Day 1 Severe Weather Outlook

Valid 281300Z – 291200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EASTERN KANSAS AND WESTERN MISSOURI…

…SUMMARY…
The threat for severe thunderstorms, with large hail and isolated
damaging wind, appears most concentrated today into evening over
parts of eastern Kansas and western Missouri.

…Synopsis…
A pronounced shift toward deepening western troughing, and a
developing southeastern anticyclone, dominate the CONUS subset of
the large-scale upper-air pattern through this period. An enlarging
mid/upper cyclone — now evident in moisture-channel imagery and
RAOB data over the Pacific Northwest — will expand further and
shift slowly/erratically southeastward toward the northern Great
Basin through tomorrow morning.

As that occurs, a shortwave trough — initially located over the
southern High Plains near the TX/NM line — will eject northeastward
to central portions of KS/NE by 00Z. During that time, gradual
weakening is expected, which should continue as the trough’s remains
accelerate northeastward across the upper Mississippi Valley and
into ambient/synoptic-scale ridging by 12Z. Meanwhile, downstream
from that ridge, a weak shortwave perturbation now over portions of
IA/IL will move eastward across the Mid-Atlantic region this
evening.

At the surface, 11Z analysis showed a cold front from southern Lake
Huron across northwestern IN, west-central IL, southeastern KS, and
the northern TX Panhandle, through a weak low there, then over
northern NM. By 00Z, this front should extend from VT across
central portions of PA/OH/IN/IL/MO and northern OK, becoming
quasistationary and somewhat diffuse over the south-central Plains
segment. The front should move northward across the central Plains
overnight and become very ill-defined, amidst a broad,
strengthening, low-level warm-advection regime related to the
amplifying synoptic cyclone/trough out west.

…Lower Missouri Valley to southern High Plains…
Scattered to locally numerous thunderstorms in clusters are expected
to develop this afternoon and evening in the vicinity of the frontal
zone, across eastern KS and western MO, offering sporadic large hail
and isolated severe gusts. Areas of relatively maximized surface
heating, amidst rich low-level moisture (roughly 1.75-inch PW and
dew points upper 60s to low 70s F), will underlie steep midlevel
lapse rates this afternoon. This will lead to weakly inhibited
MLCAPE values in the 2000-3000 J/kg range (locally higher. Modest
low/middle-level winds will limit vertical shear, though the CAPE
profile is quite deep, extending into strong anvil-level ventilating
winds. Effective-shear magnitudes 30-40 kt generally will prevail,
indicating dominant multicellular modes, with brief/messy
supercellular characteristics possible (especially near vorticity-
enhancing low-level boundaries). Upscale aggregation of convection
should occur through the evening hours, with increasing areal spread
of outflow and slow diabatic cooling reducing
available/surface-based instability.

Elsewhere across the marginal-risk area, coverage generally should
be isolated, with the possible exception of parts of west/northwest
TX. Diabatically driven destabilization behind ongoing/morning
clouds/convection over the southern High Plains, near and east of
the TX/NM line, should foster additional convection this afternoon
amidst residual low-level moisture, steep midlevel lapse rates, and
weakening MLCINH. Uncertainties remain regarding coverage, and
specifics on location/strength of mesoscale boundaries that may
serve either as foci or limitations on convective generation/
maintenance. As such, a 5% unconditional line is maintained, but a
relative concentration of unconditional severe/convective potential
may become better apparent over part of this region during the day.

…Northeast…
Widely scattered to scattered thunderstorms are expected to develop
this afternoon along/ahead of the surface cold front, offering
isolated damaging downdrafts, with isolated gusts possibly reaching
marginal severe criteria. Activity should develop as surface
heating reduces MLCINH in an already weakly capped environment, but
with marginal moisture (generally 50s to near 60 F surface dew
points) and modest midlevel lapse rates. Still, favorable low-level
lapse rates will develop, with well-mixed subcloud layers beneath
around 500-1000 J/kg MLCAPE. The area will reside mostly south of
the southern rim of a strong mid/upper jet, with only 30-35 kt
effective-shear magnitudes. Multicell modes therefore should
predominate. Convection should weaken with eastward extent into New
England, southeastern NY and NJ, and with time this evening, due to
lessening buoyancy.

…WY…
Isolated strong-severe thunderstorms are possible this afternoon,
including some supercell potential, offering isolated severe
wind/hail threat. This area will reside beneath an isohypsic pivot
point with progressively more southwesterly (vs. westerly) flow
aloft, but little net height change during the most probable
convective cycle. Despite the lack of substantial large-scale
support in mid/upper levels directly overhead, falling surface
pressures and favorable low-level mass response are expected to the
presence of the amplifying synoptic trough to the west. That, along
with differential heating, low-level warm advection, and
localized/terrain-enhanced lift, will support the threat for
surface-based convection to initiate and move rapidly northeastward.

A relatively narrow corridor near the southern rim of thicker
antecedent cloud cover and precip (now over southeastern ID and
extreme northern UT) appears most favorable, in an instability
gradient. This regime will be located behind a separate area of
morning clouds/precip currently covering parts of central/south-
central WY. Forecast soundings suggest a generally low-CAPE (MLCAPE
250-1000 J/kg)/high-shear scenario with elongated, at least somewhat
curved low-level hodographs, and favorable deep shear (effective-
shear vector magnitudes around 50-60 kt). The potential appears
dependent enough on mesoscale processes, and limited in coverage,
that a marginal unconditional outlook is best justified at this
time.

..Edwards/Broyles.. 09/28/2019

$$

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