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Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0751 AM CDT Wed Aug 21 2024

Valid 211300Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EASTERN MONTANA AND WESTERN NORTH DAKOTA...

...SUMMARY...
Severe hail and thunderstorm gusts are most probable over parts of
eastern Montana and western North Dakota.

...Synopsis...
The CONUS portion of the large-scale, mid/upper-level pattern this
period will be characterized by mean troughing over the East and
West Coasts, and ridging extending northward over the Great Plains
States from a high over west TX.  A strong cyclone -- initially
centered just south of the AK Panhandle -- is expected to dig
south-southeastward, with its 500-mb center moving to around 200 nm
west of UIL by 12Z tomorrow.  As this process reinforces the mean
western trough, a series of mostly low-amplitude shortwave
perturbations will eject through the foregoing southwest flow.

The best-defined and potentially most important of those
perturbations evident in moisture-channel imagery over portions of
northern CA, northwestern NV and southeastern OR.  This feature
should reach southeastern ID and west-central MT around 00Z, before
ejecting northeastward across the northern High Plains and losing
definition tonight.  Meanwhile, in the subtropical easterlies, a
shortwave trough will continue to drift westward across parts of
Sonora, the Gulf of California, and central/northern Baja.

At the surface, 11Z analysis showed rich Gulf-origin moisture still
confined near and south of a slowly weakening synoptic front over
northern FL, the northern Gulf shelf waters, and southeast to
north-central TX.  Nonetheless, a plume of mainly 60s F surface
dewpoints was analyzed from much of OK north-northwestward across
western/central portions of KS, NE and the Dakotas.  A lee trough
was drawn farther west, over eastern portions of MT/WY/CO.  This
trough should intensify through the day as ridging aloft shifts
somewhat eastward, and southwest flow aloft spreads over more of the
High Plains.  As part of that process, surface cyclogenesis is
expected over the northeastern WY/southeastern MT region, with the
resulting low shifting eastward into northwestern SD around 06Z.

...Northern Plains...
Widely scattered to scattered thunderstorms are expected to develop
initially over the higher terrain of south-central/southwestern MT
by midafternoon.  In this area, heating of greater elevations
preferentially removes MLCINH first, as large-scale lift ahead of
the ejecting shortwave begins to move overhead.  This activity
should move northeastward into parts of central MT atop a boundary
layer that is diurnally destabilizing/mixing, yet still favorably
moist.  Additional thunderstorms may develop by late afternoon near
the surface trough, where strong surface heating and the western rim
of the corridor of greatest moisture (surface dewpoints co*monly in
the upper 50s to mid 60s F) will remove capping at lower elevations.
 Convection of both origins may merge near the trough, over eastern
MT.  Along the way, activity should produce at least occasional
large hail (mainly with any sustained supercells) and damaging
gusts.  Some aggregation of convection should occur, leading to
forward-propagational clustering and increased potential for
swath(s) of severe to potentially significant (65+ kt) gusts from
late afternoon through much of the evening, as activity moves
through a lingering, deep, well-mixed layer left by diurnal heating.

A well-heated and deep boundary layer west of the moist plume will
support MLCAPE in the 500-1500 J/kg range, with 1000-1500 J/kg DCAPE
as well.  This will foster high-based convection for the first few
hours, with some potential for activity to cluster and beco*e
collectively outflow-dominant before reaching the grater moisture. 
weak low-level winds are expected, beneath 50-80 kt flow in the
upper half of the buoyant profile, contributing to favorable deep
shear (40-50 kt effective) for supercells, and long, relatively
straight hodographs supporting splitting storms and hail potential
where convection doesn't produce too much outflow too soon.

Thunderstorms will move into larger nominal CAPE and greater near-
surface moisture over easternmost MT into western ND, where the
moist axis should be found, with MLCAPE of 2000-3000 J/kg.  Though
MLCINH also will be stronger in the higher-CAPE corridor than
farther west, forced ascent that is driven by cold-pool processes
should sustain some of the strong-severe convection well into ND
this evening.  Activity should peak near the moist/buoyancy axis and
weaken afterword tonight, as it moves into progressively more-stable
low-level profile, both with lower ambient theta-e and nocturnal
diabatic cooling/stabilization near the surface.

...Central High Plains...
Widely scattered to scattered thunderstorms should develop this
afternoon over higher terrain from the Black Hills to the Laramie
Range and southward over the Front Range, its foothills, and
adjoining ridges such as the Cheyenne Ridge and Palmer Divide.  This
convection should move east toward the lee trough and low-level
moist axis, while crossing over a hot, deeply mixed boundary layer. 
Isolated severe gusts are expected, and severe hail also may be a
concern with relatively discrete cells.

While vertical shear will be weaker than farther north, strong
veering of flow with height (especially near the trough) should
contribute enough deep shear (30-40 kt effective mainly north of
I-70) for at least brief supercell potential.  Aggregation of
outflows may foster one or more eastward-surging clusters into this
evening from western KS to central SD, and a relatively concentrated
local severe-wind threat will exist in any such cluster.  However,
guidance is too inconsistent in convective evolution and longevity,
and in location/number of such clusters, to assign greater
unconditional probabilities within the long "marginal" area at this
time.

...Southeastern AZ...
Scattered thunderstorms are expected to develop this afternoon, at
first over higher terrain then across deserts/valleys as outflow/
differential-heating boundaries develop.  With a relatively deep
layer of south-southeasterly midlevel flow, activity should move
generally northwestward.  Though most progs keep the flow aloft
modest (15 kt or less), some mesoscale enhancement is possible east
of the track of a northwestward-moving MCV -- evident in IR
satellite imagery over north-central Sonora.  The most intense
convection will be capable of isolated severe gusts.

Clouds related both to earlier/overnight convection over southern
AZ, as well as to activity over MX, are south and west of this
region already, portending essentially a full day of insolation over
the outlook area.  Residual lower-elevation surface dewpoints in the
50s to mid 60s should decrease only slightly through the diurnal
heating/mixing cycle, offset to some extent by moist advection. 
MLCAPE in the 1000-2000 J/kg range should develop through early/mid
afternoon, atop the well-mixed subcloud/boundary layer.

..Edwards/Mosier.. 08/21/2024