Day 1 Severe Weather Outlook

Valid 261300Z – 271200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE LOWER TO MID MISSISSIPPI VALLEY REGION…

…SUMMARY…
Severe storms, with a threat for damaging winds and a few tornadoes,
are possible across parts of the lower to mid Mississippi Valley
region this afternoon through tonight.

…Synopsis…
Around a western CONUS mean trough, a progressive, strongly
amplified synoptic and subsynoptic-scale wave train will exist
through the period. The main features of interest will be a strong,
leading trough now over the central/southern Rockies, and an
initially low-amplitude Pacific shortwave perturbation, evident in
moisture-channel imagery about 400 nm west of the WA coastline.

The Rockies trough will pivot eastward then northeastward across the
central/southern High Plains, lower Missouri Valley and upper
Mississippi Valley through 12Z. An embedded 500-mb low — now
forming over northwestern CO, should deepen somewhat and track
across portions of eastern CO, northwestern KS, southeastern NE, and
IA through the period. As the trough ejects, associated mid/upper-
level gradient flow will become very intense for several hours
overnight, with 06Z jet maxima over northeastern OK attaining 175 kt
at 250 mb and 130-140 kt at 500 mb. The low/middle-level mass
response will be strong as well, with 70-80 kt expected in the
850-700-mb layer across parts of the Ozarks and western TN/KY
regions. Meanwhile, the Pacific perturbation will dig southeastward
and intensify rapidly, leading to landfall of a compact, intense,
low/midlevel cyclone across the southern OR or extreme northwestern
CA coastline around 03Z.

At the surface, 11Z analysis showed a quasistationary front from the
lower Great Lakes across central IN, southern IL, southwestern MO,
and northwestern OK, to an elongated area of low pressure in
southeastern CO. A “Pacific” cold front was drawn from there across
south-central NM and northern Sonora. The surface cyclone should
deepen/consolidate this morning across the southwestern KS/OK
Panhandle region, then eject northeastward across northeastern KS
and IA to WI through 12Z. The “Pacific” cold front should move
southeastward across the southern High Plains and most of KS/OK
through the day, merging with/reinforced by a secondary polar front
through the night. By 12Z, the largely combined cold front should
reach central/eastern IN, western/mid TN, northern MS, southwestern
LA, to near the TX coastal bend and northern/central Coahuila.

…Lower/mid Mississippi Valley and vicinity…
Multiple rounds of thunderstorms are expected along and ahead of the
cold front this afternoon through tonight. Damaging wind and a few
supercellular or QLCS tornadoes will be possible through most of the
outlook area with any of the strongest convection, while the hail
potential is greater in northern parts, under colder air aloft and
steeper midlevel lapse rates.

This presents as a classical cold-season, low-CAPE/strong-shear
scenario with the greatest uncertainties around the periphery, for
different reasons: 1) Northern/eastern fringes, where low-level
airmass recovery and buoyancy will be minimal, but under strong
midlevel destabilization; 2) Western fringes, with timing/
concentration of initiation and early convection in question; and 3)
Eastern/southern fringes, for timing of what should evolve into a
frontal squall line and perhaps separate prefrontal convective band
late overnight, near the end of the period. Deep lift also will
lessen with southward extent toward the Gulf Coast, making the
southern rim of the threat fuzzy.

Convection that is surface-based, or very nearly so, should develop
mainly in two ways:
* Warm-sector storms developing in areas of weakening MLCINH, broad
confluence, increasing boundary-layer theta-e, and strengthening
large-scale forcing for ascent (mainly warm advection), this
afternoon through overnight. More than one belt or swath of
thunderstorms may be involved in this regime from the mid
Mississippi Valley to parts of AR/MS.
* An initially separate band of evening convection forced by
frontal convergence from MO-LA, moving eastward and intensifying as
it impinges on a progressively more moisture-rich (but still
incompletely modified) return-flow layer. Late tonight over
portions of MS/AL, the southern part of this activity may overtake
trailing storms from the first regime.

With surface dewpoints increasing into the 50s over the northern
part of the outlook area and 60s south, peak preconvective MLCAPE
should reach 500-1000 J/kg across much of LA/AR, limited by some
cloud cover and modest lapse rates aloft. Stronger midlevel cooling
will offset weak boundary-layer theta-e in the MO/IL area enough to
support 200-500 J/kg MLCAPE, locally/briefly higher.
Effective-shear magnitudes will reach 50-70 kt over most of the
outlook area.

Aforementioned intense deep-layer flow and convective mechanisms for
some of that momentum to be transferred to the surface will support
the threat for damaging, occasionally severe gusts. More
uncertainty involves the tornado threat. The kinematic parameter
space, with strong deep shear and long, somewhat curved hodographs
with high SRH, conditionally favors significant-tornado potential.
However, modest low-level lapse rates, lack of stronger overall
buoyancy, and questions regarding duration of/transition from
supercell mode, are the main concerns precluding greater
unconditional probabilities at this time.

…Northern CA coast…
A band of convection is expected to form over the Pacific, in a zone
of intensifying deep lift south through southeast of the rapidly
deepening cyclone. Strong gradient winds with gusts potentially
reaching severe levels, largely unrelated to any convective
activity, are likely over coastal waters and near the coast.
Isolated to widely scattered thunderstorms may become superimposed
on already damaging ambient winds. Model-forecast soundings suggest
marine-layer theta-e may support MLCAPE in the 100-300 J/kg range,
effectively making such convection surface-based, despite cool
temperatures. The question is how much convective augmentation will
occur to already strong-severe ambient flow. That potential
contribution remains too uncertain to draw an unconditional severe
outlook at this time, but will continue to be assessed.

..Edwards/Dean.. 11/26/2019

$$

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