SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0638 AM CST Tue Dec 15 2020

Valid 151300Z - 161200Z


Thunderstorms are possible this evening and tonight over the central
Gulf Coast region.

In mid/upper levels, a split-flow pattern will prevail across the
CONUS, with the northern stream belt over southern Canada and
portions of New England through the period. The southern stream will
buckle around a strong trough -- now evident in moisture-channel
imagery from the NE Panhandle southward across eastern portions of
CO/NM.  This trough will shift eastward across the southern Plains
and gradually assume more-positive tilt with time.  By 12Z tomorrow,
the 500-mb trough should extend near an axis from UIN-LIT-GGG-SAT.

At the surface, 11Z analysis identified a low over the Llano
Estacado northwest of MAF and southwest of LBB.  This low should
remain in continental/Polar (CP) air as it moves southeastward
across central TX into this evening.  A weak low was drawn over the
Gulf southeast of CRP and northeast of BRO, with warm frontogenesis
underway to its east, and north of a synoptic front that extends
from south FL across the central/southwestern Gulf.  As the mid/
upper trough approaches, the northwest Gulf low should deepen
slightly and move to roughly south of LCH by around 00Z, still
distinct from the TX inland low.  The Gulf low will become the
dominant one overnight, while moving east-northeast generally toward
the coastal AL/FL line.  A warm front should extend southeastward
from that low, while the other frontal zone weakens from west-east,
allowing both sea-air fluxes and theta-e advection to modify the
Gulf low's marine warm sector gradually. A cold front will extend
southwestward from the low, and move eastward across the
northwestern/north-central Gulf.

...Central Gulf Coast -- LA to western FL Panhandle...
A belt of convection, with isolated to scattered embedded
thunderstorms, should develop by this evening near the low and
southwestward along the cold front.  Isolated to widely scattered
thunderstorms also are possible over the modifying Gulf warm sector,
which will be weakly capped.  Convection should extend inland from
the low along a low-level trough, atop a relatively stable near-
surface layer with modest elevated buoyancy.  This regime should
shift eastward overnight, in step with the longitudinal progression
of the low.  The strongest DCVA/cooling aloft and associated steeper
midlevel lapse rates should lag the regime, closer to the mid/upper
trough.  However, buoyancy may extend into icing layers supporting
lightning production inland.

Marginally surface-based inflow parcels may reach the immediate
coastal areas of southeastern LA to the Mississippi River mouth,
after 06Z and in a narrow plume with MLCAPE less than 500 J/kg. 
This potential is very conditional, especially given:
1.  The great amount of open-water airmass modification still needed
-- especially with ongoing, cold offshore cP flow still observed
18-24 hours out, and
2.  Potential for partly modified warm-sector trajectories to return
through precip before reaching inland, even if the warm front does
reach the coast.
While an isolated strong storm cannot be ruled out on that stretch
of coast, the potential for severe appears too low, small in scale
and conditional for an outlook area.  Strong-severe convection --
including supercells -- are possible farther south, over open Gulf
waters outside the outlook domain, and may impact aviation, shipping
and rig interests there.

..Edwards.. 12/15/2020

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