SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Thu May 12 2022
Valid 121300Z - 131200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
EASTERN SOUTH DAKOTA...PARTS OF WESTERN MINNESOTA AND SOUTHEASTERN
NORTH DAKOTA...
...SUMMARY...
Severe thunderstorm gusts (some near 75 mph), large hail and a few
tornadoes are expected today over parts of the eastern Dakotas,
eastern Nebraska, western Iowa, and central/southern Minnesota.
...Synopsis...
The mid/upper-level pattern will feature mean troughing over the
western CONUS, a lengthy but weakening ridge from west-central MX
across the Arklatex to southern ON, and a broad/retrograding
Atlantic cyclone, forecast to move westward and ashore over much of
the southeastern Atlantic Coast overnight. Within the southwest-
flow field preceding the western larger-scale trough, an intense
shortwave trough is apparent in moisture-channel imagery from near
the ID/WY border across eastern UT. This perturbation, with a
negative tilt and embedded low apparent near the southwest corner of
WY -- is expected to eject northeastward through the period,
reaching southeastern MT, western SD and the NE Panhandle by 00Z.
By 12Z tomorrow, a well-defined 500-mb low should be apparent near
the ND/SK/MB border confluence, with trough southeastward over
southeastern SD.
The 11Z surface analysis depicted a surface low over southwestern
NE, with cold front across northern CO, and warm front over
southeastern SD, extreme southern MN, and southern WI. The low
should move northeastward to a frontal triple point over
northeastern SD by 00Z, with an occluded low developing farther
northwest and near the mid/upper cyclone center over northwestern
SD/southwestern ND. By then, the warm front -- likely reinforced by
outflow from earlier/morning convection to its north -- should
extend from the triple point across southeastern ND, north-central
MN, northern WI and the western U.P. of MI. The cold front should
extend across eastern SD (likely behind a line of convection),
south-central NE, western KS, and southeastern CO. A dryline will
intersect the front over west-central KS, extending
south-southwestward over the eastern TX Panhandle, the Permian Basin
and the Big Bend region with isolated strong-severe convection
possible late this afternoon. By 12Z, the triple-point low should
lose definition, as the western low deepens and becomes nearly
stacked with its midlevel counterpart. The cold front should extend
across northern, central and southwestern MN, northwestern IA,
southeastern NE, central KS, northwestern OK, the southern TX
Panhandle, and southeastern NM.
...North-central Plains/Upper Midwest...
Scattered to numerous thunderstorms are expected to develop in an
arc near the surface low and cold front by mid/late afternoon, from
eastern SD across central/eastern NE and into at least northern KS.
A brief interval, early in the convective cycle, may support
discrete to semi-discrete supercells before the convection becomes
quasi-linear, and that accounts for potential for significant-severe
hail in western parts of the outlook area. However, the most common
severe type should evolve quickly to thunderstorm gusts -- some of
which may be significant (65+ kt) strength, especially from parts of
eastern SD and southeastern ND into western MN.
The addition of the significant-wind area technically triggers a
"moderate" categorical level, though the overall scenario hasn't
changed in a major way from that discussed in the previous outlook.
A few tornadoes also are possible -- especially near the surface
warm front, where large buoyancy, backed near-surface winds and
enlarged hodographs/SRH will yield the most favorable parameter
space. The main uncertainty regarding the density and intensity of
the tornado threat involves convective mode, which may be largely to
entirely quasi-linear by the time activity encounters the largest
combination of low-level buoyancy/shear with surface-based inflow
parcels.
The warm sector is forecast to destabilize throughout the day, with
a combination of at least weak large-scale ascent (increasing
northward), diurnal heating and related lift from below, and
ultimately frontal convergence, eliminating a basal EML inversion
and supporting convection initiation. Activity is expected to
intensify quickly as it impinges on a narrow but very favorable
prefrontal corridor where 60s to near 70 F surface dewpoints
contribute to MLCAPE in the 3500-4500 J/kg range in and near the
"moderate" area, decreasing gradually with southward extent as more
low-level moisture is mixed out. Low-level and deep shear will be
greatest near the triple point and warm front, with 300-500 J/kg
effective SRH and 45-55-kt effective-shear magnitudes, also
decreasing southward toward KS. A substantial component of
mid/upper winds parallel to the axis of convective forcing indicates
potential for fairly fast merging of early discrete and sporadically
supercellular convection, forming a QLCS. Surges of wind from
resulting LEWP/bowing segments will pose the greatest overall severe
hazard, with line-embedded mesovortices and perhaps a few associated
tornadoes also possible. With the warm sector's not being very
broad, the convective event should diminish late this evening into
early overnight hours as it outruns the most favorable instability.
...Central/southern MS and vicinity...
Widely scattered thunderstorms should develop this afternoon into
early evening, in a moist, well-heated and weakly capped
environment, along and west of a nearly north-south low-level
baroclinic zone separating the lower-theta-e air related to the
cyclone from the ambient Gulf warm sector. Activity should move
generally southwestward, offering isolated severe downbursts.
As the mid/upper low moves toward the Atlantic Coast, and associated
cyclonic flow spreads across more of the Southeast, a channel of
enhanced northeast flow aloft (40-50 kt at 500 mb, 75-85 kt at 250
mb) will shift slowly westward across GA/AL. While the outlook area
will be off the western rim of the strongest flow aloft, increasing
winds are expected to support seasonally fast southwestward movement
of multicellular clusters forming on the boundary and in the nearby
warm sector. MLCAPE of 1500-2000 J/kg (locally higher) should
develop atop a well-mixed boundary layer supporting downdraft-
acceleration potential and strong/isolated severe gusts.
..Edwards/Gleason.. 05/12/2022
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