LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1241 AM CDT Mon Jul 17 2023
Valid 171200Z - 181200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE BLACK
HILLS VICINITY INTO NEBRASKA...AND ACROSS THE LOWER OHIO VALLEY...
...SUMMARY...
Severe storms producing large hail and damaging gusts are expected
from the Black Hills area southeastward into northern Nebraska this
evening into tonight, and across portions of the Lower Ohio Valley
vicinity this afternoon. More isolated strong to severe
thunderstorms are possible across parts of the northern Great
Basin/northern High Plains vicinity, the Ozark Plateau, and the
Upper Ohio Valley.
...Synopsis...
An upper ridge centered over the northern Rockies this morning will
deamplify as it shifts east in response to a Pacific Northwest
shortwave trough progressing eastward along the international border
through Tuesday morning. Further east, an upper trough oriented over
the Midwest to the TN Valley will pivot slowly east. Moderate west
to northwest mid/upper flow will overspread the northern/central
Plains and much of the Midwest vicinity, while numerous vorticity
maxima migrating through this broad area of northwest flow will
provide focus for areas of strong to severe thunderstorms. The most
likely area of organized severe storms existing across parts of
northeast WY into SD/NE and the Lower OH Valley vicinity.
...Ohio Valley vicinity...
A seasonally moist airmass will be in place beneath moderate
mid/upper west/northwesterly flow. A surface cold front is forecast
to sag southward across MO/IL/IN/OH during the afternoon and
evening. The sagging surface front and convectively enhanced
vorticity maxima migrating through the mid/upper trough will focus
thunderstorm development as diurnal heating aids in moderate to
strong destabilization from southeast MO to southwest OH. Low-level
winds are expected to mostly be southwesterly, limiting directional
shear. However, speed shear will result in effective bulk shear
magnitudes around 40 kt. A mix of discrete cells and clusters are
expected. Steep low-level lapse rates will aid in strong outflow and
damaging gusts are expected, especially with any organizing
clusters. Otherwise, isolated cells may be capable of producing
large hail with forecast soundings indicated midlevel lapse rates
around 7-7.5 C/km and elongated hodographs.
With east and northward extent along the Upper Ohio Valley vicinity,
effective shear and instability weakens. However isolated strong
storms producing marginal hail and strong gusts are possible across
much of Ohio into western PA.
...Black Hills into Nebraska...
A deepening lee trough ahead of the northern Rockies upper trough
will result in moist upslope flow into the Black Hills/northeast
WY/southeast MT vicinity. Thunderstorms are expected to develop by
late afternoon in a moderately unstable and strongly shear
environment. These initial supercells may be elevated. However,
steep midlevel lapse rates and long, straight hodographs suggest
large to very large (greater than 2 inch diameter) hail is possible.
With time, and as a low-level jet strengthens during the evening,
upscale development into a bowing MCS is possible as convection
track southeast into northern/northeastern NE. A swath of damaging
gusts may accompany this MCS during the evening and overnight hours.
...Northern Great Basin into Central/Eastern Montana...
Moderate to strong west/southwest mid/upper flow will overspread the
region ahead of the northern Rockies trough. Meanwhile, steep
midlevel lapse rates and pockets of midlevel moistening will support
MLCAPE around 500-1250 J/kg. A well mixed boundary-layer will result
in mostly high-based, low-precipitation convection capable of
sporadic strong dry microburst winds from far northeast NV/northern
UT into central MT. With eastward extent into parts of eastern MT,
deeper boundary-layer moisture will result in potential for isolated
strong gusts as well as hail.
...Eastern KS/OK into MO/AR...
Convection may be ongoing at the start of the period as a
continuation of nighttime storms from the prior Day 1 period. If
this activity persists through the morning, some strengthening could
occur as the boundary layer destabilizes through midday. Strong
gusts and marginal hail could accompany these storms, but confidence
in this scenario is low.
..Leitman/Wendt.. 07/17/2023
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