SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0744 AM CDT Fri Sep 16 2022
Valid 161300Z - 171200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE CENTRAL PLAINS...
...SUMMARY...
Severe thunderstorms are possible across portions of the central
Plains this afternoon and evening.
...Synopsis...
Mid/upper-level troughing now over the Gulf of Alaska will dig
south-southeastward across the Northeast Pacific. By 12Z tomorrow,
this will form the southwestern node of a binary closed cyclone,
well offshore from the Columbia River mouth, while the northeastern
center meanders near the middle of the BC/AB border. This complex
process will lead to height falls and amplified cyclonic flow over
the Pacific Coast States, Great Basin and lower Colorado River
Valley. Downstream west-southwest to southwest flow aloft will
continue, with the leading perturbation now extending across central
parts of SD, NE and KS. This feature should eject northeastward
across the upper Mississippi Valley and Lake Superior region through
the period, weakening as it moves through a larger-scale ridge.
Behind that, the main feature for this forecast will be a shortwave
trough evident in moisture-channel imagery from the northern Rockies
to near the NV/UT border. This perturbation should reach central
parts of MT/WY and north-central CO by 00Z, the trough should be
located near an axis from GGW-RAP-AIA-AKO.
The 11Z surface analysis showed a low over northwestern MN between
FAR-TVF -- related to the leading shortwave trough. A surface
trough and weak cold front extended from the low across eastern SD
and central NE to northeastern CO, and should weaken through the day
as mass response occurs to strong lee troughing over the High
Plains. By 00Z, the primary lows should be located over
southeastern CO and southeastern MT. These lows -- and the trough
connecting them -- should migrate eastward tonight. By 12Z, the
lows should reach western SD and northwestern KS, along a trough
from northeastern MT to central NE to the eastern TX Panhandle.
...Central Plains...
Isolated to widely scattered showers and thunderstorms are evident
this morning over portions of UT, western CO, and western/southern
WY, in a plume of large-scale ascent that precedes the NV/UT
shortwave-trough segment. As the associated UVV field proceeds
eastward today, it will overlie a diurnally destabilizing boundary
layer to support early/mid-afternoon convection -- developing both
over the higher terrain of central WY and northern CO, and near the
lee trough on the adjoining High Plains. Initially, this activity
will be high-based, forming over weak but sufficient moisture and
deep/well-mixed subcloud layers. Wind profiles west of the lee
trough will be nearly unidirectional, limiting effective shear,
despite gradually increasing mid/upper-level speeds. Isolated
severe gusts and marginal hail will be the main concerns at first.
However, thunderstorms forming on or crossing the lee trough will
gain greater severe potential while impinging on a more-favorable
boundary layer with:
1. Surface dewpoints in the upper 40s to 50s F, but still steep
lapse rates and deep mixing, supporting MLCAPE near 500 J/kg;
2. Weak but relatively backed flow, enabling enough veering and
strengthening of wind with height for long hodographs and around
35-45 kt effective-shear magnitudes. This will support some
supercell potential with activity that has not already become too
outflow-dominant/cold-pool-driven in character atop the deep mixed
layers farther west.
Greater low-level moisture will be available to convection reaching
west-central/southwestern NE late this afternoon and this evening,
from parcels originating east and southeast of the old, decayed
front. Some of the activity may persist into the evening with a
continued wind/hail potential, before overnight stabilization in the
foregoing inflow layer reduces the severe threat. However, elevated
thunderstorms may persist or develop anew from southern/central NE
to southeastern SD and eastern NE tonight, along the northwestern
rim of a LLJ. The severe threat gradually becomes lower and more
conditional with eastward extent and time tonight.
..Edwards/Leitman.. 09/16/2022
Read more CHECK UPDATE ZOOM GRAPHIC
http://dlvr.it/SYSBL8
