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Office: JAN

FXUS64 KJAN 250546 AAA
AFDJAN

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1146 PM CST Sat Feb 24 2018

.UPDATE...
Updated for 06Z aviation discussion

&&

.AVIATION...
06Z TAF discussion:

Line of showers and embedded thunderstorms will press east through
the region the rest of the night with some brief gusty winds,
brief heavy downpours, and MVFR conditions mainly associated with
activity. Conditions should clear out to VFR in GLH/GWO area late
tonight through the morning tomorrow with rain, embedded
thunderstorms, and more MVFR conditions returning there later
tomorrow. Elsewhere, showers and a few embedded thunderstorms will
take a briefer respite tomorrow morning before returning back in
from the southwest with MVFR (to temporary IFR) conditions due
mainly to ceilings more persistent through tomorrow once they
build in tonight. Winds will be a little gusty from the south and
west along and right behind the cold front tonight but then will
be lighter and more variable tomorrow. /BB/

&&

.DISCUSSION...

Update discussion for the remainder of the night:

Convection coming into our northwest Arklamiss Delta zones is
really struggling to remain severe, in at least a widespread
fashion, and the Storm Prediction Center has lowered the risk of
severe weather in portions of the forecast area. The previous
ENHANCED risk has been pulled entirely north of our region,
although a SLIGHT risk still covers many northern zones into the
early morning hours as a squall line of showers and thunderstorms
progresses in from the east.

Despite the warmth, moisture near the ground has not been
sufficient to really break the thermal cap a few thousand feet
above the ground and has kept worrisome pre-frontal supercells
from occurring. In the past few hours there have been some
shallow, rotating pre-frontal cells in northern zones near the
I-55 corridor but these cells have diminished before getting
robust and slowly diminishing instability gives confidence any
deep convection should mainly be confined to the aforementioned
squall line along and just in advance of the incoming cold front.
Of course the squall line itself could produce severe weather but
the trends of prime juxtaposition of shear and instability well to
our north will likely continue, and in fact low level shear
should slowly weaken in our area through the night. But even
considering all this there is still enough residual instability
and shear to keep a severe concern in mentioned northern zones
into the early morning hours (or at least until the main
convective line passes). The main risk with any severe activity
will be damaging wind gusts, although in northern zones a stray
tornado still cannot be ruled out. Farther south (mainly along
and south of the I-20 corridor) we will maintain a low-end
MARGINAL risk for severe weather where some isolated reports of
severe weather due to damaging winds gusts cannot yet be ruled
out. Tornadoes are even less likely here but cannot quite yet be
determined to be absolutely no threat. Graphics and HWO are being
updated to reflect the latest thinking.

Of course pockets of heavy rain are still anticipated tonight,
especially in the far northwest Arklamiss Delta where residual
saturated conditions will make flash flooding somewhat easy to
achieve. Farther southeast the prospect of overall less vigorous
convection is also bringing down the risk for flash flooding
issues in most other areas, although a limited risk in the
flooding category still seems valid in the graphics and HWO.

Looking ahead, we anticipate the shallow boundary to push
southeast through the whole region by 7 to 8 am with another
incoming shortwave tomorrow likely picking up rain and embedded
thunderstorms (of a more isentropic nature) as we go through
Sunday. There is a MARGINAL risk in far southeastern MS zones of
an isolated severe storm tomorrow. This risk is mainly conditional
on uncertainty on exactly where the front will stall; if it
stalls in the morning just south of HBG then it opens up the
possibility of the boundary inflecting back north into the Pine
Belt by afternoon. If the latter were to happen then a
juxtaposition of decent shear and low level instability may be
able to continue a low end severe weather risk north of the coast.
/BB/

Prior discussion below:

Rest of Today into Sunday...Warm conditions and gusty southerly will
continue across the forecast area for the remainder of this
afternoon.  With wind gusts reaching upwards of 40-45 mph in the
Delta this afternoon and evening, while sustained between 25-30 mph,
a Wind Advisory remains in effect until midnight tonight.  Some
scattered showers, and perhaps an isolated thunderstorm or two, will
be possible. However, most of the CWA will remain dry until the
evening and overnight hours.

