Severe Thunderstorm Warning 450, NWS Jackson, MS

Area Forecast Discussion
National Weather Service Jackson MS
1228 PM CST Mon Dec 31 2018

.UPDATE...
Updated for 18Z aviation discussion

&&

.AVIATION...
18Z TAF discussion:
A cold front is working its way across the area and now extends
generally along the Natchez Trace corridor. Showers and
thunderstorms are occuring along the line, some of which are
severe. This cold front is quickly moving to the east and will be
east of MEI/HBG by 22-23Z. A mix of VFR/MVFR/IFR conditions are
occuring across TAF sites this afternoon and expect these
conditions to continue through the afternoon hours. By this
evening most sites will become VFR, but expect stratus clouds to
develop after 06-07Z and this will bring more widespread IFR/LIFR
conditions into the morning hours on New Years Day. The winds are
gusty out of the south out ahead of the boundary, but after
frontal passage the winds will become more westerly and begin to
diminish. /15/

&&

.DISCUSSION...
The cold front generally extends along the Mississippi River this
morning and is quickly moving to the east. Mainly showers are
developing along and ahead of the boundary, but with heating,
expect an increase in showers and thunderstorms to develop as we
progress through the morning into the afternoon hours. The winds
are becoming gusty across the area with gusts from 30 to 35 mph.
Will expand the current limited risk for gusty winds from the
delta to across the entire area. The winds at Bastrop gusted to 47
mph as the front moved across the site, so anticipate the winds
to gust from 40 to 45mph across much of the area with FROPA. After
FROPA the winds will quickly diminish as the surface gradient
weakens.

There continues to look like the potential for strong to
severe storms across most of the area as the front moves through.
We have extended the slight risk for severe further south, and it
now covers most areas along and east of the I-55 corridor. Areas
further to the west remain in an marginal risk. The main risk will
be strong winds, but tornadoes cannot be ruled out. The storms
will quickly exit the area to the east by 4-5pm with the
boundary. /15/

Prior discussion below:

Today through Tonight:

As a shortwave lifts from the Southern Plains toward the Great Lakes
today, its developing surface low should track from Arkansas toward
Indiana. This will bring a cold front through our forecast area,
with a chance for strong to severe thunderstorms as the front moves
through. Instability is currently increasing in the developing warm
sector. Latest RAP analysis fields show MLCAPE values of around 1000
J/kg over southeast LA. Expect instability and wind
shear to increase through the morning, with the greatest
contribution from instability being achieved during the after Noon
today as the squall line reaches eastern Mississippi. Deep
layer wind shear of around 60 kts will support supercells, while 0-
1km shear of 30-40 kts will support an isolated tornado threat
especially with any bowing segments along the squall line which can
become oriented perpendicular to the low-level shear vectors. Strong
winds just off the surface will make it easier for damaging wind
gusts to be mixed down with stronger thunderstorms. A Marginal Risk
for severe thunderstorms exists today for all but a small portion of
the Delta region where storms will be less organized earlier this
morning. A Slight Risk for severe thunderstorms has been expanded to
cover a little more of northeast Mississippi, where the better
overlap of instability and shear will develop during the afternoon
hours. This area is highlighted in our latest Hazardous Weather
Outlook and graphics - roughly from Greenwood to Meridian and areas
north and east from there. The threat for showers or storms to
impact New Year's Eve celebrations will be very slim for most of our
forecast area. The front and associated squall line should push
through the area by early this evening, leaving only a chance for
some lingering showers in far eastern Mississippi before midnight.

Apart from the severe weather threat across the area, tightening
pressure and height gradients will result in the potential for gusty
winds through early this afternoon ahead of the main thunderstorm
threat. Sustained wind speeds of 20-25 mph and gusts of up to 35 mph
will be possible ahead of the cold front passage. A Limited threat
for non-thunderstorm winds is in the Hazardous Weather Outlook for
those areas. Elsewhere, gusts of up to 25 to 30 mph will still be
possible apart from thunderstorms. Light winds out of the northwest
will be in place behind the cold front. Patchy fog can't be entirely
ruled out for tonight though, and better chances will probably be in
the vicinity of the frontal boundary through southeast Mississippi.
/NF/

Tuesday through Sunday:

Come Tuesday morning the cold front will continue shifting
farther south of our CWA as a large 1040mb high drops south of the
Northern Plains resulting in a cool dry northerly flow across our
region. Our winds aloft will remain southwesterly though as
another closed low moves over the Four Corners region. Models have
trended slower with the progression of this low but it will drop
over the desert southwest Tuesday night into Wednesday and help
send moisture back atop our cool and relatively dry airmass.
Models are in good agreement that light rain will begin reaching
the ground in our southwest after midnight Tuesday night and then
spreading over the whole CWA Wednesday morning. This model run the
Canadian is the fastest in moving the closed to the northeast
while the GFS is the slowest. The low is expected to track east
across the Southern Plains Thursday and across northern
Mississippi Friday morning. Once the rain starts Tuesday night it
likely will not end until Friday morning. Several inches of rain
will be likely across most of the CWA Wednesday through Thursday
with storm totals generally greatest across our northern zones.
Considering the cold temperatures aloft associated with the closed
low, the light rain may briefly mix with light snow before ending
early Friday morning across our northeast. As surface
temperatures are expected to be just above freezing when the
precipitation falls, no impacts are anticipated. Dry weather is
expected Friday night through the weekend as ridging surface and
aloft move over our region. /22/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       50  62  45  53 /  82   9   4  42
Meridian      53  64  47  53 /  60  24  11  30
Vicksburg     47  59  44  50 / 100   5   4  49
Hattiesburg   57  65  51  60 /  69  21  14  37
Natchez       49  61  46  53 / 100   5   5  60
Greenville    44  54  40  44 / 100   6   4  37
Greenwood     45  56  41  47 /  91   5   4  28

&&

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

&&

15

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