Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 241816
AFDJAN

Area Forecast Discussion
National Weather Service Jackson MS
116 PM CDT Fri Mar 24 2017

.UPDATE...
Updated for 18Z aviation discussion

&&

.AVIATION...
18Z TAF discussion:
Mainly VFR conditions will continue this afternoon through
evening. MVFR ceilings will increase in association with the TSRA
moving in after 06z in the GLH/GWO to HKS/JAN area and eventually
east to GTR/MEI/HBG before daybreak. While MVFR conditions will be
prevalent early Saturday, there will be some improvement with
areas of VFR during the late morning/afternoon outside of TSRA.
The TSRA threat should be confined to mainly the GTR/MEI/HBG
corridor in the mid to late afternoon. /EC/

&&

.DISCUSSION...
The severe threat for late tonight and Saturday is still roughly
the same, but there are some changes. First, timing of the linear
convective system is forecast by recent CAM guidance to be slower
and have adjusted accordingly in the HWO graphic, having pushed
timing back by a few hours. Also, per SPC update, have expanded
the slight risk eastward toward the I-55 corridor. Otherwise,
removed thunder this afternoon for western border given the strong
cap per morning LCH/SHV/LZK RAOB. No other significant
adjustments were necessary. /EC/

Prior discussion below:

Today through Saturday night:

Lots of low clouds rolling north through the region this morning
as moist low level southeast flow increases rapidly in advance of
a storm system taking shape in the southern High Plains. Expect
these low clouds to break up in patches through the late morning
to afternoon hours, but most areas today should probably feature
more clouds than sun. These clouds may also produce a few brief
showers, primarily over southern zones, but the potential for any
thunderstorms today will be quite low (and mainly confined to
extreme western zones) due to thermal capping and moisture depth
limitations. Boosted blended guidance ideas for highs in most
locations by a few degrees based on mixing potential and the warm
start. Brisk south to southeasterly winds will aid in the mixing
and these winds will easily gust over 30 mph at times,
particularly along and northwest of the Natchez Trace corridor
(where occasional gusts over 30 mph are worthy of a limited
gradient wind risk in the HWO and graphics).

Showers and potent thunderstorms will be erupting this afternoon
to our west in association with the strong trough of low pressure
moving into the Arklatex. Latest model consensus has an evolved
squall line of storms pushing into far western zones from the west
around 10 to 11 pm tonight and then gradually weakening and
slowing, arriving at a position roughly parallel to (and just
past) the Natchez Trace corridor by daybreak Saturday. The storm
prediction center has shifted the main severe weather risk for
today and tonight back further west, but there could be enough
instability in far western zones to produce damaging wind gusts,
hail, and even a tornado cannot be ruled out. Further east through
the overnight severe weather will be increasingly less of a
possibility as available surface-based instability wanes a great
deal. The HWO and graphics will be updated to reflect this
slightly revised thinking.

Even though the severe risk for tonight may have diminished for
some folks a bit, the potential for severe weather certainly
remains for Saturday. Confidence in minor details of what will
unfold remain somewhat low but examination of the consensus of
latest hires models suggests the likely sub-severe band of showers
and storms progressing through central zones will probably
re-intensify by midday before clearing past the I-59 corridor
(and mainly east of our forecast area). But that will not be the
end of the threat for many because the surface trough will be
edging into central zones from the west and likely trigger potent
storm clusters or supercells in the afternoon to early evening.
Fortunately, wind shear accessible to these potential cells looks
remain fairly modest (although still probably enough to organize a
supercell or two) because instability levels right ahead of the
trough will be firmly in the moderate range (with MLCAPE values in
excess of 1500 j/kg possible). In the worst case some of these
cells may be able to at least produce large hail, but due to
uncertainties of exactly how the situation will play out the
Slight Risk advertised by the Storm Prediction Center will suffice
(although may be upgraded in later outlooks).

