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

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414
FXUS64 KLIX 171302
AFDLIX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
802 AM CDT FRI APR 17 2015

...SOUNDING DISCUSSION...

PRECIPITABLE WATER VALUES REMAIN ELEVATED DESPITE DRIER AIR
BEING MIXED INTO THE MIDLEVELS THIS MORNING. THE DRY AIR IN THE
MIDLEVELS MEANS TWO THINGS FOR TODAY. 1) LESS EXPECTED COVERAGE
FOR SHOWERS AND THUNDERSTORMS AS HAS BEEN THE CASE THE PAST TWO
DAYS (COMPARED TO TUESDAY). 2) THE DRY AIR IN THE MIDLEVELS WILL
INCREASE THE DOWNBURST WIND POTENTIAL. AS FOR HAIL POTENTIAL...WET
BULB ZERO HEIGHT IS CURRENTLY ANALYZED AT 12300 FEET WITH A
FREEZING LEVEL AROUND 13000 FEET. THESE PARAMETERS ARE BORDERLINE
HIGH FOR HAIL...BUT FORECAST SOUNDINGS SHOW THAT THESE ARE
EXPECTED TO LOWER. THERE IS MORE SHEAR THAN YESTERDAY WITH 0-3KM
SURFACE RELATIVE HELICITY AT 200M2/S2 AND BULK SHEAR OF 53 KNOTS.
WINDS AT THE SURFACE WERE FROM THE SOUTHEAST AND ROTATED AROUND TO
FROM THE WEST BY 9500 FEET. A PEAK WIND SPEED OF 91 KNOTS AT 39000
FEET.

12Z BALLOON INFO: A ROUTINE FLIGHT THIS MORNING. THE BALLOON
ASCENDED TO A HEIGHT OF 21.7 MILES ABOVE THE GROUND BURSTING NEAR
VANCLEAVE 67 MILES DOWNRANGE FROM THE OFFICE. FLIGHT TIME WAS 108
MINUTES.

ANSORGE

&&

.PREV DISCUSSION... /ISSUED 420 AM CDT FRI APR 17 2015/

SHORT TERM...

MORNING SHOWERS AND THUNDERSTORMS SHOULD GIVE WAY TO LOWER RAIN
CHANCES THIS AFTERNOON AND EVENING...AS SOME DRIER AIR ADVECTS
INTO THE AREA. MODEL SOUNDINGS SHOW THAT DRY AIR IN THE MID-LEVELS
WILL SLIDE INTO THE AREA THIS AFTERNOON IN THE WAKE OF A PASSING
UPPER LEVEL VORTICITY MAXIMA. THIS REGION OF DRIER AIR WILL ALSO
BE ASSOCIATED WITH SOME WEAK UPPER LEVEL SUBSIDENCE SLIPPING
THROUGH THE LOWER MISSISSIPPI GULF COAST. WITH LESS OVERALL
MOISTURE IN PLACE...EXPECT ANY CONVECTION TO FAR MORE SCATTERED IN
NATURE THIS AFTERNOON AND EVENING. GIVEN THIS...HAVE ONLY WENT
WITH CHANCE POPS FOR COASTAL ZONES AND SLIGHT CHANCE POPS FOR FAR
INLAND ZONES LATER TODAY.

EVEN WITH LESS OVERALL COVERAGE OF THUNDERSTORMS EXPECTED...THE
SOUNDINGS ARE FAVORABLE FOR SOME STRONG TO POSSIBLY SEVERE
THUNDERSTORMS TO DEVELOP TODAY DURING THE AFTERNOON HOURS. WET
BULB ZERO HEIGHTS ARE VERY LOW AT AROUND 11K FEET...AND LAPSE
RATES ARE MARGINALLY FAVORABLE FOR SOME HAIL TO DEVELOP.
CONDITIONS ALSO LOOK PRIME FOR SOME STRONGER STRAIGHT LINE WINDS
TO DEVELOP IN ANY DEEPER CONVECTION AS NOTED BY A CLASSIC
HOURGLASS SOUNDING PROFILE DURING THE AFTERNOON HOURS.

