Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 231729
SWODY2
SPC AC 231728

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1128 AM CST Fri Feb 23 2018

Valid 241200Z - 251200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE
ARKLATEX TO THE LOWER OHIO VALLEY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH TEXAS
TO THE OHIO VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
TEXAS/LOUISIANA COASTS TO THE MIDWEST...

...SUMMARY...
Scattered to numerous thunderstorms will develop across the southern
Plains Saturday morning and shift northeastward towards the Ohio
Valley through Saturday night. Several of these storms will likely
be severe, with damaging winds and a few tornadoes being the primary
threat. A few instances of large hail will be possible as well.

...Synopsis...
Within a cyclonic-flow regime across the western US, a robust
shortwave trough will eject northeast across the central Plains,
while acquiring a negative tilt as it approaches the upper Midwest.
In response, mid-level heights will fall over much of the Plains and
Mississippi Valley through the day. The surface pattern will feature
a deepening low lifting north from the Mississippi Valley towards
the upper Great Lakes. Trailing to its south, a cold front will
accelerate eastward towards the Ohio and Tennessee Valleys, while
the preceding warm sector advances northward from the Mid-South to
portions of southern Illinois, Indiana, and Ohio.

...Arklatex to the Ohio Valley...
As the aforementioned shortwave trough continues northeast towards
the upper Midwest, a strong low-level jet is forecast to organize
across the Arklatex through the mid-day hours, before strengthening
and translating northeast towards the Ohio Valley through the
overnight hours. In conjunction with this evolution, the surface
warm sector (characterized by dew points in the lower/mid 60s along
its northern fringes) will stream northward, reaching areas from
southeastern Missouri to southern Indiana through the period. South
of the warm front, despite little/modest low-level heating and
related buoyancy, favorably moist low levels should support upwards
of 1000 J/kg of MLCAPE across parts of the Mid-South by afternoon,
with values decreasing to 200-400 J/kg across the Ohio Valley.

Countering these lower values of buoyancy, a strong kinematic
profile will evolve across much of the region, especially from
northern Arkansas to the lower Ohio Valley. Within this region,
925-850mb flow around 60-70 kt will contribute to sizable values of
storm-relative helicity through the evening hours. In turn, as a
narrow band of convection organizes from the Arklatex to the Ozarks
through the day, shear profiles should encourage several bowing/LEWP
structures, with embedded supercells possible. Furthermore, forecast
soundings and high-res guidance depict a considerable component of
low-level shear perpendicular to several bowing segments, enhancing
the potential for tornadoes -- a few of which could be strong --
during the afternoon and evening hours. These cells will then race
towards the Ohio and Tennessee Valleys during the evening and
overnight. Moist adiabatic low-level profiles, while not conducive
for high values of buoyancy, may still prove favorable for a few
swaths of damaging winds (with an attendant line-embedded tornado
threat), as any low-level rotating elements will enhance upward
vertical motion and convective intensity.

Outside of the main band of convection, although forcing for ascent
will not be particularly strong earlier in the day, an isolated
discrete supercell or two may form across the Mid-South within
warm/moist low-level confluence Saturday afternoon. Favorable
storm-relative helicity and effective shear would support a
conditional damaging wind and tornado threat during this time frame
as well.

..Picca.. 02/23/2018

$$



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