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Severe potential 2/24


rolltide_130

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Okay, it may be time to start sounding the alarm bells for a significant severe weather threat across the eastern portion of the subforum for this Saturday.. 

The Euro has been slowly trending towards a more active solution over the last 48 hours, with the warm sector rapidly becoming more favorable across the Mississippi Valley in addition to kinematics that have been strong from the start. SBCAPE has been on a massive uptick, with values of well over 2000 J/KG now in place across the region. Put that below over 60 knots of deep layer shear, and well.. problems may start arising.. 

 

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I'll be rather surprised if I don't see a 30% in the upcoming D3 outlook (at least for the damaging wind threat) given the strength of the wind fields associated with this and a seeming uptick in instability with each new model run across the suite. There's certainly potential for a notable event here, the 12z/00z Euro runs emphasize that with what appear to be several supercells breaking out across the Arklamiss and Mid-South, which would certainly be capable of tornadoes given the large hodographs and 35-45+ kts of low level shear in place.

In addition, a mid level dry slot seems to be showing up through the morning/early afternoon in these areas that may allow clearing and fairly rapid destabilization given the degree of low level moisture. 500 mb temperatures have also been coming down a bit with a bit of an increase in 700 mb temperatures leading to steeper lapse rates in that layer. In other words, this looks to be trending up thermodynamically and it was always quite strong kinematically.

Edit: No enhanced, but definitely expanded the slight and added a mention of a possible upgrade.

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This is a pretty classic cold-season setup for the area. CIPS analogs have some big events (#2 event from 12z NAM was 03/01/1997) mixed in with some lesser ones, but pretty much all of them had some tornadoes. Analogs aren't perfect, but I like to use them to get an idea of the ceiling and floor of an event. The 18z NAM maxed out around 1000 MLCAPE in east AR, which would be sufficient for tornadoes, maybe even a strong one or two. Any increase in lapse rates would really help this setup. With all that said, this is definitely the best setup so far this year, so I'll be paying close attention.

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11 minutes ago, monsoonman1 said:

This is a pretty classic cold-season setup for the area. CIPS analogs have some big events (#2 event from 12z NAM was 03/01/1997) mixed in with some lesser ones, but pretty much all of them had some tornadoes. Analogs aren't perfect, but I like to use them to get an idea of the ceiling and floor of an event. The 18z NAM maxed out around 1000 MLCAPE in east AR, which would be sufficient for tornadoes, maybe even a strong one or two. Any increase in lapse rates would really help this setup. With all that said, this is definitely the best setup so far this year, so I'll be paying close attention.

I agree

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Can't rule out QLCS tornadoes on Saturday, and/or a few embedded supercell structures, but the storm mode looks largely linear. 

Low level instability does not look as impressive as some of the more severe analogs and issues in the mid-level wind fields suggest the window for any discrete supercells will be minimal at best. Even the agressive 3km NAM shows a linear storm mode, with some cellular elements within the larger scale line. Essentially it shows no open warm sector activity, where the parameter space and backing of near-surface winds are maximized.  

A typical forecast sounding ahead of the line (late afternoon in southern/eastern Arkansas) looks like this:

IMG_6750.thumb.JPG.ebc56cd5e28fcc08c007424cfb76ceaa.JPG

The 0-2km wind fields are quite favorable, but then above that it gets junky looking.

With all of that said, such considerable low level shear, combined with seasonably impressive low-level moisture and <1km LCLs suggest an elevated potential for tornadoes, although they'll probably be short-lived and embedded within a line. 

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I'm on Quincy's side here. I don't see any reason to believe that there will be much in terms of supercellular activity. Typically what makes or breaks these cold season events is whether you get open warm sector development. Sure, the parameters may bring with them some impressive analogues, but they also bring some mediocre ones, and that's the problem with these events. The difference between a memorable tornado event and one that sucks is small. 

I'm not surprised in the 30%/hatched but I'm surprised definitely in the wording. That said, the forecaster has me less surprised.

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SHEAR PROFILES SHOULD ENCOURAGE SEVERAL BOWING/LEWP   STRUCTURES, WITH EMBEDDED SUPERCELLS POSSIBLE. FURTHERMORE, FORECAST   SOUNDINGS AND HIGH-RES GUIDANCE DEPICT A CONSIDERABLE COMPONENT OF   LOW-LEVEL SHEAR PERPENDICULAR TO SEVERAL BOWING SEGMENTS, ENHANCING   THE POTENTIAL FOR TORNADOES -- A FEW OF WHICH COULD BE STRONG --   DURING THE AFTERNOON AND EVENING HOURS.

Good tidbit from the new Day 2. Interesting that they would use the "strong tornado" wording for what is essentially a QLCS forecast. Not that you can't get strong tors from a QLCS, but typically SPC doesn't use that wording for this kind of setup. Also this:

OUTSIDE OF THE MAIN BAND OF CONVECTION, ALTHOUGH FORCING FOR ASCENT   WILL NOT BE PARTICULARLY STRONG EARLIER IN THE DAY, AN ISOLATED   DISCRETE SUPERCELL OR TWO MAY FORM ACROSS THE MID-SOUTH WITHIN   WARM/MOIST LOW-LEVEL CONFLUENCE SATURDAY AFTERNOON. FAVORABLE   STORM-RELATIVE HELICITY AND EFFECTIVE SHEAR WOULD SUPPORT A   CONDITIONAL DAMAGING WIND AND TORNADO THREAT DURING THIS TIME FRAME   AS WELL.  

 

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Can't say I strongly disagree with the 30% hatched, as a robust low-level jet (increasing to 55-65+ knots) would support a significant damaging wind threat with an evolving squall line. The potential for strong tornadoes seems more conditional.

Despite some issues in the wind fields, just looking at LCLs, most of the region highlighted is progged to stay below 500m, so it won't take much at all for tornadogenesis, especially with the degree of 0-1km shear. 

One cannot completely rule out warm sector storm development given the magnitude of shear, especially if some of the low level cloudiness can be eroded to boost buoyancy. If anything, weak forcing would favor only isolated activity. Reference 12z HRRRX for a scenario where an isolated supercell could form in such an environment. 

The 3km NAM doesn't show this and the NSSL WRF tries to, but struggles to form any deep, robust convection ahead of the line.

Stay tuned. 

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18z GFS, 18z NAM, 18z 3k NAM, and 00z HRRR are all suggesting that the warm sector might move into (at least part of) the DFW metroplex area, most of which has been under a slight risk for tomorrow due to what seems to be a hail risk from elevated thunderstorms.  It would be interesting to see if we can get surface-based convection in the vicinity of a warm front; the 18z 3k NAM and HRRR suggest that convection will develop just as the warm air moves in.

However, the more recent 00z NAM and 00z 3k NAM are keeping DFW in the cool sector, though the 01z HRRR is moving the warm sector into DFW.  So we'll see what happens.

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I'm a fan of the HREF so far (it's only been around since October IIRC) as it did pretty well on 01/22 and it nailed that weird stuff in TX on the morning of 02/20. The 00z run has a messy convective mode tomorrow but it has some decent UH in good parameters with the line, so it points to a decent QLCS tornado day for AR, maybe into TN/MS. 

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Very impressive hodograph out of KSRX. Critical angle leaves a lot to be desired, but can't get much better than 0-1KM Shear of 45kts and 0-1km SRH of 360+ m2/s2. Definitely going to have a fairly significant QLCS threat later on... In addition to any discrete cells that might be able to develop and mature.ffbe00b4835c9d5be4db4f281d1071e7.png

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