U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 180543 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1243 am CDT Thu Apr 18 2019 


Valid 181200z - 191200z 


..there is an enhanced risk of severe thunderstorms today into 
tonight across parts of the central Gulf Coast states... 


... 
Severe thunderstorms are possible today across the lower Mississippi 
Valley through the central Gulf Coast states, accompanied by the 
risk for damaging wind gusts and a few tornadoes, some of which 
could become strong. 


... 
An amplification within the large-scale mid/upper flow now 
progressing inland of the Pacific coast appears likely to translate 
downstream, through the Mississippi Valley, where a deepening 
mid-level trough is expected to evolve by 12z Friday. One 
significant short wave trough and mid-level low, already progressing 
through the Southern Plains, appears likely to become increasingly 
sheared while accelerating across and northeast of the mid/lower 
Mississippi Valley vicinity. This may include one prominent 
embedded perturbation, associated with extensive ongoing convection 
across central/eastern Texas, which is forecast to pivot through the 
lower Mississippi Valley by late this afternoon. Thereafter, a 
significant upstream short wave trough, digging to the Lee of The 
Rockies, is forecast to continue southeastward into the lower 
Mississippi Valley, where another embedded mid-level low may begin 
to evolve by the end of the period. 


Models indicate primary low-level cyclogenesis during this period 
will accompany the lead perturbation, with a deepening surface low 
center probably migrating from East Texas through the lower 
Mississippi Valley, toward the western slopes of the central 
Appalachians. Considerable convection, and associated surface 
outflow, are expected to precede the trailing cold front across the 
lower Mississippi Valley and central Gulf Coast states. 


While the low-level environment across much of the southeast may be 
initially dry and or stable, models continue to indicate an influx 
of seasonably high boundary-layer moisture off the western Gulf of 
Mexico, across parts of the lower Mississippi Valley and central 
into eastern Gulf Coast states. The extent to which this precedes 
the eastward advancing convective outflow remains unclear, but 
potential exists for sufficient destabilization to support 
re-intensification of thunderstorm activity, with the evolution of 
an organized squall line accompanied by potential damaging wind 
gusts and a few tornadoes possible. 


..lower Mississippi Valley/central Gulf Coast states... 
Some uncertainty exists concerning how quickly thunderstorms will 
redevelop and intensify along eastward advancing outflow generated 
by ongoing thunderstorm activity now spreading through eastern 
Texas. There appears likely to be at least some stabilization of 
mid-level lapse rates. However, guidance indicates that southerly 
850 mb flow may strengthen in excess of 50 kt, inland of the 
Louisiana coast through Mississippi, as early as the 18-21z time 
frame. This probably will be accompanied by a corridor of 
considerable boundary layer moistening, with guidance indicating 
surface dew points increasing through the upper 60s to near 70 f as 
far north as at least much of southern/southwestern Mississippi, 
contributing to cape of 1000-1500 j/kg. 


Aided by strongly difluent and divergent upper flow, and strong 
vertical shear, a gradually intensifying and organizing squall line 
is expected to be accompanied by increasing potential for damaging 
wind gusts, and perhaps a few embedded tornadoes. As the convective 
system advances eastward, it may be preceded by discrete supercell 
development, and a weak warm frontal zone, which may provide the 
focus for large clockwise curved low-level hodographs supportive of 
a risk for tornadoes. A couple of these could become strong, 
perhaps as far east as southeastern Alabama by late tonight. 


.Kerr/Bentley.. 04/18/2019 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 180851 
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Mesoscale discussion 0385 
National Weather Service Storm Prediction Center Norman OK 
0351 am CDT Thu Apr 18 2019 


Areas affected...East Texas...southwestern and south-central 
Arkansas...western and central Louisiana 


Concerning...Severe Thunderstorm Watch 81... 


Valid 180851z - 181045z 


The severe weather threat for Severe Thunderstorm Watch 81 
continues. 


Summary...local severe risk -- predominantly comprised of 
strong/damaging wind gusts -- continues across portions of ww 81. 


Discussion...the latest radar loop over the arklatex/Sabine River 
valley region shows a line of storms -- with embedded complex 
structures -- moving across East Texas toward the river. The line 
itself is advancing eastward at roughly 30 kt, while individual 
bowing elements within the line are moving northeastward with a more 
rapid forward motion (near 40 kt). One such bowing segment -- 
crossing the Rusk County Texas vicinity -- appears to be producing 
wind gusts near 40 kt. Per recent obs from ktyr (tyler, tx). 


Storms should maintain intensity over the short term, especially 
farther to the south where the instability axis appears broader, per 
latest objective analyses. Farther north along the line however, 
into the arklatex region, a north-northwest-south-southeast instability gradient exists, 
which suggests overall/gradual weakening of the line as it shifts 
east with time. Still, we will continue to monitor the 
environmental and convective evolution, with an eye toward possible 
downstream ww issuance as convection begins to approach the eastern 
edge of the current ww. 


.Goss.. 04/18/2019 


..please see www.Spc.NOAA.Gov for graphic product... 


Attention...WFO...Jan...lzk...lch...shv...hgx... 


Latitude...Lon 33869377 33989325 34029205 33169190 30909252 30339385 
30099549 31809466 32519494 33869377