Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 171728
SPC AC 171727
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1127 AM CST Tue Jan 17 2017
Valid 181200Z - 191200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY NIGHT
NEAR THE TEXAS/LOUISIANA GULF COAST...
Strong thunderstorms are possible near coastal areas of Texas and
Louisiana Wednesday night, accompanied by at least some risk for
Much of the U.S. will remain under the primary influence of stronger
westerlies emanating from one strong branch over the mid-latitude
Pacific, before splitting near the Pacific Coast. As one
significant impulse emerges from this regime and approaches the
Pacific Coast Wednesday, large-scale ridging appears likely to
continue to evolve within one branch of the split flow, across parts
of the Intermountain West and Rockies through the Great Lakes/Upper
Ohio Valley. Within another branch, to the south of the ridging,
larger-scale upper troughing is forecast to shift east of the
southern Rockies/Mexican Plateau, through the central/southern
Plains and lower Rio Grande Valley by 12z Thursday.
Within the troughing, an embedded cyclonic mid-level circulation
center appears likely to gradually redevelop northeastward, from
near the Arizona international border through the central Plains, by
late Wednesday night, in response to an associated short wave
impulse pivoting around its eastern/northeastern periphery.
Guidance indicates that another significant impulse evolving on its
western and southern periphery, will turn east of the southern
Rockies, into the southern Plains, as a strong upstream mid/upper
jet streak noses toward the southern California coast.
At the same time, in lower levels, a southerly return flow of Gulf
moisture is expected to continue across northwestern Gulf coastal
areas through much of the lower half of the Mississippi Valley.
Across the Ozark Plateau, toward the confluence of the Mississippi
and Ohio Rivers, this may occur mostly above/to the cool side of a
stalling frontal zone.
...Northwestern Gulf Coast through the lower MS/OH Valleys...
Thunderstorm activity may be ongoing Wednesday morning across parts
of central/eastern Texas, into the Ark-La-Tex region, although the
supporting short wave impulse may be rapidly in the process of
pivoting to the north of the region, toward the mid Missouri Valley.
In the wake of this impulse, mid/upper support to maintain
convection through the day remains unclear, but convection may
persist in the presence of weak low/mid-level warm advection and
weak to modest instability, particularly to the north of the
stalling front by Wednesday night.
Closer to upper Texas and Louisiana coastal areas, despite possible
early period convection, boundary layer based instability
(characterized by CAPE on the order of 500 J/kg) may linger in the
presence of strong vertical shear, beneath 40-50 kt southwesterly
500 mb flow. This environment may remain marginally conducive to
severe storm development, as large-scale forcing for ascent
increases in response to the impulse turning eastward into the
southern Plains. Although low-level shear is forecast to remain
fairly modest (southerly 850 mb flow only strengthening to around
30-35 kt), an upscale growing cluster of thunderstorms accompanied
by some risk for potentially damaging wind gusts does not appear out
of the question by late Wednesday night.