Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 262045
AFDJAN

Area Forecast Discussion
National Weather Service Jackson MS
345 PM CDT Wed Jul 26 2017

...DANGEROUS HEAT CONTINUES THURSDAY AND FRIDAY BEFORE STRONG COLD
FRONT MOVES IN THIS WEEKEND...

.DISCUSSION...
Tonight & Thursday:

Expect the strong mid-upper ridge to slowly build more into the area
tonight & into Thursday. Aloft the 595+DM 500mb ridge axis will
slowly pivot eastward towards the area and help keep subsidence &
drier air in the region. This will help suppress any convection
tonight and any that has developed today will weaken quickly after
sunset. Due to the surface ridging & building heat ridge, expect a
quiet night free of any convection & warm and muggy conditions
with above normal low temperatures in the low 70s to prevail.

By Thursday, expect the heat to gradually build more entrenched into
the area as the upper ridge pivots east and the heights & boundary
layer temperatures increase. Aloft a strong shortwave/jet streak
will be propagating down into the Great Lakes, with surface
low/attendant cold front will be moving towards the central Plains &
across the Ohio Valley & into the Appalachians. Out ahead of this
front, thermal profiles will warm with 850mb & 925mb temperatures
across the ArkLaMiss will approach 20-21 deg C & 24-26 deg C,
respectively. This will easily support the mid-upper 90s, with some
areas approaching the 96-98 degree range, especially in the Delta.
Models want to mix out the area somewhat, which could be possible,
especially in the east, but kept dewpoints up in the low-mid 70s
along & west of the I-55 corridor. Heat indices will peak in the 106
to 109 degree range in the Delta and up to the I-55 corridor or so.
If we don`t mix enough in the east, we could reach near criteria in
the east. Therefore, due to some potential for mixing in the Highway
45 & I-59 corridor, decided only to extend & expand the heat
advisory along & just east of I-55 corridor. Will have to monitor if
we need to expand that heat advisory up to the Alabama border.
Updated the graphics/HWO to reflect this change.

With isentropic subsidence/strong sinking air & PWs only around or
less than a half inch, any convection will be hard pressed to
develop, other than near the Pine Belt on Thursday. /DC/

Thursday night through Wednesday...

The period will start off with an elongated ridge stretched from the
Desert Southwest through the Southern Plains and into the ArkLaMiss.
A shortwave trough is also progged to be located over the western
Great Lakes and propagating to the southeast around the northeastern
edge of the aforementioned ridge. Thursday evening will be quiet
with mild overnight low temps in the mid 70s. On Friday, the
aforementioned shortwave trough will swing through the Great Lakes
region and into the Mid-Atlantic helping to initiate lee side
cyclogenesis east of the Appalachian Mountains while dragging a
surface cold front through the Midwest and Southern Plains. Timing
differences still remain in regards to the frontal passage, however
the 12z GFS has trended slower more in line with the ECMWF solution.
During the day on Friday, another round of heat stress issues is
expected with the cold front progged to remain north of the CWA
during peak heating. Warm 850mb temps, on the order of 20-21C, will
support daytime high temps in the mid to possibly upper 90s and
coupled with moisture pooling ahead of the advancing front will
result in heat index values of 105-108 degrees especially in
portions of the Delta and south of the Hwy 82 corridor. Will add an
elevated area in the HWO for heat and this area will have to be
watched for a heat advisory with future forecast issuances.

By late Friday afternoon and into the overnight hours the cold front
will start to push into the northern portions of the CWA. While
lapse rates and vertical totals are not overly impressive, strong
heating coupled with moisture pooling and increased low/mid-level
flow ahead of a 60kt upper jet will support the potential for strong
storms with isolated severe storms possible. The main threat will be
for damaging winds and is expected to be confined to the northern
portions of the area/Hwy 82 corridor with the better height falls
and upper-level diffluent flow focused to our north and east. As
such, will add a marginal risk for severe weather to the HWO for
this area. In addition to the severe threat, dewpoints in the mid
70s in concert with 2.25-2.4" PWATs, 345-350K H850 ThetaE air, and
deep moisture convergence along the front will set the stage for a
few instances of isolated flash flooding. This area will be focused
along and north of I-20 Friday afternoon and through the overnight
and then shift to the south/Hwy 84 corridor on Saturday as the front
pushes through. Will add a limited area for flash flooding to the
HWO across the entire CWA for Fri night/Saturday to account for
this. While the global models are in fairly good agreement in terms
of parameters and hazards, a few timing differences remain with the
GFS still remaining slightly faster than the ECMWF.

By Sunday morning, the front is expected to be clear of the CWA as
the shortwave trough phases with a deep low over northeast Canada.
The CWA will be located beneath northwest flow aloft with much drier
air being advected in. This will result in a period of much drier
and quiet weather through the middle of next work week with only
isolated rain chances which will be mainly confined to the southern
portions of the area. Daytime temps will remain in the low 90s, but
dewpoints mixing out into the low 60s each afternoon will preclude
any heat stress issues. /TW/

&&

.AVIATION...
18Z TAF discussion:
Expect light surface winds today from the west through northwest
today remaining mainly around 6-8 mph. Conditions should be all
VFR this afternoon, with any afternoon thunderstorms/VCTS very
isolated and limited mainly to vicinity of CBM, GTR, NMM, MEI, PIB
& HBG. Due to very isolated nature, did not mention in any TAFs
for now but any sites impacted could have brief reductions in
ceilings & visibilities. Expect convection to wind down around
27/01-02Z. Most TAF sites should remain VFR, other than some
occasional MVFR patchy fog at NMM, MEI, PIB & HBG around 27/08Z &
this will lift by mid-morning Thursday around 27/14Z. Some more
very isolated storms/VCTS may be possible near HBG by tomorrow
afternoon. /DC/BB/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       72  95  74  93 /   8   5   4  32
Meridian      72  96  74  93 /  15  12   3  34
Vicksburg     71  96  74  94 /   3   4   4  33
Hattiesburg   73  95  73  95 /  13  15   3  14
Natchez       72  94  73  92 /   3   5   3  20
Greenville    74  96  76  91 /   3   4  15  51
Greenwood     73  95  74  91 /   6   4  12  51

&&

.JAN WATCHES/WARNINGS/ADVISORIES...
MS...Heat Advisory until 7 AM CDT Friday for MSZ018-019-025>027-
     034>036-040>042-047.

     Heat Advisory from 7 AM Thursday to 7 AM CDT Friday for MSZ028-
     029-032-037-043-044-048>050-053>055-059>063.

LA...Heat Advisory until 7 AM CDT Friday for LAZ007>009-015-016-
     023>025.

     Heat Advisory from 7 AM Thursday to 7 AM CDT Friday for LAZ026.

AR...Heat Advisory until 7 AM CDT Friday for ARZ074-075.


&&


$$

TW/DC


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