LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0756 AM CDT Fri Aug 11 2023
Valid 111300Z - 121200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MID/UPPER
MISSISSIPPI TO LOWER MISSOURI/OHIO VALLEYS...
...SUMMARY...
Severe thunderstorms capable of large hail, damaging to severe gusts
and perhaps a tornado or two, are possible across portions of the
Mid and Upper Mississippi Valley and Lower Missouri/Ohio Valley
areas today.
...Synopsis...
In mid/upper levels, an elongated anticyclone from central TX across
the northern Gulf will strengthen slightly through the period. This
will lead to net height rises -- but also stronger mid/upper-
tropospheric gradient winds -- in a belt of northwest flow extending
from the northern Plains across the southern Upper Great Lakes
region. The main perturbation in this flow belt is a strong
shortwave trough, apparent in moisture-channel imagery over SK, with
a large field of difluent flow downstream across the northern U.S.
Plains and Upper Midwest. The trough is forecast to reach MN and
northwestern ON by 00Z, then eastern Upper to Lower MI and
northwestern IN 12 hours later.
A weaker preceding trough -- initially located from an MCV over
northwestern WI south-southwestward to northeastern MO -- should
shift eastward across most of Lower MI by 00Z. Farther south, even
weaker (yet still potentially consequential), convectively induced/
enhanced perturbations were moving southeastward across AR, and from
AL into portions of western/northern GA.
At the surface, 11Z analysis showed a low over northern MN with cold
front southwestward across western KS, to another low over the
northern TX Panhandle. A warm front was drawn from each low east-
southeastward, across OH and the Arklatex region respectively. The
southern boundary was interrupted by outflow from an MCS over
central AL, then became apparent again over southern/central GA.
This front should move slowly northward through the day over GA/AL
and into parts of SC. Meanwhile, the cold front will move eastward
over the upper Mississippi Valley into this evening, then parts of
Lower MI by 12Z tomorrow, in step with the eastward shift of the
surface low to its north.
...Mid/upper Mississippi to lower Missouri/Ohio Valleys...
An ongoing band of thunderstorms over parts of northern/western IL
may continue to pose a threat of locally damaging to isolated severe
gusts, as it moves southeastward through the reminder of the
morning, south of the northern warm front. See SPC mesoscale
discussion 1942 for near-term guidance. The trailing outflow
boundary should shift back northward by early/mid afternoon over the
southern IA/northern IL area.
Isolated to scattered strong-severe thunderstorms, with some
clustering, are possible this afternoon and evening from eastern KS
and the Ozarks northward to the upper Mississippi Valley, starting
this afternoon. Large hail and severe gusts both are possible from
more-discrete, early-stage convection, and a conditional
supercellular tornado threat may develop near a retreating outflow
boundary across the southern IA/northern IL region.
The severe threat should transition mainly to wind for any activity
that can organize upscale and grow a forward-propagational,
cold-pool-driven process. The most probable part of the outlook for
a longer-lasting, organized MCS of that nature may be southern areas
where inflow-layer theta-e is greatest: somewhere between eastern
KS and either the Mid-South or lower Ohio Valley tonight. The
western part of the greater boundary-layer moisture over central/
eastern KS will be characterized by strong diurnal heating/mixing
and a deep boundary layer, beneath steep midlevel lapse rates
supporting 2500-4000 J/kg MLCAPE. Though shear will be modest,
strong upper/anvil-level flow should aid in storm organization. To
the extent later guidance and mesoscale trends converge on a
more-specific corridor, a channel of greater severe-wind
probabilities may need to be drawn within the broader area.
Farther north -- near and north of the outflow boundary from the
morning activity -- stronger deep shear and weaker buoyancy are
expected, but with instability still sufficient to support at least
isolated strong-severe convection. MLCAPE should range from around
3000 J/kg just south of the boundary to 1000-2000 J/kg in parts of
central/eastern MN and northern WI, under the strongest cooling
aloft preceding the mid/upper trough. 40-50 kt effective-shear
magnitudes are progged over the upper Mississippi Valley region as
well.
...Southeastern CONUS...
Isolated to numerous thunderstorms are possible in highly variable
patches of coverage today, with damaging gusts as the main threat.
Isolated, marginally severe (50+ kt) gusts also may occur.
A broad swath of warm-sector air with rich low-level moisture (e.g.,
surface dewpoints commonly in the 70s F) will be heated diabatically
to convective temperature, minimizing MLCINH and boosting MLCAPE to
3000-4000 J/kg. Weak, nearly unidirectional low/middle-level flow
will serve two detrimental purposes: reduction of convergence and
maintaining weak vertical shear. Still, given the large amount of
moisture and buoyancy, water-loaded downdrafts will pose a local
damaging- to severe-gust threat, and perhaps broader in scale if
cold-pool aggregation/organization can build sufficiently upscale.
Any such process(es) may yield one or two relative concentrations of
potential today, depending on the downshear evolution of MCV/
trough-related large-scale lift and mesobeta-scale shear
enhancements from the perturbations now associated with currently
non-severe convective complexes over AR and AL/GA activity.
However, potential still appears rather nebulously focused, and
marginal in environmentally supported gust magnitude, for a 15%
unconditional-wind area this outlook cycle.
..Edwards/Dean.. 08/11/2023
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