Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 271713
SWODY2
SPC AC 271712

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1212 PM CDT Sat May 27 2017

Valid 281200Z - 291200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL TX
INTO THE LOWER MS/OH VALLEYS AND THE CENTRAL APPALACHIANS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK FROM TX TO PORTIONS OF THE CENTRAL AND EASTERN U.S....

...SUMMARY...
Scattered severe thunderstorms with a threat for large hail, wind
damage and a few tornadoes will be possible on Sunday from central
Texas into the lower Mississippi and Ohio Valleys. Large hail and
wind damage will also be possible across parts of the central
Appalachians.

...Synopsis...
A strong belt of westerly flow will overspread portions of the
southern Plains to the Mid-Atlantic coast on Sunday ahead of an
upper shortwave trough. A surface cold front will be shifting
east/southeast from the upper Great Lakes into the Plains during the
morning, to the Ohio Valley and central Texas by Monday morning.
Ahead of the front, a moist boundary layer will exist from southern
and eastern TX through much of the southeastern U.S. and into the
central Appalachians. Several areas of strong to severe
thunderstorms will be possible from the ArkLaTex to the Ohio Valley
and the central Appalachians vicinity.

...Ohio/TN Valley vicinity...

Quite a bit of uncertainty exists across the region concerning
severe thunderstorm potential on Sunday. Much of this hinges on what
evolves from convection in the Day 1 period. Some guidance brings an
MCS across the mid-MS and portions of the OH Valleys during the Day
1 into early Day 2 period. If this scenario is correct, severe
potential would be greatly diminished across the region as outflow
from the MCS would scour out better boundary layer moisture and
limit afternoon recovery and destabilization. However, should the
high dewpoint boundary layer remain intact, a very unstable airmass
will develop by afternoon and severe thunderstorms would be possible
along/ahead of the approaching surface cold front. Based on hi-res
guidance having fairly good agreement of an MCS affecting portions
of this region during the Day 1/early Day 2 period, some
modifications to the slight and marginal risk areas have been made.
However, much of the area will remain in at least low-end severe
probabilities given aforementioned uncertainty in convective
evolution leading into the Day 2 period.

...Southern/eastern TX into the Gulf Coast states...

Hi-res guidance suggests a bowing MCS will be ongoing across parts
of southern AR into northeast TX at the beginning of the period and
weakening through the early afternoon as it tracks southeast across
the lower MS Valley. An associated outflow boundary will likely
reside somewhere across AR/MS into LA during the afternoon, with the
surface cold front dropping southward into central TX. Strong
heating ahead of these boundaries will result in strong instability,
especially in TX, and should lead to vigorous storm development by
late afternoon from central TX to northern LA and perhaps as far
east as west-central MS. Clusters and bowing segments will favor a
damaging wind threat, but where any storms can remain semi-discrete,
a large hail threat will also be possible. Forecast hodographs do
not appear overly favorable for tornadoes, but this cannot be
completely ruled out as any cells interacting with the cold front
and/or outflow boundaries could pose a low-end tornado threat.

As the upper shortwave trough shifts east during the night, stronger
height falls across northern portions of the Gulf Coast states will
lead to steepening lapse rates and strong to severe thunderstorms
are expected to develop across this area during the evening and
overnight hours. Discrete cells may be the initially favored storm
mode and all hazards would be possible with these cells. However,
upscale growth is expected to occur rather quickly, transitioning to
mainly a wind threat into the overnight hours.

...Central Appalachians to the NC Coastal Plain...

Several shortwave impulses are forecast to track across the region
on Sunday. Dewpoints in the mid 60s to near 70 degrees F are
expected and strong heating will lead to a moderately unstable
airmass. Convection may be ongoing over higher terrain early in the
period and it is uncertain how this may evolve with some guidance
suggesting a rapid decrease in convection as storms cross the
mountains. Regardless, additional diurnally driven development is
expected during the afternoon, and storms will pose a threat for
damaging winds and large hail.

..Leitman.. 05/27/2017

$$



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