####018006801####
ACUS01 KWNS 301250

SWODY1

SPC AC 301248



Day 1 Convective Outlook  

NWS Storm Prediction Center Norman OK

0748 AM CDT Tue Apr 30 2024



Valid 301300Z - 011200Z



...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL

AND WESTERN IOWA TO NORTHEASTERN KANSAS...



...SUMMARY...

The greatest threat for severe weather (large hail severe gusts and

a few tornadoes possible) is from western and central Iowa to

northeastern Kansas.



...Synopsis...

A fairly low-amplitude, nearly zonal pattern will remain across the

CONUS east of the Rockies, and downstream from a slow-moving mid/

upper-level cyclone now over the northern Rockies.  By the end of

the period, this cyclone (containing numerous mesoscale vorticity

lobes/shortwaves) should pivot to the northern High Plains, roughly

astride the northern border of MT and ND with the Canadian Prairie

Provinces.  



The most impactful associated feature will be a shortwave trough

evident in moisture-channel imagery over portions of eastern MT into

central/southern WY.  This perturbation should shift eastward to

near a BIS-MHE-LNK axis by 00Z, then eject northeastward over the

upper Mississippi Valley tonight.  Meanwhile, in response to height

falls and synoptic trough amplification over the Intermountain West,

south of the cyclone, flow over the central and southern Rockies and

adjoining Plains should back gradually to southwesterly, but with

few, if any, notable embedded perturbations today.



At the surface, 11Z analysis showed a low over northwestern SD, with

occluded/cold front arching across western SD, western NE and

northern CO.  The low should move eastward to northeastern SD by

00Z, with a triple point over northern IA or extreme southern MN,

warm front to northern IL, and cold front to northeastern and

south-central KS, the OK Panhandle, and Raton Mesa.  That low should

move over southern MN and northern WI through tonight, possibly

reaching western Upper MI by 12Z, while the cold front reaches

northeastern IL, the central/western Ozarks, and northern OK,

stalling near a lee-side/frontal-wave low in the CAO/DHT area by

12Z.  As moisture return continues, a dryline will become better

defined today from central KS to the TX Permian Basin, shifting

eastward to east-central KS, north-central/southwestern OK,

northwest TX, and northernmost Coahuila before stalling late this

afternoon.



...Lower Missouri Valley and vicinity...

Widely scattered to scattered thunderstorms are forecast to form

this afternoon--earliest around the Siouxland area and later with

southward extent.  All forms of severe are possible, but with an

earlier lean to the greatest hail vs. greatest wind threats. 

Activity should coalesce from mid-late afternoon onward and shift

eastward over the outlook area, weakening this evening into tonight.

 

Observed thermodynamic profiles and forecast soundings from this

area southward toward the Gulf Coast reasonably show a shallow moist

layer in low levels, easily mixed via diurnal heating.  This appears

legitimate,  related to trajectories emanating from an extensive

area of outflow observed over the northwestern Gulf and adjoining

parts of east TX, LA and AR to the lower Mississippi Valley.  That

outflow came from MCS activity yesterday.  Nonetheless, diurnal

heating should help to steepen low/middle-level lapse rates

considerably and eliminate MLCINH -- by early afternoon over

northwestern parts of the outlook area (where moisture will be lower

but deep-layer lift/cooling aloft greater), and mid/late afternoon

roughly south of I-80 (stronger heating/moisture, adequate lift).  



Strengthening and veering of winds with height will yield enlarged

hodographs and favorable deep-shear profiles for supercells, albeit

with somewhat high LCL atop a deep mixed subcloud layer.  Forecast

soundings reasonably depict MLCAPE ranging from around 500-1000 J/kg

in the SUX/FSD area to over 3000 J/kg down the narrow plume of

favorable instability in eastern KS.  Effective-shear magnitudes of

50-65 kt and SRH of 200-400 J/kg are expected.  This parameter space

and profile will support severe wind and hail, both in early

supercell stages (when large to locally very large hail also is

expected), and as upscale aggregation of convection occurs with

small-scale cold pools. 



...Southern Plains...

Isolated to locally scattered thunderstorms, including a few

supercells, are possible for a few hours this afternoon into early

evening, initiating near the dryline and moving slowly east-

southeastward (right movers) to faster northeastward (left movers).

Large hail and severe gusts will be the main concerns, and a

conditional significant (2+ inch diameter) hail risk may occur with

any sustained supercell in this corridor.  However, in the absence

of large-scale support, specific foci along the dryline appear

unclear, with coverage uncertain (and probably low) over most of the

area.  This makes predictability low for any specific mesobeta-scale

patch where greater unconditional probabilities may develop. 



Strong surface heating along the dryline will support at least

isolated thunderstorm development mid/late this afternoon, in an

environment of modest low/middle-level flow but strong veering with

height.  Before dark, SRH will be small in the lowest couple km,

however, with effective shear magnitudes in the 30-40-kt range,

supporting a blend of organized multicell and transient/isolated

supercell potential.  Steep lapse rates, dewpoints commonly in the

60s F, and enough heating for a deep/well-mixed subcloud layer,

should support MLCAPE of 2000-3500 J/kg.  Activity should weaken

considerably after dark. 



...NY...

Widely scattered thunderstorms are possible today in a zone of

large-scale ascent preceding the trough aloft, superimposed on

modest low-level convergence related to low-level trough drifting

eastward over the area.  The trough may manifest at the surface as a

weak low, but projected inflow-layer parcels should remain largely

(if not entirely) elevated above the boundary layer, where moisture

will be greatest and MUCINH weakest.  Elevated MUCAPE of 500-1200

J/kg is possible, and briefly surface-based parcels may be

attainable in pockets of sustained heating over portions of the

Finger Lakes to Catskills vicinities.  The best-organized cells may

produce small hail and subsevere gusts.  Lack of greater near-

surface instability (magnitude and area), and weak midlevel lapse

rates, preclude a severe threat warranting an unconditional outlook

at this time.



..Edwards/Leitman.. 04/30/2024



$$