Special Weather Statement, NWS Jackson, MS

Area Forecast Discussion
National Weather Service Jackson MS
1252 AM CDT Sat Jul 29 2017

.UPDATE...
Updated for 06Z aviation discussion

&&

.AVIATION...
06Z TAF discussion:
The primary concerns go through the early morning hours will be
the TSRA/SHRA impacting locations south of the GLH/GWO/GTR
corridor and a several hour period of MVFR ceilings ahead of the
cold front. VFR conditions are expected to return and prevail this
afternoon into tonight. /EC/

&&

.DISCUSSION...
Convection through much of the afternoon and evening has been
limited to scattered activity over East MS and a weakening complex
coming out of AR into the Delta. However, over the last hour,
we've begun to see new development over Central MS, likely a
manifestation of considerable residual instability and increasing
deep layer moisture and subtle forcing ahead of the approaching
front. The 00z KJAN RAOB sampled a 2.28 inch PW, which is not a
daily record but is climatologically above the 90th percentile for
this date. Satellite based TPW and model guidance show this number
continuing to increase heading into the overnight. This
environment will be favorable for continued convective
development, with the potential for training storms. This will
create the potential for locally higher rainfall accumulations and
flash flooding. No changes will be made to the current FF Watch or
hazard graphics. There is still limited potential for storms to
produce a damaging wind gust over the next few hours as moderate
to strong instability lingers a while longer after sunset,
juxtaposed with 25-30 kt of 0-3 km flow. Forecast updates to
account for current radar and temperature trends have been sent.
/DL/

Prior discussion below:

Main focus for this forecast is on the potential for heavy
rainfall and resulting flash flooding. Thinking remains the same
from the morning model analysis. Nearly all the guidance is
suggesting the same thing overnight as the mid level trough edges
southward, we will see increasing low level flow help to increase
moisture advection and convergence. Moisture content in this
environment is on the rare side with 99th percentile 850/700mb
dewpts reaching 18C and 9C respectively. This results in PWs of
2.2 to 2.5 in. The will result in efficient rain processes which
will support high rainfall rates of nearly 2 in/hr. Combine that
with low level inflow supportive of backbuilding and storm
training, locations could easily see prolonged precip which would
support areas receiving 2-3 inches and locally higher which could
reach the 4-6 range. The main timeframe for this is in the
midnight to 10am window. While it's tough to pinpoint, the
primary location is across central ans S/SE portions of the
forecast area. From all of this, we have issued Flash Flood watch
for the area of greatest confidence and this is supported by
local graphics with our "elevated" risk.

As for the severe threat, this is still in play and continues to
remain a low end risk. The marginal risk was trimmed out of the N
a bit and focused across central sections of the forecast area
where the best instability is setting up and where the best combo
of lift and storm evolution will align later. The main component
from this setup is increased winds in the 850-600mb layer that
will help enhance downdrafts. The strongest storms will be those
that can organize on SE/SSE oriented outflows and become balanced
with that boundary.

As for specific forecast elements, temps and PoPs were tailored to
fit the timing and areas described above. I did go with a 1-2
degree cooler guid on lows tonight to account for precip cooled
air. As for Saturday, focused Wx/PoPs mostly across the S half and
before noon. Trended those downward for the afternoon as all
activity shifts south of the area. The lingering heavy
rain/flooding threat is what was held into Saturday. The marginal
risk for severe storms was removed as the focus will be on
rainfall/flooding. Temps were adjusted from the prev forecast to
show central and N sections getting warmer due to more sunshine.
Southern areas will be cooler due to clouds/precip. /CME/

Saturday night through next Friday...

The long term forecast is relatively quiet over the next week. The
front will continue exiting the CWA Saturday night and eventually
become stationary over the northern Gulf of Mexico. Drier air (PWs <
1 in) will move in behind it which will make for a seasonably
pleasant Sunday. With mostly clear skies, temps will warm to the
upper 80s across the area. Dewpoints will be in the 60s. With the
upper ridge remaining over the western U.S. and a trough over the
eastern U.S., we are left with north/northwest flow aloft for much
of next week. Temps will be average to slightly below average for
the beginning of August this year. As the prior front that gets hung
up south of the Gulf Coast backs up onto the coast again on
Wednesday, diurnal rain chances increase and will exist through the
rest of the week. /10/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       68  89  70  90 /   3   0   0   9
Meridian      67  89  69  90 /   4   0   2   6
Vicksburg     67  88  68  90 /   3   0   0   7
Hattiesburg   69  89  70  89 /  12   1   0  11
Natchez       67  87  68  89 /   6   0   0   6
Greenville    66  87  67  89 /   1   0   0   8
Greenwood     65  88  67  89 /   1   0   0   9

&&

.JAN WATCHES/WARNINGS/ADVISORIES...
MS...Flash Flood Watch until noon CDT today for MSZ040>044-047>051-
     053>059-061>066-072>074.

LA...Flash Flood Watch until noon CDT today for LAZ009-016-025.

AR...None.

&&

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Special Weather Statement
National Weather Service Jackson MS
244 AM CDT SAT JUL 29 2017

MSZ051-052-057-058-290845-
Lauderdale MS-Newton MS-Jasper MS-Clarke MS-
244 AM CDT SAT JUL 29 2017

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT JASPER...SOUTHEASTERN
NEWTON... CLARKE AND LAUDERDALE COUNTIES UNTIL 345 AM CDT...

At 244 AM CDT, Meteorologists were tracking strong thunderstorms
along a line extending from near Meehan to near Stonewall to near
Turnerville. Movement was east at 25 mph.

Winds in excess of 40 mph will be possible with these storms.

Strong thunderstorms will be near...
  Stonewall, Meehan and Suqualena around 255 AM CDT.
  Nellieburg and Vernon around 300 AM CDT.
  Meridian and Sable around 305 AM CDT.
  Quitman, Paulding and Savoy around 310 AM CDT.
  Marion and Middleton around 315 AM CDT.
  Heidelberg, Zero and Sykes around 325 AM CDT.
  Barnett and Stafford Springs around 330 AM CDT.
  Increase around 335 AM CDT.

Other locations impacted by these storms include Pachuta, Chunky,
Shubuta, Montrose and Louin.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with these storms, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

High winds can bring down trees and large limbs resulting in serious
injury or property damage. Exercise extreme caution when outdoors
during such strong winds...and be especially aware of older trees.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3189 8848 3187 8858 3182 8902 3189 8932
      3211 8931 3224 8902 3246 8895 3246 8841
TIME...MOT...LOC 0744Z 289DEG 23KT 3239 8892 3220 8886 3205 8924

DC