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ACUS01 KWNS 101304
SWODY1
SPC AC 101303
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0703 AM CST Fri Dec 10 2021
Valid 101300Z - 111200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
ARKANSAS AND NORTHERN MISSISSIPPI TO CENTRAL ILLINOIS AND
SOUTHWESTERN INDIANA...
CORRECTED FOR HEADLINE
...SUMMARY...
Severe thunderstorms -- with tornadoes, damaging gusts and isolated
large hail -- are likely this evening and overnight from parts of
the lower Ohio Valley and adjacent states through the middle and
lower Mississippi Valley region. A few of the nighttime tornadoes
may be strong.
...Synopsis...
A series of mid/upper-level shortwave troughs are phasing in the
western CONUS, with deepening of a major synoptic trough and related
increase in cyclonic flow aloft. the strongest of the shortwave
troughs -- apparent in moisture-channel imagery over portions of ID
and the northern Great Basin -- will dig southeastward across the
Four Corners to the central/Southern Rockies by 00Z, then pivot
eastward and northeastward. By 12Z tomorrow, the 500-mb trough
should be located from the upper Mississippi Valley to eastern KS
and northern OK, near the axis of the synoptic trough extending
southwestward from there over northwestern MX.
The 11Z surface analysis showed a frontal-wave low near the southern
CO/KS border, with a warm front arching across south-central/
southeastern KS and central portions of MO/IL. A separate warm-
frontal zone -- signifying the inland extent of a strongly modified
Gulf air mass -- was drawn across central/north-central TX to
central portions of AR, southeastward over central MS and
southwestern AL. The latter boundary should shift northward and
gradually become diffuse through the day, merging from west to east
with the synoptic warm front initially to its north. The eastern
rim of the Gulf air will remain somewhat distinct through tonight,
in an eastward-moving corridor from AL northward. Meanwhile, the
low should deepen continuously while following a curving path
eastward across KS to northwestern MO today, reaching northern Lower
MI by the end of the period. By 00Z, the cold front should reach
southeastern KS and western OK, with warm front moving northeastward
over parts of IL/IN/OH. By 12Z, the cold front should extend across
parts of Lower MI, southern IN, western KY, northwestern MS, central
LA, and south TX.
...Lower/mid Mississippi and Ohio Valleys and vicinity...
Scattered to numerous thunderstorms are forecast along and ahead of
the cold front tonight, offering the full spectrum of severe
weather. The main concerns will be for occasional tornadoes (some
potentially capable of EF2+ damage) and scattered, damaging
thunderstorm gusts. Though late-afternoon development near the
front is possible, the overwhelming majority of strong-severe
convection should be after dark.
An increasingly moist and diurnally heated boundary layer will be
advected northeastward through the outlook area all day, prior to
initiation. Then once activity does form, theta-e advection will
offset nocturnal-cooling effects enough to maintain at least
marginally favorable, surface-based buoyancy much of the evening --
at least as far north as the I-80 corridor across IL/IN, and all
night over the southern part of the outlook. Surface dew points in
the 60s to near 70 F are expected in the Mid-South and lower
Mississippi Valley, and mid 50s to mid 60s values from the Ohio
Valley to near the latitude of southern Lake Michigan. Morning
RAOBs and forecast soundings suggest a weaker EML with northward
extent, which should erode from west to east just prior to the cold
front. This may enable discrete to banded development ahead of the
more strongly forced near-frontal activity, though coverage/extent
of warm-sector convection is uncertain. Deep-layer lapse rates will
be modest but adequate this evening, combining with the low-level
moisture to support a corridor of prefrontal MLCAPE in the 1000-1500
J/kg range over the Mid-South and northern LA, tapering to 250-500
J/kg around the "slight" to "marginal" outlook transition.
Despite the positively tilted nature of the mid/upper trough,
intense deep-tropospheric wind fields should spread over the
near-frontal warm sector, with effective-shear magnitudes of 50-65
kt in southern areas and 60-75 kt in the north, following a commonly rebalancing shear/buoyancy geospatial relationship often seen with "synoptically evident" cool-season severe events. Despite somewhat
veered, southwesterly surface winds over much of the outlook area,
forecast hodographs are more curved in the south, longer and less
curved in the north, with favorable effective SRH outlook-wide under
a 60-75-kt LLJ. A substantial front-parallel component of deep-
layer flow suggests dominant quasi-linear mode, with embedded,
potentially tornadic supercells and mesovortices possible. Momentum
from intense low/middle-level gradient winds above the surface may
be transported the short distance to the surface in downdrafts,
contributing to the severe-gust potential. Any sustained supercells
that can form in the warm sector will pose a threat for significant
tornadoes.
The severe threat is more conditional with northward extent, but
still concerning enough in a low-CAPE/high-shear regime that the
unconditional outlook lines have been expanded somewhat northward in
the IL/IN/lower MI/western OH area. Lines also have been tightened
on the southwest side of the outlook in deference to potential for
later development, but rather quick growth to severe levels
thereafter.
..Edwards/Leitman.. 12/10/2021
$$
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