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jojo762

Memorial Day weekend severe threats

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9 minutes ago, MUWX said:

May 6th or may 4th? 

May 6th. May 4th was basically a textbook outbreak setup.

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God, I thought I was pretty well versed on the 2003 outbreak sequence, and I didn't even know about May 6... 63 reports?! It's hard to even comprehend a stretch like May 4-15, 2003, these days.

Anyway, the 12z NCEP HRW runs paint by far the prettiest chasing picture out of all the guidance I've seen. They hold the CF at bay late enough for a significant threat across E KS, which would be far preferable to either the SW MO/NE OK Ozarks region or the I-35 corridor from OKC to DFW and eastward. But, failing that... yeah, I just don't see the dryline play having very good odds, tornado-wise. Instead, it looks like a messy triple point/OFB focal point around the Tri-state region that, if anything big does manage to happen, harkens back to some ugly days that everyone (chasers and public) would rather forget than relive. Followed by nothing at all on the Plains for a good while after tomorrow.

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The synoptic similarities to 5/22/2011 are somewhat notable upon looking at the most recent cycle of guidance. Same sort of idea with a lumbering upper low to the north (albeit this one is centered further north) and a belt of considerable mid level flow rotating through an extremely unstable warm sector to the south with relatively veered low level flow. I believe the height falls through the afternoon were a bit more pronounced in the 2011 case though.

Adding the obvious disclaimer that nothing on that magnitude should be expected tomorrow, especially when factoring in the mesoscale nature of what happened that day, but the suggested environment (SREF mean has 76˚F+ Tds over basically all of eastern OK fwiw) certainly would suggest some pretty hellacious updrafts if anything can go up in that vicinity.

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From the National Weather Service in Tulsa, Oklahoma for Saturday...

A favored scenario is initial supercellular development capable all severe hazards with a local
tornado threat especially in vicinity of any outflow boundary. 
Storms may then evolve into broken LEWPs capable of destructive 
winds. Regardless of the exact scenario, the environment will 
favor significant severe weather potential which should be the 
focus of any awareness messages. Outdoor venues and recreational 
activities should understand the weather hazard potential and have
a severe weather safety plan.

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1 hour ago, Quincy said:

Just one possible evolution:

An early look from the 18z HRRR shows an interesting setup, in which overnight convection basically weakens and leaves an almost entirely undisrupted warm sector tomorrow. This scenario results in an unusually large area with both extreme CAPE and favorable shear. By 21-22z, cells start to blow up over Missouri near the warm front. Back west, 10m winds are backed to the south across eastern portions of Oklahoma and North Texas. 

The obvious caveat...one run from one model, but yeah, the 18Z HRRRX lights Missouri up like a Christmas tree.

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8 minutes ago, Jim Martin said:

From the National Weather Service in Tulsa, Oklahoma for Saturday...

A favored scenario is initial supercellular development capable all severe hazards with a local
tornado threat especially in vicinity of any outflow boundary. 
Storms may then evolve into broken LEWPs capable of destructive 
winds. Regardless of the exact scenario, the environment will 
favor significant severe weather potential which should be the 
focus of any awareness messages. Outdoor venues and recreational 
activities should understand the weather hazard potential and have
a severe weather safety plan.

Yea, hopefully everything avoids Rocklaohoma at minimum...

My sister and brother in law are there.

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Depending on what the various 00Z CAMs show, i'd hate to be the 06Z forecaster... Potential of multiple discrete supercells developing by afternoon/evening capable of all severe hazards including giant hail and potentially large...strong tornadoes (per 12Z NCEP HRW) across E KS/SW MO/NE OK, but also potential for a long-track derecho developing by early/mid morning across KS and sweeping east-southeastward (per 18Z 3km NAM)... All with a myriad of other convective evolutions as shown by other 12Z CAM guidance.  

