SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0656 AM CST Sat Dec 19 2020

Valid 191300Z - 201200Z


Isolated strong to severe thunderstorms may occur today across parts
of the middle/upper Texas Coast.

A generally progressive upper-air pattern will prevail through the
period, with broadly cyclonic flow spreading eastward from the
central to eastern CONUS.  The most important feature aloft will be
a positively tilted trough -- initially evident in moisture-channel
imagery from northern WI across eastern NE to far west TX and
adjoining northern MX.  This perturbation will maintain positive
tilt while moving eastward and gradually weakening.  By 00Z the
trough should extend from Lower MI across the Ozarks to south-
central TX.  By 12Z tomorrow, it should reach southeastern ON,
eastern KY and AL.

Surface analysis showed weak lows over TX between MWL-SPS and
between DRT-LRD, connected by a cold front.  The current south TX
low, or a new one, should move northeastward just of the mid/upper
TX Coast this afternoon, turning eastward along or just south of the
LA Coast tonight.  Mesoscale convective effects may modulate the low
track southward and/or somewhat faster.  The trailing cold front
will move across the TX Coast and northwestern Gulf, while extending
southwest of the low.  A weaker inland version of the front, and a
collocated surface trough, will extend northeastward from the low. 

...Mid/upper TX coastal plain...
Scattered thunderstorms are expected to develop, increase in
coverage and spread across the outlook area from mid/late morning
into the afternoon, offering the potential for isolated strong-
severe gusts and marginally severe hail.  A tornado cannot be ruled
out as well. 

The bulk of convection should occur in the form of a primary band
developing in a zone of relatively maximized low-level lift near the
front.  With weak CINH in place, isolated thunderstorms also are
possible farther east near the immediate coast and over the Gulf. 
This evolution should occur as the zone of lift encounters a
boundary layer steadily destabilizing from a combination of:
1.  Warm/moist advection of a still-modifying marine layer from the
Gulf, and 
2.  Muted diurnal heating, amidst considerable cloud cover. 

Although the CAPE-shear parameter space will be optimized offshore,
deep shear will increase over the outlook area as the mid/upper
trough approaches, with 40-50-kt effective-shear magnitudes becoming
common.  Well-curved but marginally sized hodographs are expected,
supporting 0-1-km SRH in the 100-200 J/kg range.  Modest low/middle
level lapse rates will keep peak preconvective MLCAPE around 500-800
J/kg inland near the coast, decreasing substantially northward.  A
few high-res mesoscale progs indicate that weak surface-based
buoyancy (i.e., MLCAPE 200-500 J/kg) briefly may extend as far
northward as parts of the HOU metro, immediately before the main
area of convection moves through.  As such, the marginal line has
been adjusted somewhat northward in this area.  

Late this afternoon into evening, the convective band should shift
eastward over the Gulf, Golden Triangle region of southeast TX, and
portions of southwestern LA.  This activity should outpace the
eastward extent of inland surface-based destabilization east of the
HOU area.  By then, the most intense convection (including supercell
potential) will be relegated to the Gulf -- where the most well-
modified and unstable inflow air will reside -- posing some threat
to the various offshore interests.

..Edwards/Smith.. 12/19/2020

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