Special Marine Warning 596, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
340 PM CST Sat Dec 18 2021

.SHORT TERM (Tonight through Monday Night)...A rather unsettled
period is expected through Monday night as a northern stream
positively tilted trough drives a cold front through the area and
offshore tonight, and another southern stream trough pushes
across the region on Monday. With the front lingering over the
northern Gulf of Mexico through the entire period, a persistent
northerly flow will keep cooler and more stable air in place in
the low levels. The front itself will remain stalled as the mid
and upper level flow remains west-southwesterly. This warmer and
more humid flow pattern will override the cooler low level
airmass and induce mainly overcast conditions throughout the
period. With a lack of strong solar insolation in place, the
diurnal curve will be suppressed. Highs should only warm into the
upper 50s and lower 60s tomorrow, and then further cool into the
lower to middle 50s Monday as a deep layer cold pool associated
with the southern stream upper level low moves directly over the
forecast area. Overnight lows will be closer to seasonal averages
in the lower to middle 40s as cloud cover limits radiational
cooling at night.

Concerning rain chances, in the immediate term, scattered shower
and weak thunderstorm activity will continue to develop in advance
of a cold front and main convective band currently pushing through
southwest Louisiana. This development is associated with jet
streak sliding through the region and difluent flow pattern aloft
that are promoting increased lift. Fortunately, a lack of decent
instability and shear has kept the thunderstorm activity on the
weak side this afternoon. The main convective line in Southwest
Louisiana has also started to weaken and will continue to decrease
in intensity as it pushes eastward due as it encounters a more
stable and less sheared environment. The front and line of
convection should push into the offshore waters by late this
evening with lingering low stratus and light rain showers expected
over land based portions of the CWA during the overnight hours.

A brief respite in the rain chances over the northern half of the
CWA can be expected on Sunday as a region of weak negative
vorticity and increased upper level subsidence slides through.
Skies will remain mostly cloudy given the broad region of
isentropic forcing over the shallow cold pool, but the lack of
deeper moisture will keep rain chances at bay. This respite will
be shortlived, as the southern stream shortwave trough quickly
advances into Texas. Low level cyclogenesis will take place along
the stalled front over the western Gulf and this low will quickly
advance east across the northern Gulf Monday into Monday night.
The low should deepen rapidly as the upper level trough takes a
more neutral tilt in response to a jet streak pushing through the
northwest Gulf. Deep layer moisture will feed back into the
region, and expect to see fairly widespread light to moderate
rain form by Monday morning and persist into Monday evening for
most of the forecast area. The rains should begin to ease by late
Monday night as the surface low pulls to the east. Given the
amount of forcing and fairly high precipitable water values in
place, have POP of 50 to 80 percent forecast for areas along and
south of I-10 on Monday.

.LONG TERM (Tuesday through Friday Night)...Strong negative
vorticity advection and deep layer northwest flow will take hold
of the region on Tuesday and remain in place through Wednesday. As
a result, clearing skies, low humidity, and cooler than average
temperatures are expected. Highs should only climb into the upper
50s on Tuesday before modifying a bit into the lower 60s on
Wednesday. The clear skies and lighter winds expected Tuesday
night will result in strong radiational cooling, and lows should
dip into the 30s for most locations. Increased ridging building in
from the west on Thursday and Friday will keep skies clear and
rain chances minimal, but a return flow pattern in the low levels
will lead to warmer temperatures and increasing humidity by the
end of the week. Highs on Christmas Eve should climb into the
middle to upper 70s and highs could potentially cross 80 degrees
on Christmas Day.

&&

.AVIATION...The initial concern will be bands of shower and
thunderstorm activity in advance of a cold front. This convection
could produce brief periods of IFR visibility restrictions and
gusty winds. This convective risk should peak between 00z and 06z.
After 06z, low stratus ranging from 300 to 800 feet is expected to
form at all of the terminals in the wake of the cold front. These
IFR or lower conditions will persist through late morning (around
16-17z) tomorrow before beginning to improve tomorrow afternoon.
Winds will also turn gusty at over 12 knots from the north at
KNEW, KMSY, and KGPT between 00z and 06z.

&&

.MARINE...A colder airmass will advect over the waters tonight
leading to a period of small craft conditions through tomorrow
afternoon. Winds should relax slightly tomorrow night, but will
quickly increase on Monday as a Gulf low pushes through the
northern Gulf of Mexico. High end small craft advisory conditions
over the open Gulf waters can be expected, and conditions may
potentially reach gale force if the low deepens enough. Strong
cold air advection and a fairly tight gradient will keep small
craft conditions going through Tuesday before high pressure
finally becomes centered over the area and winds relax for the
latter half of week.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  46  57  40  53 /  90   0  10  30
BTR  49  59  44  54 /  90   0  30  40
ASD  51  63  44  57 /  80  10  20  50
MSY  55  62  49  56 /  80  10  30  60
GPT  54  63  45  56 /  70  10  20  50
PQL  53  64  43  58 /  70  10  20  50

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...Small Craft Advisory from 9 PM this evening to 3 PM CST Sunday
     for GMZ536-538-550-552-555-557-570-572-575-577.

MS...None.
GM...Small Craft Advisory from 9 PM this evening to 3 PM CST Sunday
     for GMZ538-550-552-555-557-570-572-575-577.

&&

PG

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