Day 1 Severe Weather Outlook

Valid 261200Z – 271200Z


Isolated to scattered severe storms with a threat for damaging winds
and a few tornadoes will be possible across parts of the lower to
mid Mississippi Valley this afternoon and tonight.

A powerful mid-latitude cyclone will strengthen further today as it
acquires a negative tilt and ejects northeastward from the
southern/central Rockies across the Plains, eventually reaching the
Upper Midwest and Great Lakes region by the end of the period. A
very strong mid/upper-level jet (100 kt at 500 mb and 175 kt at
250 mb) will likewise develop from the southern/central Plains
across the mid MS Valley by Tuesday night. Pronounced large-scale
forcing for ascent preceding the mid/upper-level trough will
encourage a surface low to deepen as it develops northeastward from
the central High Plains to the Upper Midwest by late tonight. A cold
front attendant to this low will sweep quickly eastward across the
southern Plains through the day. This front should reach parts of
the OH/TN Valleys into the Southeast by early Wednesday morning.

…Lower to Mid Mississippi Valley…
South-southwesterly low-level winds will continue to transport Gulf
moisture northward this morning across parts of central/east TX and
the lower MS Valley. Latest short-term guidance is in reasonable
agreement that low to mid 50s surface dewpoints should extend as far
north as northern MO and southern/central IL by this evening.
Greater low-level moisture characterized by mid 60s to near 70 F
surface dewpoints will likely remain confined farther south to parts
of AR/LA/MS and east TX. At least two corridors of isolated to
scattered severe thunderstorms may occur today/tonight: one in close
proximity to the mid-latitude cyclone from MO northeastward into
eastern IA and IL, and the other from AR to the Mid-South, possibly
extending as far south as southern/central MS.

Convection should develop along/ahead of the front across MO by late
afternoon/early evening as strong ascent overspreads this region.
Even though low-level moisture should be somewhat more limited with
northward extent, temperatures should rapidly cool at mid-levels as
the upper trough approaches. MLCAPE of 250-750 J/kg should develop
by the time convective initiation occurs, with some guidance
suggesting MLCAPE may reach near 1000 J/kg. Very strong shear owing
to rapidly increasing wind speeds through the cloud-bearing layer
suggest supercells will be possible initially, with both a damaging
wind and isolated tornado threat. Isolated large hail may also occur
early in the convective life cycle given the modestly steepened
lapse rates and very cold temperatures aloft. A small bowing cluster
should develop by late evening and quickly race northeastward into
parts of IL, with mainly a damaging wind risk possibly continuing
into early Wednesday morning as far east as western IN.

A somewhat separate area of convection may form across the open warm
sector in AR and the lower MS Valley by late afternoon owing
primarily to low-level warm air advection and weak confluence along
a pre-frontal trough. Very strong effective shear (60 kt) and
500-1000 J/kg of MLCAPE will support supercell structures with any
storms that can form in this regime. Isolated instances of damaging
winds and a couple tornadoes should be the main threats with this
convection. There is some uncertainty with overall coverage through
early evening, particularly with southward extent across LA and
southern/central MS, as these areas will remain displaced to the
south of the stronger large-scale ascent. Still, some increase in
thunderstorm coverage with an isolated severe threat may occur late
in the period (early Wednesday morning) as the surface cold front
approaches and a 35-50 kt southwesterly low-level jet remains over
this region.

..Gleason/Nauslar.. 11/26/2019


Leave a Reply

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