Area Forecast Discussion
Issued by NWS Shreveport, LA

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940
FXUS64 KSHV 300251
AFDSHV

Area Forecast Discussion
National Weather Service Shreveport LA
951 PM CDT Wed Mar 29 2017

.UPDATE...
Sent an update to remove a couple counties from TOR Watch # 100
which remains in effect until midnight and to remove pops west of
the dry line which is the back line of convection to our west.
Also reduced pops east of the dryline which in compasses the
remainder of our CWA. TOR Watch # 101 for portions of Central and
Northeast LA remains in effect until 3 am. Cold front to the west
and northwest of the dryline will eventually catch the dryline
later this evening/overnight and help to advance what is left of
the convection eastward. Over the last hour, the line of
convection which developed on the dryline had weakened
considerably as we continue to lose instability this evening.

13

&&

.PREV DISCUSSION... /issued 700 PM CDT Wed Mar 29 2017/

AVIATION...
Stg tstms increasing in coverage over portions of LA, including
KMLU with large storm cells to train across terminal locations
thru 30/02-03z. Stg winds, hail, and excess ltg strikes with these
storms. Further to the north and west, scattered cells increasing
ahead of dry line, including vcnty of KTXK, KTYR and KGGG. Cells
movg quickly newd 30 to 35 kts. Skies bcmg vfr skc behind dryline
btwn 30/03-06z except for eastern sections where storms may
continue near KELD and KTXK until around 30/06z. South winds 10 to
15 kts becmg west 5 to 10 kts behind dryline, before increasing
to 10 to 20 kts with daytime mixing by around 30/14-15z. Once
again, gusty winds will decouple to lgt by around 31/00z, with
skies continued skc./VII/.

PREV DISCUSSION... /issued 531 PM CDT Wed Mar 29 2017/

UPDATE...
Just a quick update to add Tornado Watch #100 in effect until
Midnight for portions of Northeast Texas, Southeast Oklahoma,
and portions of Southwest Arkansas.

Also added Natchitoches, Winn, Caldwell, Grant and La Salle
Parishes in Central Louisiana to Tornado Watch #98 in effect until
9 pm this evening.

Convection across Tornado Watch #100 is the most uncertain as
while mesoanalysis showing that the atmosphere has recovered
instability wise across this region, water vapor imagery also
showing dry slotting working its way into that region from the
west. Thus, the severe threat across Northeast Texas, Southeast
Oklahoma and Southwest Arkansas is very conditional.

Further south across Tornado Watch #98, we are not only dealing
with a moderately unstable airmass but also strong mid level lapse
rates and descent low level shear which suggests that isolated
tornadoes, large hail and damaging wind gusts will all be
possible across this area through the remainder of the evening.

Updated products have been sent...13.

PREV DISCUSSION... /issued 352 PM CDT Wed Mar 29 2017/

DISCUSSION...

A Tornado Watch is in effect until 9 pm CDT for Angelina, San
Augustine, Sabine county and Sabine Parish...

Earlier model run iterations struggled with the speed of the line of
showers and thunderstorms from earlier this morning, as it was
exhibited to impinge on the ARKLATEX and southeast OK much later
than what actually occurred. The line has since dissipated but
scattered showers and thunderstorms persisted across southeast TX
which drifted northeast to across portions of Deep east TX and
central Louisiana late this aftn, which was a result of being co-
located with the best low level moisture axis and lingering old
outflow boundaries. Given this morning/s precip activity, the
atmosphere has been worked over/stabilized a bit, which was
validated by isolated showers struggling to develop. However, the
UA storm system is still to our west across southern KS and drier
air aloft ahead of the system, is infringing upon east TX, deep
east TX and southeast OK and portions of southwest AR, thus
resulting in thinning middle to upper level clouds. Consequently,
daytime destabilization may come to pass whilst a 35+ kt LLJ
persists leading to increased instability and moistening low-mid
levels respectively. In addition, a nearby sfc low has promoted a
tightened pressure gradient, leading to breezy southerly sustained
wind speeds of 15-25 mph. A notable decline in wind speeds is
expected shortly after sunset.

