Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 210556 AAA
AFDJAN

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1156 PM CST Fri Jan 20 2017

...SEVERE STORMS TONIGHT AND POSSIBLE AGAIN SATURDAY NIGHT...

.UPDATE...
Updated for 06Z aviation discussion

&&

.AVIATION...
06Z TAF discussion:

Patchy fog and low clouds to will affect PIB/HBG/MEI/NMM for the next
few hours but the main aviation impact to the region through early
tomorrow will come from a solid cluster of strong to severe
thunderstorms pushing in the west between midnight and daybreak,
possibly impacting sites such as HKS/JAN/NMM/MEI/PIB/HBG with strong
winds and some hail. Confidence of the worst conditions is at
PIB/HBG. Locations to the north such (such as GLH/GWO/GTR/CBM) will
have the most intense activity pass to the south. After storms clear
around or shortly after daybreak expect lower stratus clouds in the
IFR to MVFR range to take at least until midday to break up at some
sites. More storms and potential high impact will come late tomorrow
into tomorrow night, but probably remained mostly confined north of
the I-20 corridor. /BB/

&&

.DISCUSSION...
Intense convection has initiated and continued to organize to our
southwest over the last several hours in response to a potent upper
disturbance interacting with a baroclinic zone near the northwest
Gulf Coast. These storms are tending to grow upscale as expected and
should eventually manifest in a linear mesoscale convection system
(MCS) that marches through the southern half of the forecast area
overnight. Even though surface moisture will be recovering from the
south quickly in advance of this system expect surface instability to
struggle to get north of the I-20 before the thunderstorm activity
passes through. This means confidence in damaging winds (and a few
embedded tornadoes) produced by the storms will be highest over
southern areas, although locations along and north of I-20 could
still see pockets of wind to 60 mph with hail a possibility and
tornadoes a non-zero proposition. Of course, as mentioned, further
south risks of damaging hail (up to golfball size) and wind will be
higher and this will especially be the case along the Highway 98
corridor in far SE MS. Here the SPC has increased potential of
tornadoes and enough instability could develop in juxtaposition with
high shear to warrant mention that strong tornadoes cannot be ruled
out as well.

Assessed movement of developing activity to our southwest, along with
latest hires model forecasts, has lead to us slightly speeding up
timing expectations through the region overnight. Graphics are in the
process of being updated to reflect the tweaks in thinking. /BB/

Prior discussion below:

Tonight through Saturday night:

Mostly sunny skies, a breezy southwest wind, and warmer than normal
temperatures in the mid 70s were being observed over our CWA this
afternoon. This pleasant weather will fade fast this evening as
another round of severe storms is expected to move through our CWA
tonight. Mid afternoon water vapor imagery/RAP analysis had a potent
shortwave trough over the central and southern plains that will swing
northeast through Saturday. Ahead of this wave this evening, WAA will
strengthen and help increase deep moisture back across at least the
southern half of our CWA. The 12Z JAN sounding had a PWAT of 0.64in
and our dew points were running in the lower 50s this afternoon. Hi-
res models develop convection this evening along the Texas/Louisiana
coast and spread rapidly east northeast into our southwest most zones
by 11 pm. There will be the potential for a few discrete cells ahead
of the main surge but model consensus suggests more of a cluster or
line spreading east northeast across our CWA. Model parameters
haven`t changed much from previous runs including MLCAPE 500-1000j/kg
with 0-6km bulk shear of 40-60 kts, mid level lapse rates of 6-7 C/km
and 0-1 km shear values of 20 to 30 kts. Any discrete cells would be
capable of producing large hail to the size of golfballs as well as
damaging winds and tornadoes. A line or cluster would likely produce
more of a wind and tornado threat. In addition, the area south of
Interstate 20 received several inches of rain yesterday so a quick
inch or two in a short period of time may lead to runoff issues. Our
Hazardous Weather Outlook continues an elevated risk for all modes of
severe weather across the southern half of the forecast area tonight,
with a limited threat across the remainder of the area. After the
convection shifts east Saturday morning there will be a lull in
activity through Saturday afternoon. A few breaks in the afternoon
sky cover will allow temperatures to top out in the lower 70s at most
sites. This will lend to decent instability going into Saturday
evening for the advertised second round of severe storms but the
potential for additional severe storms is more of a question this
afternoon as hi-res models do not show a lot of redevelopment. The
cold core low will approach our delta region by sunrise Sunday
morning. This is expected to steepen lapse rates and initiate
convection Saturday evening. Large hail and damaging winds are
expected with the storms that develop. The northern half of our CWA
will have the more favorable conditions for severe storms. /22/

Sunday through Friday morning:

Sunday morning the deep upper trough will have dived far south into
the ArkLaTex and moving into the ArkLaMiss. Anomalously low heights
and surface pressures will be present as the surface low deepens
~990-995mb just to the north in the low-mid Mississippi Valley. In
addition, height falls will be some 10-15DM at 500mb. This with
strong upper lift, anomalously low surface pressures/upper heights
and 150kt upper jet over the northern Gulf will help continue shower
and thunderstorm development to be possible through the early morning
into the afternoon. Lapse rates could approach 7-8 deg C/km but
limited moisture will limit overall destabilization and coverage of
storms. With such cold low-mid levels (500mb temperatures around
-20), some destabilization will be still ongoing. With efficient
shear around, there is definitely a possibility of some stronger to
severe storms. Wet bulb zero heights around 5-7kft will support small
hail being achieved even in showers and any storms that could attain
enough depth to have possibility of any severe hail. In addition,
there is enough flow and height falls that there looks to be some
type of squall line progressing across early-mid morning or so across
the region, mainly east of the Mississippi River. For now, there is
quite a bit of uncertainty on how this event will pan out, limited
moisture, extremely low heights, and how far east it will have
progressed by Sunday morning. For now, left out of any mention in the
graphics but this will have to continue to be evaluated. Still have
mention in the HWO for Saturday night into Sunday morning, so some of
that has a possibility of lingering over into the daytime hours.
There could also be some gusty winds of 20-25mph or so due to a
strong pressure gradient around. Some increasing isentropic
lift/moisture transport could keep some chances of rain lingering
north of I-20 as the upper low swings through Sunday night. Expect
rain and storm chances to decrease into Sunday evening as the surface
low ejects northeast.

As the upper trough/surface low eject northeast, expect a strong
pressure gradient as surface ridging and drier air move in the wake.
Highs should top out near or slightly above normal in the wake. This
dry and cooler period will last through early-mid week as surface
ridging moves through the region. A deep trough will dive down into
the western half of the CONUS around mid-week and a strong surface
low moves into the Great Lakes. Temperatures will moderate around
mid-week around mid 60s-low 70s. Rain chances will moving back in
mid-week around Wednesday or so as an attendant cold front helps
isentropic/moisture transport to move back in, helping chances of
showers to move back in. As the cold front moves through, strong
surface high (~1030-1040mb) will be diving into the Rockies, bringing
much drier air with PW`s less than a quarter of an inch across the
country and into our region. This will usher in much cooler
conditions than recently, much closer to climatological normals. /DC/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       59  73  54  64 /  91  19  73  54
Meridian      58  73  54  64 /  99  53  71  61
Vicksburg     58  74  53  64 /  74  15  73  39
Hattiesburg   62  76  56  67 / 100  55  66  46
Natchez       59  74  54  66 / 100  18  68  31
Greenville    56  71  53  62 /  41  13  73  67
Greenwood     57  72  54  63 /  54  15  73  73

&&

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

&&

$$

22/28/BB



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