THE MAIN UPPER LEVEL LOW OVER THE SOUTHWEST WILL FINALLY BEGIN TO
PULL TO THE EAST TOMORROW...RESULTING IN A BROAD AREA OF UPPER
LEVEL DIFLUENCE AND STRONG FORCING OVERSPREADING THE AREA. THIS
PROCESS SHOULD ACTUALLY BEGIN LATE TONIGHT...AND EXPECT TO SEE
NUMEROUS SHOWERS AND THUNDERSTORMS BEGIN IMPACTING WESTERN ZONES
BEFORE DAYBREAK. FAIRLY WIDESPREAD SHOWER AND THUNDERSTORM
ACTIVITY WILL THEN PERSIST THROUGH THE DAY ON SATURDAY AS THE MAIN
UPPER LEVEL LOW MOVES INTO TEXAS. THE ATMOSPHERE WILL BE MOISTURE
LOADED...SO SOME LOCALLY HEAVY RAINFALL WILL BE POSSIBLE ON
SATURDAY. GIVEN THIS THREAT...ALONG WITH ANTECEDENT CONDITIONS
ALREADY IN PLACE...HAVE EXTENDED THE FLASH FLOOD WATCH THROUGH
SATURDAY EVENING. ADDITIONAL RAINFALL OF 2 TO 4 INCHES WILL BE
COMPLETELY POSSIBLE THROUGH SATURDAY EVENING. FORTUNATELY...LAPSE
RATES WILL NOT BE CONDUCIVE TO HAIL FORMATION..AND WET BULB ZERO
HEIGHTS WILL RISE BACK TO AROUND 13K FEET. THERE WILL BE SOME
DIRECTIONAL AND SPEED SHEAR OVER THE AREA ON SATURDAY AND SATURDAY
EVENING...BUT OVERALL SHEAR VALUES LOOK WEAKER THAN THEY DID
PREVIOUSLY. THE GREATEST VALUES OCCUR IN THE EVENING WHEN 0-3 KM
SHEAR INCREASES TO AROUND 30 KNOTS AND STORM RELATIVE HELICITY
APPROACHES 150 M2/S2.  THERE COULD STILL BE AN ISOLATED STRONG TO
SEVERE THUNDERSTORM ON SATURDAY AFTERNOON AND EVENING...BUT
OVERALL THE THREAT IS LOWER THAN IT APPEARED PREVIOUSLY.

CONDITIONS WILL BEGIN TO SLOWLY IMPROVE LATE SATURDAY NIGHT AND
EARLY SUNDAY AS THE GREATEST OMEGA AND OVERALL VORTICITY SHIFTS TO
THE EAST. THERE WILL BE SOME LINGERING RAINFALL AROUND SUNDAY
MORNING...BUT EXPECT TO SEE IMPROVING CONDITIONS DURING THE
AFTERNOON AND EVENING HOURS ON SUNDAY. IN FACT...EXPECT TO SEE DRY
CONDITIONS TAKE HOLD OVER WESTERN ZONES BY SUNDAY AFTERNOON. THIS
DRYING TREND WILL BE DRIVEN BY STRONG DRY AIR ADVECTION IN THE
MID AND UPPER LEVELS. THE MAIN UPPER LEVEL TROUGH AXIS WILL SWING
THROUGH THE REGION SUNDAY NIGHT...DRIVING A WEAK FRONTAL BOUNDARY
THROUGH THE AREA. LIGHT NORTHWEST FLOW AND A LOWER DEWPOINT
AIRMASS WILL SLIDE INTO THE AREA ON MONDAY...WITH PARTLY CLOUDY
SKIES AND DRY WEATHER EXPECTED THROUGH THE DAY. TEMPERATURES WILL
BE VERY CLOSE TO SEASONAL NORMS.

LONG TERM...