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FWIW the typically conservative TTU WRF has had multiple continuous runs showing supercell development in North Texas tomorrow afternoon into the early evening hours. It also has backing of low-level winds. Would probably be a scenario similiar to that one about a month ago where giant hail demolished parts of Montague, Cooke, Denton, and Collin counties. That storm produced a brief tornado just before sunset. However, with 7000 J/Kg of CAPE tomorrow and increasing 0-1KM SRH at sunset, can't rule out a tornado threat. All conditional on storms developing that far south on the dryline, but it is unusual for the TTU WRF to have convection while others don't. It is usually the other way around. 

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In the short-term, watch convective evolution over the central High Plains tonight. There are major discrepancies with the models, particularly with an MCS or broken line of storms surviving into central Kansas by morning. If a robust convective system does survive to I-135, then one of two scenarios is likely: 

1. MCS maintenance and eventual intensification into a significant MCS/derecho, given the extreme thermodynamic environment downstream. This scenario would also limit the supercell tornado potential across its path, which would likely be Missouri and vicinity. 

2. Gradual decay tomorrow morning, but at least a loosely organized MCS/clusters of storms gradually reintensify across eastern Kansas/western Missouri, with some potential for brief supercells within the complex before it becomes more consolidated. This scenario also poses the risk to leave one or more outflow boundaries, where supercells could still form, mainly on the southern flank of persistent convection. 

Either of the two scenarios above would likely result in an eastward/southeastward acceleration of an effective cold front, pushing the significant severe threat more toward the Ozarks vicinity and points east toward the Mississippi River, as opposed to closer to the I-35 corridor.

However, if convection generally does not survive the night across Kansas, that presents a host of other convective scenarios.

3. Convection lingers overnight, but any remaining activity has minimal effect on the thermodynamic environment across Kansas/Missouri northeastern Oklahoma. The activity remains persistent enough to lay down one or more outflow boundaries, focusing a threat for significant severe, including giant hail and intense tornadoes. 

4. Convection simply weakens tonight and effectively has no effect on the warm sector tomorrow. This scenario yields an unusually large area of extreme instability in the warm sector and rapid heating of the boundary layer would erode most convective inhibition by 21z/mid-afternoon. While outflow boundaries would be lacking across the area, the focus would shift first to the warm front for tornadic supercells, but then second toward the dryline and lastly to the cold front. Here, you'd really expect the warm front to have the most action, while the dryline might be lacking tornado potential somewhat with higher LCLs and relatively marginal storm relative helicity. The cold front might have a narrow window for supercells, but shear vectors displaying a relatively acute angle with the cold front would suggest a quick linear transition.

There's then a 5th and more complicated scenario, involving widespread convective contamination of the warm sector during the day tomorrow. I'd imagine we'd have a hot mess of mixed/clustered storm modes and a little bit of everything, but a tendency for storm mergers and HP beasts if that happens. 

As it stands, based on convective trends, I'd say that scenarios 1 or 2 seem most plausible. Even though southeastern Colorado convection never materialized today, evolution of northeastern Colorado convection into at least a somewhat organized MCS across northern Kansas overnight seems probable. While that might lower the potential for eastern Kansas/western Missouri supercells tomorrow afternoon, the warm front could still be in play, as well as the southern flank of the MCS in the northeastern Oklahoma/northwestern Arkansas vicinity. 

One key takeaway here is that any MCS may have a tendency for rapid acceleration, perhaps faster than model progs. That pushes the threats east (especially over the northern half of the SPC risk areas) and an MCS/derecho threat could extend well into the lower Ohio Valley and Tennessee Valley. 

I didn't highlight it much in the discussion above, but widespread destructive winds could become the biggest threat, with large hail given extreme instability and embedded QLCS tornadoes with the amount of low-level shear generated in the vicinity of such a convective system. 

In terms of significant tornadoes and very large hail, no matter what happens, the main focus should be on the warm front and any outflow boundaries. Dryline convective initiation is already low confidence, let alone the issue of marginal low-level shear. 

My earlier post still stands. Don't discount the ability of extreme instability to offset what might otherwise appear to be marginal low-level shear. 

