Severe Thunderstorm Warning 89, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
457 PM CDT Thu May 16 2024

...New SHORT TERM, LONG TERM...

.SHORT TERM...
(This evening through Saturday)

We are heading into quite an active period of weather with multiple
rounds of storms through Saturday morning. This will bring us an
extended risk for flash flooding and severe weather during the short
term.

A warm front was located just off the LA coast this afternoon. This
boundary marks the leading edge of a very moist, unstable airmass.
While there is a temperature gradient across the boundary aloft, it
not very noticeable at the surface owing to much stronger boundary-
layer heating on land north of the it vs over the water.

Low-level convergence and isentropic lift along the warm front will
serve as a focus for convective development through tonight. Showers
and storms has so far today has been confined to coastal LA, but
this activity will increase this evening as the MCS that is
currently over eastern TX and northwest LA moves into the area. The
environment will become increasingly supportive of heavy rainfall
producers as the deeper moisture arrives behind the warm front
(PWATs increase from 1.2 to 2.1 inches this evening). Although this
system should be progressive as it is embedded in fast zonal flow
aloft, the west-east orientation of the convection on the southern
flank of the MCS will bring the potential for storms to track over
the same areas. Additionally, the northward progress of the surface
warm front may slow once it meets up with the cold pool on the
southern periphery of the complex. This will provide a favorable
setup for flash flooding across a narrow west-east corridor of the
area. The latest CAMs continue to highlight areas along and north of
the I-10/I-12 corridor to be under the axis of heaviest rainfall
tonight. The latest guidance continues to suggest a quick 2-3" with
isolated amounts of 4-5" with the storms. The Flood Watch was
expanded southward earlier in the day to include both the Northshore
and Southshore of LA and MS Coast. There is also a severe
thunderstorm threat this evening into the early overnight hours but
it is highly conditional on convection being able to become rooted
in the boundary layer. The opportunity for surface-based convection
is narrow and dependent on storms on the southern flank of the MCS
being able to develop south of the warm front.

A "relative" lull in showers and storms is still expected behind the
departing MCS during the day Friday from about mid morning through
early afternoon as warm front lifts just north of our area. Expect
additional storms to develop during the mid to late afternoon as the
boundary layer quickly destabilizes during peak heating. Plenty of
disagreement among the CAMs regarding how widespread the
convection will become later Friday afternoon and evening.
However, a highly unstable airmass fueled by a moist boundary
layer and steep mid- level lapse rates will support a risk of
damaging wind gusts and large hail with this second round.

But wait...there's more. A third round is expected Friday night
(especially overnight) into Saturday morning. Convection will be
aided by favorable synoptic dynamics ahead of a sharpening
shortwave trough and upper jet streak. Accordingly, convection
should grow upscale, resulting in another round of heavy rainfall
and flash flooding.

&&

.LONG TERM...
(Saturday through Wednesday night)
Issued at 320 PM CDT Thu May 16 2024

The upper-level trough will be departing our area on Sunday and
northwest flow kicks in on the backside. Due to the upstream ridging
across the southern Plains, surface high pressure will filter in
across the area starting on Sunday. This will keep the area dry and
warm on Sunday.

The aforementioned ridging will slide east over our area by Monday.
This will cause temps to increase further throughout the week. The
suppression from the ridge will also keep us mostly dry, although the
onshore winds will pump moisture into the area.

&&

.AVIATION...
(18Z TAFS)
Issued at 103 PM CDT Thu May 16 2024

VFR this afternoon with mid level clouds from upstream convection
overspread the area. CU field eventually building from SW to NE
across the area later this afternoon and early evening as a warm
front progress northward. A complex of showers and storms
currently over C and NE TX will move in from the west this
evening as it interacts with a northward moving warm front.
Attempted to narrow the timing window of storms based on the
current observations and latest model guidance, but there is
still some uncertainty with the onset timing. There is some
indication that storms could arrive a couple of hours earlier than
what's advertised in the 18Z TAFs (as early as 01Z for BTR/MCB,
03Z for MSY/HDC/NEW/ASD and 04Z for GPT/HUM). The latest TAFs
also show CIGs and VSBYs lowering to MVFR shortly after the onset.
Also added a 2-3 hour TEMPO IFR group late this evening and
overnight for all TAFs except HUM as confidence of storms
making it this far is lower. In reality, conditions will
deteriorate to IFR at the onset with brief/local restrictions near
LIFR in heavy downpours.

Some of the model guidance in indicating the potential for a wake
low to develop late in the day. Easterly winds would strengthen
overnight for at least 2-3 hours as a result. Forecast wind speeds
may be underdone.

A relative lull in the showers and storms are expected Friday
morning and afternoon in wake of the overnight MCS. However, it
may not be entirely convection free with a few showers and storms
lingering near MCB and GPT during the morning. Additional storms
are expected to develop during peak heating hours, especially
during the latter half of the afternoon as it will likely take
some time to recover from the earlier activity.

