Small Craft Advisory 34, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
527 AM CDT FRI MAY 20 2016

.SYNOPSIS...

Very well defined mesoscale convective vortex (MCV) on radar and
satellite moved quickly east-northeast across northern portions
of the forecast area last evening to shortly after midnight, and
this feature was now moving into southwest Alabama. There were at
least 14 reports of severe weather that occurred with the quasi-
linear convective system (QLCS) as it raced through the forecast
area. water vapor imagery shows the main mid/upper level
shortwave trough extending from a low over Kansas into
central/eastern Texas. The upper level winds/flow are extremely
divergent from east of the shortwave trough to surrounding the
MCV, and that typically is a pattern where the mesoscale features
and thunderstorms move quite a bit faster than the models can
depict a day or even less in advance. At the surface, low pressure
over southern Mississippi has a trough extending to it's south,
however, the slow moving weak cold front appeared to extend
southwest from the low into southwest Louisiana with the drier air
still lagging well behind the front.

.SHORT TERM (Today through Sunday night)...

The upper level divergence in advance of the shortwave trough over
the southern plains will continue to support the development of
scattered showers and thunderstorms over the coastal waters off
the Louisiana coast, and these should continue to generally weaken
as they try to move inland into more stable air early this morning.
As the day progresses, the shortwave trough will continue to move
east towards the lower Mississippi valley. Daytime heating should
lead to more scattered inland development of showers and
thunderstorms by mid to late morning, then activity should
continue into the early to mid afternoon hours before drier air
decreases coverage from the northwest and west during the mid to
late afternoon. One or two strong thunderstorms can't be ruled
out, however severe thunderstorms are no longer expected. Some
localized banding and temporary training of showers and
thunderstorms could occur, but the threat of any additional
flooding appears minimal at this time.

The shortwave trough is expected to amplify over the eastern
states and east of the forecast area tonight into the weekend.
This will help push a weak cold front through the forecast area
this evening with a slight reinforcement Saturday night as surface
high pressure builds south across the mid Mississippi valley to
Gulf coast region. Saturday and Saturday night are expected to be
dry and less humid with cooler lows expected Saturday night.
Daytime highs will be near to slightly above normal however with
the strong late May sun with Saturday likely the warmest day. The
only complication is weak disturbances that will traverse over a
flat mid/upper level ridge that will be building over the southern
plains to just west of the lower Mississippi valley. The ECMWF
model is a more aggressive forecasting some QPF over portions of
the coastal waters and just west of the forecast area on Saturday
and Sunday. Have followed a blend of the wetter ECMWF and other
drier models at this time.

.LONG TERM (Monday through Thursday night)...

A series of shortwave troughs will keep a mean trough over the
western U.S. coast area early in the week which should progress
slowly east across the southwest states Wednesday and Thursday.
The downstream shortwave trough is expected to move from eastern
portions of the southern plains across the lower Mississippi
valley as the week progresses, and surface high pressure will also
move east early in the week allowing southerly surface winds to
return.

The ECMWF is more bullish with "ridge riders", or weak shortwave
troughs that move through the southern belt of westerlies across
the lower Mississippi valley, therefore the ECMWF has more QPF
than the GFS, starting in western areas Monday and Tuesday, and
across most areas Wednesday into Thursday. Have gone with a model
blend at this time. 22/TD

&&

.AVIATION...

A brief period of stability in the wake of a strong MCV that swept
through before 06Z will persist through around 15Z with the only
issue being some MVFR ceilings around 1500 feet that could form
toward daybreak.  After 15Z, some scattered convection should try to
reform in advance of an approaching cold front.  Have VCTS wording
in place to reflect this risk at all of the terminals throughout the
daytime hours.  By 06z, frontal passage should have occurred
resulting in an end to the convective risk.  Where convection is not
impacting a terminal, VFR conditions are expected through the day.
If convection does directly impact a terminal, generally an hour or
less of gusty winds and IFR conditions are expected.  32

&&

.MARINE...

