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

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361
FXUS64 KLIX 212159
AFDLIX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
359 PM CST FRI NOV 21 2014

.DISCUSSION...
MAIN FORECAST CHALLENGE WITH THIS PACKAGE CONTINUES TO BE SEVERE
WEATHER POTENTIAL FOR SATURDAY NIGHT AND SUNDAY MORNING. OVERALL
FORECAST THINKING HAS NOT CHANGED...BUT TIMING HAS BEEN ADJUSTED
BASED ON LATEST MODEL GUIDANCE.

MODEL GUIDANCE HAS SLOWED THE PROGRESSION OF THE UPPER TROUGH AND
PRE-FRONTAL SQUALL LINE BY ABOUT 6 HOURS WITH THE 12Z RUN. THIS
PUTS THE MAIN THREAT TIMING FOR THE LOCAL AREA DURING THE
OVERNIGHT HOURS SATURDAY NIGHT THROUGH SUNDAY MORNING. IN FACT...
HAVE DROPPED POPS INTO THE 20 TO 40 PERCENT RANGE WITH HIGHEST
POPS ACROSS THE LA COAST FOR AFTERNOON HOURS SATURDAY BASED ON THE
SLOWING TRENDS. CURRENT THINKING IS THAT THUNDERSTORM THREAT WILL
INCREASE FROM WEST TO EAST STARTING NEAR OR JUST AFTER MIDNIGHT.
SPC STILL INDICATING A SLIGHT RISK OF SEVERE WEATHER ACROSS THE
ENTIRE AREA AND THEY TOO ARE NOW INDICATING THAT THE RISK WILL
BLEED INTO SUNDAY MORNING.

MOISTURE WILL BE PLENTIFUL AFTER A FEW DAYS OF SOLID RETURN FLOW.
MODEL FORECAST SOUNDINGS INDICATE PW VALUES REACHING THE 1.75 TO 2
INCH RANGE BY SATURDAY EVENING. LIFT WILL ALSO BE EASY TO COME BY
WITH THE DYNAMICS ASSOCIATED WITH THE UPPER TROUGH AND JET STREAK.
INSTABILITY...HOWEVER...WILL BE MARGINAL WITH ML CAPE VALUES
TOPPING OUT LESS THAN 500 J/KG FOR NORTHERN HALF OF THE AREA.
CLOSER TO THE COAST...ML CAPE IS FORECAST IN THE 500 TO 1000 J/KG
CAPE. THIS SHOULD STILL BE ENOUGH INSTABILITY TO SUPPORT
THUNDERSTORM DEVELOPMENT AND AT LEAST A FEW SEVERE THUNDERSTORMS
ACROSS THE AREA GIVEN STRONG DYNAMICS.

THERE ARE A COUPLE OF POSSIBLE LIMITING FACTORS...HOWEVER. FIRST
IS THE POSSIBILITY OF A SHALLOW MARINE LAYER DEVELOPING OVER
COASTAL AREAS DUE TO WATER TEMPERATURES BELOW 60 DEGREES IN SOME
OF THE SHALLOWER NEAR SHORE WATERS FOLLOWING RECENT COLD SNAPS. IF
A MARINE LAYER DEVELOPS...THUNDERSTORMS COULD REMAIN ELEVATED. OF
MORE CONCERN...HOWEVER...IS THE POTENTIAL FOR ONGOING CONVECTION
OVER THE COASTAL WATERS SATURDAY/SATURDAY NIGHT TO LIMIT INFLOW
INTO ANY STORMS OVER LAND.

MAIN HAZARD CONCERN WITH THE SQUALL LINE WILL BE STRAIGHT LINE
WINDS...THOUGH PLENTY OF SHEAR WILL ALLOW FOR THE POSSIBILITY OF
EMBEDDED ROTATING CELLS THAT COULD PRODUCE AN ISOLATED TORNADO OR
TWO. ANY STORMS THAT DEVELOP AHEAD OF THE SQUALL LINE WILL HAVE
CAPABILITY TO DEVELOP ROTATING UPDRAFTS GIVEN SRH VALUES RANGING
FROM AROUND 200 M2/S2 SOUTH TO NEARLY 600 M2/S2 NORTH. ANY
STRONGER ROTATING UPDRAFTS WOULD ALSO BE CAPABLE OF SUPPORTING
DEVELOPMENT OF LARGER HAIL...THOUGH THIS IS THE LEAST LIKELY OF
THE SEVERE WEATHER MODES.

