676
ACUS01 KWNS 161248
SWODY1
SPC AC 161246

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0746 AM CDT Mon Sep 16 2024

Valid 161300Z - 171200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN THE EASTERN
CAROLINAS AND PARTS OF THE NORTHERN PLAINS...

...SUMMARY...
A tornado threat exists in the eastern Carolinas.  Thunderstorms
capable of severe gusts and large hail are expected today in parts
of the northern Plains.  Isolated severe wind and hail are expected
in parts of the Four Corners and Great Basin.

...Synopsis...
A highly amplified pattern is expected in mid/upper levels, anchored
by two cyclones:
1.  A purely cold-core, midlatitude circulation initially centered
over northern CA, with accompanying cyclonic flow covering most of
the CONUS from the Rockies westward.  Equatorward progression of
this feature is about done, and the 500-mb low should pivot eastward
across the RNO area by 00Z, then east-northeastward toward EKO
overnight.  In the downstream difluent flow, a shortwave trough is
apparent in moisture-channel imagery over the western Dakotas.  This
feature should move northeastward to southwestern MB by 00Z, then to
westernmost parts of northwest ON overnight.
2.  What is now a small, deep-layer circulation just offshore from
ILM and MYR, designated Potential Tropical Cyclone 8 by NHC as of
this writing.  This system may become subtropical or tropical before
moving inland later today, per NHC discussions.  Regardless of
whether it is named, the system will offer some tornado potential
(below).  The associated mid/upper low is progged to become better
defined through the day and moving farther inland, with the
low-level center tracking not far behind.

As the surface, aside from PTC 8 and its attached frontal zone, the
11Z analysis showed a quasistationary front extending northeastward
from a low near SHR, across southeastern MT to north-central ND,
behind the outflow of a mass of ongoing convection over ND.  The
outflow boundary should act as the effective frontal zone for this
region, as it moves slowly eastward over ND and north-central SD
before likely stalling around midday.

...Eastern Carolinas...
A zone of enhanced low-level gradient flow is apparent north through
northeast of the center of PTC 8, over coastal sections of NC and
westward past MYR.  This is related both to the cyclone itself and
antecedent/post-frontal high pressure from north of the baroclinic
zone upon which this low developed.  The synoptically driven
baroclinicity is expected to continue to decrease (frontolysis),
especially if this low becomes tropical.  However, some mesoscale
baroclinicity will remain due to persistent rain north of the old
front, and a lack of precip -- combined with a high-theta-e,
maritime/tropical airmass to its south and southeast.

This airmass should shift northward as the low moves inland, and
support potential for inland penetration of enough moisture and
limited diurnal heating to enable 500-1000 J/kg MLCAPE today, away
from areas of continual/persistent rainfall.  In an environment
where LCLs are low and low-level shear and SRH already are favorable
(per VWP from ILM and MHX), the tornado threat will be tied directly
to a combination of
1. Sufficient surface-based destabilization on the mesobeta scale,
in that transition zone where flow still is backed enough to keep
hodographs large, and
2. Formation of supercell(s) and their ability to remain in the
favorably buoyant/backed-flow slot long enough to mature and
produce.

See SPC mesoscale discussion 2070 for marginal, near-term potential
near and north of center.  Through the day, the effective warm-
frontal regime is expected to shift northward up the coastline and,
to some extent, areas around the Sounds, so a small unconditional
5%/"slight risk" has been introduced in those areas.  Tonight into
tomorrow morning, as the center penetrates deeper inland away from
the most favorable airmass and weakens considerably, and as diabatic
cooling reduces instability at the surface, the tornado potential
should diminish.

...Northern Plains...
A large area of precip and convection is ongoing across much of
west-central/central and northeastern ND, with embedded strong/
isolated severe thunderstorms capable of gusts and hail.  This
activity is reinforcing/reorienting the antecedent baroclinic zone
across the area, and may persist northeastward into the remainder of
northeastern ND and northwestern MN through midday with a continued
threat for isolated severe.  Additional development is possible
along the outflow-modulated boundary, both in the near term and
through the afternoon, ahead of the shortwave trough, and in the
area of related maximized lift (both convective-scale on the
boundary, and broader-scale in the midlevel DCVA).

The airmass south and east of the boundary should heat diurnally,
steepening the low/middle-level lapse rates amidst favorable 60 F
surface dewpoints.  This should support peak/preconvective MLCAPE in
the 1500-2500 J/kg range, along with veering flow with height.
However, lack of stronger midlevel flow limits deep shear, keeping
progged effective-shear magnitudes generally under 35 kt.  The most
probable convective mode appears to be a mix of multicells and messy
supercells.  Convection-allowing progs that most-closely depicted
the current state (which has not included the underdone hourly HRRR
for most of the night) reasonably show a lack of substantial
destabilization north of the boundary, except perhaps for some
elevated hail potential this evening.  Convective coverage on ether
side of the boundary may increase for a few hours this evening in
response to enhancement of moisture, shear and storm-relative flow
provides by the LLJ.

...4 Corners/Great Basin...
A broad arc of combined baroclinic and large-scale ascent is
expected to extend across the near-eastern to far-southeastern
sectors of the mid/upper cyclone, supporting episodic, widely
scattered to scattered thunderstorms throughout much of the period.
Where this activity encounters diurnally/diabatically destabilized
boundary layers, characterized by deep mixing with related
maximization of lapse rates and DCAPE, and at least marginal CAPE
above, isolated severe gusts will be a threat.  The potential for
high-based supercells amidst greater shear to the southeast, across
the Four Corners area, suggest isolated severe hail also will be
possible there.

..Edwards/Mosier.. 09/16/2024

$$