506
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

$$