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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