SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0254 PM CDT Mon Jul 29 2024
Valid 292000Z - 301200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN AND CENTRAL PLAINS...AND PORTIONS OF THE LOWER OH AND
TN VALLEYS...
...SUMMARY...
Scattered strong to severe thunderstorms may occur across parts of
the northern/central Plains this afternoon and evening, and also
over portions of the Lower Ohio and Tennessee Valleys mainly this
evening through the early overnight hours.
...20Z Update...
...OH/TN Valley into the Southeast...
Several disorganized convective lines are currently ongoing, one
across middle TN and the other farther southeast across central GA,
driven mainly by southeastward-progressing outflow. As mentioned in
recently issued MCD #1725, the line across central GA and western SC
is expected to continue southeastward for the next several hours. A
few stronger to severe thunderstorms are possible within this line,
with damaging gusts as the primary severe risk. A similar scenario
is anticipated across middle TN, where regional
reflectivity and velocity data has shown some tendency for
consolidating outflows and the development of a more coherent cold
pool over the past 30-60 minutes. This may be the start of a loosely
organized convective line that should propagate to the
southeast into parts of northeast AL and northern GA in the coming
hours along a diffuse buoyancy gradient. Damaging gusts are the
primary severe risk here as well.
Farther north (i.e. from southern IL/IN into KY), the airmass has
been slower to destabilize in the wake of early morning storms.
However, temperatures have climbed in to the mid 80s amid dewpoints
in the low 70s and only limited convective inhibition remains.
Additional thunderstorm development still appears likely later this
afternoon and evening as the airmass continues to destabilize and
large-scale ascent glances the region. Strong buoyancy (i.e. MLCAPE
greater than 2500 J/kg) will support robust updrafts. Shear will be
modest (i.e. effective bulk shear around 30 kt), with a largely
outflow-dominant storm mode anticipated. Cold pool amalgamation is
possible, with the resulting bowing cluster tracking southeastward
into KY. Damaging gusts are the greatest severe threat.
...Northern/Central Plains...
Forecast reasoning outlined in the previous discussion (appended
below) remains valid, no changes needed to the ongoing outlook.
Severe thunderstorms capable of large to very large hail and strong
gusts are still possible this evening and tonight.
..Mosier.. 07/29/2024
.PREV DISCUSSION... /ISSUED 1130 AM CDT Mon Jul 29 2024/
...Northern/Central Plains into the Upper Midwest...
Various 12Z observed soundings across the northern/central High
Plains show shallow/limited low-level moisture in the wake of prior
convection and a weak front, with PWAT values generally less than 1
inch. Still, sufficient moisture to support weak to moderate
instability should be in place through this afternoon along/east of
a weak surface trough near the border of ND/SD and MT/WY.
Large-scale ascent should remain fairly modest in a mostly zonal
mid/upper-level westerly flow regime, with a low-amplitude mid-level
shortwave trough over southern Saskatchewan/Manitoba and ND expected
to move eastward through the period. Another weak mid-level
perturbation evident on water vapor satellite imagery late this
morning should also move eastward from the northern Rockies across
the adjacent High Plains through this evening. Both of these
features should aid isolated to scattered thunderstorms developing
across parts of ND/SD and western NE by late afternoon, generally
along/east of the weak surface trough.
Modest low-level flow is expected to veer to westerly and gradually
strengthen with height to around 30-40 kt through mid levels.
Similar values of deep-layer shear will support organized updrafts,
including the potential for a few supercells. Large hail generally
1-2 inches in diameter will be possible with any sustained, discrete
convection through the early evening. With a long, generally
straight hodograph forecast at mid/upper levels, splitting
supercells may occur. Still, overall coverage of intense
thunderstorms remains somewhat uncertain given the modest
large-scale ascent. The potential for convection to grow upscale
this evening into one or more bowing clusters also appears rather
uncertain given the weak low-level mass response forecast. If a
cluster can develop, then severe/damaging winds, with some
potentially up to 75 mph given rather steep low/mid-level lapse
rates, would become a greater threat with time through the evening
hours across central/eastern ND/SD and northern NE. An isolated
severe wind threat may persist with eastward extent into southern MN
and IA tonight with any cluster that can persist along an axis of
greater instability.
...Midwest/Lower Ohio Valley into the Tennessee Valley/Southeast...
A large area of convection from an earlier MCS across the Midwest
has devolved into an area of disorganized showers and thunderstorms
late this morning across the lower OH Valley vicinity. Extensive
outflow from this activity has surged southward to the Ozarks to
parts of western KY based on recent visible satellite and radar
imagery. But, airmass recovery is already well underway across parts
of central MO, with southwesterly surface/low-level winds advecting
a very moist airmass northward towards the mid MS Valley and western
portions of the Midwest/OH Valley. Convective evolution across these
areas later today into tonight remains rather uncertain given the
early-day convection. Even so, most guidance continues to remain
insistent that a moderately to strongly unstable environment will
become re-established across parts of eastern MO into
southern/central IL/IN by early evening. If this destabilization can
occur, then potential for intense thunderstorms, including
supercells, and a bowing cluster could be realized.
Forcing for ascent for this robust convection will likely be tied to
southwesterly low-level warm/moist advection, and a mid-level
vorticity maximum forecast to move eastward from WI/IL to the Great
Lakes region. Sufficiently strong northwesterly mid/upper-level flow
and veering winds through the column will support generally 25-35 kt
of deep-layer shear and updraft organization. It appears possible
that initial supercells with an isolated large hail threat may
quickly transition to a linear cluster with greater severe/damaging
wind potential with time this evening/tonight across the lower OH
Valley and vicinity. Sufficient low-level shear should also be
present to support some threat for a couple of tornadoes with any
semi-discrete thunderstorms, although the mode may quickly become
messy this evening with multiple mergers/interactions. Given the
continued uncertainty in convective evolution later today into
tonight, only small expansions have been made to the Slight Risk
across this region. A broader area of isolated strong to damaging
wind potential remains apparent across parts of the TN Valley into
the Southeast, where moderate to strong instability and marginal
deep-layer shear may support loosely organized clusters spreading
generally southeastward this afternoon/evening.
...Mid-Atlantic...
Modestly enhanced northwesterly mid/upper-level flow will overspread
parts of the eastern Mid-Atlantic and Delmarva Peninsula on the
western periphery of a mid-level cyclone that will continue to
advance northward across New England today. As the boundary layer
across this region warms and becomes weakly to moderately unstable,
a few strong thunderstorms could develop within this regime during
the afternoon. Locally moderate instability and effective shear
around 30-35 kt should support some potential for isolated damaging
gusts and marginally severe hail with the more robust cores.
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