Valid 251300Z – 261200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
DAKOTAS AND NORTHWESTERN MINNESOTA THIS AFTERNOON AND EVENING…
…SUMMARY…
Thunderstorms with severe hail and severe gusts are expected over
parts of the Dakotas and northwestern Minnesota this afternoon and
evening.
…Synopsis…
The mid/upper-level pattern will continue to be dominated by a mean
negatively tilted trough from western Canada across the central
CONUS to the lower Mississippi Valley region. The associated belt
of greatest northern-stream cyclonic flow will shift eastward from
the Pacific Northwest across the northern Rockies and northern
Plains through the period, with height falls and difluent flow
spreading over the Dakotas. Several low-amplitude shortwaves are
embedded in that cyclonic-flow field. The leading one — now
apparent in moisture-channel imagery over the central Dakotas —
will move eastward across MN today and this evening, while
weakening. This feature will be followed closely by another over
eastern MT and northeastern WY, and still another initially located
over southern BC and WA. The latter trough will be the strongest by
tonight, while crossing MT.
Farther southeast near the mean trough, a series of slower-moving,
mesoscale trough in midlevels, anchored by MCVs, will populate the
skies. The best-defined MCV in satellite and composited radar
imagery is located over northeastern KS in the MHK/FRI area. The
associated perturbation should move eastward over MO through the
period. Meanwhile, a pre-existing height weakness near the LA Gulf
Coast will phase with the mean trough as heights fall south of the
MCV-related perturbation.
At the surface, 11Z analysis depicted a quasistationary frontal zone
from the Mid-South region across northern AR and southwestern MO,
intercepted by outflow over the southern KS/MO border, then evident
again across east-central portions of NE and SD. Where not
baroclinically overwhelmed by MCS outflow, this boundary should move
little through the period, except for some weak eastward-moving/
warm-frontal behavior over the eastern Dakotas/Red River of the
North region. A weak cold front, analyzed from a low NE of GGW
southwestward across eastern MT, and a prefrontal/lee trough, will
move eastward to western ND during the day.
…Dakotas/northwestern MN…
Scattered thunderstorms are expected to develop this afternoon as
convergence related to the weak front and a surface trough impinge
om a corridor of favorable moisture and buoyancy. A mix of
organized multicells and a few supercells is expected, with large
hail and severe gusts possible, while a tornado cannot be ruled out.
As height falls and cooling/destabilization aloft spread across the
warm sector, diurnal heating will further contribute to steepening
deep-layer lapse rates, atop a plume of 60s F surface dew points
near and west of the eastern frontal zone. Modified RAOBs and model
soundings indicate that the peak MLCAPE — behind the morning
clouds/convection related to the initial perturbation — should
reach 2000-2500 J/kg, despite patchy cloud cover. 35-45 kt
effective-shear magnitudes will support convective organization, as
will strong upper-level/ventilating winds. Low-level flow will be
modest and, in much of the area, veered west of southerly, limiting
hodograph size. Overall weakening of convection is expected late
evening into the overnight hours as remaining activity moves deeper
into/atop a progressively more-stable boundary layer within and east
of the slow-moving surface warm-frontal zone. In each outlook
level, slightly more room as been allotted for this process to occur
into northwestern MN.
…Southern Plains to Ozarks region…
Isolated gusts near severe limits still may occur with ongoing
convection moving southeastward from the END area over central OK,
before it weakens. See SPC mesoscale discussion 1856 for additional
near-term details.
Otherwise, widely scattered to scattered thunderstorms are expected
to develop through this afternoon over portions of the Ozarks and
vicinity, amidst rich low-level moisture (70s F surface dew points,
roughly 2-inch PW) and minimal MLCINH. Activity should be focused
along and ahead of the outflow boundaries from the morning MCS
activity over portions of KS/OK. A general westward increase in
midlevel lapse rates, combined with surface diabatic heating and
aforementioned moisture, should contribute to MLCAPE ranging from
around 3500 J/kg over parts of eastern OK to 1000-1500 J/kg in the
frontal zone across the Ozarks region. Although some large-scale
support may be provided in the form of weak DCVA and/or mesobeta-
scale midlevel gradient-flow enhancement around the MCV(s), overall
organization of the convection should be loose and multicellular in
nature given weak low-level shear and modest mid/upper winds in most
of the region. Damaging to severe gusts are possible in strongly
water-loaded downdrafts, but the unconditional severe threat appears
isolated and marginal.
..Edwards/Gleason.. 08/25/2019
$$