Special Weather Statement, NWS Jackson, MS

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1219 PM CDT Tue Jul 25 2017

.UPDATE...
Updated for 18Z aviation discussion

&&

.AVIATION...
18Z TAF discussion:
Due to varying ceilings and ongoing convection across the area, a
mix of flight categories ranging from VFR to IFR are currently
being observed at TAF sites across the area this afternoon. This
trend will continue through the remainder of the afternoon, and
perhaps into this evening as convection could linger, especially
across Southeast Mississippi at KHBG. Overnight, most locales will
improve to VFR categories, but some developing low stratus and/or
patchy fog toward day break Wednesday, could reduce some sites to
MVFR/IFR categories. These will again improve throughout the
Wednesday morning hours. Frequent lightning and torrential
rainfall is expected with storms across the area today,
especially along and south of the Interstate 20 corridor. /19/

&&

.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.

&&

.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

Hazardous Weather Outlook
National Weather Service Jackson MS
248 PM CDT Tue Jul 25 2017

ARZ074-075-LAZ007>009-015-016-023>026-MSZ018-019-025>054-056>061-
261100-
Ashley-Chicot-Morehouse-West Carroll-East Carroll-Richland-
Madison LA-Franklin LA-Catahoula-Tensas-Concordia-Bolivar-
Sunflower-Leflore-Grenada-Carroll-Montgomery-Webster-Clay-Lowndes-
Choctaw-Oktibbeha-Washington-Humphreys-Holmes-Attala-Winston-
Noxubee-Issaquena-Sharkey-Yazoo-Madison MS-Leake-Neshoba-Kemper-
Warren-Hinds-Rankin-Scott-Newton-Lauderdale-Claiborne-Copiah-
Smith-Jasper-Clarke-Jefferson-Adams-Franklin MS-
248 PM CDT Tue Jul 25 2017

This hazardous weather outlook is for portions of southeast
Arkansas, northeast Louisiana, central Mississippi, north central
Mississippi, and southwest Mississippi.

.DAY ONE...Tonight and Wednesday...

The probability for widespread hazardous weather is low.

.DAYS TWO THROUGH SEVEN...Wednesday night through Monday...

Heat stress concerns will trend upwards again Thursday into
Friday, especially along and west of the Interstate 55 corridor.
Heat advisories may eventually be needed, particularly in portions
of the Arklamiss Delta.

(www.weather.gov/images/jan/graphicast/image2.png)

.SPOTTER CALL TO ACTION STATEMENT...
The activation of storm spotters, HAM radio operators, and
emergency management personnel in support of severe weather
operations is not expected through next Monday.

MSZ055-062>066-072>074-261100-
Simpson-Lincoln-Lawrence-Jefferson Davis-Covington-Jones-Marion-
Lamar-Forrest-
248 PM CDT Tue Jul 25 2017

This hazardous weather outlook is for portions of central
Mississippi, south central Mississippi, and southeast
Mississippi.

.DAY ONE...Tonight and Wednesday...

FLOODING
THREAT...Limited
TIMING...Through the remainder of this afternoon

The threat for showers and storms producing torrential rainfall
will persist through the rest of this afternoon. Locally higher
rainfall amounts in a short period of time can lead to flash
flooding, especially in areas that saw heavy rain on Monday.

(www.weather.gov/images/jan/graphicast/image1.png)

.DAYS TWO THROUGH SEVEN...Wednesday night through Monday...

Heat stress concerns will trend upwards again Thursday into
Friday, especially along and west of the Interstate 55 corridor.
Heat advisories may eventually be needed, particularly in portions
of the Arklamiss Delta.

(www.weather.gov/images/jan/graphicast/image2.png)

.SPOTTER CALL TO ACTION STATEMENT...
The activation of storm spotters, HAM radio operators, and
emergency management personnel in support of severe weather
operations is not expected through next Monday.

19

&&

...HAZARD LEGEND...

*FLOODING*
Extreme...Confident in significant flooding.
Significant...Significant flooding possible.
Elevated...Confident in flooding.
Limited...Flooding possible.

...USEFUL WEATHER WEB PAGES (URLS)...

NWS JAN Home page...
   www.weather.gov/jan
Stay Aware, Stay Safe NWS JAN Briefing Page...
   www.weather.gov/jan/safe
Latest Weather Briefing...
   www.weather.gov/jan/weather_briefing

Tropical page...
   www.weather.gov/jan/tropical
National Hurricane Center Home Page...
   www.nhc.noaa.gov/
Find NWS JAN on Facebook...
   www.facebook.com/NWSJacksonMS
Follow NWS JAN on Twitter...
   @NWSJacksonMS
Follow NWS JAN on Youtube...
   www.youtube.com/user/NWSJacksonMiss

...ADDITIONAL URLS FOR FLOODING ISSUES...

Current Watches and Warnings...
   www.weather.gov/jan/safe_HeavyRain

Observed Precipitation Images...
   www.weather.gov/jan/precipmaps

River Forecast Center...
   www.weather.gov/lmrfc/
Significant River Flood Outlook...
   www.wpc.ncep.noaa.gov/nationalfloodoutlook/index.html
Day 1 through 3 Excessive Rainfall...
   www.wpc.ncep.noaa.gov/qpf/excess_rain.shtml
Excessive Rainfall Discussion...
   www.wpc.ncep.noaa.gov/discussions/qpferd.html
Day 1 through 7 QPF...
   www.wpc.ncep.noaa.gov/qpf/qpf2.shtml
QPF Discussion...
   www.wpc.ncep.noaa.gov/discussions/qpfpfd.html

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Special Weather Statement
National Weather Service Jackson MS
304 PM CDT TUE JUL 25 2017

MSZ042-252030-
Yazoo MS-
304 PM CDT TUE JUL 25 2017

...A STRONG THUNDERSTORM WILL AFFECT CENTRAL YAZOO COUNTY UNTIL 330
PM CDT...

At 304 PM CDT, Meteorologists were tracking a strong thunderstorm
over Benton, or 7 miles southeast of Yazoo City, moving southwest at
15 mph.

Winds in excess of 40 mph will be possible with this storm.

This strong thunderstorm will be near...
  Myrleville around 320 PM CDT.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

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

&&

LAT...LON 3284 9011 3268 9018 3266 9022 3267 9023
      3266 9026 3282 9051 3293 9030
TIME...MOT...LOC 2004Z 035DEG 15KT 3281 9029

17