Valid 252000Z – 261200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A THE CENTRAL
DAKOTAS INTO EXTREME NORTHERN NEBRASKA…
…SUMMARY…
Thunderstorms with isolated large hail and a few damaging gusts are
expected mainly over the central Dakotas this afternoon/evening into
northern Nebraska overnight.
…Discussion…
Primary change to this outlook has been to continue to trim the
eastern bound of the SLGT across ND primarily due to thermodynamic
concerns related to the widespread stratus that has been slow to
erode. Have also dropped the MRGL risk area over the lower MS Valley
region, as much of the airmass is in the process of being overturned
by numerous multicell storms.
Otherwise, expect storms to develop along the pre-frontal trough
over the Dakotas this afternoon. Here, the atmosphere has become
moderately unstable (1500-2000 J/kg MLCAPE), and vertical wind
profiles with 30-40 kt effective bulk shear should support a few
supercell structures, but eventually evolving into multicell line
segments. Reference SWOMCD 1857 for more details.
..Dial.. 08/25/2019
.PREV DISCUSSION… /ISSUED 1124 AM CDT Sun Aug 25 2019/
…Dakotas to NE this afternoon into tonight…
A northern stream shortwave trough will amplify over the northern
High Plains through tonight. An initial surface trough/cold front
across the western Dakotas will continue eastward into the eastern
Dakotas and southeastward into NE by the end of the period. A
narrow zone of surface heating along the surface trough and weak
large-scale ascent will support thunderstorm development this
afternoon/evening across the central Dakotas, and storms will
subsequently spread eastward and southeastward tonight. Steep
midlevel lapse rates will combine with the surface heating and
boundary-layer dewpoints in the mid 60s to result in MLCAPE of
1500-2500 J/kg immediately east of the surface trough this
afternoon.
Deep-layer vertical shear with straight hodographs will initially
favor supercells capable of producing large hail, though upscale
growth into clusters/line segments with damaging winds appears
probable this evening before the convection begins to spread east of
the stronger buoyancy. The southern flank of the frontal convection
is likely to maintain intensity the longest, potentially developing
southward into NE in conjunction with the surface triple point and
warm advection on the nose of 30-40 kt nocturnal low-level jet.
Other more isolated storms will be possible later this evening from
the Big Horns into southwestern SD, in the immediate post-frontal
upslope flow regime. Deep-layer vertical shear will be favorable
for supercells, though limited moisture/buoyancy should tend to
temper the hail/wind threat somewhat.
…OK/AR/MO this afternoon…
A convectively enhanced midlevel trough is moving eastward over
eastern KS and northeastern OK as of mid morning. Vertical shear
will be strongest to the west of the trough in conjunction with the
stronger northwesterly midlevel flow. However, storm initiation
beyond ongoing convection in the warm advection zone near I-35 along
the KS/OK border is unlikely given the expected onset of subsidence
in the wake of the midlevel trough. Some destabilization is
expected south through east of the convective outflow, though clouds
are relatively widespread and will likely continue to slow surface
heating. Overall, it appears that a marginal threat for damaging
wind is the primary concern with multicell clusters along the
outflow boundary this afternoon from southern MO into northwestern
AR and southeastern OK.
$$