MOVING ON TO THE HEAVY RAIN THREAT...AREA AVERAGE QPF VALUES ARE
GENERALLY IN THE 2 TO 3 INCH RANGE OVER THE ENTIRE EVENT. THIS
SHOULDN/T BE ENOUGH TO CAUSE ANY SIGNIFICANT PROBLEMS...
ESPECIALLY GIVEN THE DRY ANTECEDENT CONDITIONS. HOWEVER...WITH PW
VALUES FORECAST TO BE NEAR 2 INCHES...SOME STORMS WILL BE CAPABLE
OF PRODUCING HIGH RAINFALL RATES AND WE COULD SEE SOME NUISANCE
TYPE STREET FLOODING OR PONDING OF WATER IN LOW LYING AND POOR
DRAINAGE AREAS. HPC IS INDICATING A SLIGHT RISK OF FLASH FLOODING
FOR THE TIME FRAME...BUT GIVEN THE FACT THAT MOST AREAS SHOULD BE
ABLE TO HANDLE THE CURRENTLY FORECAST RAINFALL TOTALS...WILL HOLD
OFF ON ANY FLASH FLOOD OR FLOOD WATCH PRODUCTS. IF QPF TOTALS
BEGIN TO SHOW AN UPWARD TREND OR IF THE FORECAST HEAVIER TOTALS
FROM OVER WATER SHIFT NORTHWARD...MAY NEED TO CONSIDER A FLASH
FLOOD OR FLOOD WATCH PRODUCT WITH FUTURE FORECAST PACKAGES.

LAST HAZARD TO TALK ABOUT WITH THIS EVENT IS THE COASTAL FLOOD
THREAT. PERSISTENT MODERATE TO STRONG ONSHORE FLOW HAS ALREADY
RESULTED IN TIDAL RISES ON EAST FACING SHORES. THIS TREND SHOULD
CONTINUE THROUGH SATURDAY NIGHT AND SOME MINOR/NUISANCE COASTAL
FLOODING WILL BE POSSIBLE OUTSIDE OF LEVEE PROTECTION FOR AREAS
FROM BAY SAINT LOUIS TO THE MOUTH OF THE MISSISSIPPI RIVER. A
COASTAL FLOOD ADVISORY IS IN EFFECT FOR THESE AREAS FROM NOON
SATURDAY THROUGH 6 AM SUNDAY.

CONDITIONS SHOULD IMPROVE RAPIDLY FROM WEST TO EAST AFTER SUNRISE
SUNDAY. CURRENT THINKING IS THAT SQUALL LINE SHOULD BE MOVING OUT
OF THE LOCAL FORECAST AREA BY NOON. COLD FRONT FINALLY SWINGS
THROUGH THE AREA SUNDAY NIGHT BRINGING COOLER AND DRIER AIR WITH
IT. A REINFORCING FRONT IS FORECAST TO MOVE THROUGH THE AREA
TUESDAY NIGHT...BUT WITH VERY LITTLE MOISTURE IN PLACE...EXPECT IT
TO COME THROUGH DRYLY.

&&

.AVIATION...
VFR CONDITIONS SHOULD HOLD THROUGH MUCH OF THE NIGHT ALL LOCATIONS
THOUGH SOME EARLY RETURN OF MARINE LAYER AIR MAY LOWER CLOUD DECKS
TO HIGH END MVFR AT KHUM BEFORE OR NEAR DAYBREAK. INFLUX OF MARINE
LAYER INLAND WILL BRING INCREASED CLOUD COVER AT LOW END VFR OR HIGH
END MVFR MUCH OF SATURDAY. HELD OFF ONSET OF ANY PRECIPITATION UNTIL
PERHAPS AFTER 23/00Z WHEN BETTER UPPER DYNAMICS ARRIVES AND RAINFALL
COVERAGE INCREASES.

