Special Weather Statement, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
617 AM CDT Tue Aug 3 2021

.AVIATION (12Z TAF DISCUSSION)...

Early in the TAF forecast, patchy areas of low-clouds persist
generally across KMCB to near KBTR, expanding east across KHDC
and KASD. This has temporarily reduced flight categories in this
region. However, all low clouds will likely continue to dissipate
shortly after sunrise, with another round of spotty SHRA/TSRA
expected again today as early as later this morning through the
rest of the afternoon/evening hours. Main threats in any one storm
will be gusty, erratic downdraft winds and temporary reductions
in VIS due to locally heavy rainfall which may lead to periodic
lower flight categories. Conditions are expected to improve later
this evening and into tonight, with SHRA/TSRA focusing more along
coastal SE LA going into the rest of tonight. KLG

&&

.PREV DISCUSSION... /issued 345 AM CDT Tue Aug 3 2021/

SHORT TERM (Today through Thursday Night)...

Starting off with early this morning, a very weak nearly
stationary front has settled in across coastal areas of SE LA and
adjacent marine zones. It's a bit tricky to analyze, with no
identifiable pressure or thermal gradient. However recent trends
with SPC Mesoanalysis illustrates weak frontogenesis from subtle
cyclonic curvature in surface winds, as well as a distinct wind
speed magnitude change and attendant surface
vorticity/convergence. This has aided in an enhancement in typical
nocturnal convection across the warmer shelf waters ongoing
across SE LA. Few of these storms have been strong and could
become severe, with strong winds >34kts and isolated waterspouts
the main risks. Latest 00Z HREF members initiated this convection
quite well matched from what is already ongoing on radar, with
more recent HRRR trends initializing and attempting at spreading
some of this convection inland through the morning. This will have
to be watched, as the main influence in any northward surge would
be from any ENE to NE drifting outflow boundaries getting out
ahead of the ongoing coastal convection, attempting to spread
bands of storms north across the southshore/northshore around mid-
late AM (similar to what we saw yesterday). Eventual diurnal
destabilization later this morning/early afternoon will lead to an
increase in scattered showers and storms once again. The degree
of (potential) northward surge of boundaries and/or convection
will determine the degree of workover limiting re-developing
convection around peak diurnal heating. But in such an anomalously
high PW environment, it will not take much for re-development to
occur with many people likely seeing rain and/or storms at some
point later today. Letting the NBM ride with PoP coverage in the
70-90% range, which seems pretty good. No major modifications
needed to temperatures, with NBM ensemble guidance coming in lower
due to increased rain/storm coverage. Otherwise, taking a closer
look at the threats possible today diving deep into the model
soundings, once again not seeing anything stand out atleast in
regards to severe weather. Forecast WMSI/WindEx actually did quite
well yesterday at the sub-severe wind gust potential and going
back at yesterdays guidance shows almost no change in forecast
downdraft parameters for today. Overall, expect occasional gusty
winds 25 to 35 mph in any stronger storms, especially those taking
on a linear orientation propagating along outflow boundaries,
with higher gusts 40-45mph across marine areas similar to what we
saw yesterday. As expected, drier air did filter in at the H5
level and above squeezing in the back side of the front, but is
retained well above any lower layers that would lead to
entrainment/flux downward transport meaning this should not be an
issue leading to severe wind gusts today (but as always, got to
watch that isolated one or two storms that read my AFD and wants
to prove me wrong). Can't rule out localized flash flooding but
storm motion/propagation will be moving quick enough to keep the
threat at a minimum. However, intense rainfall rates could still
lead to some minor flood concerns.

