Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 230337

Area Forecast Discussion
National Weather Service Jackson MS
937 PM CST Thu Feb 22 2018

Updated for evening discussion.


No major changes made to this evening forecast update. Rain
chances are still on the low side, a light rain shower may pop up
in the Southern Delta but no significant rainfall will be added to
our areas that have already been hit hard with flooding. Low
level moisture will bring in some patchy fog during the early
morning hours near Hwy 84, it is expected to clear out after
sunrise. Low temperatures overnight will be in the mid-60s across
the regions that stayed dry throughout today while the Northern
Delta will linger in the mid to lower 50s./SKH/

Prior discussion below:

Tonight and Friday...

Rain is slacking off across the area this afternoon as the stalled
front will gradually lift back to the north and west this afternoon.
Areas to the north of the front have remained rather cool with
readings in the upper 40s to lower 50s. Along and south/east of the
front, temperatures have been unseasonably warm with record highs
being realized in the mid 80s. The bulk of the rain tonight should
remain in Arkansas but some could reach the northwest Delta through
the night as the front wavers in the region. The remainder of the
forecast area will be dry and unseasonably warm yet again.

As the front remains stalled just on the northwest doorstep of my
forecast area, it will be another warm and muggy day Friday. A
disturbance moving north out of the Gulf of Mexico will combine with
the anomalous moisture in place to bring potential for showers and
storms to more of the southern and central sections of the forecast
area, not as much in the Delta. Given the recent rainfall, it won`t
take much rain to cause flooding issues and will keep the
limited/elevated flooding graphic in place for Friday. Hi-res models
indicate some discrete cells, and given some decent shear and theta-
e advection, I can`t rule out some potential for strong storms.
However, confidence is low at this point and will keep out of the
HWO/graphics. /28/

Friday night through Thursday...

An active period of weather with the potential for severe
thunderstorms and additional flash flooding will persist across
portions of the area this weekend into next week. On Friday night,
the forecast will start off with a few scattered warm sector showers
and thunderstorms across the area as a subtle perturbation in the
southwesterly flow provides enough ascent in concert with H850
ThetaE advection and increasing moisture as PWATs rise to near
1.75". This convection will be in an environment characteristic of
1000-1500 J/kg of SBCAPE and marginal deep layer shear of 30-40 kts.
While widespread severe weather is not anticipated with this
activity, an isolated strong to possibly severe storm cannot be
ruled out during the early evening hours on Friday. Attention then
turns to the much larger scale and more impactful weather maker
Saturday evening into Sunday as a strong cold front moves through
the ArkLaMiss.

The stubborn subtropical ridge will break down and become suppressed
as a stout shortwave trough digs across the Desert Southwest and
ejects into the Central Plains. This will help induce lee side
cyclogenesis over eastern Colorado on Saturday morning with the low
quickly deepening and moving northeast into the Midwest as the
shortwave trough becomes negatively tilted. At the surface, a strong
cold front will move through the region Saturday night into early
Sunday morning and will bring the potential for severe weather and
another round of flash flooding. A few isolated to scattered warm
advection showers and perhaps an isolated thunderstorm will be
possible across much of the area during the afternoon Saturday but
the bulk of the severe weather and heavy rainfall will be confined
to a fast moving QLCS during the overnight to early morning hours.
Steep H500 height falls (50-60m over 12hrs) will overspread the area
with increasing wind fields as the cyclone continues to mature.
These strong wind fields and 60kt low level jet will help contribute
to 50-60kts of deep layer shear in the presence of 1000 J/kg of
SBCAPE and 6.5 C/km mid-level lapse rates. Given the strong forcing
and height falls, the predominate storm mode will be a fast moving
QLCS with embedded bowing segments capable of damaging winds. In
addition, any northeast moving line segments that can become aligned
with the 0-3km shear vectors would be favorable candidates for
mesovortex production with an attendant risk for a few tornadoes.
The main threat for severe weather will be along and north of the I-
20 corridor where the strongest forcing will be present. Will
continue to highlight this area with a slight risk in the
HWO/graphics, however as the event nears and enters the range of the
hi-res CAM guidance its possible that some of the far north/Hwy 82
corridor could require a further upgrade depending on how the
convection evolves. South of I-20, the severe weather threat will
not be as high with the convective line further removed from the
best forcing and dynamics, however isolated severe wind gusts and
perhaps a tornado will still be possible so will also continue to
highlight this with the ongoing marginal risk area.

While the line of storms will remain progressive, given the recent
heavy rainfall and flooding across the Delta it will not take much
rainfall to reinvigorate flooding issues. There has also been better
agreement between global models with regards to a second round of
anafrontal precip late Sunday afternoon through Monday as another
shortwave trough ejects through the Southern Plains and the cold
front slows/stalls over the Gulf Coast with a weak surface low
propagating along the boundary helping to keep moisture return over
the area. Will continue to advertise the inherited elevated (Delta)
and limited (along the Natchez Trace corridor) flood threat
graphic/HWO but will extend the timing from Friday afternoon into
early Monday morning to account for additional heavy rainfall and
flooding potential.

The final hazard for this weekend will be for gusty gradient winds
on Saturday ahead of the advancing cold front. Strong surface
pressure falls and efficient boundary layer mixing with temps rising
into the low 80s will help mix down gusty winds across much of the
area, especially in the ArkLaMiss Delta. Wind gusts of 30-35mph will
be common with a few gusts upwards of 40mph possible just ahead of
the cold front in the Delta where the strongest pressure falls
occur. Will go ahead and introduce a limited threat area for gusty
winds in the HWO/graphics for Saturday into Saturday night.

For next week, flow will become zonal as the subtropical ridge axis
becomes elongated across the central Gulf of Mexico. Tuesday has the
best potential to be the driest day of the week as the area remains
between the departing system from this weekend and the next system
emanating out of an active southern stream. There`s fairly decent
model agreement that another potent shortwave trough will dig
through the Desert Southwest and eject through the Plains Tuesday
into Wednesday, however guidance has struggled with respect to the
amplitude of the wave and resulting wind fields. Nevertheless, an
active period of weather looks to return to the region Wednesday
into Thursday and will need to be closely monitored for severe
weather and flooding potential over the next few days. /TW/


00Z TAF discussion:
IFR ceilings at GLH and GWO will rise to MVFR by 23/06Z as a warm
front passes through those areas. Elsewhere, VFR conditions will
prevail through midnight with IFR/MVFR ceilings developing from
the south toward dawn. Ceilings will begin slowly rising after
sunrise, becoming VFR by noon. Scattered showers and thunderstorms
will also begin developing from the west after sunrise with the
best coverage during the afternoon./26/


Jackson       64  82  66  83 /  12  57  46  30
Meridian      61  82  66  80 /   7  21  48  30
Vicksburg     65  81  66  82 /  23  55  50  41
Hattiesburg   62  81  66  82 /   9  42  44  30
Natchez       66  79  67  81 /  18  56  41  30
Greenville    52  77  64  79 /  36  45  60  55
Greenwood     57  80  65  80 /  18  49  65  49





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