Flash Flood Warning 36, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
402 AM CDT Thu Jun 30 2022

.SYNOPSIS...

The area of disturbed weather over the western Gulf of Mexico
remains poorly organized and is creeping slowly westward. Looks to
be a trough over the Florida Peninsula as well, with ridging to
the north.

Onshore flow has been a little more persistent than expected on
the Mississippi coast, and a cluster of showers/storms has been
moving northward across portions of Harrison, Hancock and Pearl
River Counties since about 130 AM CDT. Not surprisingly, with
precipitable water values greater than 2.2 inches on the 00z LIX
upper air sounding, isolated rain amounts in excess of 3 inches
have been estimated by radar. To this point, most of the rain has
occurred in areas that aren't densely populated, so not a lot of
ground truth to go by. There has been a bit of a downward trend in
rainfall rates in the last 20 minutes or so. Elsewhere,
precipitation much more isolated. Temperatures generally mid 70s
to lower 80s with dew points in the mid 70s at 3 AM.

&&

.SHORT TERM (through Saturday)...

Moist airmass isn't going anywhere over the next 3 days. Forecast
soundings (GFS BUFR) at our main sites continue to show
precipitable water values remaining greater than 2.0 inches for
almost the entire period through the holiday weekend. This is a
very moist airmass even for us. Our sounding climatology for late
June shows the 75th percentile for precipitable water around 1.85
inches and the 90th percentile at around 2.05 inches. We're going
to spend a lot of time above that 90th percentile through the
weekend. With troughing remaining across the area, daily rounds
of showers and thunderstorms are expected. No large scale forcing
mechanisms, but with the moisture and instability available, we'll
get convective development.

One point to add with potential for minor coastal flooding in
Hancock County this afternoon, if winds become stronger than
expected this morning, can't rule out a few trouble spots as the
10 percent exceedance value is about a quarter foot below our
trigger point. Will not go with an advisory at this point but will
brief day shift about possibility.

Yesterday afternoon's discussion is still applicable and
follows...

With deep moisture and slow storm motions, expect some storms to
be efficient rainmakers with rainfall rates in excess of 1-2" per
hour. Instantaneous rates will likely be significantly higher.
This will continue to lead to a threat of locally heavy rainfall
and localized nuisance or even flash flooding, and it's not
surprise that WPC keeps a daily marginal risk of excessive
rainfall across the entire area through at least Friday.

As is typical in this kind of air mass, expect showers and storms
to fire over the coastal waters overnight and into the morning
hours, waning in those areas as the morning progresses. Over land,
expect the best rain chances during the afternoon and early
evening hours most places - the main exception being in coastal
areas where marine storms could wander onto land in some areas.

Given the expected cloud cover and convection, highs should
generally top out in the upper 80s to lower 90s, though there is
certainly potential for higher temperatures if convective onset is
delayed or the cloud cover breaks for long enough. Overnight lows
will fall into the low to mid 70s north and into the mid to upper
70s south, with a few places along the immediate coast only
dropping to around 80 degrees.

Other than bumping up POPs during the first part of the evening
since convection rarely shuts off completely right at 00z, really
didn't make any significant changes to the NBM starting point.

35/95

&&

.LONG TERM (Sunday and beyond)...

While upper ridging tries to build into the area, it's not
expected to be strong enough to totally quash convective
development. Rain chances will be somewhat lower, in the 50 to 60
percent range, with similar timing of development/dissipation each
day. The lower rain chances will lead to high temperatures getting
a couple degrees warmer, primarily in the 90-95 range as we get
into Tuesday/Wednesday. 35

&&

.AVIATION...

Outside of convection, expect conditions to generally remain VFR.
Timing of showers and thunderstorms at individual TAF sites
continues to be a challenge and will likely require amendments.
Any heavier showers/storms will be capable of causing IFR
visibilities and MVFR ceilings. A few of the stronger
thunderstorms may also result in gusty/erratic winds.

&&

.MARINE...

Daily showers and thunderstorms will be the main marine concern.
Expect a typical summertime diurnal cycle, with showers and
storms firing over the open waters during the overnight hours,
then waning during the daylight hours. Protected waters will see
showers and thunderstorms mainly during the daylight hours.
Outside of convection, expect light to moderate south or
southeasterly flow resulting from high pressure to the east and
developing low pressure to the west. Winds and seas will be higher
in the vicinity of thunderstorms.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  89  72  88  72 /  60  30  80  20
BTR  91  73  90  74 /  70  40  80  20
ASD  89  74  90  75 /  80  40  80  30
MSY  88  77  89  77 /  80  30  80  30
GPT  88  76  88  76 /  80  50  80  30
PQL  87  75  86  75 /  90  60  80  40

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&