SPC MD 1897



MD 1897 CONCERNING TORNADO WATCH 520…521… FOR PARTS OF THE UPPER TEXAS AND LOUISIANA COASTAL PLAIN

MD 1897 Image


Mesoscale Discussion 1897
NWS Storm Prediction Center Norman OK
0244 PM CST Thu Dec 31 2020

Areas affected...Parts of the upper Texas and Louisiana coastal
plain

Concerning...Tornado Watch 520...521...

Valid 312044Z - 312245Z

The severe weather threat for Tornado Watch 520, 521 continues.

SUMMARY...Potential for supercell structures capable of producing
tornadoes may become primarily focused across southwestern
Louisiana, near/south of Fort Polk, by 5-7 PM CST.

DISCUSSION...The center of the surface cyclone continues to deepen
across the Greater Houston Metro area, as an 80-90 kt cyclonic
mid-level jet (around 500 mb) noses across the lower Rio Grande
Valley, toward the upper Texas coastal plain.  However, the cyclone
is in the process of occluding, and inflow of moist boundary-layer
air into the upper Texas coastal plain probably will become cut-off
within the next couple of hours.

Although strongest south/southeasterly 850 mb flow is forecast to
develop across the Ark-La-Tex into the Red River Valley between now
and 23-01Z, 40+ kt speeds may still be maintained as far
south-southeast as the southwestern Louisiana coastal plain. 
Surface temperatures across the Louisiana coastal plain have warmed
into the lower and mid 70s, with surface dew points in the upper
60s/near 70F.  Despite weak lapse rates and lingering warm layers
aloft, it appears that this is contributing to mixed-layer CAPE of
500-1000 J/kg.

Given this instability, in the presence of wind profiles
characterized by strong deep-layer shear and sizable
clockwise-curved low-level hodographs, the environment is becoming
at least conditionally conducive to supercell storms capable of
producing tornadoes.  

Although strongest mid-level forcing for ascent will remain
displaced to the northwest, strongest high-level difluence appears
likely to overspread the region through early evening.  Aided by
low-level warm advection, this may contribute to discrete supercell
development ahead of the eastward advancing cold front, probably
focused near its intersection with the surface warm front (roughly
extending east-west across the Fort Polk vicinity).

..Kerr.. 12/31/2020

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...LCH...

LAT...LON   30389422 30919361 31099269 30999209 30899188 30639179
            29969170 29609208 29819308 29829371 29829399 30389422 

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