At this time, best severe indices are noted across portions of deep
east TX: MUCAPE of 1000 j/kg, 0-6 km bulk shear of 50 kts, 0-3 km
SRH of 300-400 m2/s2, 0-1 km shear of 20-25 kts, low LCL/s and
curvature in the low levels per progged hodographs. What these
parameters suggest is that storms will have the potential to
become severe, with damaging wind gusts, hail and isolated
tornadoes being the main threats. At 20Z, a severe storms was
located approximately 30 miles south of Zavalla, TX just south of
the CWA, with a movement to the northeast. The neighboring office
issued both a Severe Thunderstorm Warning and a Tornado Warning
with this storm. This could be the start of this afternoon/s
severe storm activity, with additional storms expected. A Tornado
Watch is in effect until 9 pm CDT for Angelina, San Augustine,
Sabine county and Sabine Parish.

This evening into tonight, the UA low across the Southern Plains
is expected to lift east-northeast to across the Central Plains
and near the lower to middle Mississippi Valley region. Southwest
flow aloft will sharpen and an embedded impulse within the flow
aloft will move across the region, resulting in an additional
catalyst for new storm development moreso along and east of a
Dequeen AR to near Shreveport LA to near Natchitoches LA line.
Models hint at the resurgence of a cluster of storms/line of
storms once again posing a threat for damaging winds, hail and
isolated tornadoes. This may result in the Tornado Watch /or a
Severe Thunderstorm Watch/ being expanded to portions of the
aforementioned areas later this evening into tonight. Make sure
you stay abreast to the latest weather information by going to our
website at www.weather.gov/shv, listening to your NOAA weather
Radio or tuning in to your favorite media outlet. Concurrently,
the precipitation activity will gradually get shoved from west to
east courtesy of the UA system/s cold front/pacific front that
will usher in a stabilizing airmass. Although some wrap around
cloud cover/light rain/drizzle may linger across portions of
southwest AR and north central LA tomorrow late morning into early
afternoon, the severe threat will be diminishing as best severe
indices exit the region.

Thereafter, the progressive synoptic flow will continue with quiet
weather expected Friday /and temps rebounding from the 60s and 70s
tomorrow to the 70s and 80s on Friday/ given the passage of a low-
amplitude UA ridge. Rain chances will return for late weekend as
an UA low slowly digs east-southeast across the Desert southwest,
to across northern Old Mexico, to across northeast central TX by
Sunday night. Sharpening southwest flow aloft and increased UL
support will raise concerns for severe potential. Furthermore, the
slow movement of the storm coupled with rich Pacific moisture
being transported to across the region resulting in PWATS of
1.50-2.00 inches is why long term solutions are hinting at high
rainfall totals /several inches/. This is something we will be
paying close attention to in the coming days as we want to make
sure models are consistent from run-to-run. We will then get a
reprieve from significant wet conditions Monday-Wednesday though
an UA low passing to our north may encourage isolated to scattered
showers along its sfc counterpart Tuesday-Wednesday.

FB

&&

.PRELIMINARY POINT TEMPS/POPS...
SHV  56  72  50  84 /  50  20   0   0
MLU  61  75  50  83 /  50  40  10   0
DEQ  50  63  42  80 /  10  20   0   0
TXK  53  66  48  82 /  50  20   0   0
ELD  55  69  47  81 /  50  30  10   0
TYR  52  73  52  86 /  10   0   0   0
GGG  54  72  51  85 /  30  10   0   0
LFK  56  77  52  89 /  50  10   0   0

&&

.SHV WATCHES/WARNINGS/ADVISORIES...
AR...None.
LA...None.
OK...None.
TX...None.
&&

$$

VII/13



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