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Severe potential for 2/23-2/27


Tallis Rockwell
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day6prob.gif?1550496548702

  ZCZC SPCSWOD48 ALL
   ACUS48 KWNS 180956
   SPC AC 180956

   Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0356 AM CST Mon Feb 18 2019

   Valid 211200Z - 261200Z

   ...DISCUSSION...
   Models continue to indicate that blocking will remain prominent
   within the mid-latitude westerlies across the northeastern Pacific
   through this period.  Downstream, it appears that flow may
   transition to a broadly confluent regime across the Pacific coast,
   Rockies, and Plains, and a less amplified regime in general across
   the southern tier of the U.S., by late next weekend into early next
   week.  As this takes place, one more vigorous short wave impulse is
   forecast accelerate east-northeastward out of the Southwest.  

   There appears increasing consensus, within and among the various
   model output, that this feature will provide support for strong
   surface cyclogenesis to the lee of the southern Rockies by this
   weekend.  Guidance generally indicates that the cyclone center will
   rapidly migrate from the Texas/Oklahoma Panhandle region into the
   Great Lakes on Saturday.  This probably will include intensifying
   lower/mid tropospheric flow (to 50-70 kt in the 850-500 mb layer)
   across an evolving warm sector boundary layer that will become at
   least weakly unstable, across lower portions of the southern Plains
   through the mid to lower Mississippi Valley region.  This
   environment may become conducive to the evolution of an organized
   mesoscale convective system, and perhaps discrete supercell
   development ahead of it, accompanied by the risk for damaging wind
   gusts and tornadoes.  

   It remains unclear whether destabilization eastward and
   southeastward, toward the Mid Atlantic and Southeast, on Sunday will
   remain sufficient to support an appreciable continuing severe
   weather threat.

   ..Kerr.. 02/18/2019
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It is official

day1otlk_1200.gif?1550899890612

.THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   NORTHERN MISSISSIPPI...SOUTHWEST TENNESSEE AND FAR NORTHWEST
   ALABAMA...

   ...SUMMARY...
   Scattered severe thunderstorms are expected to develop today from
   the Arklatex region and lower to mid Mississippi Valley eastward
   into the Tennessee and Ohio Valleys. Tornadoes, wind damage and
   isolated large hail will be possible across much of the area.

   ...Significant tornado and wind damage event expected across parts
   of the lower to mid Mississippi and Tennessee Valleys today...

   ...Arklatex/Lower to Mid Mississippi Valley/Tennessee and Ohio
   Valleys...
   A well organized negatively-tilted upper-level trough will move
   across the southern Plains today as a 90 to 100 kt mid-level jet
   moves through the base of the trough. At the surface, a low will
   gradually deepen and move northeastward across the central Plains
   into the lower Missouri Valley. A surface trough is forecast to
   extend southward into the lower Mississippi Valley with a moist
   airmass located across the region. Surface dewpoints in the upper
   60s and lower 70s F will overspread northern Louisiana, southeast
   Arkansas and most of Mississippi by midday. At the start of the
   period, a cluster of thunderstorms should be ongoing from northern
   Mississippi into western and middle Tennessee. The stronger storms
   embedded in the cluster may have an isolated severe threat. This
   activity is forecast to move northeastward away the moderate risk
   area allowing for the moist sector to warm up late this morning. In
   response, a corridor of moderate instability is expected to be in
   place by midday from northeast Louisiana into far southeast Arkansas
   and western Tennessee. As large-scale ascent increases ahead of the
   southern Plains upper-level trough, thunderstorms are forecast to
   develop during the morning along the surface trough in the Arklatex
   with this convection moving northeastward into southeastern Arkansas
   by midday. More isolated thunderstorms will be possible across the
   moist sector from northern Louisiana into north-central Mississippi.
   During the early to mid afternoon, convective coverage should
   increase with scattered thunderstorms moving east-northeastward
   across the Enhanced and Moderate Risk areas.

   RAP forecast soundings across the Moderate Risk area from
   north-central Mississippi into southwest Tennessee at 21Z show
   moderate instability and impressive kinematic profiles. MLCAPE is
   forecast to be from 1000 to 1200 J/kg with 0-6 km shear in the 50 to
   55 kt range. This will support supercell formation with cells that
   remain discrete. In addition, hodographs are long and looped with
   0-3 km storm relative helicity values in the 350 to 450 m2/s2 range.
   This low-level shear environment will be favorable for tornadoes. A
   potential for long-track significant tornadoes will exist across the
   Moderate Risk area from late this morning through much of the
   afternoon. Wind damage and isolated large hail will also be possible
   with supercells. A squall-line is also expected to develop and move
   across the Enhanced and Moderate risk areas during the late
   afternoon and early evening. Wind damage, isolated large hail and a
   few tornadoes will also be possible with the stronger thunderstorms
   embedded in the line. This squall-line with wind damage potential
   should move across middle Tennessee and north-central Alabama during
   the evening.

   Further north across the Ohio Valley, scattered thunderstorms are
   forecast to move northeastward into western Kentucky and southern
   Indiana during the afternoon. Forecast soundings at 21Z for Paducah
   show MLCAPE near 800 J/kg with 0-6 km shear of 65 kt. This combined
   with strong low-level shear will be sufficient for severe storms
   capable of producing tornadoes and wind damage. Most of the severe
   convection should remain south of the Ohio River. Due to the weaker
   instability in the Ohio Valley, severe coverage is expected to be
   less.

   ..Broyles/Bentley.. 02/23/2019
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And I'm shocked hardly that people in the TN Valley or SE subforums are not really talking about this event either.  Right now the warm front appears to be in northern LA and central MS (based on the northernmost edge of the surface CAPE) but we all know that is going to move north into TN over the next several hours.  I guess I'm just getting a bit concerned about contamination of the warm sector here given the current location of storms in the Arklatex (but those aren't even in an area with surface CAPE at the moment) and the current precipitation over northern MS and TN.  I'm not worried about contamination in MS, but more so in TN.

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MLCAPE is already >1500 J/kg over a sizable chunk of the risk area at 17z so that element is basically checked off going forward. Going to come down to storm mode and whether the storm relative wind profiles are favorable enough for longer lived/strong mesocyclones (read: if the surface winds can remain at least southerly).

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In keeping with the “under the radar” theme of today: Just issued watch is 90-70 for MS, AR, and LA.

A little surprised the initial watch across the moderate risk area in MS isn’t PDS. SPC said last year that the significant tornado percentage in the watch has to be 80% or higher to warrant the PDS language.

Won’t derail the thread with the language discussion, but I think it’s worth debating at some point whether that language should be reserved for set-ups where several significant tornadoes are likely, which is what SPC says it’s for, or if we should use for it set-ups where, even if there aren’t “several,” there could still be 1-2 violent long-track tornadoes.

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1 minute ago, Upper Level LOL said:

Now

 

I saw this post on the board about 3 seconds before the warning polygon showed up on my GRLevel3 radar screen. 

I am always amazed by these things. A few days ago, the NAM most-unstable CAPE was barely 500 J/kg. Now we have the SPC Mesoanalysis saying that there is 2500 J/kg of surface based CAPE and a STP value of 3-4 in northern Mississippi. I really don't know how the models miss the instability forecast. Nevertheless, the SPC posted an enhanced risk on this yesterday, moderate risk today. So that all seems to make sense given the parameters measured today.

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