Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 232030
AFDJAN

Area Forecast Discussion
National Weather Service Jackson MS
330 PM CDT Wed Aug 23 2017

.DISCUSSION...

Through Thursday:
Convection has struggled to get going today, as a
ribbon of somewhat drier mid/upper level air remains across much of
the area. 18Z KJAN sounding showed PW continuing a downward trend
since 6Z. Nevertheless, there is still potential for scattered
convection through this evening as a summer cold front moves
southward into the area overnight. The aforementioned front is
currently approaching the Delta, with 60s dewpoints not too far
behind - already being observed in the Memphis area. Noticeably
drier and slightly cooler air will begin to filter into northern
portions of the CWA late tonight, with lows in the upper 60s by
tomorrow morning in some areas. Unfortunately, this front will begin
to stall Thursday somewhere between the I-20 and Hwy 84 corridors.
That will still be good enough for pleasant conditions tomorrow
across the upper Delta and Hwy 82 corridor. However, isolated to
scattered storms will remain possible across the south as PW values
remain near 2". Between the slightly cooler air mass to the north of
the front and more opportunities for clouds along and south of it,
high temps will be slightly below normal on Thursday. /DL/

Thursday night through Wednesday...

The period will be characterized by a complex and low confidence,
but potentially high impact forecast with respect to the evolution
of now tropical depression Harvey.

At the start of the period on Thursday evening, a trough will be
located in the Great Lakes region with a frontal boundary progged to
be located south of I-20 with showers and storms expected to be
ongoing along and south of said boundary. This convective activity
should dissipate through the early evening hours with the chance for
an isolated shower or two to linger overnight. Much drier air will
continue to filter in behind the boundary with dewpoints dropping
into the upper 50s along the Hwy 82 corridor to mid 60s further
south. This will help allow for cooler low temps overnight into
Friday morning. For Friday, the front should stay stalled out in the
far southern portions of the area where isolated to scattered
afternoon showers and storms will be possible, but overall it should
be a dry day across the vast majority of the area.

By the weekend into the middle of next week, confidence quickly
drops off as model disagreement and significant run-to-run
inconsistencies continue to plague the forecast in regard to
tropical depression Harvey. As of the 15z NHC outlook, Harvey is
located in the Bay of Campeche just off the northwest coast of the
Yucatan Peninsula. Model guidance continues to offer a widely
varying array of solutions, thus introducing large spatial and
temporal uncertainties for any impacts to the ArkLaMiss. Will stick
to the official NHC track and WPC QPF with the general thinking that
Harvey will continue to intensify as it tracks out of the Bay of
Campeche and into the northwest Gulf of Mexico before making
landfall somewhere along the south central Texas coast on Friday.
This is where uncertainty really starts to increase as Harvey
becomes located between a ridge over the Desert Southwest and
another ridge over the northeast Gulf of Mexico with very weak
steering flow through the troposphere. Latest model trends have been
to increase the amplitude of the Desert Southwest ridge and thus
keeps Harvey stalled over Texas with the potential for a
bifurcation pattern for Harvey`s track. Some guidance suggests a
turn to the left confining the impacts to Texas but these
solutions appear to be outliers at the moment with the majority of
guidance and the official NHC track favoring a turn to the right
with heavy rain potential spreading towards the ArkLaMiss.

By early to mid next week, the Desert Southwest ridge looks to
breakdown/retrograde into the Great Basin at the same time a
stronger shortwave trough dives down through the Northern Plains in
northwesterly flow. This shortwave trough looks to open a weakness
in the ridging and increase steering flow allowing Harvey, or at
least remnants, to propagate through the ArkLaMiss before getting
absorbed into the mean flow over the Ohio Valley while undergoing
extratropical transition. Explicit model QPF continues to show some
incredible amounts of rainfall from Texas into southwest Louisiana
where confidence is higher in flooding concerns, but continue to
struggle with placement and timing of the axis of heaviest rainfall
across the ArkLaMiss. A lot of moving parts and complex interactions
will govern any impacts to our area so uncertainty remains high as
previously mentioned, however given the presence of deep tropical
moisture, a slow moving tropical system, and the fact that models
have consistently painted swaths of 6-10+ inches of rain the
possibility for potentially high impact flooding cannot be ignored
but exact placement, timing, and amounts will have to be continually
refined as the event nears and it is entirely possible that the
heaviest rain occurs outside of our CWA completely. With all of
that being said, will continue to advertise the limited risk for
flooding for the entire area in the HWO/graphics but will not make
any changes at this point given the lengthy list of
uncertainties. And finally, impressive 0-1km shear and helicity
values will favor low topped miniature supercells and the
potential for severe weather but the spatial extent of this threat
will be dependent on the exact track of Harvey. Once again, this
is a low confidence forecast with the potential for high impacts
so please stay abreast of the latest forecast over the next week.
/TW/

&&

.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/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       72  88  68  86 /  26  21   9   9
Meridian      72  88  66  88 /  20  15   5   9
Vicksburg     72  87  69  85 /  33  24   9  11
Hattiesburg   73  90  71  88 /  20  33  26  14
Natchez       72  86  70  84 /  35  38  20  22
Greenville    69  86  66  87 /  30   5   4   8
Greenwood     69  87  65  87 /  19   4   3   6

&&

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

&&


$$

TW/DL



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