Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 031037

Area Forecast Discussion
National Weather Service Jackson MS
437 AM CST Sat Dec 3 2016

Today through Tonight: Rain event is beginning to take shape across
the area this morning. While radar is showing widespread rainfall
over the forecast area at the moment, the actual leading edge has
only managed to encroach upon a Camden AR to Greenville MS to Jackson
MS to Quitman MS line as of 03/10Z. East of this line, the rain
continues to erode the dry air in the lower levels and should begin
reaching the ground later this morning.

The main culprit for the widespread rain event is escalating
isentropic ascent over the region. Increasing onshore flow over the
upper TX coast will steadily feed warm/moist air from the Gulf of
Mexico into that area. As this air encounters the baroclinic zone
along the coast, it will rise and continue to rise downstream
isentropically up and over the cooler/stable air near the surface by
strengthening winds aloft. The axis of best ascent looks to setup
over northeast LA into north central MS where amounts by early
Sunday morning may approach 3 inches. Due to recent prolonged dry
conditions and the slow accumulation of these rains, am not looking
for any flooding problems at the moment.

The models are also indicating some potential for elevated
convection after midnight tonight. Pockets of MAUL aloft with
shallow CAPE look to traverse northeast LA and across central MS
with somewhat deeper CAPE/instability rooted around H85 moving over
southern sections. Not a lot expected from this convection other
than a few claps of thunder and some brief, localized, higher
rainfall rates.

With the prolonged cloudy and rainy conditions through tonight, not
expecting a lot of variation in temperatures. After morning lows in
the mid to upper 40s, highs will only crawl into 50s and just fall
back into the mid/upper 40s again tonight./26/

Sunday through Thursday: Main areas of focus next week will be on
the potential for strong storms Monday (and/or heavy rainfall Sunday
into Monday) and a much colder airmass for the end of the week.

Sunday: ...will be a rather raw day across much of the area as
isentropic ascent continues across the area north of the coastal
baroclinic zone. Synoptic-scale support for a widespread rainfall
event will increase during the day as a shortwave swings through the
Mid-Mississippi Valley and upper divergence increases in the right
rear quadrant of a broad upper jet streak. In general, rain rates
are expected to remain in check, as PWs won`t be exceptionally high
and much of the precip will be stratiform in nature. However, there
will be some elevated instability, with models still showing hints
of MAUL, so thunder and brief periods of heavier precip remain
possible. With widespread clouds and precip, there will be a smaller
diurnal range. Daytime highs were trended down toward raw guidance.
A bit of a decrease in coverage is anticipated Sunday night as the
shortwave moves away from the area, though large scale ascent will
continue to support periods of light rain.

Monday: A more potent upper shortwave (the once closed upper low
over northern Mexico) will swing into the area late Monday.
Continued strong upper divergence will support the development of a
surface low along the NW Gulf Coast, which will deepen as it tracks
NE across the area Monday afternoon and evening. This will bring
another uptick in precip across the area, including better potential
for thunderstorms as shear increases and strong height falls bring
steeper lapse rates. Severe storms will be possible, but there
remains a good deal of uncertainty over how large of an area. The
return of richer surface based moisture will be delayed until
shortly head of the surface low. This calls into question how far
north surface based instability will become available. Particularly
with cooler easterly surface flow and potential ongoing rain
reinforcing the more stable surface layer just ahead of the cyclone.
Latest ECMWF and GFS continue to suggest mid 60s dewpoints will
surge northward across east MS later in the evening. We`ll continue
to highlight a limited threat for severe in our graphics/HWO with
only minor changes planned to the area.

Some additional periods of heavy rain will be possible Monday into
Monday night. WPC has portions of the area highlighted for a slight
risk for excessive rainfall Sunday and Monday. We certainly can`t
rule out the potential for some localized flooding, but given the
still dry antecedent conditions and the fact that most of the
anticipated rain will be spread out over a longer period of time, we
plan to hold off on a flood threat graphic at this point.

Tuesday through the rest of the week: There will be no major
cool down with the passage of the Monday night system, as the upper
jet remains north of the area. However, the pattern will remain
active, and the next big upper disturbance will deliver another cold
front in the late Wednesday/early Thursday time frame - this one
accompanied by a notably colder airmass. There remain some
differences between the ECMWF and GFS on the timing and character of
this front. The GFS shows mostly post-frontal precip and even hints
at wintry precip. However, even if this solution was believed,
forecast soundings suggest very light precipitation and saturated
layers would primary exist in temps > -5 C, suggesting more of a
drizzle situation. The ECMWF shows a slightly more robust system
with precip occuring more along the front and in liquid form. Given
the strength of the upper dynamics, the ECMWF solution seems more
plausible at this point, and this was reflected in the forecast.
Strong cold advection will occur behind the front as an anomalously
strong surface high builds in across the southern Plains. After a
quick cool off by late Thursday, lows in the 20s appear likely by
Friday morning, with highs on Friday possibly struggling to reach
the 40s in some parts of the area. Meteorological winter isn`t
waiting long to make its presence felt. /DL/


06Z TAF discussion:
Widespread rain will continue to slowly spread into the region from
the southwest overnight. The majority of what reaches the ground overnight
will be light as dry air in the lower levels evaporates most of what
falls. With the relatively dry air in the lower levels, VFR ceilings
and visibilities will exist until 03/18Z. Thereafter, MVFR conditions
will increase from the southwest as ceilings lower and rainfall
increases in intensity./26/


Jackson       53  49  54  50 /  95 100  99  72
Meridian      53  48  56  51 /  77  89  89  71
Vicksburg     53  49  52  49 /  99 100 100  74
Hattiesburg   57  51  67  56 /  63  69  78  67
Natchez       54  52  57  51 /  95 100  97  78
Greenville    51  46  49  46 /  98 100  83  59
Greenwood     52  46  50  47 /  99 100  88  60




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