24, 2015.

Issued:  04/21/2015.

 

Highlights:

 

–  A few severe thunderstorms are possible Wednesday across southern Oklahoma, primary threats are large hail and strong winds.  The event looks to be on the low-end, however if temperatures warm then a more substantial threat may develop.

 

–  Active severe weather day is possible on Friday.  Numerous questions remain re dryline position, clouds and extensive storms in Texas.

 

Discussion:

 

12z model suite has done little to resolve questions re severe weather potential Wednesday and Friday.  There is enough potential relative to severe weather impacts on Friday to warrant special discussions.

 

Wednesday – Large northern stream closed low will be located over southeast Canada with a shortwave ridge just west of Oklahoma.  Southerly flow has commenced across the region, although dewpoints remain rather low.  19z surface analysis shows dewpoints in the upper 50s/low 60s along the Texas Gulf Coast and surface pressure in the 1014mb range.  Prior cold front pushed moisture well offshore and current return flow will take a while to bring even marginal moisture back north.  4km NAM appears to be the best thing going as dewpoints reach the upper 50s along and south of I-40 by Wednesday afternoon, keeping low/mid 60s in Texas.  Additionally, a cold front is progged to move into northern Oklahoma and reach I-40 by afternoon.  As such, the best chance for severe thunderstorms appears to reside in far southern Oklahoma and Texas.  Any storms that do form will have a large hail and strong wind threat.  Tornado potential would seemingly be higher along the Red River and south.  I’m not convinced this setup warrants an enhanced risk, but I’ll let SPC make that call.  4km NAM does light off storms over OKC @2-3pm Wednesday.  Although these are likely to be elevated hailers.

 

Friday – Southeast Canadian closed low shifts eastward as shortwave ridging moves over Oklahoma on Thursday.  A southern stream medium wavelength trof is expected to dig southward over the Baja Peninsula and then eject east-northeast towards the region.  Models offer a variety of solutions, mainly regarding timing, of how this system will evolve.  12z GFS/NAM are fastest with the upper system and hence dryline while the 12z EC/UKMET are slower with the upper system.  The JMA from 12z yesterday also is slower than the GFS/NAM.  The slower approach appears reasonable given the southern latitude this system will take before reach Oklahoma.  All that being said, the new JMA is now faster and more inline with the GFS/NAM.  Ultimately the difference is a matter of hours and probably will not be resolved until Thursday.  Further, the upper system will not be sampled by the upper air network until Friday morning.  For now much of this is a guessing game.  Models do indicate that moisture will return to Oklahoma late Thursday into Friday, with mid 60s dewpoints across central and eastern Oklahoma.  Wind fields will be impressive with deep layer share across the region.  Dryline movement remains one of the issues complicating the forecast.  My thinking is a dryline near HWY 81 by 2-3pm, with convection kicking off near that time.  This is in best agreement with the EC/UKMET and potentially the JMA.  The other issue to address is potential for early morning convection in Oklahoma and robust east-west oriented convection in Texas.  Admittedly the NAM is not near as robust with QPF in Texas as the EC.  So, again tough to determine what will happen.  Given all the questions (not really uncommon) there is the potential for an active severe weather day on Friday, impacting parts of central and eastern Oklahoma.

 

Upper system moves by and apparently another develops near southern California.  This latter system ejects in a similar manner to Friday’s system but appears to have a more southern approach (Texas).  Since models do not seem to have Friday handled, I do not expect much agreement about Sunday.  However, additional severe weather appears possible.

 

Probabilities:

 

Risk area for any part of OK (valid: 04/22 Day 1 and Day 3):

 

Slight:  100%, 100%

Enhanced:  75%, 75%

Moderate:  0%, 20%

High:  0%, N/A

 

Chase Status:

 

Level 4 – Monitoring Wednesday and Friday

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