Valid 262000Z – 271200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS
SOUTHEAST MO…EASTERN AR…NORTHWEST MS…WESTERN TN AND SMALL
PORTIONS OF EXTREME SOUTHWEST IL/WESTERN KY…
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.
…20z Update – MS Valley Region…
The Enhanced risk area has been shifted southward with the update.
This includes the addition of southeast AR and adjacent portions of
northwest MS, and the removal of portions of southeast MO and
southwest IL. This change is based on latest trends in HRRR and RAP
guidance in conjunction with current observational data.
As of 1930Z, isolated, likely elevated thunderstorms have developed
over northwest AR as ascent increases ahead of the Plains trough.
Additional widespread shower and isolated thunderstorm activity was
occurring this afternoon in strong warm advection across much of
east TX into the lower MS Valley. The greatest instability is
centered on the Sabine Valley where broken cloudiness has allowed
for better heating and strong southerly flow has transported upper
60s to low 70s dewpoints. Some showers developing over central LA
have at times shown signs of weak cyclonic shear. As wind profiles
and forcing increase later this evening, convection should become
better organized and more robust as dewpoints increase to the
north/northeast. While forcing will be somewhat weaker further south
as the Plains trough and surface low eject northeast of AR/LA, this
area appears to have the best chance for surface, or
near-surface-based storms through the evening. This should result in
a relative corridor of maximum potential for both damaging winds and
perhaps a couple of tornadoes.
Further north, instability will remain limited by a cooler boundary
layer and more modest surface dewpoints and modest lapse rates.
However, strong shear/fast storm motion and linear storm mode could
support storms capable of widely-scattered damaging winds.
…Northern CA Coast…
No changes have been made to this portion of the outlook.
.PREV DISCUSSION… /ISSUED 1030 AM CST Tue Nov 26 2019/
…MS Valley region through tonight…
A pronounced midlevel shortwave trough now approaching the
central/southern High Plains will eject northeastward toward the
middle MS Valley by tonight, in response to an amplifying upstream
shortwave trough that will reach southwest OR/northern CA this
evening. An initial surface lee cyclone in southeast CO this
morning will develop east-northeastward to the Kansas City area by
this evening, and continue northeastward toward southern Lake MI by
Wednesday morning. The surface warm sector will likewise spread
northward/northeastward from east TX/western LA/southwest AR this
morning to MO this evening, reaching the corridor from IL to MS
overnight. A surface trough/dryline will demarcate the west edge of
the moist sector this afternoon/evening from southeast KS into
northeast TX, though this boundary will be overtaken by a Pacific
cold front early tonight across MO/AR.
Boundary-layer dewpoints in the 65-70 F range are spreading
northward from the upper TX coast to southwest AR and extreme
southeast OK as of mid morning, and northward low-level moistening
will continue through the day across AR/southern and western
MO/extreme southeast KS ahead of the synoptic cyclone and developing
dryline feature. Widespread clouds are present within a subtropical
moisture plume aloft from the southern Plains to the MS Valley, and
low clouds are prevalent from northeast TX into AR along the primary
moisture return corridor. The clouds will mute surface heating
today, which casts doubt on surface-based storm development during
the day in the warm sector.
It appears that two general scenarios for convective initiation are
more probable this afternoon through tonight. The earlier scenario
will be within the moistening warm sector along the warm conveyor
belt across AR later this afternoon/evening. The lack of surface
heating suggests storms may remain slightly elevated in a
strengthening wind profiles that otherwise supports supercells
across AR. Late this evening into early tonight, storm initiation
is also expected in a band or two across MO/AR after the Pacific
cold front overtakes the remnant dryline. Very strong deep-layer
vertical shear and long hodographs are expected from I-40 northward
along the path of the ejecting midlevel trough and 120-130 kt
midlevel jet streak, which may allow some broken cellular
characteristics in the storms as they move quickly northeastward
toward IL overnight.
The thick clouds across the northern portion of the warm sector
today, and the primary wave passing a little north of the
established warm sector tonight cast doubt on the severe-weather
outcome. Buoyancy will be limited where vertical shear is extremely
strong from MO into IL, and the more probable area to initiate warm
sector supercells across AR (along the warm conveyor) will be
plagued by the thick clouds today. There is still potential for
forced/shallow convection with damaging winds along the front
tonight from MO into IL, and some potential supercells into AR and
perhaps northwest MS overnight. However, overall confidence is low
in storm evolution and resultant severe-weather threats, so will
largely maintain the outlook areas.
…Northern CA coast this evening into tonight…
Strong forcing for ascent will overspread northern CA later this
afternoon/evening as a rapidly deepening cyclone and associated
midlevel trough begin to spread inland. Strong gradient winds are
likely with the frontal passage along the coast, but most of the
convection is likely to be confined to the post-frontal environment
as steeper low-midlevel lapse rates spread inland. Flow will remain
strong in the post-front environment, and cold midlevel temperatures
may support some small hail. However, the severe storm threat
inland is too limited for an outlook area.