&&

.MARINE...
Issued at 103 PM CDT Thu May 16 2024

A warm front was located over the coastal waters of LA. A slight
wind shift from easterly to southeasterly is occurring with the
frontal passage. Wind speeds will also increase from around 10 kt
to 15 kt with occasional gusts to 20 kt. This boundary will
continue to lift northeastward through the rest of the waters
through the evening.

Storms have already developed just behind the surface boundary
across our far southwestern zones early this afternoon. Locally
strong wind gusts of 30-40 kt is possible with the strongest
convection.

Additional showers and storms are expected to move in this
evening and continue overnight. The highest coverage will be over
the lakes and MS Sound as convection becomes focused along the
warm front. Have ise SCA for tonight owing to the presence of a
wake low behind the storms. There is a potential for a brief 1-3
hour period of gales depending on where the wake low tracks and
how strong it is as it approaches our area.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  65  83  67  84 /  90  70  70  50
BTR  71  89  70  89 /  80  70  60  50
ASD  71  89  70  87 /  70  60  60  70
MSY  75  89  73  87 /  60  50  70  60
GPT  71  85  70  85 /  70  50  70  70
PQL  70  87  70  85 /  70  50  60  80

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Small Craft Advisory until 6 AM CDT Saturday for LAZ068.

     Flood Watch from 7 PM CDT this evening through Saturday morning
     for LAZ034>037-039-046>048-057-058-060-064-071-076>087-089.

GM...Small Craft Advisory until 6 AM CDT Saturday for GMZ530-534-550-
     552-570-572.

     Small Craft Advisory from 7 PM this evening to 6 AM CDT Saturday
     for GMZ532-536-538-555-557-575-577.

MS...Flood Watch from 7 PM CDT this evening through Saturday morning
     for MSZ068>071-077-083>088.

GM...Small Craft Advisory until 6 AM CDT Saturday for GMZ534-550-552-
     570-572.

     Small Craft Advisory from 7 PM this evening to 6 AM CDT Saturday
     for GMZ536-538-555-557-575-577.

&&

SHORT TERM...JK
LONG TERM....JZ
AVIATION...JK
MARINE...JK

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Preliminary Local Storm Report
National Weather Service New Orleans LA
550 PM CDT Thu May 16 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0723 PM     Tstm Wnd Dmg     Donaldsonville          30.11N 91.00W
05/13/2024                   Ascension          LA   Emergency Mngr

            Tree down on residence.

&&

JZ

Special Weather Statement
National Weather Service New Orleans LA
554 PM CDT Thu May 16 2024

LAZ057-058-086-162330-
St. John The Baptist LA-St. James LA-Eastern Ascension LA-
554 PM CDT Thu May 16 2024

...A STRONG THUNDERSTORM WILL IMPACT PORTIONS OF NORTHWESTERN ST.
JOHN THE BAPTIST, SOUTHEASTERN ASCENSION AND NORTHERN ST. JAMES
PARISHES THROUGH 630 PM CDT
...

At 554 PM CDT, Doppler radar was tracking a strong thunderstorm near
Convent, or 11 miles east of Donaldsonville, moving northeast at 40
mph.

HAZARD...Winds in excess of 30 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Gramercy, Lutcher, Sorrento, Convent, and Garyville.

This includes Interstate 10 in Louisiana between mile markers 184 and
197.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3012 9091 3012 9087 3016 9088 3028 9080
      3023 9074 3017 9070 3018 9068 3023 9066
      3021 9061 3023 9058 3025 9058 3024 9055
      3020 9055 3000 9076 3005 9095
TIME...MOT...LOC 2254Z 214DEG 36KT 3008 9081

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...30 MPH

Zeringue

Special Weather Statement
National Weather Service New Orleans LA
556 PM CDT Thu May 16 2024

LAZ046-162330-
Iberville LA-
556 PM CDT Thu May 16 2024

...A STRONG THUNDERSTORM WILL IMPACT PORTIONS OF NORTHWESTERN
IBERVILLE PARISH THROUGH 630 PM CDT
...

At 556 PM CDT, Doppler radar was tracking a strong thunderstorm near
Butte La Rose, or 9 miles northeast of Catahoula, moving northeast at
35 mph.

HAZARD...Winds in excess of 30 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Maringouin, Rosedale, Grosse Tete, and Ramah.

This includes Interstate 10 in Louisiana between mile markers 127 and
142.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3038 9163 3039 9163 3049 9154 3050 9148
      3043 9140 3040 9139 3038 9136 3035 9133
      3023 9149 3024 9157 3032 9163 3035 9163
      3037 9165
TIME...MOT...LOC 2256Z 223DEG 33KT 3030 9158

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...30 MPH

Zeringue