Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 220545 AAA

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1245 AM CDT Sun Oct 22 2017

Updated for 06Z aviation discussion


06Z TAF discussion:
Despite some ongoing scattered showers across mainly central and
southern portions of the area, VFR flight categories are prevailing
at TAF sites, and will continue to do so through much of the
overnight hours.  The only exception being KHBG, where some low
stratus could build across that locale toward day break this
morning.  Increasing rain chances across the area today, along and
ahead of a cold front that`ll push east through the region during
the course of the today.  This increasing coverage of showers and
storms will result in a degradation of both visibilities and
ceilings at sites are affected by convection, especially in the
vicinity of the front itself. Behind the front, light to moderate
rain will likely persist for a few hours, with ceilings likely
remaining at MVFR status for quite some time into tonight.  Winds
will generally be from the south between 10-15 knots ahead of the
front, with a few higher gusts. However behind the front, winds will
become northwesterly between 5-10 knots. /19/


Local radars only showed a few showers over portions of the Delta
region and our southwest zones at this update but additional
showers and thunderstorm activity developing over southern
Louisiana is poised to move across our western zones during the
next several hours. The potential for a few showers will continue
over our western zones up to sunrise ahead of an approaching cold
front that will near our northwest most zones shortly after
sunrise Sunday. Adjusted pops to current radar trends but no
significant changes were needed to the current forecast with this
update. /22/

Prior discussion below:
Tonight and Sunday: Warm advection showers and a few
thunderstorms continue to break out over south and east central LA
as deep moisture (dewpoints in the mid 70s) pools along the Gulf
Coast. All of this occurring ahead of a storm system taking shape
over the Plains which looks to deepen as it approaches the MS
valley. The scattered warm advection showers will continue in the
west overnight as a QLCS gets going over OK/TX and approaches the
westernmost parts of the forecast area by sunrise Sunday.

By Sunday morning deep moisture will be in place as PWs approach
1.85 inches and surface dewpoints rise into the lower 70s. With such
moisture, very efficient rainfall will occur likely producing 2-3
inches before ending late Sunday night.

Also of concern will be a marginal risk of severe thunderstorms over
southeast portions Sunday afternoon. CAPEs of 1500-2500J/kg will
nudge northward into southeast MS while the deep shear increases to
30-40 knots. This may allow some of the more robust storms to
produce some damaging winds and/or hail. /26/

Sunday night through Friday:

By Sunday night, expect the deep longwave trough to be amplifying
& diving through the ArkLaTex & quickly towards & through the
ArkLaMiss. This will help cold front to quickly move through the
area overnight. There will likely be some anafrontal showers & a
few thunderstorms, with best PoPs & storms residing across eastern
Mississippi. This will be due to the fact of better lift & jet
dynamics/diffluence over that area. PWs will be just high enough
in the wake to help spark some shower & thunderstorm development
before midnight or so before drier air quickly filters in the
wake. Due to some lingering moisture in the +1.5-1.75 inch range
across eastern & southern zones (90th+ percentile), expect some
heavy downpours to still be possible. Kept the HWO going into the
night but likely will be earlier for any flooding threat across
the east.

As the trough ejects through & strong high pressure builds down
into western to southern Plains, this will help drive much drier &
cooler air into the region. This will help drive down PWs around
to half to three quarters of an inch. However, another deep &
anomalous trough will dig down even drier & colder environment in
the wake by mid-week. This trough is extremely anomalous, with
850mb temperatures & 500mb heights approaching only around the 1st
percentile or so. With only 1-4+ deg C at 850mb, this will bring
highs possibly some 8-10 degrees below normal by Wednesday
afternoon in the low-mid 60s. Went cooler for highs on Wednesday
due to that setup. Wet soils should limit some mixing but we will
be very dry with dewpoints falling into the mid 40s by Tuesday
with them falling into the mid-upper 30s by Thursday. Fire weather
concerns should be lower due to recent heavy rainfall but due to
humidity levels being below critical levels & strong pressure
gradient, this will have to be monitored around early-mid week.
Due to ~1025mb surface high over Texas & strong gradient on
Wednesday & developing low over the northern Plains Thursday
morning, winds will stay up enough in this environment to keep
lows from falling off too much. Went slightly cooler on lows
(still 5+ degrees below normals) but not too much due to factors
previously mentioned.

Another anomalous trough will dig down by mid-late week over the
western half of the nation, with the GFS & Canadian being slightly
faster & more progressive to move east. This is likely due to the
injection of a strong typhoon over the west Pacific, helping
buckle the pattern downstream. Similar to the upcoming cold front,
this has potential to be pretty chilly in the wake but likely in
store sometime next week. The Euro is slower with this cold
frontal passage & sided towards this slower solution due to
strong longwave amplification & strengthening high-latitude
blocking. Due to this slowed down PoPs moving in on Friday,
especially eastern half of the area until after the period. But it
could be even later into the weekend more rain & storms would be
possible if the new slower 12Z Euro solution pans out. /DC/


Jackson       54  72  50  70 /  76  12   4   0
Meridian      59  73  50  69 /  92  19   6   0
Vicksburg     52  74  50  71 /  60   7   4   0
Hattiesburg   59  74  51  71 /  84  11   4   0
Natchez       52  72  52  70 /  54   6   3   0
Greenville    52  73  50  69 /  56  10   4   0
Greenwood     53  72  49  68 /  66  16   5   0





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