243
ACUS01 KWNS 212007
SWODY1
SPC AC 212006

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0306 PM CDT Sat Sep 21 2024

Valid 212000Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS...

...SUMMARY...
Scattered severe storms capable of all hazards are possible this
afternoon and evening across portions of the southern High Plains.

...20Z Update...
Severe probabilities have been trimmed behind convection in parts of
western Pennsylvania as it appears potential for severe storms will
be diminished through the remainder of the afternoon. In
north-central/northeast Iowa, temperatures have risen into the low
90s ahead of the cold front and near an outflow boundary in the
southeast Minnesota/west-central Wisconsin. This area could see a
locally higher potential for large hail/damaging winds. Still,
linear forcing along the front and very modest mid-level ascent
reduces confidence in a discrete storm mode and sufficient storm
coverage, precluding any increase in severe probabilities.

..Wendt.. 09/21/2024

.PREV DISCUSSION... /ISSUED 1127 AM CDT Sat Sep 21 2024/

...Southern High Plains...
A potent upper low and associated 65-75 kt mid-level jet max
centered over AZ are expected to quickly move eastward,
overspreading the southern Rockies/High Plains this afternoon and
evening. At the surface, sustained southerly flow ahead of a
deepening surface low and trailing cold front over western NM will
support moderate destabilization (MLCAPE 1500-2000 J/kg) with
afternoon heating and continued low-level upslope. As the mid-level
system approaches, mid-level height falls and flow aloft will
intensify, elongating low and mid-level hodographs, allowing for
convective development/organization in the form of supercells.

One source of continued uncertainty is the ongoing elevated
convection and cloud debris centered north of a diffuse surface
boundary stretching from central NM to the western TX Panhandle. So
far, this boundary has been slowly lifting north with pronounced
low-level theta-e advection taking place to the south. As deep-layer
ascent intensifies through the late morning, continued convective
development to the north may support reinforcement and stalling of
the boundary. The northward extent of destabilization remains
unclear, though this boundary could serve as a focus for renewed
development of more robust convection expected later this afternoon.


Most CAM guidance shows scattered thunderstorms initiating off the
higher terrain of eastern NM, or ahead of the cold front farther
west by early this afternoon. These storms should quickly mature
with supportive thermodynamics and kinematics, eventually spreading
eastward into the TX Panhandle. The risk for large to very large
hail/severe gusts will likely accompany the stronger supercells, and
a threat for a couple tornadoes may be realized later this
afternoon/early evening from eastern NM into parts of adjacent west
TX, especially near the boundary where low-level shear may be
locally enhanced.

...Upper Midwest and MO Valley...
A second upper low near the international border is forecast to
quickly move eastward reaching the western Great Lakes by early this
evening. Weak ascent is expected ahead of a surging cold front and
diffuse wave cyclone across the northern MO valley and southern MN.
Surface moistening with a warm front lifting north near the IA/MN
border may act as a focus for more robust convective development
along the cold front late this afternoon. Increasingly orthogonal
deep-layer flow and steeping low/mid-level lapse rates could support
a mix of supercells and multi-cell clusters. A few of the stronger
storms may pose some hail/wind threat, which should peak in
intensity during the early evening. However, somewhat limited
heating and the lack of broader forcing for ascent suggests storm
coverage should remain isolated.

...Western PA into the Mid Atlantic...
Short-wave troughing across the upper OH Valley within mid-level
northwesterly flow is expected to support isolated to widely
scattered thunderstorms across western PA/VA later this afternoon.
Model forecast soundings with moderate buoyancy and modest
deep-layer shear would imply some of the stronger storms being
capable of an isolated risk for wind damage (50-60 mph) and perhaps
some hail. This activity will likely diminish by early evening
coincident with nocturnal cooling.

$$