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ACUS01 KWNS 081256

SWODY1

SPC AC 081255



Day 1 Convective Outlook  

NWS Storm Prediction Center Norman OK

0755 AM CDT Wed May 08 2024



Valid 081300Z - 091200Z



...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVE PORTIONS OF

SOUTHERN/SOUTHEASTERN MISSOURI...EXTREME NORTHEASTERN AR...EXTREME

SOUTHERN ILLINOIS...WESTERN/SOUTH-CENTRAL KENTUCKY...AND

NORTHWESTERN TO MIDDLE TENNESSEE...



...SUMMARY...

Severe thunderstorms appear likely from parts of the mid

Mississippi, Ohio, and Tennessee Valleys into the southern Plains. 

All severe hazards, including tornadoes, very large to giant hail,

and potentially significant damaging winds are possible.  Some

tornadoes may be strong.



...Synopsis...

In mid/upper levels, a temporarily blocky pattern has evolved around

a cutoff cyclone over the northern Great Plains.  The associated

500-mb low is expected to drift erratically southeastward and fill

slowly through the remainder of the period, while a series of

shortwaves amplify associated synoptic troughing extending

southwestward across the Intermountain West and Great Basin. 

Meanwhile, a northern-stream perturbation over the Upper Great

Lakes, that peeled eastward off the cyclone's influence, has a

compact cyclone aloft centered near ANJ.  This feature should

devolve into an open shortwave trough and accelerate eastward as it

enters the northern stream today, reaching eastern NY and western

New England around 00Z.



In the southern stream, broadly cyclonic flow will prevail over the

western CONUS.  A weak perturbation -- apparent in moisture-channel

imagery over the central Gulf of California -- should move

east-northeastward through the period, reaching northern Coahuila

and adjoining southwest TX around 00Z, then parts of LA/northern MS

by the end of the period.  Downstream perturbations over the

Southeast and central/southern Appalachians should be predominantly

convectively derived.



At 11Z, a surface lows were evident over southwestern ND and between

OFK-YKN, along an occluded front.  An occluded/cold front arched

from the NE low across northeastern KS to another weak low near CNU,

then southwestward over southwestern OK and the TX South Plains. 

Today, the cold front will overtake the northern part of a dryline

now over northwest TX to the Big Bend region.  By 00Z, the front and

dryline together will act as a western limit of substantial

convective/severe potential, from eastern OK to

north-central/central TX.  A Gulf warm front over northeastern OK

and central/northeastern AR was moving northward and should cross

the Ozarks this morning, moving into southern MO.  A progressive

outflow boundary was apparent from northern VA across southern WV,

southeastern KY, to northern parts of middle/western TN and

southeastern MO.  The western part of this boundary is stalling, and

soon should retreat northward through the rest of the morning across

parts of western KY and southeastern MO, essentially linking with

the warm front to form a baroclinic zone focusing the greatest of

today's severe risks.



...Ozarks to Mid Mississippi, lower Ohio and Tennessee Valleys...

A complicated, potentially dangerous severe-weather event is

possible today into this evening.  Multiple rounds of strong-severe

thunderstorms are expected today across this region, additively

contributing enough severe hazard for the "moderate" (level 4 of 5)

risk area.  A swath of damaging to severe gusts and at least a few

embedded tornadoes are likely to accompany a complex of

thunderstorms from southern MO across parts of the multi-state area

near the Ohio/Mississippi River junction, to at least northern parts

of the Tennessee Valley region.  The main MCS should move

east-southeastward to southeastward along and near the instability

gradient associated with the aforementioned outflow boundary. 

Before and during that process, several supercells are expected,

offering tornadoes (a few potentially significant/EF2+), large to

very large hail, and localized severe gusts.



An ongoing area of strong-severe thunderstorms across portions of MO

should pose mainly a severe-hail threat for another couple hours,

though the gust potential will increase as it grows upscale.  See

severe thunderstorm watch 202 and related mesoscale discussions for

the near-term threat.  As the morning progresses, the wind threat

will ramp up further, with some tornado potential starting as well,

as this activity and/or additional convection developing later over

western MO interacts with 1) a northward-moving plume of

surface-based inflow air and 2) rich low-level moisture now found

south of the outflow boundary.  Meanwhile, diurnal destabilization

of an already richly moist airmass south of the boundary will grow

MLCAPE commonly into the 3000-4000 J/kg range, beneath favorable 

(50-60 kt) deep shear.  Low-level shear/vorticity, SRH and hodograph

size will peak near the boundary, supporting a corridor of

relatively maximized tornado potential with any sustained/discrete

or semi-discrete supercells in the area, as well as a focus for MCS

forward propagation from the northwest.  MCS activity should merge

upscale further this evening and tonight, maintaining severe-wind

potential into parts of the Tennessee Valley region before moving

into gradually less instability late tonight.



