Area Forecast Discussion
Issued by NWS Shreveport, LA

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000
FXUS64 KSHV 171614
AFDSHV

Area Forecast Discussion
National Weather Service Shreveport LA
1114 AM CDT Thu Aug 17 2017

.UPDATE...
To lower the temperatures, dew points, heat idices for some
locations, raise the pops for many locations and lower for
others.

&&

.DISCUSSION...
The leading edge of the outflow boundary had shifted into
North Central Louisiana and extended Southwest into parts of Deep
East Texas and the East Texas Lakes Region. The resultant lower
temperatures will affect the daytime high readings and have
dropped the heat advisory for a row of zones and parishes across
Texas and Louisiana. With some scattered showers and possibly
a thunderstorm, other locations in the heat advisory area may not
reach criteria but opted to keep in for now and let expire at
7 PM CDT. /06/

&&

.PREV DISCUSSION... /issued 645 AM CDT Thu Aug 17 2017/

AVIATION...
For the 17/12z TAFs, a convective complex currently moving through
the Interstate 30 corridor will continue east-southeast across the
area through the period. Terminals along and north of Interstate
20 will likely be affected by this convective complex today. South
of Interstate 20, including KLFK, rain chances are lower and more
uncertain, but scattered showers and thunderstorms may also develop
ahead of the line. Convection should gradually weaken and exit
most of the area by 18/00z. Abundant mid-level clouds are expected
to persist through the end of the period.

/09/

PREV DISCUSSION... /issued 525 AM CDT Thu Aug 17 2017/

DISCUSSION...
Currently watching the MCS dropping SE across N and NE TX/SE OK
and SW AR this morning, driven primarily by the trailing upper
trough from the classic mid-latitude cyclone over the upper
Midwest, extending SW into OK. This cold pool remains fairly
progressive, given the 2-3mb/3 hour pressure rises noted behind
this convection over Srn OK, with the mesoscale analysis depicting
strong moisture convergence along the leading edge of the gust
front near the DFW Metroplex, extending NE along the Red River.
All of the short term progs are struggling with the ongoing
convection, with the HRRR depicting the convection better to some
extent, albeit a couple of hours too slow, as is usually the case
with more organized MCS`s. However, analysis depicts that the
best convergence/pressure rises are occurring across the Wrn flank
of the MCS, with a notable cooling of cloud tops per IR imagery
over extreme NE TX/SE OK/SW AR, which is indicative of weakening
convergence along the lead gust front as this bndry is becoming
more unidirectional with the low level flow. The HRRR is correctly
depicting this, which suggests that the associated mesoscale
bndry could lay up somewhere over N LA late this morning/early
afternoon, but could make more progress SE into E TX around the
same time before weakening. Have had to expand likely/high chance
pops farther SE into NE TX/SW AR and extreme NW LA this morning,
before this complex completely weakens. Thus, confidence regarding
the current Heat Advisory in effect for E TX and N LA S of I-20
is low, but is quite likely that at least the nrn fringe of the
Advisory area over portions of Lower E TX/NW LA will need to be
cancelled early as temps should remain a bit cooler than forecast.
Did leave the Advisory area in place as is, and will continue to
evaluate based on the SE progression of this storm complex.
However, the potential for compressional warming and low level
moisture pooling near and S of the bndry should yield a great
potential for heat indices at/above 105 degrees across Ncntrl LA
and portions of Deep E TX.

The residual convective bndry may play a role for additional sct
convective development later this afternoon especially over E TX/N
LA near and S of I-20, where moderate instability develops along
differential heating bndrys in VC of the tail end of the upper
trough as it lies along the I-20 corridor. Did keep mid chance
pops going this afternoon for these areas, and carried over chance
pops along/S of I-20 this evening as the convection could
continue, tapping into the lingering instability in place.

The short term progs continue to advertise the H850-500 ridge
along the Gulf coast E of the mouth of the MS River retrograding
W and amplifying into extreme SE TX and across much of Srn LA by
this evening, and persisting in place through Friday. Meanwhile,
the remains of the weak trough/shear axis appear to linger along
the I-20 corridor Friday, with the potential for lingering
mesoscale bndrys focusing sct convective development especially by
afternoon in VC of a NW to SE oriented H850 theta-e ridge.
However, the progs are not in good agreement with QPF, with the
GFS completely dry across the region. Given the low confidence,
have toned down pops to low chance for Ncntrl LA until the progs
come into better agreement. However, with max temps climbing to
the mid 90s across much of the area, as the presence of the ridge
to our S, there is the potential that heat indices will near
Advisory criteria across portions of Ern TX/N LA, but will defer
this decision to later shifts.

The upper ridge remains forecast to retrograde W into TX Saturday
as another shortwave trough begins to dig SE across OK and into
the MS Valley. Another MCS is progged to develop over Wrn and
Cntrl OK Friday afternoon and evening, and progress ESE along a
weak sfc front progged to become stationary tonight from Ern OK
into Srn AR late tonight/Friday morning. As was the case with this
morning`s MCS, this next one could follow on its heels, affecting
the Nrn sections of the region after 06Z Saturday, Have beefed up
pops to mid chance for these areas, with mesoscale bndrys with
this complex likely focusing additional convective development
during the day Saturday over much of the region. Again, given the
low confidence in placement of where the best focus will be, have
expanded chance pops over much of the region, with mid chance pops
over the Ern sections of N LA further away from the center of the
upper ridge to our W. The convection should remain mostly diurnal
in nature, with the unsettled weather pattern continuing into
Sunday as the remnants of this shortwave may linger over the
region.

Afterwards, our focus will turn to our SE as a tropical wave will
migrate W across the Gulf, which should result in widely sct
convection over the region Monday, before diminishing to just the
Srn zones Tuesday as this wave lingers. Fortunately, it appears
the ridging aloft will remain to our W over TX, with a continued
influence in NW flow through the end of the week, which could
yield the potential for at least isolated convection and temps
remaining near climo through the period.

Prelims to follow below...

15

&&

.PRELIMINARY POINT TEMPS/POPS...
SHV  92  77  94  77 /  60  30  30  20
MLU  92  75  94  75 /  40  30  30  20
DEQ  88  74  92  74 /  30  20  30  40
TXK  88  75  93  75 /  80  20  30  30
ELD  89  75  93  75 /  60  20  30  30
TYR  91  77  94  77 / 100  30  30  10
GGG  92  77  94  77 / 100  30  30  10
LFK  96  76  95  76 /  30  30  20  10

&&

.SHV WATCHES/WARNINGS/ADVISORIES...
AR...None.
LA...Heat Advisory until 7 PM CDT this evening for LAZ017>022.

OK...None.
TX...Heat Advisory until 7 PM CDT this evening for TXZ152-165>167.

&&

$$

06/09


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