ACUS01 KWNS 041253
SWODY1
SPC AC 041252

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Thu Jul 04 2024

Valid 041300Z - 051200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MISSISSIPPI VALLEY...AND LOWER OHIO VALLEY TO SOUTHERN
PLAINS...

...SUMMARY...
Severe thunderstorms will be possible across parts of the upper
Mississippi Valley, and lower Ohio Valley to southern Plains.

...Synopsis...
In mid/upper levels, strengthening ridging over the Pacific Coast
will result in height rises over much of the Northwest, while the
subtropical ridge weakens slightly across the Gulf Coast States and
GA.  Broadly cyclonic flow still will prevail from the northern
Rockies across the central/northern Plains and Great Lakes.  Within
that flow, a well-developed shortwave trough is evident in moisture-
channel imagery from the Dakotas to northeastern CO, preceded
closely by a smaller but convectively augmented perturbation over
parts of IA, northwestern MO and eastern KS.  As these features
pivot through the broader cyclonic flow today, the leading
perturbation's MCV should move across northern parts of IL/IN/OH and
perhaps extreme southern Lower MI.  The trailing, stronger trough
should extend from central MN to central NE by 00Z, then cross the
remainder of MN/NE  and much of IA overnight with an at least
intermittently closed 500-mb low.

At the surface, the 11Z analysis showed a frontal-wave low over
southwestern IA, with some possible augmentation from wake effects
related to an area od precip/convection to the east and southeast.
A wavy cold front, with several weak lows attached, was drawn from
there across eastern/southern KS, the TX Panhandle, and north-
central NM, preceded by outflow boundaries from northern OK across
southern MO.  By 00Z, the cold front should reach northwestern IL,
central MO, southwestern OK, and central NM, with diffuse warm front
over central parts of IL/IN/OH near or north of a convective
boundary.  By 12Z, the cold front should reach northwestern IN,
southern IL, southern MO, southeastern OK, north-central/northwest
TX, and southern NM.

...Southern Plains to lower Ohio Valley...
Multiple rounds of convection are expected across this corridor
today, contributing to the overall severe potential, without sharp
demarcation between the three main regimes numbered below.  The
outlook has been expanded northeastward up the Ohio Valley in
deference to an increased severe threat related both directly and
indirectly to MCV-aided convection.

1.  Ahead of the MCV and accompanying perturbation, a broad area of
clouds and precip is apparent from central/northern MO across
southern/central IL and extending into southern IN.  The southern
rim of this precip area contains scattered, initially non-severe
thunderstorms, utilizing a broad warm-advection/moisture-transport
plume with parcels isentropically lifted to an LFC.  All this
activity will continue to shift eastward through the remainder of
the morning, helping to reinforce the ambient baroclinic zone across
the area, which in turn should help to focus later/afternoon
development.  Meanwhile, some of the morning activity may encounter
a diurnally destabilizing boundary layer with weakening MLCINH, more
purely surface-based effective-inflow parcels, and rich boundary-
layer moisture.  The associated outflow/differential-heating
boundary may shift somewhat northward today over southern/central
parts of IL/IN before the leading perturbation passes, and also, can
act to back low-level flow and enlarge hodographs, in a moisture-
rich, low-LCL environment.  As such, some supercell/tornado
potential has become evident, with damaging gusts and isolated large
hail also possible.

2.  Closely following that regime, and perhaps blending with the
western part of it, a round of widely scattered to scattered
thunderstorms is expected along/ahead of the front from central/
southern MO to the lower Ohio Valley this afternoon into evening, as
large-scale ascent from the stronger/trailing shortwave trough
overspreads a still very moist air mass.  Strong insolation and
surface dewpoints in the low/mid 70s F will offset modest mid/upper-
level lapse rates to yield MLCAPE in the 1500-2500 J/kg range south
of the front and outflow boundary.  While warm-sector surface flow
will be weak, and more veered than on the boundary, sufficient
deep/speed shear still should be present for a blend of multicells
and at least transient supercells, with some clustering/bowing of
convection possible as well.  Damaging to severe gusts will be the
main threats with this activity, though a tornado and isolated large
hail may occur.

3.  Vertical shear generally will weaken with westward extent near
and ahead of the front, but substantially hotter surface conditions
will contribute to very steep boundary-layer lapse rates and
"inverted-v" thermodynamic profiles in the subcloud layer.  A plume
of 2000-2500 J/kg MLCAPE is expected over northern OK, decreasing
southwestward across northwest TX into drier and still more deeply
well-mixed conditions.  As such, severe downdrafts and isolated
large hail will be possible, with greatest potential concentration
of the gust threat in the OK to southeast KS corridor.  Activity
forming in this regime also may persist and move/merge into the MO
environment from late afternoon into evening.

...Upper Mississippi Valley region...
Scattered thunderstorms are expected to develop as early as midday,
and continue through the afternoon, east through southeast of the
developing mid/upper low and primary vorticity maximum.  Some of
this convection will become supercells, capable of large hail and
damaging gusts, with clustering or line segments also possible.

The most favorable low-level parameter space for that convection to
become severe still appears to be across southern MN, northern IA
and western WI, where residual moisture north of the optimal warm
sector still will support surface dewpoints in the mid 60s F, amid
pockets of favorable diurnal heating and low-level warm advection
behind the morning cloud/precip plume.  Large-scale ascent will
occur in relation to midlevel DCVA preceding the trough aloft, and
the left-exit region of a cyclonically curved, 250-300-mb speed max.
Though low-level flow should be modest, favorable deep shear
(effective-shear magnitudes in the 45-55-kt range) will support
organized convection for several hours.  Overall, this activity
should diminish through the evening as nocturnal cooling and
expansive outflow air collectively stabilize the boundary layer.

..Edwards/Kerr.. 07/04/2024

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