SPC Dec 26, 2016 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0647 AM CST Mon Dec 26 2016

Valid 261300Z - 271200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
SOUTHWESTERN AR TO PARTS OF THE MID-SOUTH...

...SUMMARY...
Isolated strong to locally severe thunderstorms may occur from
southwestern Arkansas to parts of the Mid-South, mainly through this
afternoon.

...Synopsis...
In middle/upper levels...the strong cyclone now centered over
northwestern MN is forecast to eject northeastward across Ontario to
north-central Quebec through the period.  This will leave behind a
regime of gently cyclonic mean flow over roughly the northern 1/2 of
the contiguous U.S., through which numerous low-amplitude shortwaves
will travel.  One of these perturbations -- now evident in
moisture-channel imagery over eastern UT -- should move eastward to
western/central parts of KS/NEB by 00Z, then deamplify and eject to
OH and Lake Erie by 12Z.  A Pacific cyclone aloft will remain
offshore from CA and move slowly southward as a cut-off low.

At the surface, an 11Z analysis showed a 986-mb low near FAR, with
an occluded front curving across northern MN, northern/eastern WI to
southernmost Lake Michigan.  From there a cold front was drawn near
a STL-HRO-FTW-JCT line, becoming quasistationary southwestward into
Coahuila.  The surface cyclone is forecast to fill and move
northeastward to near the southern tip of James Bay by 06z and
across northern Quebec afterward.  By 00Z the cold front should
extend from the lower Ohio Valley region southwestward across the
Arklatex to south-central TX, becoming more diffuse with
southwestward extent in TX.  A reinforcing cold front initially over
southern Kansas should be located across the Red River region
west-southwestward over southeastern NM.  By the end of the period,
the combined frontal zone should extend from New England
southwestward over the central and southern Appalachians to central
portions of MS/LA and the middle/upper TX coastal plain. 

...Southwestern AR to parts of the Mid-South...
Strong to severe thunderstorms are possible through this afternoon
in and near the marginal-risk area; damaging gusts are possible and
a tornado cannot be ruled out.  Most of the convection should occur
in a prefrontal/low-level convergence zone already apparent from
northeast TX northeastward across central/northeastern AR, and
forecast to shift eastward gradually through the daylight hours. 
The front should act as more of a western bound to the threat with
time, though the convergence zone should remain the primary focus,
given relatively backed flow to its southeast.  A deep layer of
favorable low-level moisture, with surface dew points 60s F and
1.1-1.4-inch PW, will offset modest midlevel lapse rates enough to
support 500-1200 J/Kg MLCAPE, amidst 35-40 kt effective-shear
magnitude.  

Given the geometry of the deep-layer flow and especially the lack of
stronger linear forcing, at least short-lived discrete to
semi-discrete storm structures are possible, in an environment where
forecast soundings suggest enlarged low-level hodographs and
favorable SRH for supercells.  However, the same lack of intense
low-level lift that precludes rapid upscale/linear growth also may
hamper storm sustenance when encountering stable layers aloft, such
as those observed in the 12Z SHV/LZK soundings.  Given these
offsetting factors, unconditional severe probabilities are kept at
marginal categorical levels for now.

Severe potential will become even more limited with northeastward
extent along/ahead of the front due to weakness of instability, and
southwestward from a decrease of both low-level shear and deep-layer
lift.

..Edwards.. 12/26/2016

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