Rinse/repeat going into tonight and into early Wednesday with the
same nocturnal convection expected across warmer shelf waters yet
again. However, we will start to see some of the drier
tropospheric air win over helping to suppress convection more
towards coastal areas. Model soundings illustrate the same mid-
level drying starting to sink and warm/compress into right around
H7. Contrary to what was said about the dry air being too far
aloft for today, this would be low enough to help entrain/mix down
to the surface with some drying noted in the entire lower
tropospheric layer to the surface. This pulls calculated DCAPE
values into the 1000 to 1100J/kg range. If storms can stay
offshore in some of the deeper tropospheric air, this won't be a
concern. But any storms that try to develop at the edge of this
airmass near and around coastal SE LA could become strong to
perhaps severe, just something to watch. Less overall convection
will lead to slightly warmer temperatures building many areas back
to the lower 90's. May see some upper to perhaps mid 60 degree
dewpoints after afternoon mixing, which may feel just a notch
nicer compared to previous weeks. Even going into Thursday, we
stay in a bit of a drier pattern with only isolated shower/storm
chances along I-10/12, with better scattered coverage towards the
south - generally a typical summertime pattern awaits going
towards and beyond mid-week. I dont know about you but I am ok
with that. KLG

LONG TERM (Friday through Monday)...

A sharp but narrow shortwave trough will move southeast from the
central/southern Plains into the lower Mississippi Valley and
central Gulf Coast region Friday through Saturday morning. A weak
residual surface boundary, mainly a result of a bit of enhanced
drainage flow from inland areas from Thursday night, will move
inland and become diffuse by midday Friday, then this is expected to
begin a pretty much consistent light southerly wind period through
early next week. A model blend looks best at this point,
particularly on Friday when the ECMWF focuses more QPF near the base
of the 500 mb trough axis just west of our area while the GFS shows
the rain more focused to the east in central to southern portions of
the forecast area. This results in forecasting a bit higher rain
chances to reach more inland areas Friday, but still in the chance
range. Over the weekend into Monday, a more typical but moist summer
pattern we've had much of this summer is expected with daytime and
particularly afternoon PoP going back up to 50 to 60% across all
land areas according to the NBM and this forecast. There is a bit of
uncertainty since the GFS is downplaying the PoP during this period.
Lows each night are expected to go back to low to mid 70s in
northern areas and mid 70s to near 80 in southern areas. Highs are
expected to be mostly in the lower 90s with some mid 90s possible.
22/TD

MARINE...

A weak front draped across nearshore marine areas will continue
to enhance early morning showers and storms. Main threats in any
stronger storm will be wind gusts >34 knots, isolated waterspouts
and locally enhanced waves/seas. Outside of any thunderstorm
development, winds will remain light to breezy from the west.
Waves will be light at 1-2ft for protected waters to 2-3ft for
outer Gulf waters. Showers and storms will continue through mid-
week and later portions of the week, with more coverage in the
early morning hours each day. KLG

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  87  69  90  68 /  60  20  10   0
BTR  87  71  90  70 /  80  30  20   0
ASD  88  72  91  71 /  90  50  20  10
MSY  89  79  92  78 /  80  50  30  10
GPT  87  71  91  71 /  80  50  30  10
PQL  87  71  91  71 /  80  40  30  10

&&

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

Special Weather Statement
National Weather Service New Orleans LA
633 AM CDT Tue Aug 3 2021

LAZ069-031200-
Lower Plaquemines-
633 AM CDT Tue Aug 3 2021

...A STRONG THUNDERSTORM WILL AFFECT SOUTHEASTERN PLAQUEMINES
PARISH...

At 633 AM CDT, a strong thunderstorm was located over Venice, or 26
miles southeast of Port Sulphur, moving east at 20 mph.

Winds in excess of 40 mph are possible with this storm.

Locations impacted include...
Boothville-venice, Boothville, Venice, Buras-triumph and Boothville
Asos.

