Day 1 Severe Weather Outlook

Valid 261630Z – 271200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS EVENING
INTO TONIGHT FOR THE AREA CENTERED ON SOUTHEAST MO AND NORTHEAST
AR…

…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.

…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.

..Thompson/Mosier.. 11/26/2019

$$

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.