Severe Weather Outlook

Severe Weather outlook

Storm Prediction Center

SPC Sep 15, 2024 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0750 AM CDT Sun Sep 15 2024

Valid 151300Z - 161200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL AND NORTHERN HIGH PLAINS...

...SUMMARY...
Thunderstorms capable of isolated severe gusts and hail will be
possible today across parts of the central and northern High Plains.

...Synopsis...
In mid/upper levels, a blocky longwave pattern will remain over the
CONUS, with amplifying troughing in the West and a Rex configuration
in the East.  Cyclonic flow already covers most of the land west of
the Rockies.  That will continue but with backing of the flow aloft,
as a cyclone now centered over Vancouver Island strengthens and digs
south-southeastward down the West Coast.  By 12Z tomorrow, the
associated 500-mb low should be located near OAK, with strongly
difluent flow and modest ridging over the High Plains from eastern
NM to MT.  The southern part of a weak shortwave trough -- evident
in moisture-channel imagery from UT to eastern AZ -- should cross CO
today and reach the eastern border of WY to near GLD by 12Z, likely
enhanced from this evening onward by convectively generated
vorticity.

A mid/upper anticyclone now over the Lower Great Lakes will stretch
eastward and open up to that direction around the end of the period.
The base of the eastern pattern block will be composed of a broad,
nearly zonal height weakness anchored by:
1. The decaying remains of Francine, long merged with a mid/upper
low, and forecast to drift erratically over the Mid-South while
weakening further;
2.  An intensifying, deep-layer cyclone with low-level manifestation
as a deepening wave cyclone along a frontal zone, offshore from the
Carolinas.

The 11Z surface analysis showed a wavy, quasistationary frontal zone
attached to a strengthening low located roughly south of EWN and
east of SAV.  The front extended across northern FL to the western
FL Panhandle and southwestern AL.  A remnant low was drawn well
inland over northern MS, connecting to the Gulf boundary via an
occluded front.  These features should move little through the
period, but with further weakening of the western low, and
intensification of the eastern one.  Elsewhere, a lee trough and
dryline either overlapped, or were drawn within less than 100 nm
apart, near the eastern border of WY, eastern CO, the northern/
western TX Panhandle, and east-central/southeastern NM.  The dryline
should shift somewhat eastward today amid diurnal heating.

...Central/northern High Plains...
Isolated to widely scattered thunderstorms are possible across a
large part of the Rockies, Intermountain West, and Desert Southwest
today.  Some of this activity will mature atop deep, well-mixed
boundary layers of desert valleys and the High Plains, with non-zero
potential for a strong-severe gust.  The most concentrated/organized
potential for gusts near to above severe limits, as well as marginal
hail, still appears to be over portions of the central High Plains
into the Black Hills and vicinity, near the lee trough/dryline. 
East of that, low-level moisture will be relatively maximized in a
return-flow airmass of predominantly continental origin, with a
north-south belt of 50s to mid 60s F surface dewpoints.  Where the
western part of that moisture coincides with strong surface heating,
and underlies the eastern part of the EML, a north-south corridor of
MLCAPE in the 1500-2500 J/kg range should develop, with 500-1500
J/kg extending into the more deeply mixed regime nearest the lee
trough.  East of the trough, considerable veering of winds with
height is expected, with some low-level hodograph enhancement
potentially moving 0-3-km SRH to near 150 J/kg, but lack of stronger
midlevel flow will temper overall shear.

...Coastal NC, extreme eastern SC...
Many progs deepen the low-level cyclone and transition it to
subtropical or tropical character through the end of day 2, while
eroding the frontal zone's baroclinicity; however, inconsistency
among models is high.  The most aggressive among the synoptic and
CAM guidance with the speed of the transition and approach to the
coast (e.g., 06Z GFS and 00Z ARW high-res window respectively) show
favorably large low-level shear, but barely have surface-based
effective inflow onshore at the end of the period, with greatest
convective coverage and buoyancy still offshore.  As such, the
associated coastal tornado threat still appears too low and
conditional to outlook until day 2 (after 16/12Z).  Potential for
faster/deeper development and shoreward translation of the low will
continue to be monitored for onshore supercell threat the last few
hours of day 1.  See latest NHC tropical outlook for guidance on
overall development potential with this system.

..Edwards/Mosier.. 09/15/2024

SPC Sep 15, 2024 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1128 AM CDT Sun Sep 15 2024

Valid 151630Z - 161200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN/CENTRAL PLAINS...

...SUMMARY...
Thunderstorms capable of producing isolated severe gusts and hail
may occur today and tonight across parts of the northern/central
Plains.

...Northern/Central Plains...
An amplified upper trough along/near the West Coast this morning
will further evolve into a closed upper low over northern CA and
vicinity by late tonight. Ahead of this feature, modestly enhanced
south-southwesterly mid-level flow will be in place over much of the
western states and Rockies through the period. Although low-level
moisture is expected to remain limited over much of this region,
isolated strong to severe gusts may occur with any thunderstorms
that can develop in a deeply mixed boundary layer favorable for
gusty downdraft winds. 

Latest guidance continues to suggest that a relatively greater
concentration of severe potential will be focused across parts of
the northern/central Plains late this afternoon through tonight.
Across this area, a weak surface lee trough and greater low-level
moisture to its east should support weak to moderate instability
with daytime heating. Steep mid-level lapse rates should also aid
updraft accelerations/intensities, with around 20-30 kt of
deep-layer shear supporting modest thunderstorm organization.
Isolated severe hail may occur with initially more discrete
convection, but a tendency for clustering should lead to a
relatively greater wind threat with time as convection spreads
generally east-northeastward through the evening. Regardless, the
overall severe threat from northeast CO into western NE/SD and
vicinity should remain fairly isolated/marginal given the modest
deep-layer shear.

Low-level warm advection is forecast to increase across much of the
northern/central Plains tonight as a southerly low-level jet
strengthens. A surface warm front is also expected to lift northward
over ND late tonight into early Monday morning. With steepening
mid-level lapse rates overspreading the northern Plains in tandem
with increasing low-level moisture, ample MUCAPE should be
available. Most guidance shows at least isolated convection
developing across parts of ND tonight in this favorable
thermodynamic environment. Although deep-layer shear may remain
fairly modest, it may still be sufficient for updraft organization
and an isolated threat for severe hail with any elevated convection
that can form. Have therefore expanded the Marginal Risk northward
to account for this potential.

...Coastal Carolinas...
Latest NHC forecast continues to indicate some potential for
sub-tropical or tropical development of a surface low currently off
the SC Coast. Regardless of development, it still appears that the
more moist/tropical low-level airmass over the Gulf Stream will
struggle to advance inland over coastal portions of NC/SC through
the end of the Day 1 forecast period early Monday morning (16/12Z).
Accordingly, tornado potential with any low-topped supercells on the
north to northeast periphery of the surface low should tend to
remain offshore until Day 2/Monday (after 16/12Z).

..Gleason/Moore.. 09/15/2024