SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0648 AM CST Wed Jan 06 2021

Valid 061300Z - 071200Z


Severe thunderstorm winds and a couple tornadoes are possible today
into tonight along parts of the western Gulf Coastal Plain.

In mid/upper levels, a highly amplified, progressive synoptic
pattern is dominated by:
1.  A broad, slow-moving cyclone over the North Atlantic, centered
south of NS,
2.  Troughing from the length of MB south-southwestward across the
Plains states to Chihuahua, and
3.  A strong shortwave trough offshore from the Pacific Northwest
and northern CA that will contribute to isolated thunder potential
near the coast.

In the southern part of the second synoptic trough, a strong, basal
vorticity max/PV anomaly and accompanying shortwave trough are
located initially over eastern NM, based on moisture-channel imagery
and objective analyses.  This perturbation is forecast to dig
southeastward to central TX by 00Z, then eastward to northern LA by
12Z.  This will form a closed 500-mb cyclone in the process, while
the accompanying 250-300-mb speed maxima blend with a nearly zonal
subtropical jet preexisting across northern MX and south TX.

As that occurs, a surface low -- initially analyzed near SPS with
cold front across the Edwards Plateau -- will move east-
southeastward over northeast TX and northern LA, while occluding. 
By 00Z, the cold front should arc from the low near TYR across east
TX then southwestward over the middle/lower TX Coastal Plain and
deep south TX.  By 12Z, the cold front should extend from a triple
point in the SIL/BIX area across the mouth of the Mississippi River
to the west-central Gulf.  A diffuse/marine warm front will extend
eastward close to the AL coast then southeastward over the eastern
Gulf, while a synoptic warm front in cooler/less-modified air should
extend across the northern/central FL Peninsula. 

...Near-coastal TX/LA...
Scattered thunderstorms should develop close to the front -- as
early as late morning over central TX -- and build southward with
time to the middle TX Coastal Plain.  The entire resulting, arching
MCS should shift eastward and southeastward across the western
outlook area through this evening, offering damaging to locally
severe gusts, isolated/marginally severe hail, and some tornado
potential.  A related but separate area of more conditional,
marginal severe potential may develop late tonight into early
tomorrow morning over extreme southeastern LA and vicinity, as the
convection (which may continue in elevated form across the
intervening gap) encounters a surge of sufficiently moist, unstable
boundary-layer air.

With surface dew points now only in the mid 50s to low 60s F across
parts of south-central TX, modified RAOBs and RAP soundings already
indicate minimal MLCINH, related to a combination of cooling air
aloft and lack of a stronger EML.  This partially modified warm
sector will shift eastward and destabilize further through the day,
from a combination of continued theta-e advection and slow diabatic
surface heating (blunted by cloud cover).  That should support
around 1000 J/kg MLCAPE over western parts of the outlook area,
decreasing northeastward and eastward over east-central/southeast TX
where deep-layer lapse rates will be weaker overall.  Effective-
shear magnitudes around 40-50 kt and effective SRH in the 100-200
J/k range will support supercells and line-embedded LEWPs/
mesovortices, locally boosting the wind/tornado threat.

As the MCS proceeds eastward through the Golden Triangle region into
southwestern LA, it will encounter a progressively more-stable
boundary layer.  This will be related to substantial,
isallobarically forced easterly component to surface winds that
emanates from (and helps to maintain) a relatively stable
continental boundary layer.  With convective inflow accordingly
becoming elevated, the potential for severe gusts and tornadoes will
diminish over land, though severe weather may still affect offshore
interests southeast of GLS and south of the western/central LA
coastline.  Though the most favorable thermodynamic support will
remain over the Gulf, non-negligible wind/tornado threat with the
remains of the MCS may extend northward across parts of southeastern
LA and coastal MS the last few hours of the period.

..Edwards/Leitman.. 01/06/2021

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