Onshore flow of 15 to 20 knots and seas of 3 to 5 feet are forecast
through the afternoon hours in response to a continued strong
pressure gradient over the coastal waters.  The onshore flow will
diminish and shift to an offshore component by tomorrow morning in
the wake of a passing cold front.  A surface high will then become
parked over the coastal waters for the remainder of the weekend and
into Monday.  Winds will generally range from 5 to 15 knots and seas
will decrease to 3 feet or less.  By the middle of next week, the
high should shift to the east of the area allowing for a more
sustained and stronger onshore flow of 10 to 15 knots to develop.
32

&&

.DECISION SUPPORT...

DSS code: Green.
Deployed: None.
Activation: None.
Activities: None.

Decision Support Services (DSS) Code Legend
Green  = No weather impacts that require action.
Blue   = Long-fused watch, warning, or advisory in effect or
         high visibility event.
Yellow = Heightened impacts with short-fused watch, warning
         or advisory issuances; radar support.
Orange = High Impacts; Slight to Moderate risk severe; nearby
         tropical events; HazMat or other large episodes.
Red    = Full engagement for Moderate risk of severe and/or
         direct tropical threats; Events of National Significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  82  67  87  62 /  50  10   0   0
BTR  85  69  89  67 /  50  10  10   0
ASD  84  68  88  66 /  70  10  10   0
MSY  85  72  89  71 /  60  10  10  10
GPT  83  70  89  66 /  70  10  10   0
PQL  84  68  90  63 /  70  20  10   0

&&

.LIX Watches/Warnings/Advisories...
LA...None.
GM...None.
MS...None.
GM...None.
&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE NEW ORLEANS LA
819 PM CDT THU MAY 19 2016

GMZ530-532-534-536-538-550-552-555-557-570-572-575-577-LAZ034>037-
039-040-046>050-056>072-MSZ068>071-077-080>082-201330-
LAKE PONTCHARTRAIN AND LAKE MAUREPAS-MISSISSIPPI SOUND-
LAKE BORGNE-CHANDELEUR SOUND-BRETON SOUND-
COASTAL WATERS FROM PORT FOURCHON LA TO LOWER ATCHAFALAYA RIVER
LA OUT 20 NM-
COASTAL WATERS FROM THE SOUTHWEST PASS OF THE MISSISSIPPI RIVER
TO PORT FOURCHON LOUISIANA OUT 20 NM-
COASTAL WATERS FROM BOOTHVILLE LA TO SOUTHWEST PASS OF THE
MISSISSIPPI RIVER OUT 20 NM-
COASTAL WATERS FROM PASCAGOULA MISSISSIPPI TO STAKE ISLAND OUT
20 NM-
COASTAL WATERS FROM PORT FOURCHON LOUISIANA TO LOWER ATCHAFALAYA
RIVER LA FROM 20 TO 60 NM-
COASTAL WATERS FROM SOUTHWEST PASS OF THE MISSISSIPPI RIVER TO
PORT FOURCHON LOUISIANA FROM 20 TO 60 NM-
COASTAL WATERS FROM STAKE ISLAND LA TO SOUTHWEST PASS OF THE
MISSISSIPPI RIVER FROM 20 TO 60 NM-
COASTAL WATERS FROM PASCAGOULA MISSISSIPPI TO STAKE ISLAND
LOUISIANA OUT 20 TO 60 NM-POINTE COUPEE-WEST FELICIANA-
EAST FELICIANA-ST. HELENA-WASHINGTON-ST. TAMMANY-IBERVILLE-
WEST BATON ROUGE-EAST BATON ROUGE-ASCENSION-LIVINGSTON-ASSUMPTION-
ST. JAMES-ST. JOHN THE BAPTIST-UPPER LAFOURCHE-ST. CHARLES-
UPPER JEFFERSON-ORLEANS-UPPER PLAQUEMINES-UPPER ST. BERNARD-
UPPER TERREBONNE-LOWER TERREBONNE-LOWER LAFOURCHE-LOWER JEFFERSON-
LOWER PLAQUEMINES-LOWER ST. BERNARD-NORTHERN TANGIPAHOA-
SOUTHERN TANGIPAHOA-WILKINSON-AMITE-PIKE-WALTHALL-PEARL RIVER-
HANCOCK-HARRISON-JACKSON-
819 PM CDT THU MAY 19 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHEAST
LOUISIANA...SOUTH MISSISSIPPI AND THE ADJACENT COASTAL WATERS.

