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ACUS48 KWNS 280827
SWOD48
SPC AC 280826
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0326 AM CDT Sat Apr 28 2018
Valid 011200Z - 061200Z
...DISCUSSION...
Models are in relatively good agreement for the D4-D6 period with
the evolution of an upper trough as it moves from the southwestern
states across the Plains.
On Tuesday/D4, the main upper jet energy will be located over
northern Baja CA, with a broad belt of moderate southwest flow aloft
across the Plains. Some height rises will occur over the Plains as
this western trough digs, but small perturbations within the flow
may exist. Low pressure is forecast to exist along the CO/KS border
during the afternoon, with a dryline from central KS across western
OK and into west-central TX. Low-level moisture will be greater than
on the previous day, with mid 60s F dewpoints across OK, and upper
60s F south of the Red River. Lowers 60s F will exist farther
northward toward the upper MS Valley, where a secondary low will
exist near the MN Arrowhead. Strong heating will occur west of the
dryline, with a plume of low-level lapse rates expanding northeast
into eastern NE/western IA. Storms are likely to form along the
front over NE and IA, in the form of supercells and/or an eventual
MCS, then moving across northeast KS and northwest MO. Severe wind,
hail, and a few tornadoes will be possible. Farther south, a
conditional threat will exist along the dryline. Wind profiles will
clearly favor supercells, but rising heights late in the day and a
retreating dryline suggest a less than favorable large-scale
environment. Still, isolated cells are expected with large hail and
perhaps a tornado or two.
On Wednesday/D5, cooling aloft along with strong winds will emerge
into the southern and central Plains. A dryline will extend from a
southwest KS low across western OK into west-central TX, with upper
60s F dewpoints. Meanwhile, the synoptic front may be reinforced by
outflow across northeast KS and northwest MO. Strong instability
will exist across the warm sector with MUCAPE of 2000-3000 J/kg from
TX into OK and southern KS. Timing of storms in KS is in question
due to possible outflow, and some models suggest midday activity is
possible. However, the greatest threat will be from the low
southward along the dryline, from KS into OK, where a few tornadic
supercells are expected, along with very large hail supported by
steep lapse rates aloft and robust boundary layer moisture. Some
tornadoes may be strong, although forecast hodographs do vary among
the models.
By Thursday/D6, the upper trough will lose amplitude and take on a
positive tilt, with strong flow aloft extending from the southern
High Plains toward the Mid and Upper MS Valley. At the surface, the
front is expected to extend from northern MO across central OK into
central TX, however, much uncertainty exists due to the possibility
of widespread thunderstorms that could accelerate the front and
scour the air mass. Thus, while some severe threat may persist into
D6, predictability is too low to attempt a severe area. Storms will
likely continue in some fashion across TX into the lower MS Valley
on D7, likely with a lesser severe threat than on previous days.
..Jewell.. 04/28/2018
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