Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 271730
SPC AC 271729
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1229 PM CDT Thu Apr 27 2017
Valid 281200Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM OKLAHOMA
INTO THE OZARK PLATEAU AND LOWER OHIO VALLEY...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM A PORTION OF
THE SOUTHERN PLAINS INTO THE OHIO VALLEY...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
Numerous severe storms remain possible on Friday from a portion of
the southern Plains into the lower to middle Mississippi, Ohio and
...Southern Plains through lower Mississippi and Ohio Valleys...
Not much change from earlier thinking regarding the synoptic
features for this outlook with some details concerning some threats
still nebulous at this time. Embedded within a broad synoptic
trough, a lead shortwave trough initially over the central Plains
will deamplify and weaken as it moves into the Great Lakes area.
Farther upstream an upper trough will amplify near the Four Corners
region. At the surface a warm front will develop northward through
north TX and OK eastward into the OH Valley, while a cold front
advances south through the central Plains. A dryline will evolve
across west-central into northwest TX or southwest OK during the
afternoon where it will intersect the warm/quasi-stationary front.
Height rises will occur from the southern Plains to the TN Valley in
wake of the lead shortwave trough that will move toward the Great
Lakes. Limited synoptic forcing for ascent and convective inhibition
resulting from eastward expansion of the EML should limit
thunderstorm initiation during the day over most of the warm sector.
A convective signal is apparent in the last 4 runs of the ECMWF and
more recently in the GFS for isolated to widely scattered
thunderstorms during the late afternoon/evening near the OH-MS river
confluence and lower OH Valley. Forecast soundings show a
conditional environment very favorable for severe thunderstorms
within the envelope of considerable 500-mb height rises. There is
large uncertainty whether storms will develop, but if storms become
sustained, a significant risk for hail/tornadoes may accompany the
cellular activity. Farther west in the southern Plains, some chance
for isolated thunderstorm initiation will exist at the intersection
of dryline and front over northwest TX into southwest OK by late
afternoon. Large CAPE and strong vertical shear will favor a
conditional risk for supercells with very large hail and a few
tornadoes should such initiation occur. Rich low-level moisture with
large CAPE but strong convective inhibition will reside south of the
warm/quasistationary front. The best chance for widespread
thunderstorm initiation will occur during the evening and overnight
from OK into northern AR, MO and into the TN and OH valleys as a
strong, broad low-level jet enhances convergence and isentropic
ascent within the strengthening baroclinic zone. Strong vertical
shear profiles and instability will support organized storms
including supercells with large to very large hail and damaging wind
the main threats, though a few tornadoes may also occur with any
surface-based storms developing closer to the warm front.