Special Marine Warning 430, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
520 AM CDT Sun Jul 25 2021

.SHORT TERM (Sunday through Tuesday)...

Water vapor imagery depicts a cyclonic shear axis extending from
the southwest Louisiana coast east-southeastward across the north
Gulf with easterlies and drier air in the upper levels having
moved through the forecast area. One other notable feature this
morning is an axis of low level moisture pooling with isolated
showers extending from east of Tylertown to metro New Orleans.

The expansive mid to upper level high pressure area will remain
just off to the north and west of the region which will keep the
forecast area in a battle between very warm/hot low level
temperatures and scattered diurnally driven convection with
precipitable water values of 1.8 to 2.1 inches expected throughout
the daytime hours. High temperatures are once again expected to
reach the mid 90s in most areas just inland from the immediate
coast to go along with continued high dewpoint temperatures in the
mid to upper 70s with some locations again up around 80. Since
yesterday overachieved with respect to the highs and dewpoints in
some areas, have adjusted the hourly NBM values up to account for
the recent bias. The forecast maximum heat index today is in the
107 to 112 degree range across most areas except the far southeast
coastal areas from Jackson and Harrison Counties in Mississippi
to the extreme southeast coastal areas of Louisiana and some areas
on the west side of metro Baton Rouge. There is the chance some
areas could see earlier development of scattered showers and
thunderstorms which could keep the duration and level of heat a
bit lower, however confidence is low on if and where that will
happen. Therefore, have expanded the Heat Advisory to include all
of metro New Orleans and Hancock County. Like yesterday, some of
the stronger thunderstorms that develop could produce frequent
lightning strikes and some gusty winds as high as 30 to 40 mph
along with mostly brief heavy downpours. A few of the showers and
storms will likely linger into the evening hours before
dissipating by late evening.

The models still indicate some drier air will likely move into
portions of the forecast area from east to west on Monday. Have
continued to follow the NBM guidance keeping the higher 40 to 60%
chances of showers and thunderstorms in northern/western areas
and 20 to 30% in areas south of the tidal lakes and near the
coast. High temperatures should remain hot with plenty of mid 90s
expected for most inland areas. Maximum heat index values are
forecast to remain below the Heat Advisory criteria of 108, but
cannot rule out some hotter locations. Once again, there will be
plenty of instability for some thunderstorms to become strong and
produce frequent lightning, gusty winds and some torrential
downpours.

Monday night into Tuesday morning should be mostly dry with the
lower precipitable water lingering over the forecast area,
however many areas should see higher moisture content in the
afternoon and have a 30 to 50% chance of showers and
thunderstorms. Temperatures will continue to be hot for highs with
mid 90s inland and lower 90s on the coast.

.LONG TERM (Tuesday night and beyond)...

Main forecast concern during the extended period will be the daily
threat of thunderstorms. Medium range solutions have shifted a
little bit eastward with the center of the upper ridge, near the
Kansas-Colorado border. The question is whether that will be far
enough east to put the local area in subsidence to suppress
convection. At this time, it looks like daily diurnal convection
will continue to be a threat for the 2nd half of the week into the
weekend. The 12z medium range operational solutions would indicate
that the NBM PoPs are too high, but with the models going back and
forth on the ridge the last couple days, won't back off the PoPs
quite yet.

Somewhat concerned about the high temperature forecast, as we've
been trending above guidance, especially the last couple of days. If
the drying trend continues, we'll need to bump up highs, possibly
beyond what the 12z ECMWF would suggest. Right now, we're consistent
with neighboring offices, just something to keep in mind.

&&

.AVIATION...

Primarily a VFR forecast. All indications are that convection
during the day Sunday will be mainly an afternoon affair, and will
carry VCTS in terminals until it becomes more clear where the
direct impacts will be. Most likely direct impacts would be IFR
visibilities and wind gusts to 25-30 knots. Most storms should
dissipate near or before sunset again Sunday evening.

&&

.MARINE...

Very benign conditions are expected across the coastal waters
through next week. Light winds of 10 knots or less and seas mostly
less than 3 feet are forecast through the period. Scattered
thunderstorm activity each day will be the primary concern. These
storms could produce brief periods of gusty winds, choppy seas,
and possibly a few waterspouts.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  95  76  93  74 /  50  20  50  10
BTR  95  76  94  76 /  50  20  60  10
ASD  95  76  95  76 /  50  20  30  10
MSY  97  81  95  79 /  50  20  30  10
GPT  93  77  93  76 /  50  20  20  10
PQL  95  75  94  76 /  50  10  20  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Heat Advisory from 10 AM this morning to 7 PM CDT this evening
     for LAZ034>037-039-046>050-056>061-063>067-071-072-075>078.

GM...None.
MS...Heat Advisory from 10 AM this morning to 7 PM CDT this evening
     for MSZ068>071-077-080.

GM...None.
&&

Short Term/Marine...22/TD
Long Term/Aviation...35