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ACUS01 KWNS 181625

SWODY1

SPC AC 181624



Day 1 Convective Outlook  

NWS Storm Prediction Center Norman OK

1124 AM CDT Thu Apr 18 2024



Valid 181630Z - 191200Z



...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS MAINLY THIS

AFTERNOON INTO EARLY TONIGHT FOR PARTS OF THE MID MS AND LOWER OH

VALLEYS...



...SUMMARY...

Damaging gusts to 70 mph, large hail up to 2 inches in diameter, and

a couple of tornadoes are expected mainly this afternoon into early

tonight across the middle Mississippi and lower Ohio Valleys.  More

isolated large hail and damaging winds will be possible farther

south from northern Mississippi and Arkansas into central Texas.



...Mid MS/lower OH Valleys through tonight...

Recent surface analysis places a low just south of MCI along the

KS/MO border vicinity. A warm front extends eastward from this low

across central MO, far southern IL and into KY. Airmass south of

this warm front is moderately moist, with observations sampling mid

60s dewpoints along much of the AR/MO border, with some upper 60s

farther east over the MO Bootheel. Expectation is for this moist

airmass to continue progressing northward throughout the day, likely

reaching the I-70 corridor by the mid-afternoon. A cold front also

extends southwestward from this low through north-central to

southwest OK, continuing through northwest TX and southwest TX. An

outflow boundary from overnight convection precedes this cold front,

although by only about 30-50 miles or so.



General expectation is for thunderstorm intensity and coverage to

increase early this afternoon as these surface boundaries interact

with the increasingly moist and unstable boundary layer downstream. 

Overall convective evolution is a bit more uncertain, particularly

regarding when the storms become more balanced with the cold front

and/or outflow no longer undercutting updrafts. Current thinking is

that this occurs during the early afternoon across central MO, with

quick upscale growth promoting an organized convective line. Primary

risk within this line will be damaging wind gusts from 45 to 65 mph.

Large hail from 1 to 2" is also possible, particularly with any more

cellular development ahead of the front and/or outflow.



A similar convective evolution appears likely farther north (from

central MO into central/southern IL and western IN) near the surface

low, but with augmented mesoscale ascent near the low. This

augmented ascent could contribute to more vigorous updrafts and a

somewhat more organized convective structure, despite being

displaced north of the better low-level moisture and buoyancy.

Additionally, the eastward-progressing low will allow storms here to

be more favorably aligned with the deep-layer vertical shear. This

could contribute to a more forward-propagating structure and the

potential for damaging gusts, despite modest thermodynamics. As a

result, the outlook probabilities were shifted a bit northeastward

across IL and IN. Primary threat here is wind gusts, although the

backed low-level flow near the surface low could contribute some

tornado threat as well.



...Much of AR through southeast OK and into North/Central TX this

afternoon/evening...

A cold front extends southwestward from a low near the KS/MO border

southwestward through north-central to southwest OK, continuing

through northwest TX and southwest TX. Surface heating ahead of this

front is expected to result in strong buoyancy within the moist

airmass that is already in place across the region. Widely scattered

to scattered thunderstorm development is expected by about 21z along

the front from northwest AR across southeast OK into north and

central TX.  Deep-layer shear will be strong enough for some

supercell structures capable of producing isolated very large hail

(up to 2.5 inches in diameter), along with damaging outflow gusts

given the potential for strong downdrafts.  These storms will last a

few hours into late evening and then weaken as convective inhibition

increases.



...Southeast AR into Northern MS this afternoon/evening...

As mentioned in MCD #480, slightly elevated convection continues to

evolve across the Arkansas/Louisiana border area this morning,

within a zone of QG ascent ahead of a subtle shortwave trough moving

across east Texas. This shortwave is expected to continue

progressing northeastward, with the ongoing storms likely persisting

as they move northeastward as well. General expectation is for this

cluster to maintain its intensity for the next few hours, with some

hail possible, before likely waning thereafter as the low-level jet

weakens and veers. However, that is a fairly low confidence scenario

given that this cluster developed about 4 hours ahead of when much

of the guidance indicated.

 

Additional development appears probable in the wake of this cluster

over northern LA/southern AR this afternoon. In this area, weak

ascent preceding the shortwave will interact with the unstable warm

sector for widely scattered thunderstorm development this afternoon.

 If a few storms do form, the environment will conditionally favor

supercells capable of producing large hail/damaging winds, with the

threat expanding eastward into northern/central MS over time.



..Mosier/Bentley.. 04/18/2024



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