&&

.MARINE...
WINDS AND SEAS ARE STEADILY INCREASING DUE TO EASTERLY GRADIENT AT
THIS TIME. THIS TREND WILL CONTINUE AND BECOME MORE PRONOUNCED
SATURDAY AFTERNOON IN RESPONSE TO DEVELOPING AND INTENSIFYING SYSTEM
OVER TEXAS DURING THE DAY SATURDAY. ONSHORE FLOW IS EXPECTED TO
FUNNEL INTO EAST-FACING SHORES FOR ENHANCED TIDE LEVELS UNTIL AT
LEAST SQUALL LINE PASSAGE SUNDAY MORNING. MEANWHILE...SEAS ARE
EXPECTED TO BUILD IN THE OPEN WATERS 6 TO 8 FEET WITH RANDOM 10-11
FEET ON LONG PERIOD SWELL TRAIN BY SATURDAY NIGHT INTO SUNDAY
MORNING. FRONTAL PASSSAGE MONDAY WILL THEN BRING NORTHERLY FLOW TO
DAMPEN SWELL COMPONENT.

&&

.DECISION SUPPORT...
DSS CODE...BLUE.
DEPLOYED...NONE.
ACTIVATION...NONE.
ACTIVITIES...SLURRY SUPPORT
             ASSESSING SEVERE WEATHER THREAT FOR SATURDAY/SUNDAY
             MONITORING COASTAL FLOOD THREAT FOR SATURDAY NIGHT.

DECISION SUPPORT SERVICES (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 SEVERE; NEARBY
         TROPICAL EVENTS; HAZMAT OR OTHER LARGE EPISODES
RED    = FULL ENGAGEMENT FOR MODERATE TO HIGH RISK SVR
         AND/OR DIRECT TROPICAL THREATS; EVENTS OF NATIONAL
         SIGNIFICANCE.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  49  70  60  77 /  10  20  90  60
BTR  49  73  63  78 /  10  30  90  50
ASD  49  72  62  78 /  10  30  90  70
MSY  57  73  65  78 /  10  40  90  60
GPT  51  69  64  76 /  10  30  90  80
PQL  46  69  62  76 /  10  30  80  90

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...COASTAL FLOOD ADVISORY FROM NOON SATURDAY TO 6 AM SUNDAY FOR THE
     FOLLOWING ZONES: LOWER PLAQUEMINES...LOWER ST.
     BERNARD...ORLEANS...AND UPPER ST. BERNARD.

MS...COASTAL FLOOD ADVISORY FROM NOON SATURDAY TO 6 AM SUNDAY FOR THE
     FOLLOWING ZONES: AND HANCOCK.

GM...SMALL CRAFT ADVISORY FROM 6 AM SATURDAY TO NOON SUNDAY FOR THE
     FOLLOWING ZONES: BRETON SOUND...CHANDELEUR SOUND...LAKE
     BORGNE...LAKE PONTCHARTRAIN AND LAKE MAUREPAS...AND
     MISSISSIPPI SOUND.

     SMALL CRAFT ADVISORY UNTIL NOON SUNDAY FOR THE FOLLOWING ZONES:
     COASTAL WATERS FROM BOOTHVILLE LA TO THE SOUTHWEST PASS OF
     THE MISSISSIPPI RIVER OUT 20 NM...COASTAL WATERS FROM
     PASCAGOULA MS TO STAKE ISLAND LA FROM 20 TO 60 NM...COASTAL
     WATERS FROM PASCAGOULA MS TO STAKE ISLAND LA OUT 20
     NM...COASTAL WATERS FROM PORT FOURCHON TO LOWER ATCHAFALAYA
     RIVER FROM 20 TO 60 NM...COASTAL WATERS FROM PORT FOURCHON
     TO LOWER ATCHAFALAYA RIVER OUT 20 NM...COASTAL WATERS FROM
     STAKE ISLAND LA TO THE SOUTHWEST PASS OF THE MISSISSIPPI
     RIVER FROM 20 TO 60 NM...COASTAL WATERS FROM THE SOUTHWEST
     PASS OF THE MISSISSIPPI RIVER TO PORT FOURCHON FROM 20 TO
     60 NM...AND COASTAL WATERS FROM THE SOUTHWEST PASS OF THE
     MISSISSIPPI RIVER TO PORT FOURCHON OUT 20 NM.

&&

$$

AVIATION/MARINE...24/RR
REST OF DISCUSSION...95/DM






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