Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 280207 AAA

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
907 PM CDT Thu Jul 27 2017

Updated for evening discussion.


As for the overnight period, the forecast looks pretty good. Our
northern counties may see some scattered showers move in after 06Z
as the precip that formed along the front in the mid-MS valley
this afternoon continues its push southward overnight. The clouds
associated with that activity to our north will filter through our
area tonight in the form of mid to high cloud cover. These clouds
will help to keep lows from sinking not much lower than the mid
70s. /10/

Prior discussion below:

Tonight through Friday night:

Humid & muggy airmass with dangerous heat are possible through
tomorrow before a cold front begins to move towards the area. In
advance, heat will prevail Friday before heavy rain & severe
weather are possible with the cold front.

The heat & humidity will slowly weaken the grip on the region
beginning Friday as the strong upper ridging begins to slowly
weaken and retrograde to the west towards the central Plains &
Rockies. This will slowly begin to weaken the grip of the heat
over the region. Aloft a shortwave/jet streak will be quickly
ejecting through the northern Great Lakes into the northern
Appalachians & northeast through Friday. This will help the upper
jet over the top of the ridge to amplify & height falls to
gradually impend over our area. This will help the attendant cold
front to approach our area by late tonight into tomorrow as it slowly
progresses south into the mid-Mississippi Valley by early Friday
morning into Friday afternoon. Out ahead, anomalously high PWs
near 2.25-2.5 inches & 345-350+K 850mb Theta E moisture will be
feeding in from the west lined parallel to the front. This will
help increase rainfall around 12Z up in the Delta. There is some
potential for MCVs in this environment and global models show some
potential for some confluence axis near and west of the I-55
corridor. This will help some potential for locally heavy rainfall
& flash flooding potential especially in the Highway 82 corridor &
into the Delta. Will have to monitor for some potential for
issuing a flash flood watch in the Highway 82 corridor tonight.
Due to the uncertainty on the area & time of heaviest rain, held
off on any flash flood watch for now. Will have to monitor. Some
of these storms may move south the Highway 82 corridor before
slowly weakening as they propagates to the south through early
morning into the late morning.

In advance of these storms & cold front, with nearly 20-21+ deg C
at 850mb & 23-25+ deg C at 925mb, we should be able to warm
efficiently into the low-mid 90s. With dewpoints pooling in the
mid-upper 70s, even this will help areas from along & southwest of
a line from southern Bolivar County through northern Clarke
County in Mississippi heat stress to rise. Heat indices will
climb near to just above 105 degrees in those areas in central and
southwestern areas of the ArkLaMiss. Depending on morning &
afternoon convection, some high clouds & storms could hold back
temperatures some. Regardless, we should still be able to warm in
areas that clouds & storms are not as prevalent. With potential
for some clouds & storms, only kept mention of limited heat in
the graphics/HWO in the previously mentioned areas.

As the cold front advances south late Friday afternoon & into the
evening & overnight, local hi-res guidance, including the NAM 3KM,
WRF-ARW & NMM show more redevelopment of thunderstorms in the
Highway 82 corridor. With such high dewpoints pooling ahead of
the front, near mid-upper 70s, if we get near 90 degrees or so
before the second redevelopment Friday afternoon to evening, we
could destabilize very well. Modified forecast soundings show
nearly 4000+ J/kg sfc based CAPE & ~26 deg C vertical totals in
that environment. Due to anomalous 850-700mb flow (25-30kts) &
nearly the same 0-3km bulk shear (25-30kts) along the cold front
and mainly oriented from the north-northwest, any storms that
propagate south-southeast along these vectors will have potential
for damaging winds. With such destabilization, flow, low level
moisture & heating (if not too much cloud cover limits it), we
should easily support isolated strong to severe thunderstorms.
Expecting these to gradually propagate south through the
overnight & through around midnight or so along & north of the
I-20. In addition, these will bring potential for heavy rainfall &
flooding down towards the I-20 corridor overnight. Muggy
temperatures in the low-mid 70s with clouds & gradually decreasing
convection after midnight will be possible through Friday night.
Kept mention of isolated severe storms & flooding in the
graphics/HWO, mainly along & north of I-20, Friday afternoon
through Friday night. As the cold front moves south, expect
convection to redevelop Saturday morning into the afternoon with
another round of severe weather & flooding potential shifted to
the south of the front. /DC/

Saturday through Thursday:

The period will start off with a cold front dropping through the
ArkLaMiss as a low moves through the Mid-Atlantic states with an
associated trough and cyclonic flow over much of the eastern CONUS.
Meanwhile, the ridge that has brought heat to the area the last
few days will become more centered over the western Plains. Timing
uncertainties remain with the cold frontal passage and hi-res
CAMs have a multitude of varying solutions which are driven by
mesoscale interactions from previous rounds of convection. While
confidence is high in convection occurring along the front on
Saturday, less confidence exists regarding the temporal and
spatial details. Taking a blend of guidance, some degree of
showers and storms are expected to be ongoing 12z Saturday morning
along and south of the I-20 corridor. 2.25-2.5" PWATs, 345-350K
H850 ThetaE, and deep moisture convergence along the front will
support the threat for isolated flash flooding with any storms
through the day Saturday so will continue to advertise a limited
area in the HWO. The severe potential will be greatly dependent on
1) the evolution of convection Friday afternoon/evening and
resulting outflow boundaries, and 2) the timing of the front
itself. Moisture pooling ahead of the front and resulting
instability combined with low-level flow will support the
potential for a few isolated damaging wind gusts, however this
will be highly dependent on the aforementioned uncertainties. Will
keep the current marginal risk advertised for now, but given the
morning timing and potential lack of destabilization this area
could be trimmed and limited to the far south or removed
completely. Both the flash flooding and severe threats will be
confined along and south of I-20 on Saturday ahead of the cold
front. By Saturday evening, the front is progged to be south of
the CWA with much drier air, characteristic of PWATs less than 1.5
inches, quickly filtering in behind it.

For Sunday through Wednesday, the ArkLaMiss will remain under the
influence of northwest flow aloft with PWATs remaining at or below
1.5 inches. This will keep a period of quiet weather across the area
with the only rain chances confined to the far south on Tue and
Wed where slight chance PoPs were carried. Daytime high temps will
be in the mid to upper 80s with a few low 90s, but dewpoints
remaining in the upper 60s will preclude any heat stress issues.
Towards the end of next work week and into next weekend, the
pattern will begin to change with a return to typical diurnally
driven showers and storms on Thursday. /TW/


00Z TAF discussion:
VFR conditions will prevail through the period. However, VCTS will
beging increasing from the north after 28/06Z and spread southward
through the day. Where storms impact TAF sites, a brief lowering
of ceilings and visibilities can be expected./26/


Jackson       73  93  75  89 /   5  37  50  52
Meridian      72  92  74  89 /   5  42  55  51
Vicksburg     74  93  74  89 /   5  37  53  52
Hattiesburg   73  94  76  87 /   5  21  55  65
Natchez       74  92  75  88 /   5  23  55  64
Greenville    77  91  73  88 /  24  56  49  16
Greenwood     75  90  72  88 /  26  55  53  15




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