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FXUS64 KJAN 171800 AAA

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1200 PM CST Tue Jan 17 2017

Updated for 18Z aviation discussion


18Z TAF discussion: 
MVFR to VFR conditions will continue through the
afternoon. Scattered showers this afternoon will be most
concentrated along a slow moving cold front extending from KIER-K5A6-
KTUP line at 17/18Z. After 18/00Z, IFR conditions will increase as
low clouds and fog develop. LIFR conditions will develop once again
at HBG as the fog becomes locaally dense./26/


Today and tonight...An upper level low and it's attendant surface
low, will continue tracking northeast into the Great Lakes Region
today. This will cause a cold front to shift east and into the
forecast area. As this front loses its upper level support, it'll
eventually become quasi-stationary on a northeast to southwest
trajectory across the forecast area tonight, in the general vicinity
of the Natchez Trace Parkway.

Aloft and over us, persistent ridging wedged into the region from
the east southeast will maintain southwest flow, thus increasing
moisture aloft over the region.  At the surface and ahead of the
front, south to southwest winds will also result in a continual
stream of moisture over much of the forecast area.  All of the
above, in addition to a couple of weak impulses embedded in the
before mentioned southwest flow aloft traversing the region, will
result in good rain chances across much of the forecast area today
and into this evening. Overnight, while there will be some some
showers across the CWA, there should be a an overall lull in
activity.  However, another disturbance aloft is progged to begin
impeding upon the region from the southwest Wednesday morning.

Some storms will also be possible across the CWA today, primarily
along the front itself.  However, these storms are expected to
remain well below severe limits.  That said, some heavy downpours
will be possible with the activity ahead of and along the front,
with some training of convection possible.  Behind the front, west
northwest winds will advect in some slightly cooler drier air.

Otherwise, highs across much of the forecast area will top out in
the low and middle 70s.  The only exception being in the Delta were
low to upper 60s are expected behind the front.  Lows tonight will
range from around 50 in the Delta, to the lower 60s across South
Mississippi. /19/

Wednesday through Monday...It will continue to feel more like late
March or early April than mid January in the ArkLaMiss this period.
Anomalously warm and stormy weather will be the general rule as a
strong southern stream upper level jet interacts with unseasonably
moist and unstable airmass surging northward into our region. The
most impactful weather should center on Thursday, when there will be
a limited threat for flooding and severe weather, and from Saturday
into Sunday when there will be the potential for a more significant
severe threat.

Wednesday should see somewhat of a lull in the action as the frontal
boundary stalls and weakens over our area beneath a continued strong
southeast CONUS ridge while greater forcing and moisture convergence
refocus to the west with the approach of more meaningful height
falls. Expect the potential for heavier convective rainfall to
develop Wednesday night over the Delta region as low level flow
increases to 30kts helping to transport a 1.5+ inch precipitable
water airmass northward while most unstable CAPE increases to ~500

This heavy convective rainfall potential will shift east across the
remainder of the forecast area Thursday with increasing deep layer
shear and instability supporting the possibility for a few strong to
severe storms especially along/se of the Natchez Trace corridor.  It
looks like the widespread nature of clouds/precip will limit
instability and overall severe threat.  The moisture convergence
axis should achieve longest duration over roughly the same area
resulting in heavy rainfall as well.  The long spell of drier than
normal conditions and unseasonable warmth have reduced soil moisture
so that flooding is not a big concern for now...but localized
rainfall rates could certainly become heavy enough for at least
minor flooding given the anomalous moisture. The trough axis will
sweep activity quickly east of the area late Thursday and expect
another break in the action Friday into Friday night. 

Global models continue to indicate that a more significant threat
for severe weather should evolve this weekend.  Expect rapid onset
of moisture transport early Saturday into southern portions of the
ArkLaMiss, and with steepening mid level lapse rates > 8 deg C/km
and H850 theta-e values ~330 K, mixed layer CAPE is forecast to
increase to near 2000 j/kg, an extreme value for January. Along with
strong deep layer shear, there is little doubt that the environment
will be supportive of significant severe weather.  But due to the
very fast upper level flow and how surface features will respond,
there is a lot of uncertainty regarding lift and specific areas of
deep convection, especially with the tendency for coastal convection
in this sort of pattern with big height falls well to our south.

After coordination with SPC, will indicate a limited severe threat
for mainly the southern half of the area and hold off on elevating
the threat until model details can become more consistent with
convective evolution. Will also broad brush the time window to
include both Saturday/Sunday. Look for quieter and cooler weather
beyond Sunday as the first significant intrusion of CP air tries to
move into the region. /EC/


Jackson       76  60  71  63 /  25  32  38  48 
Meridian      76  60  72  61 /  15  25  25  36 
Vicksburg     71  57  68  62 /  47  38  57  62 
Hattiesburg   78  61  74  63 /  16  12  25  27 
Natchez       75  61  72  64 /  36  28  53  58 
Greenville    61  50  61  56 /  65  52  71  70 
Greenwood     67  52  65  61 /  81  49  56  63 



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