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000
FXUS64 KJAN 251533 AAA
AFDJAN

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1033 AM CDT Tue Jul 25 2017

.UPDATE...
Updated for morning discussion.

&&

.DISCUSSION...
Somewhat of the same scenario as observed yesterday, continues
across the southern half of the forecast area this morning, although
to a slightly lesser extent.  A weak upper level trough layed out
over the Lower Mississippi River Valley, coupled with a general west
northwest flow along a theta-e axis through the trough, continues to
be responsible for an axis of moderate to heavy rain across mainly
South Central and Southeast Mississippi. Fortunately, this isn`t to
the extent to what it was on Monday.  However, this is happening
over the same locations where it happened on Monday.  Thus, further
promoting the threat for flash flooding over these same areas.

This scenario will ease through the remainder of this morning and
into this afternoon South Central and Southeast Mississippi.
However, showers and storms will eventually develop across the
remainder of the forecast area during the afternoon hours.  Given
the rich moisture content of our airmass, noted by the 2.19 inch
precipitable H2O value on today`s KJAN 12Z RAOB, any of today`s
convection will be capable of producing some torrential downpours,
perhaps resulting in some localized flash flooding, especially along
and south of the Interstate 20 corridor.

Due to convection and clouds, highs this afternoon will be lower
across central and southern portions of the CWA, when compared to
northern portions of the area.  Therefore, highs will range from the
lower 90s to middle 80s.  Despite the slighty cooler temperatures,
muggy conditions will continue as dew points sit in the middle 70s.

I`ll adjust POPs across the area to better fit ongoing trends.
Otherwise, no further changes will be made to the forecast on this
update. /19/

Prior discussion below:


Today through tonight:

The upper level low feature that helped to bring central and
especially southern portions of the region locally heavy rain and
thunderstorms yesterday has evolved into more of a weak upper
level trough laying across the southeast third of the forecast
area and up into central AL. Low level flow (specifically at H850)
continues to promote decent thetaE advection into the trough axis
along and south of I-20 this morning, although flow is lighter
than was the case yesterday and thus less prone to organize
training bands of thunderstorms. Expect this low level flow to
veer around a bit more to the northwest as we go through the
afternoon and the associated warmth and moisture advection this
will promote should instigate more showers and storms
(particularly over central and southern areas of MS).

Many locations across S-C and southeastern MS yesterday received
multiple inches of rainfall from training thunderstorm activity,
which instigated scattered reports of flooding. Debated issuing a
flash flood watch in these same areas today since locally heavy
rain is again possible. However, we decided against it since
ingredients for concentrated prolonged torrential downpours is not
as good and available hires model guidance is all over the place
with convective evolution given lighter flow regime. The end
result is not enough confidence to go with a flash flood watch,
despite the very saturated ground. Instead, we will cover the risk
a more limited risk of flooding in the HWO and graphics across a
good chunk of central and southern MS. Maximum temperatures across
these latter areas will be kept in check of 90 degrees due to
clouds and precip, but elsewhere highs will at least make a run at
90 degrees (and possible up towards the mid 90s along the NW
outskirts of the CWA in Ashley County AR. In the aforementioned
area heat index values may manage to peak near 105 degrees but all
locations should remain shy of making a heat advisory necessary
at this point.

Tonight, expect isolated to scattered evening showers and storms
to diminish mainly before midnight. However, there are still some
lingering signals that the nearly departing regime of low level
thetaE advection may still kick up some isolated storms late in
the night in far east/southeast portions of the CWA. At this time
we are not anticipating any hazardous weather impact from this
late night activity. Otherwise, expect warm and muggy conditions
with lows ranging from the low to mid 70s. /BB/

Wednesday through early next week...

A warming trend will commence by midweek, resulting in a return to
hazardous conditions due to the increasing heat stress. Relief
from the heat will come by the way of a rare late July cold front
that will push the region late in the week and into the early
parts of the weekend.

By Wednesday, a drier airmass will be in place over the region as
the broad upper level ridge centered over the Western Plains
nudges into our region. Any diurnal convective activity on
Wednesday or Thursday will be limited to the far eastern counties
of Mississippi. 850 temps will approach 20 degrees by Wednesday
and increase by another degree by Thursday, anomalously high
values, even for this time of year. Expect highs to to range from
the 93-96 degree range on Wednesday and a degree or two warmer on
Thursday with the warmers temps occuring in the Delta. Dewpoints
could mix down to the upper 60`s during the hours of peak heating
on Wednesday in the warmest areas resulting in max heat index
values between 100-105 degrees. There is greater confidence in
heat indices surpassing the 105 degree mark for a good portion of
the ArkLaMiss on Thursday as there will be more moisture present
throughout the day in the lower levels. This will likely prompt
the issuance of heat advisories in the near future.

An upper level closed low will move from west to east along the top
of the upper ridge through the week, dragging an associated
surface front through the Plains and eventually approaching our
region by Friday. Even though models have come into better
agreement on the timing of the front, some temporal ambiguity
still exists. With a faster solution resulting in cooler
temperatures on Friday due to storms and rain associated with the
front beginning to move through our region. A slower solution
could result in another day of oppressive temperatures and even
warmer heat index values due to moisture pooling ahead of the
front. Frontal timing will also play a great deal in the coverage
and severity of storms along and ahead of it. However, at this
point, widespread severe weather seems unlikely due to marginal
instability and insufficient flow. Chances of rain and storms
will linger at least through Saturday evening as the front
continues to push slowly southward through the region.

A cooler pattern will prevail late in the weekend through early
next week as cooler, drier air filters into the region behind the
front. Models are in good agreement of NW flow aloft pattern
prevailing early next week. This should result in continued below
average temperatures for the last few days of July and into the
first few days of August.

&&

.AVIATION...
12Z TAF discussion:

Isolated to scattered showers and thunderstorms are redeveloping
along and south of the I-20 corridor this morning with activity
increasing and getting focused more towards
HKS/JAN/MEI/NMM/PIB/HBG sites after daybreak and through the
afternoon hours. Some locally heavy rains and potential for
associated prolonged IFR conditions with downpours is more likely
early through late morning around HBG/PIB, but also not out of the
question at other central MS sites. Away from storms expect IFR
to MVFR ceilings to be possible through 8 to 9 am in the morning
at most sites, although GLH should be MVFR at worst. Winds away
from storms through tomorrow will be light and rather variable.
/BB/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       87  73  95  73 /  52  19  11   6
Meridian      87  73  93  74 /  55  24  20  14
Vicksburg     89  73  95  72 /  49  16   4   3
Hattiesburg   83  73  93  73 /  90  24  27  14
Natchez       88  72  94  72 /  58  12   7   3
Greenville    92  74  96  74 /  34  17   5   2
Greenwood     90  74  95  74 /  37  19   7   3

&&

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

&&


$$

19