SPC AC 191216
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0716 AM CDT Sat Aug 19 2017
Valid 191300Z - 201200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON INTO EARLY TONIGHT FOR PARTS OF CENTRAL NE AND NORTHERN
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
NE/KS SLIGHT RISK AREA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
FROM EASTERN OH TO CENTRAL PA...
Scattered severe thunderstorms are possible across parts of Nebraska
and Kansas late this afternoon and through the evening. Isolated
severe storms are possible from Ohio into central Pennsylvania
during the afternoon and ending by the early evening.
...Central Plains area this afternoon through tonight...
Lee cyclogenesis is expected later today across the central High
Plains, along the southern fringe of the stronger mid-upper
westerlies. This will induce strengthening low-level southerly flow
and associated northward moisture transport from KS/OK to NE by this
afternoon. Strong surface heating near the weak lee cyclone/trough
and along the west edge of the returning moisture will lead to
weakening convective inhibition by mid-late afternoon. Near this
same time, a low-amplitude shortwave trough (over south central WY
this morning) will reach western NE, and thunderstorm development is
expected by late afternoon.
A thermodynamic environment characterized by steep low-midlevel
lapse rates, MLCAPE of 2000-3000 J/kg, and DCAPE approaching 1500
J/kg will favor storms with both strong updrafts and downdrafts.
Effective bulk shear near 30 kt will favor some marginal supercell
structures early in the storm evolution. Upscale growth into a
cluster is expected this evening/early tonight as a result of storm
mergers/interactions, as well as increasing low-level warm advection
with a strengthening low-level jet. A few large hail and
strong/damaging outflow instances can be expected this evening into
early tonight, prior to storms slowly weakening by 04-06z.
...Eastern OH to central PA this afternoon...
A pronounced midlevel trough over the mid MS and lower OH Valley
regions will progress eastward to the central Appalachians by this
evening. In the wake of weak morning convection, scattered
thunderstorms will form in advance of the midlevel trough and along
a diffuse surface front early this afternoon across eastern
OH/northwestern PA, and then spread eastward across central PA
through the afternoon. Residual boundary layer dewpoints in the 60s
beneath relatively cool midlevel temperatures (-12 C at 500 mb), as
well as surface heating in cloud breaks, will contribute to MLCAPE
of 500-1000 J/kg this afternoon. With midlevel westerly flow and
effective bulk shear near 30 kt, a few organized clusters could
produce strong outflow gusts and marginally severe hail.
...NC coast this afternoon...
Scattered thunderstorm development is expected this afternoon along
a surface trough/weak cold front across coastal NC, aided by local
sea breeze circulations. The 12z MHX sounding revealed rather poor
lapse rates and modest DCAPE around 600 J/kg, despite MLCAPE near
2500 J/kg. Though an isolated downburst cannot be ruled out, the
overall damaging-wind threat appears too low to introduce a 5% wind
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