SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0747 AM CDT Mon Aug 16 2021
Valid 161300Z - 171200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WITH A TORNADO
THREAT OVER PARTS OF THE FLORIDA PANHANDLE INTO SOUTHEAST ALABAMA
AND SOUTHWEST GEORGIA...
...SUMMARY...
The threat for a few tornadoes should increase across the Florida
Panhandle into southeast Alabama and southwest Georgia today. Other
storms may produce isolated severe wind gusts and some hail from
parts of the central and southern High Plains into the Southwest.
...Synopsis...
The main belt of midlatitude westerlies aloft will remain north of
the CONUS this period, except across portions of the Pacific
Northwest. A series of 500-mb vorticity maxima over southern/
western BC, and the northeastern Pacific west of BC, will phase late
today and tonight while moving southeastward. By 12Z, a strong,
positively tilted shortwave trough should result from the Canadian
Rockies across south-central BC, northern/western WA and
northwestern OR, with a weaker perturbation to the south over parts
of northern CA and northwestern NV. In response to these
developments, height falls will spread southeastward across much of
CA and the northern/western Great Basin, with the ridge breaking
down over the southern Great Basin and desert Southwest. A
collection of weak vorticity maxima and shortwaves -- evident in
moisture-channel imagery over CO, NM, AZ and eastward across KS --
should move generally southward over the remainder of CO/KS, the
southern High Plains, and AZ/NM through the period.
At the surface, a wavy, quasistationary frontal zone was analyzed at
11Z near an ILG-HTS-OWB-LIT-TUL line, becoming a warm front
northwestward over central KS. Lee troughing will persist over the
High Plains today, with mostly weak surface flow on either side. A
cold front associated with the strengthening mid/upper trough will
move southeastward through the Pacific Northwest and northern
Rockies, reaching north-central MT, southwestern IA and northern CA
by 12Z.
...Northeastern Gulf Coast -- T.S. Fred...
See SPC tornado watch 441 and related mesoscale discussions for the
latest near-term coverage of the tornado threat with T.S. Fred. The
center of the cyclone is forecast by NHC to reach the FL Panhandle
coastline near PFN sometime early this evening, then move inland and
decay. Fred's relatively small circulation has a northeastward tilt
with height, as well as a decidedly asymmetric, northeastward-skewed
convective field, related physically to strong deep (850-200-mb)
shear impinging on the system. This should continue, rendering the
bulk of broader-scale to convective-scale ascent, deep convection,
favorable low-level hodographs, and potentially supercell-bearing
convective bands in that sector arching from inner-north through
middle/outer southeast of center, as often seen in such deep-shear
settings.
Diurnal heating will be significantly limited over the area of
most-suitable lift and low-level shear, given the thickness and
northeastward spread of Fred's cloud shield. As such,
destabilization will be optimized more by advective processes and
moisture transport, in the most purely maritime/tropical,
highest-theta-e air mass. This air mass should spread inland to
within about 100 nm of the coast through this evening. Potential
will be more marginal with inland extent from there tonight due to
weaker instability and smaller low-level shear with northward
extent.
...AZ...
Widely scattered to scattered thunderstorms are expected to form
this afternoon and early evening over the Mogollon Rim and nearby
higher terrain, once enough time has elapsed for favorable
destabilization from residual cloud cover left behind from
convection last night and this morning. Isolated strong/locally
severe gusts and hail are possible early in the convective cycle.
Some of this activity may aggregate outflow as it moves southward to
southwestward off the higher terrain, evolving a discretely
forward-propagational cold pool that would concentrate wind
potential on the mesoscale. Moisture will remain favorable across
the area, beneath a deep layer of initially northeasterly midlevel
winds that should become more northerly with time. Peak
preconvective MLCAPE in the 1500-2000 J/kg range should develop,
locally higher. If confidence increases in a mesoscale corridor of
greatest severe-wind threat, a 15%/slight area may be introduced in
a succeeding outlook.
...Central Plains to southern High Plains...
Isolated to scattered thunderstorms are possible this afternoon and
evening -- both near the lee trough and around outflow/
differential-heating boundaries related to clouds/precip ongoing
this morning. Convection should move predominantly southward,
offering isolated severe gusts/hail. Some localized clustering and
concentration of severe potential may occur, conditional on
mesoscale boundary effects. Activity will be encouraged by
steepening low/middle level lapse rates and weakening MLCINH,
related mainly to boundary-layer heating, but also to subtle
large-scale lift in midlevels associated with the perturbations
aloft. With surface dew points commonly in the 60s F, areas of
most-sustained diurnal heating will foster preconvective MLCAPE
values of 1500-2500 J/kg over much of the Plains part of the outlook
area. Although strong veering with height will be noted from
surface through midlevels, weak absolute speeds will keep deep-shear
values and hodograph sizes small for the most part. Still, brief
supercell structures may develop where favorably oriented boundaries
provide an augmenting source of low-level vorticity. Otherwise,
multicell modes should prevail, with some clustering and localized
cold-pool generation possible.
..Edwards/Kerr.. 08/16/2021
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