Severe Weather Discussion for May 18 – 21, 2013

Severe Weather Discussion for May 18 – 21, 2013.
Issued:  05/17/2013.
Highlights:
–  Severe thunderstorms are possible Saturday afternoon and evening across western Oklahoma.  Primary threats are large hail to tennis ball size and winds to 70mph.  A tornado or two certainly seems possible.
–  Severe thunderstorms remain possible Sunday and Monday, with the continued potential for one or more significant severe weather episodes.  Sunday will be strongly governed by Saturday’s storms and Monday by Sunday’s.  Confused yet?  I am!
Discussion:
00z NAM is arriving and appears to be a keeper.  Southerly winds have transported rich Gulf moisture northward with upper 60s/low 70s common across Oklahoma.  Despite my prior thinking, moisture is very robust across the state.  Even though much of central Oklahoma didn’t see sunshine today, instability with still very impressive.  OUN 00z sounding with 75/72 had CAPE value 3778 J/Kg and LI -9.  Holding this down was our friend the cap, with a Tc of 100F.  That’s a spicy meatball!  Low 850mb dewpoints across Texas have now been sampled by the models and this carries northward into Saturday morning.  While period runs gave this a lot of credit, the latest run of the NAM appears to discount the impacts.  As such, CAPE values Saturday afternoon are in the 4000-5000 J/Kg range.  Not sure I’ll go that extreme, but wow.  Medium wavelength trof is sampled over the southwest U.S. this evening with 45-50kt flow on the downstream and upstream sides.
18z GFS, 12z ECMWF, 00z NAM are all considered for this forecast package.  The models are in decent agreement overall, but there are critical differences regarding surface features and convection.  The latter mainly related to impacts on the atmosphere and subsequent convection.  It is bad enough that Sunday and Monday will change as a function of prior days but worse given the multiple severe weather episodes expected across parts of Oklahoma.
For Saturday, models indicate a dryline will form in the Texas panhandle and surge eastward into Oklahoma during the afternoon hours.  The atmosphere to the east will be characterized by dewpoints in the low 70s with temperatures likely in the 80s and 90s.  This may yield the CAPE values noted above.  CIN will remain rather high but dryline convergence, peak heating, and a well-timed shortwave trof should overcome this feature.  As such, thunderstorms are likely in west-central and northwest Oklahoma late Saturday afternoon.  NAM indicates the storms will dissipate by midnight.
The general idea for Sunday remains the same but the big issue is the development of a cold front as shown by the NAM.  The GFS does not show this feature at all while the EC has to some extent.  This remains one of the more significant forecast concerns Sunday and especially Monday.  Relative to Sunday, the front may not be that big of a deal as strong instability once again develops across the state.  The dryline/front may move into central Oklahoma by mid-afternoon.  Models indicate an early morning shortwave trof ejecting through northern Oklahoma with another feature rotating around by late afternoon.  As such, sudden onset of convection is likely by late Sunday afternoon into the evening hours.  The models develop an MCS that moves across northern Oklahoma.  The NAM seemingly uses this feature to reinforce the front and push it southward.  The new run is also farther south than the prior run with the cold front placement.  Either way, supercells are expected to develop by 4-5pm just west of Oklahoma City.  These storms will have the potential to produce all severe weather types, including destructive hail greater than baseballs and tornadoes.
Despite the overall southern change to the cold front, the NAM still lifts this feature back north on Monday and it is north of OKC by early afternoon.  NAM has convection starting by late afternoon, although I think it will be earlier.  Once again, the front location, should it occur, will be key.  Along with any prior convection and associated cold pools.  The current forecast CAPE values of 4000 J/Kg are very much on the “we’ll see plan” given likely atmospheric modification late Sunday.  For now, the only thing to do is trend the models.  Favorable winds will again provide support for supercells presenting all severe weather threats.
Chasing wise, I have toyed back and forth with Saturday.  Given the latest NAM, I’m pretty much in the “go” mode right now.  If anything, we’ll travel west to Weatherford and just hang out.  Been there enough, plenty of good places to eat.
SPC has been rather interesting on the outlooks for this event.  Today’s product suite looked good.  I do think we’ll see some modifications for the new Day 1.  Will that include a moderate for Oklahoma, I’m not sure.  The NAM was seemingly support such an issuance, maybe down to I-40 or a tad north.  I’d think they’ll go to a moderate for Sunday.  The risk for Monday will likely be a slight given that they probably don’t want to lock themselves in given Sunday’s convection and some big differences among the models.
Probabilities:
Risk area for any part of Oklahoma (valid:  05/18 Day 1, Day 2, Day 3):
Slight:  100%, 100%, 100%
Moderate:  75%, 75%, 25%
High:  0%, 10%, N/A
Chase Status:
Level 2 – Watching  Saturday closely, I’m modifying my target area to Clinton.  Sunday, also very interested and target area is El Reno.
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