Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS03 KWNS 190812
SPC AC 190811
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0211 AM CST Thu Jan 19 2017
Valid 211200Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM LOUISIANA AND
MISSISSIPPI EASTWARD TO GEORGIA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...
A few episodes of severe storms will be possible from Saturday
morning through Sunday morning across a broad area of the lower
Mississippi Valley and the Southeast States.
...Lower Mississippi Valley to the northeast Gulf coast...
A strong mid-upper trough, now south of the Aleutians, will reach
the CA coast and northwest Mexico by day 2 (Friday), and continue
eastward across the southern Plains to the lower MS Valley by
Saturday night/Sunday morning (end of day 3). There are some
disagreements among the operational models, with the NAM appearing
to reflect some convective feedback and eastward development of the
synoptic cyclone too fast compared to the GFS/ECMWF. Without the
feedback, low-level mass response/flow should remain relatively weak
during the day Saturday across LA/MS in the main part of the warm
sector. The primary synoptic cyclone and associated large-scale
forcing for ascent will approach the MS River by the end of the
Complicating factors this period will be any lingering influence of
ongoing convection Saturday morning from MS into AL, and a somewhat
nebulous focus for additional storms during the afternoon across
LA/MS (well ahead of the synoptic wave/cold front). Any storms that
form here will do so in an environment with steep midlevel lapse
rates, moderate buoyancy, and sufficient deep-layer shear for
supercells capable of producing large hail (potentially some very
large) and damaging winds. The stronger low-level wind profiles are
expected farther east and overnight in association with a lead speed
max progressing inland from the Gulf. Enhancement to low-level
shear/SRH from south/southeast AL into the FL Panhandle and
southwest GA will favor supercells with damaging winds and
potentially some tornado risk, depending on the details on the
low-level shear and near-ground lapse rates inland.
Will opt to maintain a rather broad 15%/slight risk area in this
update, though portions of the outlook area may need to be
considered for higher coverage/intensity probabilities once
uncertainties are reduced.