SPC AC 311250
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0750 AM CDT Sat Jul 31 2021
Valid 311300Z - 011200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF CENTRAL AND SOUTHERN MISSOURI INTO THE SOUTH-CENTRAL HIGH PLAINS
AND WESTERN PORTION OF THE TENNESSEE VALLEY...WISCONSIN AND
VICINITY...THE COASTAL CAROLINAS...AND SOUTHERN VIRGINIA...
Severe storms with damaging gusts will be possible today, mainly
across a portion of central and southern Missouri into the
south-central High Plains and western portion of the Tennessee
Valley. Other strong storms will be possible over Wisconsin and
vicinity, the coastal Carolinas, and southern Virginia.
In mid/upper levels, the longstanding large-scale pattern of western
ridge and eastern trough will continue, but with important internal
changes. The ridging from the northern/central Rockies to the
Mid-South will break down somewhat -- especially on the east side.
By the end of the period, this will render a poorly defined, more
north-south-aligned and elongated closed anticyclone -- with
multiple, embedded, subtle 500-mb highs -- from the Canadian Rockies
to north-central MX. To the east, some trough amplification/height
falls on the synoptic scale are expected over the Great Lakes, Ohio
Valley and central Appalachians. The biggest contributor to that
will be a strong shortwave trough -- apparent in moisture-channel
imagery over northwestern ON that should reach Lake Superior between
00-03Z, and Lower MI by 12Z. A weaker perturbation -- initially
over western Lake Superior and western Upper MI but not well-
depicted by most models -- that should move southeastward to the
OH/WV area by 12Z.
The 11Z surface analysis showed a wavy frontal zone from near the
coastal NC/VA border across northwestern SC, middle TN, to a weak
low near MKC, then across northern KS to southeastern CO. The front
west of the Appalachians generally should move southward through the
period, reaching southern MO, southern KS and northeastern NM by
00Z. The boundary will oscillate over the NC/southern VA region.
Farther north, a cold front associated with the ON shortwave trough
-- initially analyzed over northeastern MN, northern SD and
southeastern MT -- will move southward/southeastward to northern
Lake Michigan, eastern WI, eastern IA and northern MO by 00Z.
...South-central Plains to Ozarks/Mid-South...
Isolated to widely scattered thunderstorms should develop this
afternoon along and south of the front, offering isolated damaging
wind and severe hail.
30-40-kt effective-shear magnitudes are expected under the
southwestern rim of the stronger northwest to west-northwest flow
aloft, over the southern MO, lower Ohio Valley and Mid-South areas.
This will support organized multicells, with potential clustering of
convection locally fostering a strong-severe gust threat. Somewhat
less coverage -- but still amidst strong warm-sector heating, rich
low-level moisture and frontal lift, will support isolated strong-
severe gust potential westward across the southern KS/northern OK
portion of the outlook. Localized strong-severe gusts also will be
possible with activity over the south-central high Plains, where
well-mixed subcloud layers and steep low-level lapse rates are
possible, even in the post-frontal surface northeasterlies.
However, weak deep-layer flow and related lack of shear will limit
organization/duration of any severe threat over the Plains.
...Western Upper Great Lakes...
Isolated to scattered thunderstorms should develop this afternoon
along/ahead of the northern frontal zone, as well as potentially on
lake-breeze boundaries. Isolated development also may occur in the
free warm sector. The main concerns will be marginally severe hail
and isolated damaging to severe gusts.
Despite thick smoke over much of the area, diurnal heating should be
sufficient to reach near convective temperatures shown in forecast
soundings, with aid from lift along the boundaries. Peak MLCAPE of
1000-1500 J/kg is possible, beneath midlevel instability that will
be increased by DCVA preceding the approaching ON perturbation.
Little change in direction with height is expected, limiting
effective-shear magnitudes to around 30-35 kt in combination with a
seasonally shallow buoyant profile. However, favorable cloud-layer
shear and anvil-level winds may aid in organization of some of the
convection. Activity should diminish in and near the eastern rim of
the outlook area by mid/late evening.
...Coastal Carolinas this afternoon/early evening...
Widely scattered multicell thunderstorms should form initially this
afternoon east and south of the front, near a surface trough and
perhaps sea-breeze boundaries. The most persistent/intense
convection may yield strong to isolated severe gusts in water-loaded
downdrafts. However, weak deep-layer flow and lack of substantial
vertical shear should limit both duration and organization of any
severe potential. Strong overland diurnal heating will steepen
low-level lapse rates in a regime of rich boundary-layer moisture--
with dew points commonly in the 70s F. This will contribute to a
corridor of MLCAPE in the 2000-3000 J/kg range over the outlook
...Central/southern VA late overnight...
Widely scattered to scattered thunderstorms may develop across
central and southern VA the last few hours of the period. Some of
this activity may pose a threat for isolated severe gusts and a
conditional/low-end tornado potential. This also may constitute the
earliest stage of the following convective day's severe threat
regionally; see the SPC day-2 outlook for more details past 12Z.
Though not directly affected by the aforementioned cyclonic-flow
shortwaves, associated height falls and difluent flow will spread
overhead. Meanwhile, low-level lift will increase late tonight into
early tomorrow morning local time, near an inverted trough and
northward-bulging, surface frontal inflection area. Along and south
of the front, moisture advection will yield upper 60s to 70s F
surface dew points, yielding weak MLCINH, as well as effective-
inflow parcels that are surface-based or only slightly elevated.
Wind profiles will veer strongly with height, yielding around
35-45-kt effective-shear magnitudes.
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z