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
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