.DAY ONE...TONIGHT

A LINE THUNDERSTORMS WILL MOVE THROUGH SOUTHEAST LOUISIANA AND
SOUTHERN MISSISSIPPI TONIGHT. A FEW STORMS COULD BE SEVERE WITH
THE PRIMARY THREATS BEING LARGE HAIL AND DAMAGING WIND GUSTS. AN
ISOLATED SHORT LIVED TORNADO OR TWO CANNOT BE RULED OUT.

LOCALLY HEAVY RAINFALL OF 1 TO 2 INCHES IS EXPECTED TONIGHT.
ISOLATED HIGHER AMOUNTS WILL BE POSSIBLE AND COULD RESULT IN SOME
LOCALIZED FLOODING OF LOW LYING AND POOR DRAINAGE AREAS.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ON FRIDAY FOR
MUCH OF THE REGION. THE PRIMARY THREAT FROM ANY SEVERE
THUNDERSTORM ACTIVITY WILL BE STRONG DAMAGING WIND GUSTS AND LARGE
HAIL.

ADDITIONAL RAINFALL OF 1 TO 2 INCHES WITH LOCALLY HIGHER AMOUNTS
MAY OCCUR ON FRIDAY. IF THIS HEAVY RAINFALL OCCURS OVER A SHORT
PERIOD OF TIME...SOME LOCALIZED FLOODING COULD OCCUR IN LOW LYING
AND POOR DRAINAGE AREAS.

THE THREAT OF HAZARDOUS WEATHER IS LOW SATURDAY THROUGH
WEDNESDAY.

.SPOTTER INFORMATION STATEMENT...

PLEASE RELAY ANY REPORTS OF SEVERE WEATHER TO THE NATIONAL WEATHER
SERVICE OR LOCAL LAW ENFORCEMENT.

95/DM

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE NEW ORLEANS LA
559 AM CDT FRI MAY 20 2016

LAZ067-201145-
LOWER LAFOURCHE LA-
559 AM CDT FRI MAY 20 2016

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN LAFOURCHE PARISH
UNTIL 645 AM CDT...

AT 558 AM CDT...DOPPLER RADAR WAS TRACKING A STRONG THUNDERSTORM 14
MILES SOUTHWEST OF LEEVILLE...OR 23 MILES SOUTH OF GALLIANO...MOVING
EAST AT 25 MPH.

DIME SIZE HAIL AND WINDS IN EXCESS OF 40 MPH WILL BE POSSIBLE WITH
THIS STORM.

LOCATIONS IMPACTED INCLUDE...
LEEVILLE.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

TORRENTIAL RAINFALL IS ALSO OCCURRING WITH THIS STORM...AND MAY LEAD
TO LOCALIZED FLOODING. DO NOT DRIVE YOUR VEHICLE THROUGH FLOODED
ROADWAYS.

&&

LAT...LON 2913 9024 2914 9028 2915 9024 2916 9027
      2915 9028 2920 9026 2922 9028 2929 9002
      2927 9008 2924 9011 2924 9010 2927 9004
      2921 9008 2923 9011 2920 9008 2928 9002
      2918 9007 2909 9024
TIME...MOT...LOC 1058Z 257DEG 21KT 2909 9038

32

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