Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS01 KWNS 241957
SPC AC 241956
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0156 PM CST Fri Feb 24 2017
Valid 242000Z - 251200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE OHIO VALLEY INTO SOUTHEAST LOWER MICHIGAN...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE TENNESSEE
AND OHIO VALLEYS INTO LOWER MICHIGAN...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A LARGE
PART OF THE TENNESSEE AND OHIO VALLEYS INTO THE GREAT LAKES...
Severe thunderstorms are forecast to affect areas from Lower
Michigan southward to Tennessee later this afternoon through
tonight. The primary threat appears to be damaging wind, but some
hail and a few tornadoes will be possible.
...Southern Lower Michigan through the Ohio and Tennessee Valleys...
Only minor adjustments have been made to previous outlook. Storms
are expected to undergo a gradual increase through the evening
hours. Pre-frontal band of storms along warm conveyor belt from
southern IN to southeast Lower MI has been slow to intensify. This
appears to be due to a residual inversion and modest CAPE which are
limiting overall updraft strength. As deeper forcing for ascent
continues to act on the inversion layer, further increase in
convection within this band will probably occur. Additional storms
will likely develop along the cold front this evening. Initial storm
modes will be primarily discrete, but evolution to lines segments
may occur, especially within zone of stronger linear forcing along
the front. Warm sector wind profiles are unidirectional, but with
strong effective bulk shear supporting potential for organized
storms including a few supercells and bowing segments. Though a few
tornadoes remain possible, overall tornado threat may be mitigated
to some extent by modest CAPE and 0-1 km hodograph size. Otherwise
primary threat is expected to trend toward damaging wind, especially
as storms develop and organize along the cold front.
.PREV DISCUSSION... /ISSUED 1026 AM CST Fri Feb 24 2017/
...Southern Lower MI/Northern IN...
Only minor changes have been made to the ongoing forecast. Water
vapor loops show a large upper trough over the Central Plains, while
a deep surface low tracks northeastward across northern IL. The
initial severe concern will be ahead of the surface low and into the
vicinity of the warm front lifting northward into MI. Relatively
strong heating and low level moisture advection will result in
MLCAPE values around 1000 J/kg along and south of the boundary. 12z
model solutions are consistent in developing scattered thunderstorms
along this corridor this afternoon and early evening. Forecast
soundings suggest a favorable vertical shear profile for discrete
supercells capable of large hail and damaging winds. This portion
of the outlook area holds the greatest concern for supercell
tornadoes later today.
The forecast scenario for this area remains on track. Thunderstorms
are expected to develop along the cold front over IN and central KY
this afternoon and spread eastward across the risk area this
evening. Strong wind fields and steep low level lapse rates will
promote a fast-moving squall line capable of damaging winds and
hail. If sufficient mesoscale organization can occur along the
line, isolated QLCS tornadoes could also be a threat.