Area Forecast Discussion
Issued by NWS Shreveport, LA

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242
FXUS64 KSHV 281220
AFDSHV

Area Forecast Discussion
National Weather Service Shreveport LA
720 AM CDT Fri Apr 28 2017

.AVIATION...
For the 28/12Z TAF period, MVFR and occasional IFR cigs blanket
all sites this morning as southerly flow increases with a warm
front advancing north of I-20 as of this TAF issuance. These cigs
will linger throughout the morning, slowly lifting and scattering
to VFR into the afternoon with southerly winds increasing up to
15-20 kts with gusts near or exceeding 25-30 kts. Any convection
that is able to develop this afternoon and evening will be very
isolated so have held off any mention in the TAFs at this time.
Cigs will drop back to MVFR around 29/03Z as moisture advection
increases ahead of a slow moving frontal boundary to the west.
Gusty south winds will maintain through the overnight period as
the pressure gradient holds steady in advance of the approaching
front.

/19/


&&

.PREV DISCUSSION... /issued 614 AM CDT Fri Apr 28 2017/

DISCUSSION...
A busy forecast period is taking shape today through this weekend
as low level moisture advection continues to increase in the short
term as upper trough over the Great Basin and Desert SW this
morning continues to amplify, eventually closing off near the
Four Corners Region later today. The morning sfc analysis
indicates a warm front beginning to surge N across N and Ern TX
into Cntrl LA, extending from a sfc low just SW of SPS to near the
DFW metroplex, to TYR/GGG, to near IER and HEZ as of 10Z. This
warm front will continue to mix Nwd quickly this morning,
eventually reaching McCurtain County OK and into the extreme Nrn
counties of SW AR by 00Z Saturday, with much warmer and more humid
air expected to surge N with this fropa. Already seeing extensive
areas of stratocu this morning with and even N of the front this
morning, which may eventually scatter out later this afternoon
over E TX/N LA such that max temps will near the 90 degree mark,
with some areas possibly reaching this benchmark for this first
time this year. As the cigs lift and possibly scatter out later
today, a portion of the 40-50kt SWrly LLJ will mis down to the
sfc, resulting in gusty Srly winds over much of E TX/N LA and
extreme Srn AR. Have issued a Lake Wind Advisory for these areas
from 16Z through 12Z Saturday, with wind gusts to 25-30+ kts
possible. These strong winds look to persist overnight and
strengthen even further Saturday, thus necessitating an expansion
of the advisory across the remainder of the region by that time.

Even with the warm frontal passage today, the overall moisture
profile looks to remain quite shallow over much of the region,
although the short term progs are in agreement with H850 theta-e
ridging developing this afternoon across extreme NE TX into SE
OK/adjacent sections of SW AR, where weak perturbations in the SW
flow aloft may result in isolated to widely sct convection near
the nwd moving warm front. Have trimmed back pops significantly
this afternoon for these aforementioned areas, although an uptick
in convection may occur by early evening near the front as a
strongly unstable air mass develops S of the front and steep
H700-500 lapse rates advect NE with the broad SW flow aloft. Have
trimmed back svr mention to just McCurtain County this evening,
as the warm front will continue to mix N out of the area prior to
06Z. Given the winds, cloud cover, and very muggy air mass
advecting N, temps tonight will not fall off much, with most areas
only falling into the mid 70s, characteristic of summer.

The short term progs continue to suggest the closed low digging SE
through NM Saturday, with the potential for shortwave energy
ejecting NE well ahead of the low across extreme SE TX into
Ncntrl LA by midday/early afternoon, which should yield an
increase in sct convection over much of the area. However, the
more organized convection will remain to our N and NW closer to
the retreating warm front and cold front intersection from Srn MO
SW into NE/Cntrl and Srn OK, especially as large scale forcing
increases along the cold front/dry line once the closed low enters
W TX/OK Panhandle. Both the GFS and ECMWF are in good agreement
with convection expanding/deepening Saturday afternoon and
especially in the evening ahead of the cold front, with strong
frontal convergence along the nose of a 50kt Srly LLJ contributing
to an enhanced heavy rainfall threat over SE OK/portions of SW
AR. Given the deep SWrly flow near and ahead of the front, and
above normal PW`s of 1.7-1.8+ inches, some cell training is
possible, with these progs suggesting widespread 3-5+ QPF amounts
over these areas, where soils remain nearly saturated in wake of
heavy rain that has fallen earlier this month. Thus, have hoisted
a Flash Flood Watch from 17Z Saturday through 12Z Sunday, with an
expansion of the watch possible farther S into extreme NE TX/SW AR
should the progs suggest backbuilding may occur before slowly
advancing E with the fropa.

Also of concern is the SVR threat, with the very warm/moist air
mass contributing to MLCapes of 2500-3500 J/Kg across the broad
warm sector and strongly sheared environment. Can`t rule out at
least an isolated svr threat Saturday afternoon across portions of
extreme Ern TX/N LA/Srn AR with the lead shortwave ejecting NE
ahead of the closed low, although forecast soundings suggest that
capping may hold over these areas until height falls ahead of the
approaching closed low are able to weaken this Saturday night. The
convection to our W should organize along along/just ahead of the
front Saturday afternoon and evening along an area of strong
diffluence ahead of the low, before marching E across the region
Saturday night. Damaging winds and locally heavy rainfall are
possible with the squall line as it organizes and pushes E,
although the threat for embedded lewps along the line will result
in an isolated tornado threat as well. The progs suggest that this
line may weaken late as instability continues to diminish ahead of
the line across Ncntrl LA, with the strongest convection along
the line exiting the area shortly after 12Z Sunday. Should see
convection linger even behind the front Sunday morning before dry
slotting aloft quickly entrains E ahead of the closed low. Have
taken out severe wording Sunday, with cooler and drier conditions
expected to filter back into the region on Wrly winds.

Not much time to work on the extended this morning, but quiet
conditions are expected through at least the first half of the
week as a zonal flow aloft continues over the Srn Plains in wake
of the closed low departure. However, convection looks to increase
as the flow transitions back to SW ahead of the next upper trough
set to develop/dive SE across the Plains late Wednesday/Thursday.
Thus, have maintained high chance pops areawide with the
associated cold fropa before a return to slightly cooler but drier
air spills back S into the region for late week.

Prelims to follow below...

15

&&

.PRELIMINARY POINT TEMPS/POPS...
SHV  89  75  87  61 /  10  20  50  90
MLU  86  74  88  70 /  10  20  50  70
DEQ  81  72  81  57 /  20  40  90 100
TXK  84  74  85  60 /  20  20  70 100
ELD  85  74  86  65 /  10  20  50  90
TYR  89  75  86  57 /  10  20  60  90
GGG  89  75  86  58 /  10  20  50  90
LFK  90  76  88  61 /  10  20  50  90

&&

.SHV WATCHES/WARNINGS/ADVISORIES...
AR...Lake Wind Advisory from 11 AM this morning to 7 AM CDT Saturday
     for ARZ070>073.

     Flash Flood Watch from Saturday afternoon through Sunday morning
     for ARZ050-051-059>061.

LA...Lake Wind Advisory from 11 AM this morning to 7 AM CDT Saturday
     for LAZ001>006-010>014-017>022.

OK...Flash Flood Watch from Saturday afternoon through Sunday morning
     for OKZ077.

TX...Lake Wind Advisory from 11 AM this morning to 7 AM CDT Saturday
     for TXZ097-108>112-124>126-136>138-149>153-165>167.

&&

$$

19/15



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