Flash Flood Warning 27, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
723 PM CDT Mon Jun 19 2023

...New UPDATE, AVIATION...

.UPDATE...
Issued at 659 PM CDT Mon Jun 19 2023

Issued Flash Flood Watch a little while ago through 7 PM CDT
tomorrow for much of the area. Admittedly, confidence in specifics
is not particularly high, but considering what's occurred in areas
to the east over the last few days, and even over Jackson County
today, it's the path of least regret.

As has been said, it's a conditional situation with potential for
a high impact event. We can see on radar outflow moving westward
toward the Pearl River and St. Bernard Parish with thunderstorms
firing to the east of the boundary. These storms have been
producing hail and very heavy rainfall. Convection allowing models
keep producing storms through late evening, and then fire more
off on Tuesday morning. Multiple reports of 4-7 inches of rain
over Jackson County this afternoon. The question has been how far
westward to carry the watch, but with the upper ridge retreating
somewhat on Tuesday, opted to carry it westward to the Mississippi
River. Adjustments are likely to be made overnight to exclude some
portions of the area based on eventual evolution of convection, as
some portions of the area may end up with little or no
precipitation.

&&

.SHORT TERM...
(This evening through Tuesday night)
Issued at 350 PM CDT Mon Jun 19 2023

A fairly active afternoon/evening is expected across portions of the
CWA in terms of heavy rainfall potential. In the upper levels, a
very amplified ridge axis extends across the entire midsection of
the country stretching from the Rio Grande to well past the Great
Lakes. East of there, a deepening low has become cutoff near the
Ohio River Valley. A weak but significant surface boundary lies
between those large scale features near the LIX and MOB border along
the MS/AL coastline. The 12Z LIX sounding showed ample moisture
throughout the column outside of a little drying in the mid levels.
More concerning is the extremely steep lapse rates above the
850mb height. 7-8C/KM, and probably steeper now, is very rare for
this time of year. But that's just one aspect of ongoing very
robust convection in coastal AL thats spreading west into coastal
MS. The existence of the nearly stalled boundary continues to be a
major focal point of development and resulting in what will be
continued training of thunderstorms. Additionally, strong southern
flow noted on MOB's radar areal surface obs indicate a meso low
is potentially developing near the MS/AL border. This has been and
will continue to enhance precip development and efficiency.
Upwards of 5 to 10 inches of rain has fallen in Baldwin County AL
today and numbers in that realm are certainly possible on the MS
Coast. Weaker tornadoes will be possible as well as mesoscale
local environments in the vicinity of the boundary create an
environment with increased shear and helicity. This is similar to
what occurred yesterday along the I- 10 Twins Span bridge between
New Orleans East and Slidell.

Moving into the late evening and overnight, CAMs suggest a line of
storms will potentially develop roughly along a line from Baton
Rouge to New Orleans. Its not clear if this will be a new line that
initiates along 80 dewpoint line or just the current line in MS that
is driven southwestward by convection over time. Either way, will
once again be looking at the possibility of flash flooding and an
isolated tornado or 2.

Going into Tuesday, the shear axis between ridge to the west and
trough/low to the east will remain active. CAMs and medium range
models are all pinging on numerous showers and storms, but moreso
along BR/NOLA line instead of coastal/SW MS as it has been over the
last few days. Flash Flood Watch did cross my mind, but wasn't
confident enough to go with one that far out due to uncertainty.
Until that convection does develop, it'll still be hot although not
as hot has it has been. With the upper low to the east nudging
southwestward, temps will not be able to warm as much as previous
days. Thus, will not be reissuing the Excessive Heat Warning for any
zones but will send out another Heat Advisory (for a trimmed down
area).

