Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 222030

Area Forecast Discussion
National Weather Service Jackson MS
330 PM CDT Tue Aug 22 2017


Through Wednesday:
Isolated convection continues this afternoon as a
weak shortwave passes through the Ohio Valley and a weak upper low
sits over the western Gulf. The local microburst checklist indicated
low potential for robust storms today, and so far this has been the
case. Much of this activity will die off with the loss of diurnal
heating. However, some limited potential for precip will persist
over northern portions of the forecast area overnight ahead of an
approaching cold front. This front currently extends from the
Central Plains through the Mid-MS Valley and is sliding southward
toward the region. It should reach North MS during the day tomorrow,
bringing a sharper focusing mechanism for scattered to numerous
showers and thunderstorms, particularly during the afternoon when
instability will be greatest. Given fairly typical (or somewhat
weaker due to clouds) instability and unimpressive deep layer shear,
no notable severe weather threat is anticipated. A few instances of
locally heavy rainfall can`t be ruled out given sufficient deep
layer moisture and potentially slow storm motions, but this threat
remains too low for mention in the HWO. /DL/

Wednesday night through Tuesday...

The period will start off with subtropical ridging across the
eastern Gulf of Mexico, troughing across the Great Lakes and a
building ridge over the Desert Southwest. A surface cold front
will also be dropping south through the ArkLaMiss in association
with a departing low over southeast Canada. Areas of convection
are expected to be ongoing during the early evening hours on
Wednesday as the front drops through the area. Expect a few
isolated to possibly scattered showers and storms to continue
overnight along and ahead of the front. By Thursday morning,
latest global models are in agreement on the front draped along or
just to the south of I- 20 which will help to focus shower and
storm chances mainly south of I-20 during the day on Thursday. A
few strong storms with isolated gusty winds cannot be ruled out
both early Wednesday evening and during the day Thursday but
severe weather is not expected with lapse rates, vertical totals
and shear remaking less than ideal for robust convection.
Moisture pooling ahead of the front will help to result in 2 inch
PWATs and when combined with weak flow could result in locally
higher rainfall totals, however any flash flooding looks to remain
isolated so will not mention in the HWO. Much drier air will
filter in behind the front Thursday into Friday with dewpoints
dropping into the upper 50s along the Hwy 82 corridor and low 60s
further south. This drier air should keep any afternoon showers or
storms limited to the extreme southern portion of the area or
outside of our CWA completely depending on where the front stalls
out at.

Forecast uncertainty starts to increase by the weekend and
especially into early next week as the models continue to try and
resolve the evolution of the remnants of tropical storm Harvey.
These remnants are currently located over the Yucatan Peninsula and
are progged to move into the Bay of Campeche and then Gulf of Mexico
Thursday and Friday. There`s good consensus in that strengthening
will occur with a tropical system developing off the coast of
Southeast Texas. This system will be moving around the western edge
of the subtropical ridging in the eastern Gulf of Mexico while the
Desert Southwest ridge expands into portions of west Texas and
strengthens over the Great Basin. This synoptic evolution would
slow down the system movement until a weakness in the ridging
develops as a shortwave trough drops into the Southern Plains and
picks up/phases with the tropical system after making potential
landfall in Texas. Global models offer a plethora of widely
varying solutions, as would be expected since models often
struggle with systems that haven`t developed yet, however there is
some increasing concern for flash flooding across at least some
portion of the ArkLaMiss during the late weekend and early next
week time frame as deep tropical moisture gets lifted through the
area along the retreating frontal boundary/baroclinic zone.
Explicit model QPF products paint swaths of 10-15 inches of rain
anywhere from coastal Texas through the ArkLaMiss and into
Tennessee but have lacked run- to-run consistency. Exact placement
of the axis of heaviest rainfall totals will be highly dependent
on the timing and evolution of the key synoptic features and given
the inconsistencies displayed in the last few runs of model
guidance the reader is encouraged to stay abreast of the latest
forecast and not to focus on any one model solution. With the
timing favoring the day 6-9 time frame will also hold off on
mentioning in the HWO/graphics for now, but will closely monitor
the trends for inclusion in later forecasts. /TW/


18Z TAF discussion:
VFR conditions will prevail through the TAF period. Isolated to
scattered SHRA/TSRA will be possible this afternoon, mainly over
east and south MS, tapering off by around sunset. A cold front
will begin to move southward into the area tomorrow, with light
winds shifting around to N/NW across most of the area during the
daytime. With the front moving into the area, rain chances will
increase on Wednesday. /DL/


Jackson       74  91  71  87 /  13  52  28  30
Meridian      74  91  71  87 /  18  57  30  27
Vicksburg     74  91  71  87 /   9  46  29  31
Hattiesburg   73  92  73  89 /  11  51  32  50
Natchez       73  90  72  86 /   7  44  32  48
Greenville    74  88  68  86 /  20  53  14   9
Greenwood     74  89  68  86 /  21  56  16  11





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