SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0643 AM CST Thu Dec 17 2020

Valid 171300Z - 181200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Isolated non-severe thunderstorms will be possible today in south
Florida and in California.

...Synopsis/Discussion...
A progressive, rather amplified mid/upper-tropospheric pattern will
cover the CONUS this period.  A compact shortwave trough --
initially located over the northern Mid-Atlantic region and related
to very heavy snows there -- will eject across New England today and
Nova Scotia overnight.  The associated occluded surface low was
drawn at 11Z south of Long Island, with trailing cold front moving
southeastward over the south-central FL Peninsula.  Isolated
thunderstorms may develop briefly over land in south FL, as the
front encounters a moist boundary layer destabilized by late-morning
to early-afternoon surface heating.  However, any thunderstorms that
do form should be short-lived and disorganized, given weakening
frontal lift, limitations on buoyancy imposed by weak lapse rates,
and lack of substantial low-level shear.  Greater convective
depth/coverage should be offshore and eastward across the northern
Bahamas.

Farther west, a series of shortwaves now over and near the
Mississippi Valley, and related area of broadly cyclonic flow, will
move to the East Coast by the end of the period, atop the
continental/Polar air mass left behind the front.  An upstream
trough -- apparent in moisture-channel imagery from southern BC
southward across northwestern CA -- consists of phased northern and
southern components.  These will split apart through the period,
with the southern vorticity lobe  -- now crossing northern CA --
digging east-southeastward over the Sierra today.  As this occurs,
DCVA and related steepening low/middle-level lapse rates will occur
atop marginal low-level moisture/theta-e and weak CINH. This will
contribute to the potential for convection.  MUCAPE should remain
under about 150 J/kg, but potentially extending deep enough into
suitable icing layers for low-topped cumulonimbi and isolated
lightning production this morning into midday.

..Edwards.. 12/17/2020

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