&&

.LONG TERM...
(Tuesday night through Sunday)
Issued at 343 AM CDT Mon Jun 19 2023

The stacked nature of these features(high and low) will also be
reflected at the sfc and will begin to send a front, although very
weak, into the area by Tuesday night into Wed. This will be a slow
process though. And while this boundary transits the area slowly,
training of storms could occur Tuesday and again Wed. These could
also last well into the evening hours each day. The front looks to
be over the gulf before stalling late Wed. This will take most of
this activity to that area as well. Dew pt temps look to cool back
to the upper 60s lower 70s by Wed night and temps look to cool also
into the lower 70s north(maybe even a 69F for MCB) to mid/upper 70s
south by Wed night. Another dry air surge will move in Fri keeping
storms over the coastal areas and offshore. This dry air surge will
be from the last short wave moving through the base of the upper
trough allowing this feature to begin moving north then NE. The
migration of this upper low looks to start late Wed night into
Thursday. But the main shift northward occurs as this short wave
moves to the east side of the trough Thu night into Friday. This
will cause the dew pt gradient to begin shifting to the NW bringing
back the deep moisture by the weekend. An easterly wave will move
through the gulf over the weekend providing moisture with a more
normal summer distribution of sh/ts. But the H7 high will have
ridged into the area as the upper trough departs, a hostile
environment will remain over the gulf as this easterly wave moves
through. Now, the good part of this is 2 fold. The first is less
severe storms by late Wed into Thu and possibly Fri, but we will
take this a day at a time. The second is the heat wave may get
broken and could be as early as Tuesday. The conditions will be
similar Tue but sine the trough will be over the area, it will bring
a lot more cloud cover keeping temps lower and as the relatively
dry air moves in, it could also keep the heat wave down Wed
through Friday. Maybe the Heat advisory now and again but the
excessive heat warnings may be going away at least for a short
while. By the way, the only normal thing in this fcast is the
easterly wave for this time of year. We get heat waves in June,
but it is not common to see them to this degree this early, these
usually come in July and August. And NW flow this long is insane.
And eventually, the H7 high will plant itself right over our area
by next week. But that's a ways out still. This migration of the
stacked high over Mexico is quite normal as it should eventually
find itself over the SW. And it is this process that causes the
NAMS(North American Monsoon System) to occur as it brings deep
moisture out of the Pacific into the SW on the western periphery
of the high. This is a normal process as well and if it carries
out like it should, then this heat will eventually subside here
and move west until the stacked high moves east, which is normally
does in late July/August. This high may be a lot more expansive
for this time of year, which is something worth looking into.

&&

.AVIATION...
(00Z TAFS)
Issued at 659 PM CDT Mon Jun 19 2023

Most terminals currently VFR, but KGPT currently has thunderstorms
in the vicinity, with potential to produce 40 knot gusts or more
and IFR visibilities. Also mentioning storms for KASD, and based
on recent discussions with SPC, as well as satellite, we'll
probably need to add TSRA threat to KMCB/KHDC/KNEW/KMSY in the
next few hours. Threat through around midnight, then refiring
around 14-15z. In between, MVFR ceilings likely around sunrise.

&&

.MARINE...
Issued at 343 AM CDT Mon Jun 19 2023

A broad ridge of surface high pressure centered over the southern
and eastern Gulf will continue to produce southwest flow of 10 to 20
knots over the coastal waters through Tuesday. Heading into
Wednesday and Thursday, a weak frontal boundary will slip into the
waters and stall. This will lead to weak northerly and easterly flow
of 10 knots or less during this period time. By the end of the week,
the boundary is expected to weaken and move NW, and a return to
prevailing onshore flow of 10 to 15 knots is expected as the surface
ridge over the southern and eastern Gulf once again becomes more
dominant.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  74  91  72  91 /  50  70  40  50
BTR  81  95  78  94 /  50  80  50  60
ASD  78  93  76  92 /  60  80  60  70
MSY  81  94  79  91 /  60  80  60  80
GPT  76  90  75  89 /  50  70  60  70
PQL  74  93  73  91 /  40  60  50  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.

     Flood Watch through Tuesday evening for LAZ034>037-039-046>048-
     056>060-064-065-069>071-076>090.

     Heat Advisory from 11 AM to 7 PM CDT Tuesday for LAZ034-035-
     046>048-056>060-064>070-076>090.

GM...None.
MS...Flood Watch through Tuesday evening for MSZ068>071-077-083>088.

     Heat Advisory until 7 PM CDT this evening for MSZ068>071-077-
     083>088.

GM...None.
&&

SHORT TERM...ME
LONG TERM....TE
AVIATION/UPDATE...RW
MARINE...TE

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