SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Thu Oct 14 2021
Valid 141300Z - 151200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
NORTHEASTERN ARKANSAS TO THE LOWER OHIO VALLEY REGION...AND
SEPARATELY ACROSS PORTIONS OF OKLAHOMA...
...SUMMARY...
A few strong/marginally severe thunderstorms are possible today from
northeastern Arkansas to the lower Ohio Valley region, and later
this evening across portions of Oklahoma.
...Synopsis...
While remaining highly amplified, the pattern aloft will shift this
period, primarily related to the longstanding mid/upper-level
troughing now centered over the Rockies. An amplifying shortwave
trough -- initially evident in moisture-channel imagery over eastern
OR and northern NV -- will move southeastward to the Four Corners
vicinity by 00Z. This trough then should pivot eastward across the
central/southern Rockies to the adjoining High Plains by 12Z
tomorrow. Downstream, a compact cyclone now centered near the
northwestern corner of MN will eject roughly northward on a wobbly
path toward northern MB by the end of the period. In the southern-
stream southwest-flow field southeast of the major trough, a
perturbation related to the remnants of Pamela aloft will eject from
south TX across the Tennessee Valley and weaken considerably.
The surface analysis at 11Z showed a cold front related to the
ejecting MN/ND/MB cyclone, extending across portions of eastern WI,
northwestern IL and central MO, becoming quasistationary
southwestward to a weak frontal-wave low over the TX Panhandle. The
front was preceded by a swath of multilayered clouds, precip and
some embedded thunderstorms from the southern Lake Michigan area
southwestward across MO to south TX.
By 00Z, the cold front should reach parts of Lower MI, central IN
and southern IL. The boundary should stall today and then move
slowly northward as a warm front across parts of southwestern MO and
southeastern KS, to a frontal-wave low over south-central KS, then
to another frontal-wave low over northwest TX. By 12Z, the KS low
should migrate roughly eastward over MO and southern IL, with the
trailing boundary again a cold front across southwestern MO,
eastern/southern OK, and west-central/southwest TX.
...AR to the lower Ohio Valley region...
Widely scattered to scattered thunderstorms are expected to move
northeastward across this region today, both in the existing, slowly
eastward-shifting plume and developing anew near its leading edges
(where best access to diurnally destabilized boundary-layer air
should be). Damaging to isolated severe gusts are possible, and a
marginal/brief tornado potential is apparent.
A corridor of moisture advection/transport will move over the area
today, sourced from the mid/upper remnants of Pamela over south TX,
as well as rich moisture from the Gulf in low levels. The result
should be a weakly capped boundary layer with modest but still
somewhat unstable low/middle-level lapse rates, and strong upper/
anvil-level flow. Forecast soundings show effective-shear magnitudes
around 30-40 kt, suggesting mainly variably organized multicells,
with sporadic/transient supercell potential. Cloud cover will keep
diurnal heating from yielding steep low-level lapse rates, but
MLCAPE in the 500-1000 J/kg range will be supported by the abundant
moisture. Elsewhere in the main precip/convective plume from south
TX to Lower MI, a strong/damaging gust or brief tornado cannot be
ruled out; however, with weaker lift and storm coverage south, and
weak instability north, the threat is too conditional, isolated and
poorly focused for a large categorical severe area.
...OK...
Capping may delay substantial thunderstorm initiation and growth
until after 00Z -- perhaps even as late as 03-06z. However,
isolated to scattered thunderstorms this evening into tonight will
offer a threat for hail and strong gusts. Marginal, weak tornado
potential also is apparent along/ahead of the front, given the
strengthening deep shear and available moisture.
Large-scale lift should increase over the region this afternoon and
tonight with the approach of the main mid/upper trough, reinforced
by the digging shortwave perturbation. The returning warm sector
has been altered somewhat unfavorably by extensive precip/convection
upstream to the south, yet still should contain enough theta-e to
support strong/isolated severe potential as it is lifted along and
over the surface front. Even with a shallow/near-surface stable
layer apparent in forecast soundings, MLCAPE of 500-1200 J/kg
appears possible in the warm sector, along with similar values of
elevated MUCAPE within about 50-100 nm behind the boundary.
500-250-mb layer winds will strengthen over this area from late
afternoon into tonight, contributing to 50-60-kt effective-shear
magnitudes in support of supercell potential. Limiting factors for
the severe threat will include lack of greater low- and middle-level
lapse rates tonight, limiting buoyancy and near-surface acceleration
potential for both updrafts and downdrafts.
..Edwards/Gleason.. 10/14/2021
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