LAT...LON 2919 8934 2916 8937 2917 8944 2935 8945
      2936 8938 2933 8937 2936 8937 2935 8930
      2931 8924 2935 8920 2929 8914 2930 8918
      2927 8921 2925 8912 2918 8910 2920 8914
      2919 8914 2917 8910 2914 8911 2914 8925
TIME...MOT...LOC 1133Z 268DEG 16KT 2927 8937

Special Weather Statement
National Weather Service New Orleans LA
644 AM CDT Tue Aug 3 2021

LAZ046-049-056>059-065>067-031245-
Lower Terrebonne-Iberville-Ascension-Upper Terrebonne-Lower
Lafourche-Assumption-St. John The Baptist-St. James-Upper Lafourche-
644 AM CDT Tue Aug 3 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT SOUTHWESTERN ST. JOHN
THE BAPTIST...WESTERN ASCENSION...ST. JAMES...NORTHEASTERN
TERREBONNE...SOUTHEASTERN IBERVILLE...NORTHWESTERN LAFOURCHE AND
NORTHEASTERN ASSUMPTION PARISHES...

At 644 AM CDT, radar indicated strong thunderstorms were located
along a line extending from near Belle Rose to near Chauvin. Movement
was northeast at 10 mph.

Winds in excess of 30 mph are possible with these storms.

Locations impacted include...
Houma, Thibodaux, Gonzales, Donaldsonville, Napoleonville, Galliano,
Larose, Cut Off, Lockport, Golden Meadow, White Castle, Sorrento,
Mathews, Schriever, Chauvin, Geismar, Supreme, Raceland, Chackbay and
Montegut.

This includes Interstate 10 in Louisiana between mile markers 174 and
187.

LAT...LON 2931 9063 3011 9121 3031 9091 2939 9024
      2934 9046 2934 9049 2935 9051 2936 9050
      2936 9052 2933 9052
TIME...MOT...LOC 1144Z 244DEG 11KT 3008 9110 2940 9060

Special Weather Statement
National Weather Service New Orleans LA
747 AM CDT Tue Aug 3 2021

LAZ048>050-057>061-063-064-067>070-072-077-031415-
St. Charles-Livingston-Ascension-Lower Jefferson-Southern
Tangipahoa-Lower St. Bernard-St. James-East Baton Rouge-Lower
Plaquemines-Western Orleans-Upper St. Bernard-Lower Lafourche-St.
John The Baptist-Upper Plaquemines-Upper Lafourche-Upper Jefferson-
747 AM CDT Tue Aug 3 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT ST. JOHN THE BAPTIST...
CENTRAL ASCENSION...NORTHEASTERN ST. JAMES...SOUTHWESTERN ST.
BERNARD...ST. CHARLES...SOUTHWESTERN ORLEANS...SOUTHERN LIVINGSTON...
NORTHEASTERN LAFOURCHE...JEFFERSON...SOUTHERN TANGIPAHOA...
NORTHWESTERN PLAQUEMINES AND SOUTH CENTRAL EAST BATON ROUGE
PARISHES...

At 746 AM CDT, radar indicated strong thunderstorms were located
along a line extending from near Prairieville to 7 miles northeast of
Cut Off. Movement was east at 20 mph.

Winds in excess of 30 mph are possible with these storms.

Locations impacted include...
New Orleans, Hammond, Gonzales, Reserve, Metairie, Hahnville,
Laplace, Marrero, Chalmette, Avondale, Harvey, Timberlane, Belle
Chasse, Port Sulphur, Jefferson, Gretna, Harahan, Westwego,
Ponchatoula and Gramercy.

This includes the following Interstates...
 Interstate 10 in Louisiana between mile markers 166 and 170, and
between mile markers 184 and 245.
 Interstate 12 between mile markers 36 and 45.
 Interstate 55 in Louisiana between mile markers 1 and 30.
 Interstate 310 between mile markers 1 and 11.
 Interstate 610 between mile markers 1 and 4.

LAT...LON 3037 9105 3051 9039 3036 9025 3019 9043
      3009 9041 3006 9037 3002 9018 3005 8997
      2966 8961 2968 8964 2968 8966 2964 8966
      2961 8963 2963 8968 2961 8973 2953 8966
      2951 8967 2949 8998 2952 9000 2949 9024
TIME...MOT...LOC 1246Z 262DEG 17KT 3032 9092 2956 9021