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39 minutes ago, David Reimer said:

FWIW the typically conservative TTU WRF has had multiple continuous runs showing supercell development in North Texas tomorrow afternoon into the early evening hours. It also has backing of low-level winds. Would probably be a scenario similiar to that one about a month ago where giant hail demolished parts of Montague, Cooke, Denton, and Collin counties. That storm produced a brief tornado just before sunset. However, with 7000 J/Kg of CAPE tomorrow and increasing 0-1KM SRH at sunset, can't rule out a tornado threat. All conditional on storms developing that far south on the dryline, but it is unusual for the TTU WRF to have convection while others don't. It is usually the other way around. 

Here is 00z from today and then 18z below:

 

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40 minutes ago, David Reimer said:

FWIW the typically conservative TTU WRF has had multiple continuous runs showing supercell development in North Texas tomorrow afternoon into the early evening hours. It also has backing of low-level winds. Would probably be a scenario similiar to that one about a month ago where giant hail demolished parts of Montague, Cooke, Denton, and Collin counties. That storm produced a brief tornado just before sunset. However, with 7000 J/Kg of CAPE tomorrow and increasing 0-1KM SRH at sunset, can't rule out a tornado threat. All conditional on storms developing that far south on the dryline, but it is unusual for the TTU WRF to have convection while others don't. It is usually the other way around. 

It's also unusual for 7000 J/kg of CAPE to juxtapose with such favorable shear. I'm not saying it's wrong, but one has wonder if such high-end instability may overexaggerate the likelihood of convective initiation within the model progs.

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FWIW, the TTU WRF is not verifiying very well this evening with that phantom convection over SE CO/SW KS. I'm not sure that means much, since we'd be comparing an extreme CAPE/modest shear environment with a modest CAPE/modest shear environment, but still... The TTU WRF was one of several models I considered when picking today's chase target. 

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4 minutes ago, David Reimer said:

3KM NAM was certainly a bit frightening for OKC tomorrow evening. 

FWIW, 01Z HRRR is in fairly good agreement with 0Z and 18Z 3km NAM in developing an intense bowing MCS fairly early on tomorrow.

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3 minutes ago, David Reimer said:

3KM NAM was certainly a bit frightening for OKC tomorrow evening. 

Kinda. Looks like an MCS-driven surging front plowing 100-150 mi. southward just between 21z-00z. Lots of maxi UH with that activity. Derecho threat, for sure, though. FWIW, the 21z HRRRx has a similar scenario, albeit farther east. If other 00z CAMs are in agreement with the midday MCS scenario in KS, it's probably time to shift the emphasis away from strong tornadoes (other than a true mesoscale accident) and toward a major wind threat centered over the Ozarks.

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Elevated convection has developed from Weleetka to Okmulgee about 75 miles east of OKC. Will have to watch for any boundaries that push west/north for tomorrow. 

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39 minutes ago, David Reimer said:

Elevated convection has developed from Weleetka to Okmulgee about 75 miles east of OKC. Will have to watch for any boundaries that push west/north for tomorrow. 

Storm W of Checotah has tops near 54KFT... storm to the northwest has tops approaching 60+KFT. Quite impressive for elevated convection, and representative of the explosive amount of CAPE.

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5 minutes ago, jojo762 said:

Storm W of Checotah has tops near 54KFT... storm to the northwest has tops approaching 60+KFT. Quite impressive for elevated convection, and representative of the explosive amount of CAPE.

This is not being handled well by recent HRRR runs either.

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Also, the upper 70s Dts shown by the NAM/GFS tomorrow seem quite possible given the 00z KCRP sounding. Current obs also indicate widespread mid/upper 70s DPs across the eastern half of TX with upper 70s across the northeastern GOM.

00z KCRP RAOB:

CRP.gif

00z KBRO RAOB:

BRO.gif

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7 minutes ago, Quincy said:

This is not being handled well by recent HRRR runs either.

