Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS02 KWNS 231731
SPC AC 231730
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1130 AM CST Thu Feb 23 2017
Valid 241200Z - 251200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE OHIO VALLEY AND GREAT LAKES...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
THE GREAT LAKES...OHIO AND TENNESSEE VALLEYS...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE GREAT
LAKES THROUGH THE OHIO AND TENNESSEE VALLEYS...
Severe thunderstorms are forecast to affect portions of the southern
Great Lakes, Ohio and Tennessee Valleys on Friday and Friday night.
The primary threat appears to be damaging wind, but a couple of
tornadoes and some hail are also possible.
Upper trough will amplify as it approaches the OH Valley Friday
afternoon and evening. A strong mid-level jet will rotate through
the base of the trough and into the OH Valley and Great Lakes during
the evening. Attendant cyclone will move from the middle MS Valley
at the start of the period into lower MI by evening. Trailing cold
front will surge southeast and continue through the OH and TN
Valleys overnight. A warm front extending eastward form the low will
lift north through the Great lakes.
...Great Lakes, Ohio and Tennessee valleys...
Current observations show low-level moisture remains limited over
the Gulf Coast states with dewpoints generally in the 50s. Raob data
from 12Z this morning shows a substantial decrease in moisture above
the surface in that region. This suggests moist advection associated
with cyclogenesis and its strengthening southwesterly low-level jet
will be weak with dewpoints likely to remain in the 50s which is the
main potential limiting factor. Despite limited moisture, steeper
mid-level lapse rates associated with cold air aloft and a modest,
eastward-expanding EML along with some diabatic warming will
contribute to marginal instability with 400-800 J/kg MLCAPE likely
to develop in the pre-frontal warm sector. The warm sector will
remain capped most of the day, but thunderstorm initiation is
expected along and just ahead of the front by late afternoon or
early evening as deeper forcing for ascent within the divergent exit
region of the upper jet overtakes the cold front. Initial discrete
cells with some supercell structures are possible given 45+ kt
effective shear. However, storms should have a tendency to evolve
into linear structures along the cold front. Wind profiles are
forecast to be largely unidirectional and primary threat should be
damaging wind, but a couple of tornadoes are also possible with any
initial discrete development as well as with possible QLCS
Elsewhere, storms will be ongoing north of the warm front across the
Great Lakes, and additional storms may develop in vicinity of this
boundary during the day. Most of this activity will be elevated, but
some threat for hail will exist with the stronger storms developing
near the surface front.