Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 231736 AAC
AFDJAN

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1236 PM CDT Wed Aug 23 2017

.UPDATE...
Updated for 18Z aviation discussion

&&

.AVIATION...
18Z TAF discussion:
VFR conditions will prevail through the majority of the TAF
period. Scattered SHRA/TSRA, potentially resulting in brief
categorical reductions, will be possible as a cold front moves
slowly southward into the area this afternoon into tonight. As the
front stalls over the south part of the area, precip will remain
possible mainly at PIB/HBG into Thursday. There is some potential
for MVFR fog in the south as well early Thursday morning, but
confidence is too low to include in TAF at this time. /DL/

&&

.DISCUSSION...
We`re off to a relatively quiet start this morning, with only a
dissipating area of showers moving across the Delta at this point.
A cold front continues to approach the area today. Though winds
have already shifted to northerly across much of the CWA, the
actual front and associated theta-e discontinuity/airmass change
lag back along a Northern AR/MO Bootheel/KY-TN border corridor.
Morning WV imagery and model analysis show two different pooling
areas of deeper layer moisture over our area - one in the north
near the actual front and a second area over the southern half of
the area extending out into the Gulf in association with a
lingering weak shortwave upper trough along the coast. Given the
dearth of clouds over the south half of the area, I`m inclined to
believe there is greater potential for development with the
greater instability that will exist with the moisture pool in
that part of the area. Meanwhile, cloud debris from remnant/dying
showers north of the area may hamper or delay development a bit
in the northern part of the area.

Given these thoughts, we have shifted higher POPs for the daytime
period a bit farther south. There will still be a decent chance
for rain in the north as the front continues to move southward.
/DL/

Prior discussion below:

Today and Tonight: A cold front over far north MS looks to
slowly drop into the area by late morning. The forcing with the
boundary will combine with the deep moisture in place and increasing
heating for scattered to numerous showers and thunderstorms. The
best coverage will be over the north as the front will arrive here
during prime heating. Convection will continue along the boundary
moving into tonight, but coverage will be less with the loss of
heating. Meager lapse rates and weak deep layer shear should
preclude the severe threat with the storms this afternoon.

Most sections will still see another muggy night tonight, however,
drier air arriving over the north after midnight will allow minimums
by Thursday morning to drop into the mid 60s here.

Thursday through Tuesday: Overall more seasonable conditions will
move in the wake of a cold front. By early next week, remnants
the remnants of the slow-moving tropical system (Harvey) looks to
bring very heavy rainfall, flash flooding & severe weather
potential.

By Thursday & Friday, a cold front will have moved through the
region, with strong ridging & drier air filtering, mainly in the
Highway 82 corridor & some drier air extending down into the Highway
20 corridor. PWs will be closer to 2 inches closer to the Gulf Coast
while an inch and a three quarters or less will be possible along &
north of I-20, with some potential of less than an inch PWs in the
north. This & dewpoints falling into the low-upper 60s will make for
a cooler day. Ensembles don`t hit on as much cooling Thursday before
cooler thermal profiles on Friday, so went slightly warmer in the
mid-upper 80s on Thursday with slightly cooler conditions on Friday.
This will be a few degrees below normal. As that front slowly drifts
south & stalls along the Gulf Coast, some isolated-scattered showers
& storms are possible along & south of the I-20 corridor &
especially towards the Highway 84 corridor by late week. Due to some
drier air around, less clouds & strong ridging, we could fall into
the mid 60s, with coolest conditions on Friday.

Main forecast challenge/impacts in the long term looks to be the
eventual track of the potential tropical development in the western
Gulf (formerly Harvey). NHC is giving this very high likelihood of
development over the next couple of days & consensus of
deterministic guidance all show redevelopment of Harvey as it moves
off of the Yucatan Peninsula & into the Bay of Campeche & west-
northwestern Gulf of Mexico. For now, by Thursday-early weekend, our
area should be protected from any impacts of Harvey as it is progged
to landfall along the Texas coastline around Friday. Where along the
Texas coastline this will occur is uncertain as deterministic &
ensemble guidance are having a hard time with initializing the
system center. In addition, the synoptic evolution will become
important in the overall timing & eventual track. Aloft a
subtropical ridge over the Great Basin & mainly zonal flow is
progged to be moving through the southern Great Plains. The exact
landfall location will be important as the GFS seems to want to
bring it just enough north near Galveston. This would help it phase
quicker with flow aloft & swing towards our area quicker. However,
the Euro, Canadian & ensembles show a slower pattern with it
slightly further south along the Texas coastline, thus helping it
miss the phasing to the north & stall out over Texas. Canadian seems
the furthest south. Ensembles & Euro seem to fit a slightly in
between idea, helping it miss the weakness along the baroclinic
zone to the north & move slower towards our area by around early
next week (Monday-Tuesday). Regardless, our area will experience
impacts as nearly 2-2.5+ inch PWs & increasing shear profiles will
bring the risk for heavy rain, flash flooding & severe weather
being possible by late weekend & especially into early next week
(Monday- Wednesday). Leaning towards slower solutions but the
overall synoptic influence & development will be key in the track
& overall forward speed. Explicit deterministic QPF is hitting
hard on the rainfall totals, with the GFS & Euro showing upwards
of 10-20 inches over the Texas coastline, with some 5-7+ inches
over portions of our area. There has be run-to-run inconsistencies
on exact placement, timing & totals. However, there seems to be
enough confidence to at least begin mentioning heavy rain & flash
flooding possible in the HWO/graphics. /DC/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       90  71  88  68 /  58  28  20   6
Meridian      91  71  88  66 /  48  33  15   5
Vicksburg     90  71  88  70 /  57  28  23   6
Hattiesburg   91  73  90  71 /  48  42  38  15
Natchez       89  72  87  71 /  57  39  40  20
Greenville    88  69  86  65 /  58  14   4   4
Greenwood     88  69  87  65 /  34  13   4   4

&&

.JAN WATCHES/WARNINGS/ADVISORIES...
MS...None.
LA...None.
AR...None.

&&


$$

DL/26/DC


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