Severe Weather Discussion for May 19-25, 2011

Severe Weather Discussion for May 19-25, 2011.
Issued:  05/17/2011.

Highlights:

–  Severe thunderstorms are possible Thursday afternoon and evening across western Oklahoma.

–  Severe thunderstorms are possible Friday across central and eastern Oklahoma, although overall evolution may reduce the threat in central sections.

–  Severe thunderstorms are possible Saturday in eastern Oklahoma.

–  Additional storm chances are likely Sunday-Wednesday, but way beyond the forecast period of this product.

Discussion:

Lots to review this evening, so I’ll stick with the typical time period of this product.  This covers the severe weather threat through Friday.  Ongoing Omega block across the lower 48 will continue to feature ridging in the central U.S. with closed lows in the western U.S. and in the southeast U.S.  This blocking pattern will slowly breakdown over the coming days, but not before Oklahoma faces severe weather threats Thursday and Friday.  This is a similar setup to last week, which produced widespread rainfall across the state and severe weather (including two tornadoes) in eastern sections.  Question becomes, how does this event differ from last week.  There seems to be two primary differences 1) system will not result in meridional flow hence no impact from the subtropical jet and 2) cap does not appear to be as strong.  The latter will work for and against us during the initial part of the forecast period.

On Thursday, deep layer moisture will advect into Oklahoma with surface dewpoints at least 65F and 850mb near or slightly above 15C.  A dryline is located in western Oklahoma with CAPE values @2000 J/Kg in a narrow strip along it.  This is the NAM, GFS has stronger instability across the state.  Enough instability looks to develop, allowing the cap to breach along the dryline.  Models show this occurring at peak heating, with best use of potential instability.  Upper level flow is currently progged to be best in northwest Oklahoma as the associated lows wrap up.  Mid-level flow (500mb) is impressive across much of the state with an 80-90kt jet max in northwest OK.  850-700mb winds are equally impressive along with crossover values, especially northwest Oklahoma.  LCL heights might be an issue in southwest Oklahoma due to the hot temperatures (T-Td spreads) in this area.  This can be mitigated if storms develop in northwest TX hence cooling conditions downstream in Oklahoma and lowering LCL heights.  While some evolution questions will remain, there seems to be enough confidence of a medium-end event to focus on Thursday.  Additionally, we are strongly considering a chase.  Target area would be west of Watonga.

Friday’s evolution is quite elusive from a thermodynamic prospective but the general setup seems agreeable.  Upper low over Colorado on Thursday will translate slowly east-northeast on Friday.  Models agree that strong lift will impinge on the state early in the day, which will likely impede surface heating in western Oklahoma.  Farther east in central/eastern sections there appears some opportunity for surface heating and potentially the creation of a potentially unstable airmass.  Models are in disagreement here with the GFS more robust on CAPE values in central sections than the NAM.  Although this was the case last week and numerous severe thunderstorms still occurred.  So, I think keeping severe chances in for Friday is very much warranted and the threat for a medium-end event remains.  We really won’t know how Friday will evolve until that day.  Suffice it to say, wind speed, directional shear at all levels will be favorable for organized severe thunderstorms.

Saturday was originally focused in southeast Oklahoma, but the 18z NAM shows a threat for the eastern 1/3rd of the state.  Wind fields will weaken but directional shear remains and CAPE values are around 3000 J/Kg with CIN generally -30 J/Kg.  Numerous boundaries should be leftover from prior convection, indicating convective initiation is possible in numerous areas.  GFS does show a line of convection in Texas to Arkansas, this could have mesoscale influences on the threat and modulate it downward should moisture inflow be disrupted.

Sunday-Wednesday, another shortwave trof enters the western U.S. and moves towards the state.  Models prog this system to also close off as it advances towards Oklahoma.  Wind fields look relatively weaker Sunday-Monday but CAPE values are impressive at greater than 4000 J/Kg (per 18z GFS).  Any storms that form in this environment would likely be severe.  Initial look at the 18z GFS indicates a potentially significant severe weather day on Tuesday.  Granted this will likely change and forecast etiquette really frowns on such statements, however the potential cannot be ignored.  Timing will likely change and would seemingly modulate the threat for Tuesday and subsequently Wednesday.

I went with medium-end chances on Thursday given recent progs.  Friday will likely change one way or the other, I doubt SPC will do a moderate on the new Day 3 but hey we’ll see.  They’ve been on the ball recently, so I look forward to their products tomorrow morning.

Probabilities:

Risk area for any part of OK (valid:  05/18 Day 2, Day 3):

–  Slight:  100%, 100%
–  Moderate:  80%, 30%
–  High:  0%, N/A

Chase Status:

Level 3 – monitoring Thursday, interested in northwest Oklahoma.

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