Valid 151300Z – 161200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
INTO TONIGHT FROM PARTS OF KS TO SD AND WY…
…SUMMARY…
Scattered severe storms are expected across parts of the central
Plains through tonight. Large to very large hail and severe gusts
are forecast with the stronger storms.
…KS to SD and WY through tonight…
A thunderstorm cluster in central NE has been maintained this
morning by warm advection on the nose of a 30-40 kt low-level jet
and frontogenesis in the 850-700 mb layer. This convection will
likely persist into early afternoon while moving southeastward into
southeast NE/northeast KS. Convective evolution thereafter is
somewhat in question, with the possibility of the storms weakening,
or for additional development by mid afternoon along the southwest
flank of the outflow across central KS. The latter scenario would
continuously reinforce the cold pool and shunt the severe hail/wind
threat to the south. If the convection weakens across northeast KS
by mid afternoon, there will be potential for redevelopment on the
edge of the modifying cold pool by this evening. The potential for
supercells with very large hail and a tornado or two will depend
heavily on the MCS weakening by early afternoon, and then the cold
pool having several hours to modify by this evening such that
surface-based storms can form along the edge of the remnant cold
pool, where low-level shear will be strongest.
The MCS should stabilize central NE enough to lower the severe
threat through the afternoon/evening. Outflow will reinforce a
baroclinic zone and upslope flow into WY this afternoon, though the
boundary layer will likely remain relatively cool/capped. Late this
evening, isolated storms may form in upslope flow regime, in an
environment conditionally favorable for supercells with large hail.
Farther north in SD, some moistening and surface heating
west-northwest of the morning convection could result in sufficient
destabilization for a few severe storms this afternoon. The focus
for thunderstorm development will be a weak surface trough in
advance of a midlevel shortwave trough moving east-southeastward
from MT. Assuming MLCAPE can increase to 1500 J/kg this afternoon,
deep-layer shear will be sufficient for organized/supercell storms
capable of producing large hail and damaging winds from about
20-00z.
…OH/PA area this afternoon/evening…
A series of low-amplitude speed maxima aloft will progress eastward
this afternoon from roughly northern IN to western PA. Similar to
the previous two days farther west, surface heating within a
residual moist boundary layer and a weak surface low/trough, plus
lake-enhanced boundaries, will help focus thunderstorm development
this afternoon from eastern OH/WV into central PA and far western
NY. Though low-level flow/shear will be weak, there will be
sufficient deep-layer vertical shear for a low-end supercell threat,
based on moderate buoyancy (MLCAPE near 1500 J/kg). A few damaging
winds and isolated large hail will be the main threats.
..Thompson/Kerr.. 08/15/2019
$$