Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 250531 AAA

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1231 AM CDT Tue Jul 25 2017

Updated for 06Z aviation discussion


06Z TAF discussion:

Expect isolated to scattered showers and thunderstorms to
redevelop along and south of the I-20 corridor overnight, with
activity increasing and getting focused more towards
HKS/JAN/MEI/NMM/PIB/HBG sites after daybreak and through the
afternoon hours. Some locally heavy rains and potential for
associated prolonged IFR conditions with downpours is more likely
early through late morning around HBG/PIB, but also not out of the
question at other central MS sites. Away from storms expect IFR to
MVFR ceilings to be possible through 8 to 9 am in the morning at
most sites, although GLH should be MVFR at worst. Winds away from
storms through tomorrow will be light and rather variable. /BB/


As of 215z, latest RAP analysis indicates a mid/upper-level cold
core low centered over east central Louisiana with a ridge
centered over the western CONUS and a trough swinging through the
Great Lakes and into the Northeast. A deep moisture axis,
characteristic of 2-2.3 inch PWATs, is centered across central
Mississippi keeping a soupy airmass locked in across the CWA. The
main headline today was torrential rainfall and areas of flash
flooding, especially along the Hwy 84 corridor where 340-345 K
ThetaE air combined with the aforementioned PWAT axis and
convergent flow to produce an area of training storms with some
locally higher rainfall totals before activity spread further
north into the Hwy 82 corridor where sufficient daytime heating
allowed for robust convection and more heavy rain. Any remaining
convection has been quickly dissipating over the last few hours
with only a few showers left on area radars. Expect a lull in the
activity during most of the overnight before precip chances ramp
back up Tuesday morning. Some patchy fog cannot be ruled out
either with the main focus being in the Hwy 82 corridor.

By tomorrow morning, the upper-low that has been plaguing the area
will finally get picked up by the departing trough over New
England and lifted through the area. Latest guidance is in
agreement showing a similar setup to what we saw today across the
southern portion of the area. 2.25+ inch PWATs in concert with an
axis of deep moisture convergence and ThetaE advection, and
forcing from the vort max associated with the low will lead to an
early start to shower and storm activity Tuesday morning along and
south of I-20. This activity is expected to spread east during the
morning hours before tapering off during the early to mid
afternoon as the better moisture and forcing pull northeast out of
the area. Given the axis of heavy rain totals observed today from
Claiborne to Franklin counties and points east opted to go ahead
and introduce a limited risk for flash flooding in this area as
it won`t take much rain to lead to another round of flooding
issues. This setup will have to be closely monitored as a flash
flood watch could possibly be needed given the already saturated
soils from Monday`s heavy rain. /TW/

Prior discussion below:

Tonight and Tuesday:

An upper level disturbance swinging east through the region,
coupled with a very moist airmass, and westerly low level flow
along a theta-e axis, continues to cause showers and storms to
develop and train over the same areas while producing heavy
rainfall across South Central and Southeast Mississippi this
afternoon. This prompted a Flash Flood Watch, that remains in
effect for South Central and Southeast Mississippi until 7 PM,
along with several Flash Flood Warnings and reports of flash
flooding over the area today. My current thinking is, and latest
high-res models agree, that this activity will subside and shift
east southeast out of the area through the afternoon and evening
as low level flow becomes more northwesterly and this better area
of convergence responsible for the redevelopment and training of
convection shifts out of the CWA. As a result, I`m currently not
anticipating having to extend the current Flash Flood Watch. In
addition, simply with the loss of daytime heating, convection as a
whole is expected to wane over the CWA during the evening hours.
However, some isolated activity looks to remain possible across
the southern half of the CWA overnight.

On Tuesday, strong high pressure aloft will begin to build east and
back into the forecast area.  This will cause the better rain
chances to shift more to the eastern half of the CWA.  Still, with
the warm moist and unstable airmass over the CWA during the
afternoon, showers and storms are expected to develop across the
forecast area during the heat of the afternoon.  Highs in the low to
mid 90s across locales west of the Mississippi River, primarily
across the Delta, combined with dew points in the middle 70s will
cause heat indices to peak around 100F during the afternoon.  Again,
any convection that develops during the afternoon, will begin to
wane around sunset. /19/

Tuesday night through next Monday morning:

The period will begin with convection slowly winding down through
mid- week & heat stress will be on the increase by mid-late week
before a cold front brings in increasing rain & storm chances &
relief to the recent heat stress issues.

At the start of the period, a strong deep upper ridge will be
centered over the western Plains into the Texas panhandle while
strong zonal flow/disturbance on the top of the mid-upper level
ridge will be moving through the northern Plains/Great Lakes &
through southern Canada while the troughing will be departing the
eastern seaboard. At the surface, a strong surface high will be
situated over the northeast with a lingering frontal boundary
extending southwest well into the Appalachians. Any disturbance
in the Gulf or any perturbations will be slowly on the decrease as
we approach mid-week. As the surface low and cold front shift well
off into the Atlantic Ocean, expect the stalled frontal boundary
to gradually shift east. This will help convection to slowly pull
east by Wednesday & into Thursday. PWs will slowly fall from
around 2+ inches currently on Monday to nearly 1.5-1.75 inches by
late Wednesday & into Thursday. This will help convection to
become more scattered and diurnal. Lapse rates/low-mid level flow
are weak so not expecting much in the way of strong storms.

Heat looks to be the main story as the strong ridging becomes more
amplified & pivots the ridge axis and increasing heights building
from west to east by Thursday & into Friday. This and increasing
boundary layer temperatures at 850mb, 20-21+ deg C each evening,
(some 90-97th+ percentile) will support warming back into the mid
90s or so. We should be able under full sun to reach up to around
95+ degrees, especially around Thursday and with the best warming
near 95-97 degrees in the Delta. With dewpoints peaking in the
low-mid 70s, heat stress will be on the increase. Expanded the
area in the graphic closer to I-55 but kept the HWO the same.
Couldn`t rule out some areas being expanded even more or a heat
advisory being possible as we get closer. The hottest and
muggiest day is expected on Thursday. Heat stress & heat related
illnesses will be possible. Make sure to drink plenty of water &
stay cool!

The pattern will change by late week & into the weekend as a cold
front moves into the area. The upper ridge axis will slowly
weaken its grip on our area and begin to build back to the west
over the western Plains & into the Rockies. In addition, a strong
deep trough will amplify over the eastern half of the nation,
east of the Rockies and into the Plains, as a strong surface low &
associated cold front traverse into the northeast CONUS. This
will help drive a more significant cold front with anomalous
moisture, near 2+ inch PWs & 345+K 850mb Theta E, in by around
Friday & especially Saturday. This will help more convection moving
in by Friday afternoon, with the bulk of the rain & storm chances
moving in by the weekend. This will help temper down our heat
stress issues. Lapse rates/flow look meager, but due to high
moisture, there could be some locally heavy downpours with this
cold front. Temperatures will slowly cool down into the mid-upper
80s due to increasing convection Saturday. As a strong high
pressure filters in the wake of the cold front, much drier air,
potentially near 1 inch or so, could move in by Sunday & into
Monday. This will bring temperatures a few degrees below average
and much drier conditions at least through the end of the period.


Jackson       74  94  74  95 /  15  22   8  19
Meridian      73  93  74  94 /  20  32  13  24
Vicksburg     73  95  73  96 /  12  11   4   9
Hattiesburg   73  92  73  94 /  20  35  13  25
Natchez       73  92  72  94 /  10  11   5   8
Greenville    75  96  74  96 /  12  11   4   9
Greenwood     74  94  74  95 /  15  16   5  17





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