Area Forecast Discussion
Issued by NWS Memphis, TN

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601
FXUS64 KMEG 182024
AFDMEG

Area Forecast Discussion
National Weather Service Memphis TN
224 PM CST Mon Dec 18 2017

.DISCUSSION...

Currently...A 1025 mb surface high is positioned over the lower
Mississippi Valley. The pressure gradient is a little stronger
over northeast Arkansas and as a result low clouds are starting to
mix out. Meanwhile toward the south where the gradient is weak...
low clouds are hanging in from the Delta through Memphis and along
I-40. Temps range from the mid 40s in areas where low clouds are
thickest to around 60 across NE Mississippi where the sun is also
breaking out.

Tonight...Should continue to see some improvement in sky cover
across northern sections into the early evening. A warm front over
the lower Mississippi Valley will then start lifting north
tonight with low clouds spreading north and redeveloping. So even
if it clears out briefly it will be clouding right back up. Expect
areas of fog and patchy drizzle to develop as well with a few
showers possible across the Delta by morning as the warm front
lifts toward the area.

Tuesday into Wednesday...A potent mid level trough will move from
the SW U.S. through Texas into the southern plains on Tuesday.
Meanwhile a warm front will lift north into northern Mississippi
while surface low pressure develops over the ARKLATEX and then
tracks along the front. Showers and embedded thunderstorms will
develop Tuesday morning and continue into Tuesday night. Looks
like the heaviest rainfall will occur along and just north of the
surface low track where very high UVVs are in place for an
extended period of time. This area encompasses northern
Mississippi into extreme southwest Tennessee and east central
Arkansas where 3-4 inches is likely. Given the potentially
convective nature of the rainfall expect that some rain rates may
be high enough for some isolated flash flooding despite the dry
antecedent conditions. Will add this mention to the HWO. As the
surface low tracks into the north Mississippi Tuesday night and
the cold front pushes across the lower Mississippi valley there is
some concern for severe weather mainly south of the region where
some instability overlays significant wind shear. Will need to
monitor this but for now will leave any mention of strong/severe
storms out of the HWO. Rain will taper off Wednesday morning as
the upper support moves east though clouds will likely linger.

Wednesday night into Thursday...In between systems this period
with above normal temperatures expected as winds turn southerly
on Thursday. Expect highs in the lower to mid 60s.

Thursday night through Friday...The last period in which the
confidence in the models is fairly high. A warm front will start
lifting north ahead of an approaching cold front moving across the
southern plains Thursday night. Expect clouds to increase with
showers developing by morning...maybe some thunder over the Delta.
Cold front pushes through on Friday with another round of
potentially heavy rain. Will need to watch severe weather threat
across north Mississippi since that area may destabilize some
before the front moves through but too early too tell.

Saturday through Monday...Major discrepancies continue in the
medium range. The 12z GFS continues to push the front well east
and weaken the upper ridge across Florida. As a result the cold
air floods south and the main axis of moisture remains well to
the east. Other than a few flurries in the arctic air it is
mostly dry through the period. The 12z ECMWF keeps a stronger
ridge over Florida and hangs up the moisture axis. As the next
piece of energy rotates around the mean upper trough over the
central U.S. precipitation continues over the Mid-South mainly
east of the MS River. Meanwhile colder air is sinking south at the
low levels setting up the possibility of some winter weather. The
ensemble systems for both models still exhibit a wide ridge of
solutions so confidence is low. We could be dealing with a cold
and dry weekend and Christmas or it cold be slightly warmer with
more precipitation...some potentially of the winter variety. For
now the plan is...turning colder on Saturday with a chance of rain
east of the river. Went with a chance of rain areawide on Sunday
though temps will be warm enough at this point for all rain
despite the colder air working south. Sunday night into
Monday...chance of precipitation with perhaps a wintry mix
spreading south overnight into Monday morning as the cold air
continues to filter in. Again confidence is very low in these
details.

SJM

.AVIATION...

18Z TAFs

Late morning GOES visible imagery showed a clearing area over
north central AR. This area was expanding east with daytime
heating/mixing and will likely spread into the MEM area during the
afternoon. Mid/high cloud coverage was greater over north MS/west
TN than over AR, this may limit or slow the eastward expansion of
VFR this afternoon.

Expect IFR to gradually build in areawide this evening. NAM Bufr
soundings depict SFC-FL010 nearly saturated but well-mixed into
the late evening, leading to reduced confidence in IFR forecast.
By 09Z, a steep saturated elevated inversion is depicted around
FL008, north of a developing surface warm front over north MS.

Flight conditions may be slow to improve Tuesday morning/early Tuesday
afternoon, given continued -DZ/-SHRA and continued influx of tropical
maritime air lifting over the stationary surface front across
northwest MS.

PWB

&&

.MEG WATCHES/WARNINGS/ADVISORIES...
AR...None.
MO...None.
MS...None.
TN...None.
&&

$$



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