Interestingly supercell near Wainwright, OK (the northern one) has a fairly classic bird wing/kidney bean shape, along with some modest low-level rotation. Doubt much comes of it as CINH appears to be quite strong per 00z OUN RAOB.

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9 minutes ago, Quincy said:

This is not being handled well by recent HRRR runs either.

HRRRx runs have had it since earlier this afternoon. I'll be using that big-time tomorrow.

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Muskogee storm has awfully tight rotation at ~1400 ft. on TTUL for an unwarned storm. Odds of these being nicer sups than anything tomorrow? lol

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9 minutes ago, David Reimer said:

HRRRx runs have had it since earlier this afternoon. I'll be using that big-time tomorrow.

00z NSSL-WRF seems to be handling Plains/Ozarks convection fairly well so far, so we'll see what that shows for later tomorrow, coming up shortly. 

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37 minutes ago, Quincy said:

00z NSSL-WRF seems to be handling Plains/Ozarks convection fairly well so far, so we'll see what that shows for later tomorrow, coming up shortly. 

Its verdict is that I'd have about a 30 minute drive from home. 45 if you count traffic out by Lake Dirtybird.

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02Z HRRR backed off the idea of a sweeping bow by early afternoon on the western periphery of the ENH risk area... 00z NSSL WRF has a bit of a complex convective evolution with initial supercells in Kansas during the afternoon eventually forming into a convective cluster/bowing segment or two later in the evening... Further south down the dryline the NSSL WRF shows supercell development by late afternoon/early evening with eventual upscale growth into a convective cluster/linear segments.

Meanwhile: HRW-NMMB...HRW WRF-ARW...and the NAM NEST all show the development of a long-lived intense bowing segment across Kansas during the afternoon hours propagating east-southeast/southeastward into Missouri and areas further east/south... Have to believe that if confidence becomes higher in a high-end wind event across Eastern Kansas, a large portion of Missouri, and areas further south/east that an increase in severe probabilities from SPC may be necessary. Seems to be nearly unanimous agreement imo from the CAMs on the development of an intense bow somewhere across the region.

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  Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1257 AM CDT Sat May 27 2017

   Valid 271200Z - 281200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
   OKLAHOMA TO THE LOWER OHIO VALLEY...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE
   SOUTHERN PLAINS TO THE MIDDLE OHIO VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH-CENTRAL
   TEXAS TO THE MID-ATLANTIC...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
   SOUTHWESTERN TEXAS TO THE ATLANTIC SEABOARD...

   ...SUMMARY...
   Widespread severe wind gusts are forecast from the Ozarks eastward
   to the lower Ohio Valley today. Additionally, very large to giant
   hail and tornadoes will be possible from the Red River Valley
   northeastward to the Ozark Plateau. Damaging winds, large hail, and
   a couple tornadoes will also be possible across the Tennessee
   Valley, middle Ohio Valley, and portions of the Mid-Atlantic.

   ...Synopsis...
   A mid-level trough will stretch from the Great Basin northeastward
   to the northern Plains early in the day, before it advances east
   towards the upper Mississippi and Missouri Valleys. Along the
   southeastern periphery of this trough, several impulses embedded in
   southwesterly flow will transit across the Plains states and
   mid/upper Mississippi Valley. Farther east, a shortwave trough will
   progress east across the Tennessee Valley during the day.

   ...Ozark Plateau to the lower Ohio Valley...
   The potential exists for a swath of damaging wind gusts to occur
   from eastern Kansas to the mid Mississippi Valley this afternoon and
   evening. However, as is typical with higher-end potential, several
   uncertainties exist with the forecast genesis/evolution. Severe
   convection currently over northwest Kansas is forecast to push east
   during the overnight hours. Dependent upon the eastward progression
   of these cells, an outflow boundary will likely be positioned from
   west to east across portions of northern/central Kansas this
   morning. This boundary may serve as a focus for thunderstorm
   development during the first half of the day, which could lead to
   some pockets of convective overturning across eastern Kansas.
   Despite this potential, convective inhibition should keep early-day
   storms relatively sparse. In turn, as a southerly low-level jet
   shifts towards the Kansas/Missouri border during the afternoon,
   thunderstorms are forecast to initiate near a warm front positioned
   from northeast Kansas eastward to the mid Mississippi Valley.

