SPC AC 191255
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Wed Sep 19 2018
Valid 191300Z - 201200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTH-CENTRAL PLAINS TO UPPER MISSISSIPPI VALLEY...
A few severe storms, mainly with a threat for hail and damaging
winds, are possible across parts of the north-central Plains to
upper Mississippi Valley.
In mid/upper levels, ridging is forecast to shift eastward from the
southern Plains and Ozarks across the Mississippi Valley to the
Tennessee and Ohio Valleys. A synoptic-scale trough -- now evident
in moisture-channel imagery over the Pacific Coast states -- is
forecast to shift eastward to western MT, UT, and the AZ/NM border
region by 12Z. To its southeast, several minor shortwave troughs
and vorticity lobes will eject northeastward across the central/
southern Rockies and central/northern Plains.
At the surface, 11Z analysis showed a low over northeastern CO, with
warm front roughly eastward across southern NE and northern MO. A
weaker, quasistationary frontal zone was drawn across southern MN,
southeastern SD, and northwestern NE. The low is expected to
move/redevelop to central/southern NE by the end of the period as
the mid/upper trough approaches, with the frontal zone consolidating
toward the current position of the northern boundary, but with
slight effective displacement southward by convective processes
possible across the southern MN/northern IA and southern WI areas.
...North-central Plains to upper Mississippi Valley...
An extensive area of clouds, rain and embedded clusters of
thunderstorms is present across portions of northern IA, southern
MN, northern and eastern SD, and northern NE. Being near the
southern rim of the stronger mid/upper flow, but north of the
surface front, a combination of elevated frontal lift and warm
advection will support isolated strong-severe convection in roughly
the southeastern half of this precip shield for a few more hours,
with pulse hail and strong gusts possible.
The morning activity will reinforce antecedent baroclinicity
accompanying the front, and another round of convection should
develop later today, perhaps beginning late this afternoon but more
likely evening, just poleward of the surface boundary. Modified 12Z
RAOBs, model forecast soundings and planar progs show the surface
front and warm sector may remain diurnally capped, though an
isolated surface-based storm with supercell characteristics, large
hail and severe gusts cannot be ruled out over parts of central/
northern NE. That potential is very conditional, too uncertain and
low in coverage to justify affixing a 15%/slight area to the broader
marginal outlook at this time.
The evening/overnight elevated convection will occur as large-scale
ascent strengthens ahead of the synoptic trough -- primarily in the
form of warm advection but also mesoscale enhancements to midlevel
DCVA immediately preceding ejecting southern-stream perturbations.
Coverage may become scattered to numerous north of the boundary this
evening over the corridor from southeastern SD to southern WI near
the nose of a strengthening LLJ, with related intensification of
moisture transport and isentropic lift. Large hail will be the main
concern, though a few strong-severe gusts are possible, especially
with any convection that can access the warm-sector boundary layer
before too much nocturnal stabilization occurs. 40-50 kt effective-
shear magnitudes and 2000-2500 J/kg MLCAPE will support some hail
risk wherever relatively discrete storms can mature before modes get
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