Area Forecast Discussion
Issued by NWS New Orleans/Baton Rouge, LA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS64 KLIX 291422
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
922 AM CDT Sat Apr 29 2017

.SOUNDING DISCUSSION...
12z sounding is complete and shows less of a cap compared
to the previous run. There is still a cap to limit convection
through much of the day, however convective parameters are
high enough that if the cap breaks, storms would become
strong quickly. We will watch close for late this afternoon
into the evening mainly in the western portion of our area. PW
measured at 1.32 inches with impressive low level winds of 40kt at
945mb, then weakens until 282mb with 43kt. A 3400 CAPE is
impressive on a 12Z sounding with lapse rates approaching 7C/km.
Shear not significant and could be influenced by early morning
light and variable surface winds at the surface just at release
time. Will be tricky how much of the cap erodes Sunday ahead of
a strong line of thunderstorms forecast to move through during
the afternoon. This could highly impact the forecast timing of
rain on Sunday.

Did not change anything in the coastal marine forecast yet. Still
looking at windy with a Small Craft Advisory in effect through
Sunday evening. Will spend some meteorologist hudle time today
looking at the gradient to see if gale force criteria will be
met as it will be close. [KEG]

&&

.PREV DISCUSSION... /issued 423 AM CDT Sat Apr 29 2017/

SHORT TERM...
What a difference 12 hours makes. Deep moisture has once again
and rapidly moved through the area as it transits northward to
help maintain strong and severe thunderstorms throughout the
center of the country. This is also causing the pressure gradient
to tighten significantly which will be noticed today and Sunday.
Wind speeds today should be within the 20-25mph range gusting to
35mph. Sunday should see 25mph with gusts to 35mph and possibly as
high as 40mph at times. Wind advisories have been posted for
today and they will likely be posted again for Sunday by this
evening.

Severe weather variables are very high today into Sunday but they
are mostly stuck beneath a strong 4c subsident inversion. This
inversion will remain relatively in tact while weakening slowly
through Sunday morning as the cold front approaches. This
inversion should keep the majority of thunderstorm activity from
forming until the front approaches causing the inversion to lift
and weaken enough to allow sh/ts to erupt as the front moves
through. Advection of these severe wx variables will also move
beneath this inversion toward the volatile boundary well to the
north of the area until Sunday. Moisture loading within the
boundary layer will help cause cloud cover at the base of the
inversion as frictional convergence helps the moisture gently rise
as it moves out of the gulf. This could cause light showers to
develop again today.

A 50kt jet at about 870mb will also set up tonight. This will help
lift the boundary layer moisture as well and cause the low level
clouds to move very fast to the northwest tonight and Sunday.
This is what will provide one ingredient for any severe weather
Sunday. Any thunderstorm activity that can produce 20-30mph winds
by momentum transfer to the sfc coupled with the already gradient
wind speeds of 25mph could easily produce 50+mph wind speeds
along the boundary Sunday. This could cause damaging wind gusts,
and this should be the main hazard with any strong or severe
thunderstorms Sunday. The other severe weather parameters will
not be discounted but should not be as prevalent. Since wind
speeds are strong at the sfc and vertical wind direction becomes
more uniform as the boundary approaches, both the vertical and
speed shear variables are expected to be moderate at best once the
inversion gives way. This could cause some weak spin ups that
would be rain wrapped as the line of sh/ts moves through. Wet Bulb
Zero heights will be around 12k+ft as very warm air produces
temps around -3c at 500mb; so hail, if any, looks to be small.

Rainfall amounts are still expected to be from 2 to 4 inches total
through Sunday night. Most of this is expected with the boundary
as it moves through which would cause this to fall with a
relatively short period. Minor flooding of low lying and poor
drainage areas would also be possible Sunday.