The potential for severe storms will increase from west to east this
evening, persisting through the overnight hours, before finally
coming to an end early Sunday morning.  This will come about as a
strong storm system lifts northeast out of the Southern Plains,
across the Mid-South region, and into the Ohio River Valley over the
next 24 hours.  This will drag a strong cold front into the region,
where a warm and unstable airmass resides.  With sufficient deep-
layer wind shear and adequate instability, some storms that develop
along, and possibly just ahead of, the front could be severe.

With high-res model data in agreement, it appears this severe
potential will come in the form of a squall-line of storms, where
both breaks in the line and/or bowing segments are possible.  This
line currently looks most defined, due to better forcing, across
the northern half of the CWA, but specifically in the vicinity of
the Highway 82 corridor as it races east across the region. Damaging
winds will be the primary concern with these storms, but some
tornados will also be possible. Currently, the best potential for
such storms is in the Delta region, or generally along and north of
a Bastrop, LA to Grenada, MS line, where an "Enhanced" risk of
severe storms exists. This is where severe storms are expected.
South of this line, but staying north of a Jonesville, LA to
Jackson, MS to DeKalb, MS line, a "Slight" risk for severe storms
exists and severe storms are possible.  Then south of this line, and
essentially covering the remaining east and southern portions of the
forecast area, a "Marginal" risk for isolated severe storms exists.

In addition to all of this, the potential for heavy rain, potentially
resulting in flash flooding and additional river flooding, will
continue across mainly northwestern portions of the CWA this evening
and into early Sunday morning.  With an additional one to two inches
of rain expected over already saturated soils from recent heavy
rains, localized flash flooding could become a concern across the
Delta region once again.

The potential for severe storms will quickly come to an end after
the leading edge of the squall-line pushes through an area.  Also,
the further east this system shifts across the ArkLaMiss, the
forcing associated with it will lift further north and out of our
neck of the woods.  Thus, the overall severe potential should wane
through early Sunday morning, when the front should reside in the
vicinity of a Columbus, MS to Brookhaven, MS line.

Winds will then shift out of the north behind the front, with cooler
drier air beginning to advect south into the region.  The front will
lose its upper level support, and essentially stall across mainly
the southern half of the CWA Sunday morning.  Though severe storms
are not expected with this, another disturbance will lift northeast
in the vicinity of the front and through the forecast area late
Sunday afternoon.  This will cause rain to once again overspread the
forecast area from southwest to northeast late in the day.  Again,
although an isolated storm of two will be possible Sunday afternoon,
these storms are currently not expected to be severe.  However, some
heavy rain could lead to the ponding of water on area roads, as well
as in low-lying and poor drainage areas, as well as further
aggravate already high rivers across much of the area. /19/

Sunday night through Friday...Sunday evening PWATs will still be
around an inch and a half when a shortwave within our southwest
flow aloft will lead to redevelopment of showers and thunderstorms
with heavy rainfall to spread back to the north across our CWA. A
weak surface low may also develop along the old frontal boundary
and track east just south of our CWA. Additional rainfall amounts
of one to two inches will be possible Sunday night into Monday
morning before drier air moves into our area in the wake of the
shortwave. Dry weather is expected by noon Monday and will
continue Monday night but WAA will increase deep moisture and lead
to the old cold front returning north as a warm front Tuesday. This
will occur in response to a closed low over Southern California
Tuesday morning that will swing across the desert southwest
Tuesday night and lift over the Southern Plains Wednesday. Daytime
heating Tuesday will contribute to convective development from
the southwest by afternoon. Inch and a half PWATs will be back
over our CWA Wednesday afternoon but the bulk of the heavy rain is
expected to fall just north of Highway 82 near the warm front
until Wednesday night when the cold front will drop into our CWA.
Windy conditions will likely develop over our west Wednesday
afternoon and evening ahead of the front. There are some timing
differences with the front but it should move into our northwest
most zones prior to midnight and then clear our CWA by noon
Thursday. Cooler and drier weather is expected Thursday night
through Friday night. /22/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       55  62  51  70 /  96  76  90  13
Meridian      57  63  54  70 /  95  85  94  42
Vicksburg     55  60  49  69 /  98  78  82   8
Hattiesburg   62  68  57  71 /  57  89  95  55
Natchez       56  64  52  70 /  97  91  89  10
Greenville    51  59  46  66 /  99  37  49   5
Greenwood     53  61  47  68 /  97  43  72   7

&&

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

&&


$$

19/22/BB