Storms confined mainly to eastern zones by Saturday evening should
gradually dissipate through the overnight hours with potential for
severe weather likely ending shortly after sundown owing to
instability decrease. May have to watch for the potential of
training cells in central and eastern zones in future forecasts
because some hires model runs hint at this potential. Of course
any cell training will raise the flash flooding concern, even
though antecedent conditions are certainly not overly wet. As
said, something to watch in future updates. /BB/

Sunday through next Friday morning:

An active period of inclement weather looks to remain in store for
the extended period. As we go into late weekend into early next
week, expect the cold front that brought the severe weather
potential to be quickly exiting the area by Sunday morning. Most
of the rain and any lingering storms will have cleared the area,
with any lingering in east Mississippi early-midday Sunday.
Shortwave ridging will build in briefly in the wake Sunday
afternoon to evening, leading to briefly drier conditions.
Temperatures could warm into the low 80s as skies clear out
somewhat. Expect this to be a brief drier period before more
moisture advects back into the area.

A strong Pacific jet energy will help dive down multiple
disturbances and strong cold fronts into the area through the work
week. The first of these will be moving into the lower-mid Plains by
Sunday night into Monday morning. The trough and surface low and
associated front will be moving into the ArkLaTex by Monday morning,
bringing our PWs back above an inch. This system will bring a more
anticyclonic look to the jet with the strongest jet on the western
periphery. Overall lapse rates are fairly steep, approaching 7.5-8.5
deg C at 700-500mb and 850-500mb, with the GFS parameters pretty
aggressive (SHIP approaching 1-2 and large hail parameter
approaching 10+ in the south). Vertical totals on the GFS/Euro and
Canadian are all near 30 and the GFS/Euro both show near 1500-2000
J/kg of MUCAPE areawide. With 20-30kts of 0-3km bulk shear and 40kts
of 0=6km bulk shear, this is supportive of severe weather and
possibly some supercell structures, with large hail and damaging
winds the most likely threats. The aforementioned anticyclonic
structure will support subsidence in the left entrance while
divergence/lift in the right entrance region. This progression will
move through Monday, with more lift in the afternoon after some
increased subsidence in the morning hours. This could support more
isolated convection, especially in the mid-late afternoon hours.
Kept the same slight risk from the SPC for the northern portions of
the region in the HWO/graphics, but this could be expanded and
upgraded as we draw closer to Monday.

Shortwave ridging will again build in the wake of the system on
Monday, bringing warmer temperatures. Adjusted highs up slightly
Tuesday and Wednesday due to increasing boundary layer temperatures.
Again, this will be another brief drier period. There is some
disagreement on rain chances on Tuesday. The GFS brings in somewhat
drier air while the Euro/Canadian keep some slight chance of some
rain/isolated storm around Tuesday afternoon. Kept some rain chances
around, which is above MOS.

Another stronger and more compact upper system and strong upper jet
will dive into the lower Plains and ArkLaMiss by mid-late week. This
system is more vertically stacked and will bring a stronger surface
low into the Plains (997-1000mb) and strong cold front through the
region around next Thursday. Rain and storm chances will begin to
move back in Wednesday afternoon as isentropic lift/moisture
transport kick up quite a bit ahead of this next system. This will
help increase rain and storm chances by beginning Wednesday night
and best chances Thursday-Friday of next week. With a more compact
surface low and stronger cold front, strong height falls, lapse
rates and shear/clockwise curving hodographs will be plenty for
severe weather potential. This system looks somewhat similar to this
weekend as you get QLCS moving in early, possibly hindering
destabilization in the afternoon. These types of uncertainties will
have to be ironed out as we get closer. Thus, kept this out of the
HWO/graphics for now but will likely have to be added later if run-
to-run consistency continues. /DC/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       65  79  60  82 /  56  74  42   9
Meridian      62  77  60  81 /  24  77  55  16
Vicksburg     64  81  58  83 /  84  72  16   5
Hattiesburg   65  78  62  84 /  28  77  39  11
Natchez       65  81  60  83 /  82  68  23   5
Greenville    62  79  55  79 /  90  66   9   4
Greenwood     63  78  57  79 /  77  73  23   4

&&

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

&&


$$


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