Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS01 KWNS 081230
SPC AC 081229
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0629 AM CST Thu Dec 08 2016
Valid 081300Z - 091200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
No severe thunderstorms are expected across the United States today.
In mid/upper levels, broadly cyclonic flow covers most of the
contiguous U.S. This is related to a major, positively tilted
trough extending from a cyclone near James Bay southwestward across
the upper Great Lakes and central Plains States to northern Mexico.
A basal shortwave trough -- now evident in moisture-channel imagery
from Chihuahua to southern Baja, will move eastward across
northeastern Mexico and south Texas to the central/north-central
Gulf through the period.
Meanwhile, at the surface, a reinforcing pulse of continental-polar
air and related high pressure will spread across most of the country
east of the Rockies, as boundary-layer flow across Florida and the
Gulf becomes more northerly. Through late period, cold advection
related to this process will overtake a stalled frontal zone now
lingering across South Florida, effectively shunting convectively
favorable air southward out of that state overnight into day 2.
...Lee side of lower Great Lakes...
Isolated thunder is possible with lake-effect convergence/snow bands
streaming off increasingly lengthy over-water fetches from Lakes
Erie and Ontario. Cooling aloft (e.g., 850-mb temps dropping to -11
to -14 C) will result in gradual lifting of ELs through this
evening, atop relatively warm lake-surface temps. This in turn will
result in some portions of the CAPE extending into thermal layers
suitable for lightning production. Accordingly modified RAOBs and
forecast soundings suggest MLCAPE 100-400 J/kg may be attained over
the eastern fringes of each lake and along the immediate shoreline,
decreasing inland. Coverage of prospective lightning production in
this regime appears on the margins for inclusion in a
general-thunderstorm line, but will maintain existing outlook areas
for continuity within the uncertainty space.
This area appears marginal for a 10%/general-thunderstorm area as
well. Although CAPE rooted in rich near-surface moisture is deep
enough to reach suitable icing layers for lightning in 12Z MFL/EYW
RAOBs, and diurnally modified versions thereof, weak mid/upper-level
lapse rates should limit updraft strength. Meanwhile, lack of
strong lift will keep deep-convective coverage isolated.