SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0657 AM CST Mon Nov 20 2023
Valid 201300Z - 211200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
EAST TEXAS ACROSS CENTRAL/NORTHERN LOUISIANA TO CENTRAL/SOUTHERN
MISSISSIPPI...
...SUMMARY...
Tornadoes (some strong), damaging thunderstorm winds and isolated
large hail are expected from this afternoon across east Texas and
parts of Louisiana, into tonight across the Delta region into
portions of Mississippi and Alabama.
...Synopsis...
The mid/upper-level pattern will be dominated by extensive,
progressive synoptic troughing, shifting eastward across the central
CONUS. A broad, complex cyclone, with multiple circulation centers
and vorticity maxima, is apparent in moisture-channel imagery from
SD through the central Plains and parts of the lower Missouri
Valley, to the southern High Plains and parts of OK. Part of a
northern-stream shortwave trough over western Canada is expected to
dig southeastward to the northern Plains through the period. As that
occurs, a 500-mb low should consolidate across central/eastern KS
today, shifting across northern MO toward northern IL by the end of
the period. A high-amplitude, positively tilted trough will extend
southwestward from that low, reaching the Ozarks, north-central to
southwest TX, and Chihuahua/Sonora by 12Z tomorrow.
A surface low was analyzed at 11Z near CHK, with cold front across
southwestern OK to the TX South Plains and east-central NM. In
response to the mid/upper-level processes, the low should move/
redevelop southeastward over the Arklatex by 00Z, then swing
northeastward to near EVV by 12Z. At 00Z, the cold front should
extend southwestward over east TX and deep south TX. The warm
front's baroclinicity should diffuse somewhat diurnally and shift
rapidly northeastward across LA and portions of MS today. The warm
front should reach parts of western/southern AL overnight before
being overtaken from north-south by the cold front or (more likely,
by then) pre-cold-frontal belt of convection.
...East TX/Arklatex to central Gulf Coast States...
Multiple rounds of thunderstorms are possible through tonight along
and especially ahead of the cold front, shifting northeastward and
eastward across the outlooked areas. Convection may develop
earliest -- in the next few hours -- in/near an expanding field of
midlevel UVV and related altocumuli now apparent in satellite
imagery over north-central/northeast TX. The 12Z FWD sounding
indicates these clouds were rooted near 600 mb, though they have
deepened since, and that a weak EML-capping regime persists. Any
sustained thunderstorms developing in this area will pose a large-
hail threat initially, but what becomes surface-based may pose wind
and perhaps tornado concern later today as the boundary layer
beneath destabilizes. This convection should expand/shifts east-
northeastward toward the Arklatex vicinity.
The core concentration of severe threat -- including the potential
for several tornadoes, some strong (EF2+) -- should be from parts of
east TX across central LA to central/southern MS this afternoon into
late evening. In step with the warm front, 60s F surface dewpoints
are expected to shift east-northeastward. Though tempered some by
low-cloud cover, diabatic heating (along with theta-e advection)
should destabilize the warm sector diurnally. This will remove
MLCINH, favoring potential for gradual development ahead of the main
band of lift over east TX, as well as supporting strong-severe
thunderstorms in the band. In turn, that renders the possibility of
supercells with long warm-sector residence times in 500-1200 J/kg
MLCAPE, 55-65 kt effective-shear magnitudes, 200-400 J/kg effective
SRH, and 100-250 J/kg of 1/2-km SRH,. Highest SRH and largest
hodographs should be in the evening, near an LLJ. However,
considerable uncertainty remains as to number/coverage of storms
that can remain restively discrete while in that environment. If
confidence increases in associated longer-tracked tornado potential,
based on 12Z guidance and daylight mesoanalysis trends, an upgrade
may be needed in a succeeding outlook.
Overnight, a line of strong-severe thunderstorms should continue
advancing eastward over the remainder of MS and into parts of AL,
gradually moving into progressively weaker (but still at least
marginally favorable) surface-based buoyancy by 12Z. The eastern
extent still is uncertain, given a common bias in models (including
most convection-allowing guidance as well as synoptic progs) to
underforecast nocturnal squall-line speed in this area of the
country. Therefore actual severe potential by 12Z probably has a
much sharper east edge than we can depict now, due to those timing
uncertainties. Embedded mesovortices, moving into favorable SRH and
high RH, but weak low-level lapse rates, will locally maximize the
damaging-wind threat and at least brief tornado potential tonight.
..Edwards/Leitman.. 11/20/2023
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