...Eastern OK to AR, Mid-South and central TX...

Thunderstorms should develop by late afternoon near the front and

dryline, as a combination of lift along those boundaries and strong

surface heating combine with very rich low-level moisture to erode

the EML cap sampled well by the 12Z FWD RAOB.  Large to giant hail,

occasional severe gusts, and a few tornadoes are possible as

activity shifts eastward to northeastward across central/east TX,

the Arklatex, and AR through the evening.  The northern part of this

convection may merge with or be overtaken by MCS activity moving out

of MO and into parts of the Mid-South.



The afternoon/preconvective environment should be characterized by

steep low/middle level lapse rates on either side of the weakening

cap, along with surface dewpoints commonly in the 70s F.  This will

contribute to strong buoyancy with the peak/preconvective MLCAPE

reaching 4500-5500 J/kg, and SBCAPE values topping 6000 J/kg. 

Although near-surface flow generally will be 10 kt or less, limiting

lowest-km hodographs/shear, effective-shear magnitudes in the

40-55-kt range indicate supercells will be possible.  These will be

capable of very large/destructive hail exceeding 3 inches in

diameter.  Clusters or upscale mergers of convection also may offer

deep, precip-loaded, hail-cooled downdrafts with locally severe

gusts.  Cell mergers and interactions with boundaries will factor

into tornado potential on the storm scale, since the environmental

low-level shear appears on the margins.



...Southern Appalachians/Cumberland Plateau into Carolinas...

Thunderstorms are ongoing along and behind the outflow boundary near

the TN/KY border area, and may pose a severe threat through the

remainder of the morning while moving generally southeastward.  See

SPC mesoscale discussion 693 for near-term details.  Further

intensification and/or additional development is possible into the

afternoon over and east of the mountains, moving into an environment

of favorable moisture and diurnal destabilization.  MLCAPE in the

1500-2500 J/kg range appears attainable, amidst roughly 35-45-kt

effective-shear magnitudes.  A mix of supercell and multicell modes

is expected, with small bows possible, supporting localized threats

for damaging wind, large hail and at least marginal tornado

potential.



...Northeastern CONUS...

Widely scattered to scattered thunderstorms may develop as soon as

mid/late morning over parts of eastern NY and New England.  Activity

should form atop a diurnally destabilizing and weakly capped

boundary layer, along/ahead of a prefrontal surface trough and

behind a plume of non-severe thunderstorms and precip now crossing

parts of the region.  Thunderstorms should move generally eastward,

offering damaging to isolated severe gusts and isolated large hail. 

A combination of low-level warm advection, spotty/irregular diurnal

surface heating, and cooling aloft related to the approaching Great

Lakes shortwave trough, will steepen low/middle-level lapse rates

substantially.  With favorable moisture (surface dewpoints generally

50s to low 60s F), a plume of 500-1000 J/kg MLCAPE should develop,

amidst effective-shear magnitudes that reach 40-50 kt, and despite

strongly veered/westerly surface wind components.  A mix of

multicell and modest supercell characteristics should be observed

before activity moves over more-stable air and/or offshore, and

weakens by early evening.



...Eastern SD to eastern IA and vicinity...

Widely scattered to scattered thunderstorms are possible from early

afternoon in western parts to evening closer to the Mississippi

River, with isolated damaging gusts, large hail and possibly a

tornado or two.  Activity should form in an area of deep-layer lift

(including low-level convergence/mass response) related to a

vorticity lobe in the southeastern part of the mid/upper-level

cyclone.  Associated cooling aloft will combine with residual

low-level moisture (dewpoints mid 40s to mid 50s F) to yield pockets

of around 500-1000 J/kg MLCAPE.  Backed low-level winds will aid

convergence and storm-relative low-level flow, and may contribute to

locally enlarged hodographs.



..Edwards.. 05/08/2024



$$