Coastal flooding would be the next biggest item on the list. Areas
that normally receive salt water inundation during times of
abnormal high tide levels will receive water during this scenario
starting today. Tide levels are expected to be 1-2ft above normal
levels through Sunday. Highest levels will be during times of high
tides which will be from late morning through the early afternoon
hours each day. Some of the lowest beach and secondary routes may
observe as much as 3" of salt water over those surfaces today and
as much as 7" Sunday.

LONG TERM...
Another round of sh/ts are expected Wednesday into Thursday
morning ahead of another cold front that should move through the
area Thursday morning.

AVIATION...

Expect MVFR to IFR conditions to prevail this morning as abundant
moisture is in place yielding low cigs across the forecast area. Low
clouds should break up some and lift for some areas to get into the
VFR range but expect mostly MVFR conditions to prevail throughout
the forecast period. Winds will be elevated today as winds at most
terminals will be around 20 knots out of the south across the entire
area. A few gusts up to 30 knots cannot be ruled out...as well
as an isolated shower or thunderstorm this afternoon. 13/MH

MARINE...

Not much change in the forecast this morning. The pressure gradient
will remain tight across the coastal waters through the weekend. A
deepening low pressure system over the Plains will yield southerly
winds of 20 to 30 knots through Sunday. Small Craft Advisories are
posted for the entire coastal waters through Sunday evening as the
waters will be hazardous all weekend. There is also an outside
chance that the Small Craft Advisory may have to be upgraded to a
Gale Warning, especially east of the Mississippi River this
afternoon through early Sunday morning. This will have to be watched
as this system evolves. Gulf waters due to these winds and seas of
up to 7 feet. The tightening pressure gradient will result in strong
onshore winds of 25 to 30 knots impacting the coastal waters this
afternoon through Sunday evening. Very rough seas of up to 12 feet
could also impact the open Gulf waters during this period. Over the
sounds and lakes waves of up to 6 feet will be possible.

Winds will shift to the northwest late Sunday night and Monday in
the wake of the low pressure system and cold front. Winds should
remain elevated at around 20 knots through morning hours on Monday
and seas will remain rough. Conditions will begin to improve Monday
night and Tuesday as high pressure settles directly over the waters.
Winds will veer back to the east by Tuesday and decrease to between
10 and 15 knots. Seas should also fall back to 2 to 4 feet by
Tuesday morning. 13/MH

DECISION SUPPORT...
DSS CODE...Blue.
DEPLOYED...None.
ACTIVATION...None.
ACTIVITIES...Monitoring severe potential for Sunday.

Decision Support Service (DSS) Code Legend:
GREEN  = No weather impacts that require action
BLUE   = Long fused Watch/Warning/Advisory in effect or high
         visibility event
YELLOW = Heightened impacts with short fused
         Watch/Warning/Advisory issuances; Radar support
ORANGE = High Impacts - Slight to Moderate risk of severe weather;
         Nearby tropical events, HAZMAT or other large episodes
RED    = Full engagement for moderate risk of severe weather; Direct
         tropical threats; events of national significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  88  73  78  52 /  30  20 100  50
BTR  89  75  78  53 /  30  40 100  40
ASD  86  75  80  60 /  20  10 100  70
MSY  87  75  82  61 /  20  20 100  60
GPT  84  76  79  62 /  20  10 100  80
PQL  84  74  81  63 /  20  10 100  90

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Wind Advisory until 10 PM CDT this evening for LAZ034>037-039-
     040-046>050-056>072.

     Coastal Flood Advisory until midnight CDT Sunday night for
     LAZ040-050-058-060>064-066>070-072.

GM...Small Craft Advisory until midnight CDT Sunday night for GMZ530-
     532-534-536-538-550-552-555-557-570-572-575-577.

MS...Wind Advisory until 10 PM CDT this evening for MSZ068>071-077-
     080>082.

     Coastal Flood Advisory until midnight CDT Sunday night for
     MSZ080>082.

GM...Small Craft Advisory until midnight CDT Sunday night for GMZ532-
     534-536-538-550-552-555-557-570-572-575-577.

&&

$$


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.