Flash Flood Warning 23, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
405 PM CDT Sat Jun 17 2023

...New SHORT TERM, LONG TERM, AVIATION, MARINE...

.SHORT TERM...
(This evening through Sunday night)
Issued at 241 PM CDT Sat Jun 17 2023

A subtle shortwave feature is currently moving across the region,
and this will continue to bring a risk of convective activity to
the northern and eastern portions of the forecast area through the
evening hours. Steep mid-level lapse rates and marginal
directional shear will continue to support the development of
discrete supercells capable of producing very large hail and
strong damaging wind gusts. A severe thunderstorm watch will be in
effect through 8 PM this evening for areas along and north of the
I-10/12 corridor. Outside of the convective risk, the influence of
a strong ridge centered over southern Texas will continue to
induce warming throughout the entirety of the atmospheric column
through the overnight hours. Heat index values are currently
ranging from 105 to 115 degrees, and are only expected to cool
into the 90s during the overnight hours. This is due to overnight
lows struggling to fall below 80 degrees and dewpoints also
remaining in the upper 70s tonight.

Tonight into tomorrow will see a much stronger southern stream
shortwave trough push into the Southern Plains. Model consensus
indicates that a strong MCS will develop over northern Texas
tonight and then sweep to the east along the I-20 corridor into
central Mississippi by early tomorrow morning. The MCS should
then follow the theta-e axis southeastward, and could impact the
Mississippi coast during the mid-morning hours tomorrow. Currently
have chance PoP in place for the Mississippi Coast to reflect
this risk, but will likely need to raise PoP in later forecast
updates as confidence on the evolution of the MCS increases. Rain
chances could then decrease into the afternoon hours as the region
experiences a period of relative stability as a cold pool lingers
in the wake of the MCS over northern and eastern portions of the
CWA. A heat advisory remains in effect for these areas, but there
is less confidence that criteria will be met tomorrow afternoon.
Further to the southwest, the influence of the upper level ridge
will still be in full control, and another day of temperatures in
the mid to upper 90s and heat index values approaching 115 degrees
can be expected. One final night of excessively warm temperatures
in anticipated for Monday night, and have continued to go with NBM
95th percentile values for the overnight lows.

&&

.LONG TERM...
(Monday through Friday night)
Issued at 241 PM CDT Sat Jun 17 2023

A closed low is expected to develop over the Tennessee Valley as
the southern stream trough in the southern Plains merges with a
northern stream shortwave feature diving down from the upper
Plains. This low will be cutoff from the mean flow and effectively
wedged between strong ridging to the southwest, north, and
southeast. This is related directly to a continued omega blocking
pattern dominating the entire CONUS. Guidance consensus has this
upper low slowly tracking southward toward the Deep South on
Wednesday and Thursday. Cooler mid and upper level temperatures
will allow for temperatures to finally cool back to more normal
levels in the low 90s and the excessive heat indices will no
longer be an issue by Tuesday.

Monday and Tuesday will continue to see steep mid-level lapse
rates and the risk of scattered strong to severe convective cells
developing within deep layer northerly flow on the western
periphery of the upper low. Any convection will tend to develop
late in the day after peak heating and will be driven by short
wave features sliding through the region. Higher precipitable
water content and less steep lapse rates of around 6.0 C/km will
help limit severe potential from Wednesday through Friday, but
limited storm motion could lead to higher flash flood threat later
in the week.

&&

.AVIATION...
(18Z TAFS)
Issued at 241 PM CDT Sat Jun 17 2023

Convective concerns will be the main issue for MCB, HDC, ASD, BTR
and GPT this evening. Have included a TEMPO group at MCB and GPT
where convective probabilities are highest. The peak timing
appears to be between 21z and 00z as convection fires up along a
boundary sinking southward into the region this evening. If a
storm directly impacts these terminals, IFR visibilities and gusty
winds can be expected. By 02z, the convective threat should
diminish as the storms push offshore and weaken. Later into the
night, an elevated inversion is expected to redevelop over inland
areas, and this will bring a period of MVFR and IFR ceilings to
BTR and MCB between 10z and 14z. After 14z, increased thermal
mixing of the boundary layer will allow the stratus deck to being
scattering out. Additionally, an MCS passing north of the forecast
area could lead to some convective impacts at MCB between 12z and
18z.

&&

.MARINE...
Issued at 241 PM CDT Sat Jun 17 2023

High pressure centered to the south and east of the forecast area
will continue to keep a southwest flow of 10 to 15 knots in place
across the waters through Tuesday. A low pressure system moving
into the area by mid-week will lead to more variable and lighter
winds. Overall, no significant maritime impacts outside of the
typical thunderstorm threats of gusty winds, waterspouts, and
lightning are expected next week.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  76  94  77  95 /  40  40  40  60
BTR  80  97  80  98 /  20  30  20  40
ASD  79  95  80  96 /  50  30  20  60
MSY  82  97  82  97 /  30  30  10  50
GPT  79  90  81  91 /  70  40  30  70
PQL  77  90  79  91 /  70  50  30  70

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Excessive Heat Warning until 7 PM CDT this evening for
     LAZ034>037-039-046>048-056>060-064>071-076>090.

     Excessive Heat Warning from 11 AM to 7 PM CDT Sunday for
     LAZ034>037-039-046>048-056>060-064>071-076>090.

GM...None.
MS...Heat Advisory until 7 PM CDT this evening for MSZ068>071-077-
     083>088.

     Heat Advisory from 11 AM to 7 PM CDT Sunday for MSZ068>071-077-
     083>088.

GM...None.
&&

SHORT TERM...PG
LONG TERM....PG
AVIATION...PG
MARINE...PG

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