   With very rich boundary-layer moisture streaming northward
   (characterized by surface dew points in the upper 60s and lower/mid
   70s) and steep mid-level lapse rates overlying much of the region,
   the thermodynamic environment will be quite volatile across the
   region. Related to the elevated mixed layer, some capping will
   initially be present over eastern Kansas and Missouri during the
   day. However, with increasing forcing for ascent, and surface
   convergence along both a southward-surging cold front and the warm
   front to the east, deep convection is forecast to develop during the
   afternoon. Ample effective shear upwards of 50kt will combine with
   strong mixed-layer CAPE (upwards of 3500-4500 J/kg) to foster the
   rapid development of severe storms across eastern Kansas and parts
   of Missouri. Several updrafts will likely rotate, with an attendant
   threat of very large/giant hail, damaging winds, and tornadoes.
   Through the afternoon and evening, however, linear forcing along the
   cold front and shear vectors somewhat parallel to the warm front
   will promote quick upscale growth into an east/southeastward-surging
   convective system, with a swath of damaging winds (some significant)
   forecast. Additionally, line-embedded circulations will support a
   few tornadoes as the line pushes towards the Mississippi Valley
   during the evening. Damaging gusts will remain possible into the
   Ohio Valley thereafter, but the line should gradually weaken during
   the overnight hours with eastward extent.

   ...Portions of Oklahoma and north Texas...
   A more conditional severe threat exists over parts of
   central/eastern Oklahoma and north Texas during the afternoon and
   evening. Stronger mid-level capping is forecast to be positioned
   over the region during the day, likely precluding initiation during
   much of the day. However, as forcing for ascent gradually increases,
   a few thunderstorms are forecast to develop from the Red River
   Valley northeastward to northeast Oklahoma. Strong/extreme
   mixed-layer CAPE and favorable effective shear profiles will likely
   encourage rapid organization of these cells. While some uncertainty
   remains regarding the overall mode (due to capping potentially
   inhibiting convection until the cold front/linear forcing arrives),
   any discrete cells will be capable of very large/giant hail,
   damaging winds, and tornadoes (which could be strong). While
   low-level shear is not particularly impressive, extreme
   buoyancy/updraft velocities may support modulation of the near-storm
   environment, locally enhancing helicity and favoring a higher
   tornado threat. Additionally, if convection initiates along the
   dryline, a more perpendicular orientation of shear vectors (to the
   initiating boundary) would encourage more discrete storms, further
   enhancing the threat. Farther south into north Texas, storms may
   remain quite isolated, but any stronger cell will be capable of very
   large hail, damaging winds, and perhaps a tornado or two.

   ...Portions of the Tennessee Valley...
   As an impulse progresses from the Ozarks to the Tennessee Valley
   today, a southwesterly low-level jet over the region will likely
   promote the development of scattered thunderstorms across the
   Tennessee Valley. Aided by sufficient deep-layer shear and
   surface-based buoyancy over 2000 J/kg, several storms will likely be
   severe, with an attendant threat of damaging winds, large hail, and
   perhaps a couple tornadoes during the afternoon/evening. Moreover,
   some upscale growth of these cells will further enhance the threat
   of damaging winds.

   ...Portions of the Mid-Atlantic...
   Weak impulses embedded in westerly flow aloft, heating along higher
   terrain, and adequate low-level moisture (e.g., surface dew points
   in the upper 50s to lower/mid 60s) will support widely scattered
   thunderstorm development this afternoon. Relatively straight
   hodographs will likely yield splitting storms, with occasional
   updraft rotation enhancing the threat of large hail. Additionally,
   dry air aloft will enhance downdraft momentum and the possibility of
   strong/damaging winds.

   ..Picca/Dean.. 05/27/2017
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