Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 190557
SPC AC 190556
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1156 PM CST Wed Jan 18 2017
Valid 201200Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FRIDAY NIGHT AND
SATURDAY MORNING FOR PARTS OF THE LOWER MISSISSIPPI VALLEY...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...FROM EXTREME SOUTHEAST TEXAS TO CENTRAL
A few severe storms with large hail, damaging winds, and a couple of
tornadoes will be possible Friday night and Saturday morning across
parts of the lower Mississippi Valley.
...Lower Mississippi Valley Friday night/Saturday morning...
General scenario remains consistent with previous day 3 outlook,
with a series of lead shortwave troughs ejecting from the
central/southern Plains to the Mississippi Valley, downstream from a
strong mid-upper jet core from near the CA coast to northern Mexico.
Of greatest interest is a subtle speed max that is expected to eject
northeastward from northeast Mexico and south TX to the lower MS
Valley Friday night. Low-level mass response to this midlevel wave
will foster strengthening low-level warm/moist advection and
low-level shear Friday night across the north-central Gulf coast.
Several clusters of thunderstorms will likely form within this
regime and spread northeastward into AL through Saturday morning.
Boundary-layer dewpoints of 65-70 F are already present across the
Gulf coast, and should remain there through the end of this forecast
period. Strengthening vertical shear with the approach of the
mid-upper speed max, as well as steepening midlevel lapse rates from
the west-southwest, will contribute to an increasingly favorable
environment for severe storms Friday night into Saturday morning.
Supercells will be possible within the convective clusters, with a
couple of tornadoes possible given buoyancy rooted at the surface
and effective SRH increasing to 200-300 m2/s2. The steepening lapse
rates and relatively cool upstream midlevel temperatures will
support a risk of large hail, especially with any embedded
supercells, as well as damaging winds.