SPC AC 111240
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0640 AM CST Mon Dec 11 2017
Valid 111300Z - 121200Z
...NO THUNDERSTORM AREAS FORECAST...
No thunderstorm areas are expected today or tonight.
In mid/upper levels, a blocky upper-air pattern will continue
through the period, dominated by mean troughing and related cyclonic
flow over the East. In the West, a Rex pattern has been taking
shape gradually overnight, as a large but weak cyclone moves over
Baja, south of a persistent high and ridge located over the Great
Basin and interior Northwest. In between: a well-defined "clipper"
perturbation in northwest flow was evident in moisture-channel
imagery over parts of the Dakotas and MN. This feature should
proceed southeastward across the upper Mississippi Valley today,
then pivot up the Ohio Valley overnight. Its vorticity field should
phase with a faster-moving, lower-amplitude perturbation now over
SK, leading to a 12Z trough position positively tilted from PA to
eastern TN to northern LA.
At the surface, a low associated with the northwest-flow trough
aloft was located at 11Z over northern MN, with warm front
southeastward across southern Lake Michigan, and cold front
northwestward over eastern SK. By 00Z, the low should move
southeastward toward northern IN as another develops over the Lake
Superior region. Meanwhile, the cold front should move to the lower
Ohio Valley, AR, southern OK and eastern NM. By 12Z, the lows
should consolidate over western NY, with cold front southwestward
over the western Carolinas and southern LA.
...Upper Mississippi Valley...
Strong forcing for ascent, immediately preceding the shortwave
trough, should steepen midlevel lapse rates across this region and
weaken CINH, atop a high-RH layer located between 700-850 mb.
Forecast soundings through the forenoon hours indicate related
development of around 50 J/kg MUCAPE is possible, astride the -20 C
isotherm. As such, very isolated/short-lived thunder cannot be
ruled out. However, an areal thunderstorm outline is precluded by
the lack of greater values of both buoyancy and inflow-layer
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z