SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0831 AM CDT Wed May 11 2022
Valid 111300Z - 121200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
CENTRAL/SOUTHERN MINNESOTA AND SURROUNDING STATES...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
SOUTHERN HIGH PLAINS...AND SURROUNDING THE ENHANCED RISK IN THE
UPPER MIDWEST...
CORRECTED FOR INCORRECT WIND PROBABILITY LINE
...SUMMARY...
Severe thunderstorms are expected this afternoon/evening over the
north-central states. Very large hail, wind, and a threat for
tornadoes is expected. Additionally, widely scattered severe
thunderstorms are anticipated across the southern High Plains.
Wind/hail are the primary threats.
...Synopsis...
The large-scale mid/upper-level pattern will continue to feature
mean troughing over western North America, a lengthy and positively
tilted ridge from west of the central MX Pacific Coast across the
Arklatex to the St. Lawrence Valley, and a retrograding/cutoff
synoptic-scale cyclone offshore from the Carolinas and GA. For this
period, the two most convectively crucial features aloft will be
ejecting northeastward from or ahead of the mean trough, in
southwesterly flow:
1. A strong shortwave trough -- now including a compact closed
cyclone over the Sierra of northern CA and NV. This feature will
pivot slowly eastward over the Great Basin today, then turn
northeastward tonight, reaching the Star Valley area of western WY
and south-southeastward near the CO/UT line by 12Z tomorrow.
2. A former southern-stream shortwave trough that ejected
northeastward out of northwestern MX yesterday, with convectively
induced vorticity supplementation last night into early this morning
over the southern High Plains. A primary/embedded MCV is evident in
satellite and especially composited radar-reflectivity animations
over west-central/southwestern KS, with a vorticity lobe and lesser
MCV(s) across western OK. This perturbation, as a whole, will move
northeastward across the central Plains this morning, reaching
southwestern MN, western IA and northeastern KS by 00Z. The trough
should reach Lake Superior and western Upper MI by the end of the
period.
At the surface, 11Z analysis showed a low over southeastern CO near
the KS line, with wavy warm front northeastward across northeastern
KS, south-central/eastern NE, central IA, and southern WI. A
dryline was drawn from the low across the northwestern TX Panhandle,
southeastern NM and far west TX, and this boundary should move
little today before retreating deeper into NM overnight. The low
should merge with another initially drawn over southeastern WY, then
migrate erratically around northeastern CO, extreme southeastern WY
and the southwestern NE Panhandle through 06Z, before moving
northeastward to the western Sandhills by 12Z. The warm front
should move northward through the remainder of eastern NE and IA,
and into eastern SD, southern/central MN and western/southern WI by
00Z. This boundary then should decelerate over northeastern SD and
central MN before being overtaken by convection.
...Upper Midwest...
See SPC mesoscale discussion 719 for near-term coverage of
isolated/elevated convection with hail risk over southeastern SD.
Otherwise, scattered thunderstorms are expected to develop mainly
this afternoon, in an arc, over southwestern parts of the outlook
area, and move northward to northeastward, offering all severe
hazards, including at least a few tornadoes and some large to very
large hail. Early-stage supercells are expected, with evolution to
an MCS over parts of southern/central MN late this afternoon into
this evening. As the upscale growth occurs, the hail hazard will
diminish (especially in magnitude), while the wind threat increases
and becomes more dense.
The severe threat will increase with time and northward extent this
afternoon across eastern NE, western IA, the Siouxland area, and
southwestern MN as the MCV and its accompanying wind/UVV fields
impinge on a very moist, well-heated, increasingly weakly capped
boundary layer, along and south of the warm front. Accompanying the
ejecting shortwave trough/MCV will be mesoscale midlevel speed
maxima -- roughly 55-65 kt at 500 mb and 55-65 kt at 700 mb, as
evident in regional VWP and reasonably progged by models that
resolve the perturbation best. These features should contribute to
a corridor of enhanced vertical shear, both directly via
strengthening/veering wind profiles with height, and through
low-level mass response along the warm front and nearby warm sector.
As these winds shift into the convective transition regime, they
may enhance the damaging-gust threat with the upscale clustering
across southern/central MN as well, near and south of the warm front
where large CAPE still will remain available to storm inflow.
Supercellular tornadoes will be possible in the warm sector, where
LCLs may be lower than forecast by RAP-based models that tend to
overmix a bit. However, this threat is more probable close to the
warm front for any relatively discrete cells and/or QLCS segments
that can interact with the front's backed flow, larger hodographs
and enhanced low-level vorticity. Surface dew points in the upper
60s to low 70s F will support MLCAPE in the 3500-4000 J/kg range
along and south of the warm front, with 40-50-kt effective-shear
magnitudes and 200-400 J/kg effective SRH. The transition toward
more-clustered convective mode should occur as flow aloft backs in
response to the passing perturbation, becoming less orthogonal to
the orientation of the axis of convective lift. The resulting MCS
should have the potential for downward momentum transfer from strong
midlevel flow, while being maintained by forced ascent of very moist
inflow air along and south of the warm front. Activity should
diminish overnight farther east across parts of central/northern WI
and perhaps western Upper MI as inflow-layer instability gets
weaker.
...Southern High Plains...
Widely scattered to scattered thunderstorms are forecast to develop
this afternoon near the dryline, with large hail expected. The
potential also exists for a few smaller-scale clusters to evolve
upscale for a few hours and offer localized concentrations of
damaging gusts.
Initiation and maturation of this convection should be strongly tied
to diurnal heating/lift processes, and should diminish after about
03Z as the boundary layer stabilizes downshear from the convection.
Strong boundary-layer heating/mixing, and resultant presence of
steep low-level lapse rates and favorable DCAPE, will support
maintenance of hail and acceleration of downdrafts to ground.
Peak/preconvective MLCAPE in the 1000-1500 J/kg range will be
attained beneath modest mid/upper-level flow, but amidst enough
veering with height in the wind profile to support 30-35-kt
effective-shear magnitudes. Mostly multicells, with some transient
supercellular structures, are possible, also supporting the threat
for large hail.
...WY/MT/western to central Dakotas...
Isolated to widely scattered thunderstorms are possible late this
afternoon into evening, beginning over areas of modest but sustained
upslope lift in northeastern WY, and spreading/developing
northeastward in sporadic fashion, potentially as far northeast as
the western/central ND/SD border region overnight. The main threats
will be isolated large hail and strong/briefly severe gusts.
Large-scale ascent will increase this evening and overnight across
the region, as the strong shortwave trough pivots out of the Great
Basin and approaches. This will destabilize midlevels, and by
extension with an improved warm-advection response, low levels as
well. Areas of surface heating/mixing may permit surface-based
development late this afternoon over northeastern WY north/northwest
of the surface low, with additional/elevated initiation probable
thereafter. Though buoyancy will be meager (MUCAPE generally
300-800 J/kg), favorable deep shear (effective-shear magnitudes
around 50-65 kt) will favor supercell potential.
Any late-developing activity overnight near the SD/ND line will have
access to somewhat richer low-level moisture, in an elevated
warm-advection plume extending westward from the regime related to
the ENH outlook described above. Elevated MUCAPE may reach
1500-2500 J/kg amidst 50-60-kt effective-shear magnitude, suggesting
a large-hail threat, conditionally risking significant-severe (2+
inches) hail in any sustained activity. Convective coverage
concerns and elevated-initiation uncertainties preclude an upgrade
for that region at this time.
..Edwards/Gleason.. 05/11/2022
Read more
