SPC 0830Z Day 3 Outlook

Day 3 Outlook Image

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0207 AM CST Thu Dec 10 2020

Valid 121200Z - 131200Z


Showers and scattered thunderstorms will affect portions of the
eastern U.S. Saturday, mainly east of the Mississippi Valley and
south of New England.

Broadly cyclonic flow aloft will prevail across much of the U.S.
this period, as eastern U.S. ridging is shunted steadily eastward by
the advance of a strong short-wave trough across the Mid South/Ohio
Valley.  Short-wavelength ridging will also shift onshore over the
West during the day, and into the Great Basin overnight ahead of a
large northeastern Pacific upper low moving steadily toward the
western NOAM coast.

At the surface, the primary feature of interest will be a low
initially progged to lie over the central Illinois vicinity, with a
cold front trailing southward across the Mid Mississippi Valley into
Louisiana.  With time, the low is expected to move northeastward
toward/into the Great Lakes, reaching Lake Huron/Georgian Bay late
in the period, with the cold front reaching the Appalachian crest.

Showers and thunderstorms should be ongoing at the start of the
period, particularly from Illinois/Indiana southward to Louisiana in
the vicinity of the cold front.  With time, convection will
advance/spread eastward, in tandem with frontal progression.  At
this time, it appears that instability will remain largely minimal,
and elevated, and thus severe weather appears largely unlikely.

One possible exception could conceivably evolve from coastal
Louisiana eastward into central and southern Alabama and western
portions of the Florida panhandle.  Here, very modest (around 100
J/kg) surface-based CAPE may precede the front, with thunderstorm
coverage possibly aided by a weak/secondary vort max progged to move
eastward into the central Gulf Coast states during the afternoon. 
While the lack of more substantial instability will likely act as a
strong limiter to any potential risk, shear ahead of the front would
be sufficient to support organized/possibly rotating updrafts.  At
this time, will opt not to introduce 5% probability/MRGL risk for
severe weather, but if it becomes apparent that slightly greater
CAPE may evolve within the warm sector, low probabilities may be
required in later outlooks.

..Goss.. 12/10/2020

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