SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0652 AM CST Fri Jan 01 2021

Valid 011300Z - 021200Z


Isolated strong-severe gusts and perhaps a tornado are possible from
eastern Alabama and parts of the Florida Panhandle to parts of South

In mid/upper levels, a binary cyclone system is evident in moisture-
channel imagery, within a large area of cyclonic flow covering most
of the Rockies and central CONUS.  The leading cyclone is centered
over southeastern OK, and is expected to eject northeastward and
weaken through the period.  By 00Z, the cyclone should be located
over IL, while beginning to phase with a northern-stream shortwave
trough now moving southeastward across northern SK.  The low will
devolve into an open-wave trough thereafter, accelerating east-
northeastward to western NY and Lake Ontario by 12Z tomorrow. 
Meanwhile, the upstream 500-mb low -- now over Sonora -- will pivot
across northern MX, becoming a strong, open-wave trough over the
southern Plains and northeastern MX by 12Z.

At the surface, 11Z analysis showed the main low (related to the
eastern mid/upper cyclone) over eastern OK between MLC-FSM, with an
elongated area of low pressure winding eastward along a wavy
occluded front over central AR.  The triple point was drawn near the
northeastern corner of MS, with warm front across northern/eastern
AL, central GA, southern SC and offshore NC.  The cold front
extended across western AL to the central Gulf.  The original/
occluded low should move northeastward across the Ozarks to IL,
close to the mid/upper cyclone center, while continuing to fill.

Meanwhile, a low should form today over the northeastern AR/western
KY/MO Bootheel/southern IL region and move northeastward toward the
lower Great lakes overnight, itself becoming the anchor of the
primary surface cyclone, while also increasingly stacked with
respect to the circulation aloft.  The trailing cold front will move
slowly eastward as the upper low weakens and pulls away.  This
boundary should be located over eastern TN, southwestern AL and the
central Gulf by 00Z.  By 12Z the front should reach the western
Carolinas, GA and the central/eastern FL Panhandle.  Cold-air
damming will retard warm-frontal advance over the southern
Appalachians/western Carolinas region, but the front should move
northeastward up the coastal Tidewater area through the period.

Isolated to widely scattered thunderstorms will remain possible
within a loosely organized band of convection along/ahead of the
front over portions of AL, GA and the FL Panhandle, in a nominally
favorable CAPE/shear parameter space.  The northeastern extent of
this activity may reach parts of SC this evening before weakening. 
A tornado cannot be ruled out in any sustained supercell or wrapping
notch on an embedded quasi-linear segment, and isolated strong-
severe gusts also are possible.

Warm and moist advection will continue in the boundary layer ahead
of the primary convective swath, offsetting the modest lapse rates
aloft enough to yield a narrow corridor of around 500 J/kg of
preconvective MLCAPE today.  Hodographs will be less elongated and
more looped with southeastward extent, but also smaller with time
and eastward extent over GA/SC.  Effective shear will diminish
southward and southeastward into higher theta-e as well, as the
mid/upper cyclone fills and ejects northeastward away from the
region as a deamplifying wave.  The resulting dampening mass
response will decrease low-level convergence with time, leading to
concerns about coverage and persistence of the strongest convection.
 Given those factors, weaknesses of both low/middle-level lapse
rates and lift at various scales may preclude sufficiently
persistent deep convection to organize more than a marginal/
conditional severe threat.

..Edwards/Leitman.. 01/01/2021

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