SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Mon May 09 2022
Valid 091300Z - 101200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE UPPER
MISSISSIPPI VALLEY REGION...
...SUMMARY...
Large hail, damaging gusts, and a few tornadoes are all possible
over the upper Mississippi Valley region today into this evening.
...Synopsis...
The basic mid/upper-level pattern through the period will feature a
positively tilted mean trough over western parts of Canada and the
CONUS, with numerous embedded shortwaves/vorticity maxima.
Meanwhile, an already high-amplitude downstream ridge -- now
extending from central MX across the Arklatex, lower Ohio Valley,
southern ON, and southern QC -- will amplify further, while moving
little. A long-lived synoptic cyclone will remain over Atlantic
waters east of the Carolinas.
The southwest-flow belt downstream from the western mean trough will
cover most of the Rockies, Great Plains, mid/upper Mississippi
Valley, and Upper Midwest. An embedded/leading shortwave trough
with convective vorticity augmentation -- now apparent in
moisture-channel imagery and composited radar fields over SD -- will
eject northeastward across ND and northern MN today. By 00Z, a
closed 500-mb low should develop along the shortwave trough, over
southern MB, with the trough extending southeastward to northern MN.
The perturbation will eject northeastward over MB and
northwesternmost ON tonight, leaving behind rising heights across
the central/northern Plains and Upper Midwest.
At the surface, 11Z analysis showed a low over northern NE near ONL,
with cold front southwestward across northwestern KS, becoming
quasistationary over east-central CO. A warm front was drawn
southeastward across eastern NE, northeastern KS, southwestern MO,
and eastern AR. A dryline intersected the cold front over southern
NE, and extended southward to western OK, then south-southwestward
across the TX Permian Basin. That low is expected to move
north-northeastward across southeastern MB by 00Z, with cold front
trailing through eastern MN, southwestern IA, southeastern NE, and
northwestern KS, to another low related to both lee troughing and
baroclinic processes over northeastern CO. By 12Z, the cold front
should be decelerating across parts of Lake Superior, Upper MI, WI,
and southern IA, then quasistationary to a low over central or
north-central KS, then a cold front again southwestward across parts
of the OK/TX Panhandles. By mid/late afternoon, the dryline should
shift eastward to eastern KS, central OK, western north TX, and the
Edwards Plateau, before retreating westward/northwestward overnight.
...Upper Mississippi Valley region...
Multiple convective episodes are expected over the region through
late evening, collectively contributing to the potential for large
hail, damaging gusts and a few tornadoes.
The initial episode is manifest as an arc of strong/isolated severe
thunderstorms with mainly a hail threat over portions of
northeastern SD, southeastern ND and west-central/southwestern MN.
This activity is occurring in a zone of strong low-level warm
advection and moisture transport, occurring as part of the mass
response to the deepening low-level cyclone. See SPC mesoscale
discussion 697 and severe-thunderstorm watch 192 for near-term
details. For at least a few hours, this activity will move over a
relatively stable boundary layer left from the nocturnal
near-surface cooling, reinforced by precip. However, with favorable
effective shear and buoyancy above the boundary layer, sporadic
large hail will remain possible. A conditional threat will develop
for some of this complex's gusts to penetrate to the surface later
this morning, as a combination of warm advection, vertical mixing
and weak diurnal heating destabilize the boundary layer.
Behind the initial activity, a narrow, fairly fast-moving plume of
superimposed low- and middle-level destabilization may develop this
afternoon. Activity could be strongly tilted amidst effective-shear
magnitudes of 60-75 kt, based on modified forecast soundings.
Meanwhile, even with only meager low-level airmass recovery via warm
advection and brief diurnal heating, cooling aloft should steepen
deep-layer lapse rates enough for surface-based convection as far
north as at least some of northern MN, with weak CINH and frontal
lift forcing convection. Offering specifics on buoyancy always is
a low-predictability endeavor in these limited-recovery, fast-flow,
"bent-back arc" scenarios. Still, 250-500 J/kg MLCAPE appears
possible in a narrow plume just ahead of the front over northern MN,
increasing to 1500-3000 J/kg in a narrow corridor shifting eastward
across southern MN and western WI. This may support a mix of
multicells and supercells in an arc over MN, perhaps
shifting/backbuilding into northwestern WI. A few tornadoes,
occasional hail, and strong-damaging gusts all will be possible.
Either backbuilding of the MN/WI arc, or initially separate
development, will be possible late this afternoon into this evening
along the cold front, over parts of WI into IA, also offering a
multi-hazard severe threat. Boundary-layer moisture and lapse rates
will be most favorable in this regime, but with only glancing
influence of large-scale support aloft from the shortwave trough,
before large-scale subsidence increases. Still, a line of
convection may build along the boundary for a few hours, as frontal
lift takes advantage of MLCINH minimized by both theta-e advection
and residual diurnal warmth. 35-50 kt effective-shear magnitudes
suggest blended supercell/multicell modes within the convective
band, with well-mixed subcloud layers supporting survival of
strong-severe gusts and large hail to the surface, before activity
diminishes overnight.
..Edwards/Gleason.. 05/09/2022
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