Area Forecast Discussion
Issued by NWS Jackson, MS

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000
FXUS64 KJAN 181135
AFDJAN

Area Forecast Discussion
National Weather Service Jackson MS
535 AM CST Mon Dec 18 2017

...DENSE FOG WILL BE A CONCERN THROUGH THE NIGHT...

.UPDATE...
Updated for 12Z aviation discussion

&&

.AVIATION...
12Z TAF discussion:
While visibiltiy reductions have been widespread, the dense fog
has been more patchy with only brief LIFR visibility issues.
However, low ceilings have been more prevalent and will continue
to persist through the mid morning. Expect the areas of fog to
slowly dissipate through the mid to late morning hours at all
locations but the ceilings will remain IFR/LIFR for most TAF
sites, especially across the east and south. GLH/GWO/GTR may
briefly reach MVFR and potentially VFR this afternoon. A band of
showers was also located from Brookhaven to MEI this morning but
this activity is expected to gradually shift south and east with
any impacts lessening through the morning. More dense fog and low
ceilings with widespread IFR/LIFR are expected again tonight into
tomorrow morning, especially across the south and east. /TW/

&&

.DISCUSSION...

Today and tonight...

The ArkLaMiss remains entrenched beneath deep persistent
southwesterly flow as a sub-tropical ridge remains centered over the
Bahamas with a split flow pattern over the CONUS. A closed upper-
level southern stream low was analyzed over far southern Arizona
along the international border with faster and more progressive
northern stream flow from the Pacific Northwest across the Northern
Plains. Widespread dense fog and low clouds are currently ongoing
across much of the area and the ongoing dense fog advisory remains
on track with no adjustments needed through the early morning hours.
The ArkLaMiss is being bisected by a moist airmass with a sharp
gradient separating warmer and moist air along and south of I-20 and
much drier air to the north along the Hwy 82 corridor. On the moist
side, PWATs are running upwards of 1.5 inches with H850 ThetaE
values of 330K+ and 25-30kts of flow through the cloud bearing
layer. These conditions combined with isentropic ascent has
supported an area of showers and isolated thunderstorms early this
morning from Natchez to Meridian. These showers are expected to
continue through the mid morning hours but will become increasingly
confined to the far south and east by early afternoon as the deep
moisture axis pivots south. Extensive cloud cover will keep high
temperatures on the cool side, so went on the lower end of guidance
for today.

Shortwave ridging will act to veer flow to more westerly tonight and
when combined with weak subsidence will be enough to limit any
shower activity to isolated through the overnight period and
confined to the far southern portion of the area. Rain chances will
increase across the Delta at the end of the period as the
aforementioned upper-level low over southern Arizona begins to
interact with a northern stream trough and speed max and becomes an
open shortwave trough while lifting into eastern New Mexico and
western Texas. Winds through the column will once again back and
become southwesterly as moist ascent commences and precip
overspreads the ArkLaTex and into the Louisiana/Mississippi Delta
region around daybreak Tuesday morning. With cool soil temps and a
persistent moist airmass in place across much of the area dense fog
and low clouds are once again expected to form again tonight through
early Tuesday morning. The greatest chance and coverage for dense
fog will be across the south and east portions of the Mississippi.
Its a little too early to mention in the HWO/graphics so will let
the next shift evaluate the latest model trends. /TW/

Tuesday through Christmas Day...

The active weather pattern will persist through the days leading
up to Christmas. Several frontal systems will push through the
region through the next 7 days, and as always when a period
features multiple dynamic systems, confidence declines in
specific model solutions as we advance further into the period.

By Tuesday, an upper closed low meandering just east of the Gulf
of California will begin to push eastward across the Southern
Plains towards the Lower MS Valley region. Low level southerly
flow ahead of this system will continue to pull warm, moist air
into the region from the Gulf of Mexico. Temperatures could reach
as high as the mid to upper 70`s in the southern portions of the
ArkLaMiss on Tuesday. Rain chances will exist through much of the
day along the a slowly advancing warm front mainly in the north
portions of the region. However, storm chances won`t begin to
increase until the surface boundary associated with this system
begins to approach the region. As of now, the best chances of
storms will exist late Tuesday into early Wednesday. A high
shear, low CAPE, prefrontal environment will exist throughout the
region with CAPE values ranging between 200-600 j/kg and 0-6km
shear values approaching 60-65kt. A marginal risk of severe
weather has been introduced into the HWO to account for the
potential of damaging wind gusts associated with any severe storms
that may develop. Precipitation chances will quickly decrease on
Wednesday as the disturbance continues to push east and out of our
region with ridging building into the upper levels behind it.
Temperatures will remain well above average through the work week
as low level southerly flow remains in place.

The next system will begin to approach the region on Friday as a
deep upper trough crosses the Rockies. Ample moisture will be
available ahead of the system as PWats increase to greater than
1.5" areawide. Model solutions are still in disagreement
temporally with this system with the euro quite slower than the
GFS. Nonetheless, both models agree on a swing in temperatures as
much cooler air enters the regions behind the front. This should
lead to a weekend with temperatures more reflective of the season.
There is still too much uncertainty to make a mention of
precipitation chances on Christmas Eve and Christmas Day.

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       65  59  72  61 /  18  21  48  74
Meridian      64  57  71  61 /  27  20  39  67
Vicksburg     64  58  73  59 /   8  30  60  78
Hattiesburg   67  61  74  63 /  53  21  27  55
Natchez       66  61  75  62 /  22  29  48  74
Greenville    60  52  65  55 /   1  34  79  90
Greenwood     62  54  67  57 /   5  28  74  85

&&

.JAN WATCHES/WARNINGS/ADVISORIES...
MS...Dense Fog Advisory until 9 AM CST this morning for MSZ018-019-
     025>066-072>074.

LA...Dense Fog Advisory until 9 AM CST this morning for LAZ007>009-
     015-016-023>026.

AR...Dense Fog Advisory until 9 AM CST this morning for ARZ074-075.


&&


$$

TW/JPM3


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