Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 241522 AAA
AFDJAN

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1022 AM CDT Mon Jul 24 2017

.UPDATE...
Updated for morning discussion.

&&

.DISCUSSION...
Thanks to a subtle upper level disturbance swinging east across
the region and some sufficient low level flow along a theta-e
axis, a band of moderate to heavy rain has set up across the
southern half of the CWA this morning, particularly across
Southeast Mississippi in the vicinity of the Highway 84 corridor.
This has resulted in rainfall totals in some spots in the two to
four inch range over the past three to five hours, which in turn
has prompted the issuance of some flash flood warnings in our
area this morning, as well as in surrounding areas.

While little in this setup will change through the day as the
aforementioned disturbance swings through, with daytime heating
and the soupy air mass over the region (noted on the 12Z KJAN
RAOB where a 2.10 inch precipitable H2O value was observed),
convection today will continue to have the potential of producing
heavy rainfall that could potentially result in some localized
flash flooding. This potential for flash flooding could be further
enhanced by any training of convection over the same locales, but
radar trends are currently indicating less training potential at
the moment.

Across the remainder of the CWA, despite the trough swinging
through, convection will still primarily be more scattered in
nature and diurnally driven. While heavy rain will remain possible
given this moisture rich airmass, convection looks more
progressive with a much less likelihood of slow-moving training
activity leading to flash flooding. Much of this activity will
dissipate around sunset this evening as daytime heating wanes.

Despite the better rain chances today, compared to recent days,
overall warm muggy conditions will persist as highs range from the
mid 80s to lower 90s.

Outside of some minor adjustments based on current trends, no major
changes will be made to the forecast on this update. /19/

Prior discussion below:


Through Tonight:

Scattered showers and thunderstorms have already started
developing at this early hour over southern zones thanks to subtle
atmospheric lift brought by a upper level low migrating east,
just north of the coast. The weather across the whole Lower MS
Valley will be influenced by this feature through the short term
but the largest share of the impact should be on areas along and
south of I-20 today where mostly cloudy skies and numerous showers
and scattered storms will keep temperatures down below 90 degrees
in almost all cases. Further north there will be more sunny
breaks by midday after morning low clouds disperse and highs in
the upper 80s to lower 90s there will work with ample atmospheric
moisture (precipitable water values 2+ inches) to generate
moderate instability by afternoon. In these areas it may take a
while for thunderstorms to develop but expect various slight
surface boundaries propagating in from the north and south to
eventually trigger a good deal of activity before the day is out.
We inherited a limited risk of flooding in the HWO for central and
southern zones and opted to keep it going for this forecast.
While explicit short range numerical guidance suggests the best
potential for torrential rains may be focused more toward the
coast there are still enough suggestions of training storms or
very slow moving storms in the previously-outlined area to
warrant a low-end potential for at least minor localized flooding
issues.

The slow-moving upper low mentioned initially should be
transitioning more east of the region tonight but westerly
convergent low level wind vectors into the backside of the feature
will continue to provide impetus for shower and storm development
even past the evening (when afternoon convection typically
diminishes). The best thetaE advection axis several thousand feet
above the ground should be aimed at the southeast half to third
of zones where it is not impossible some issues with locally heavy
rain might continue at least intermittently through the night.
The next shift will watch this aspect of the forecast closely and
update mentioned heavy rain threats as warranted. Otherwise,
anticipate another warm and muggy night with shower and
thunderstorm activity generally diminishing after midnight
elsewhere and widespread temp minimums in the low to mid 70s.
/BB/

Tuesday through next weekend...

A very moist airmass will exist throughout the ArkLaMiss on
Tuesday as moisture is fed into the region from the Gulf due to
high pressure over the eastern Gulf. Forecast soundings depict
PWats of above 2". Expect the best coverage of storms and showers
to occur during the the mid morning through afternoon hours.
Flash flooding due to heavy rainfall will be the primary threat on
Tuesday. Drier conditions will begin to prevail late Tuesday into
Wednesday as high pressure over the Gulf expands, cutting off the
low level moisture flow into most of the region. Upper level
heights will begin to increase also on Tuesday as ridging builds
over the eastern CONUS. The increased moisture levels and
precipitation will keep temperatures at bay on Tuesday but
temperatures will once again approach hazardous levels on
Wednesday as some drying occurs, especially in the western
portions of the region.

The region will be even drier on Thursday, where convective
activity will be limited to the our easternmost counties. 850
temperatures will increase to above 20 degrees by Thursday. This
usually corresponds to temperatures in the upper 90`s in late
July. Although widespread temps in the upper 90`s aren`t expected
on Thursday, some may be sampled in areas along and west of the
MS River. These temps combined with dewpoints near 70 will result
in heat index values surpassing 105 degrees in the aforementioned
zones. Friday could be the warmest day of the period depending on
the timing of an approaching surface front dropping into the
region from the north and associated with an upper level low that
will cross through Canada. If the Euro`s solution of a slower
front holds true, temperatures could reach the upper 90`s areawide
ahead of the front. The front will result in a stormy start to
the weekend but cooler temperatures and drier air in it`s wake
will provide much needed relief from the heat and humidity of this
upcoming week.

&&

.AVIATION...
12Z TAF discussion:

Locally heavy showers and a few thunderstorms will be impacting
HBG/PIB sites directly over the next few hours with convection
gradually spreading north to cover other sites along and south of
the I-20 corridor through late morning to midday. GLH/GWO/CBM/GTR
may take until afternoon to get much thunderstorm development, but
at least scattered activity is expected in those areas as well. Of
course locally IFR conditions will be briefly possible in all
areas getting underneath a thunderstorm today. This morning away
from storms expect IFR to MVFR conditions (mainly due to ceilings)
through 9 to 10 am. Conditions should be mainly VFR away from
heavier precipitation by the time we get to the afternoon. Surface
winds will be from the southeast generally and less than 10 mph
(except stronger gusts of course will be possible around
tstorms). /BB/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       88  74  89  73 /  65  31  52  17
Meridian      88  73  87  73 /  65  35  57  23
Vicksburg     89  73  91  73 /  65  34  40  13
Hattiesburg   85  73  88  73 /  66  39  59  23
Natchez       85  73  89  72 /  65  35  37  10
Greenville    91  74  91  74 /  46  34  38  13
Greenwood     91  75  90  74 /  47  28  44  16

&&

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

&&


$$

19



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