HEADING INTO THE MIDDLE OF NEXT WEEK...A MORE ZONAL FLOW PATTERN
WILL TAKE HOLD OF THE GULF SOUTH. ANOTHER WEAK IMPULSE WILL RIDE
ALONG THIS FLOW THROUGH THE CENTRAL AND NORTHERN GULF MONDAY NIGHT
INTO TUESDAY. THERE WILL BE LITTLE IN THE WAY OF IMPACTS TO THE
FORECAST AREA OTHER THAN SOME INCREASED CLOUDS AND A FEW OFFSHORE
SHOWERS AND THUNDERSTORMS. CONDITIONS WILL QUICKLY TURN TRANQUIL
TUESDAY AFTERNOON AND REMAIN SO THROUGH WEDNESDAY...AS A FAIRLY
DRY AND STABLE AIRMASS DOMINATES THE GULF SOUTH. SKIES WILL REMAIN
PARTLY CLOUDY AND DEWPOINTS WILL REMAIN FAIRLY LOW ACROSS THE
AREA. LOWS WILL DIP INTO THE UPPER 50S AND LOWER 60S BOTH MONDAY
AND TUESDAY NIGHTS...AND HIGHS WILL RISE TO SEASONAL NORMS IN THE
UPPER 70S AND LOWER 80S. OVERALL...THE MIDDLE OF THE WEEK LOOKS
FAIRLY PLEASANT.

UNFORTUNATELY...ANOTHER WEAK IMPULSE SLIDING ALONG THROUGH THE
ZONAL FLOW ALOFT WILL SLIDE THROUGH THE REGION LATE WEDNESDAY
NIGHT AND THURSDAY. THIS FEATURE WILL FAIRLY WEAK...BUT WILL BE
STRONG ENOUGH TO SPARK OFF SOME ISOLATED TO SCATTERED SHOWERS AND
THUNDERSTORMS ON THURSDAY. HAVE PUT IN LOW END CHANCE POPS TO
REFLECT THIS RISK. THIS FEATURE WILL QUICKLY PULL TO THE EAST
THURSDAY NIGHT...AND AN UPPER LEVEL RIDGE SHOULD BEGIN TO BUILD
OVER THE AREA ON FRIDAY. A DRY AND WARM DAY IS EXPECTED AS THIS
RIDGE BUILDS IN.

AVIATION...

NEARLY ALL TERMINALS CURRENTLY EITHER IFR OR LOW END MVFR BUT
CONVECTION FREE EXCEPT FOR KBTR WITH MODERATE SHRA UNDERWAY. THIS
APPEARS TO BE LEADING EDGE OF NEXT IMPULSE MOVING OUT OF SE TEXAS.
THERE APPEARS TO BE SOME GRAVITY WAVE BEHAVIOR TO THE RADAR
RETURNS IN THAT THE CONVECTION IS CYCLING THROUGH LINEAR WAVES OF
ENHANCEMENT AND DECAY WHILE PROGRESSING EASTWARD. THIS WILL MAKE
IT DIFFICULT TO TIME ONSET AT ANY PARTICULAR TAF LOCATION. UPPER
LEVEL LARGE SCALE DIVERGENCE WILL PRIME THE ENTIRE FORECAST AREA
FOR CONVECTIVE DEVELOPMENTS TO WARRANT PREVAILING SHRA/TSRA FOR
MUCH OF THE NEXT 18-24 HOURS. CIGS/VSBYS WILL BE CONTINGENT UPON
CONVECTION WITH GENERAL MVFR CONDITIONS OUTSIDE RAIN AREAS AND IFR
TO LOW END MVFR WITHIN BANDS OF HEAVIER RAIN THROUGH AT LEAST 12Z
SAT. 24/RR

MARINE...

THROUGH TOMORROW...A GENERAL ONSHORE FLOW OF AROUND 10 KNOTS AND
SEAS OF 1 TO 3 FEET CAN BE EXPECTED. THIS ONSHORE FLOW WILL
INCREASE TO AROUND 15 KNOTS SATURDAY NIGHT AS A LOW PRESSURE
SYSTEM APPROACHES THE REGION. THE GRADIENT FLOW WILL FURTHER
INCREASE AND WILL APPROACH SMALL CRAFT ADVISORY RANGE OF AROUND
20 KNOTS ON SUNDAY AS THE LOW PASSES THROUGH THE LOWER MISSISSIPPI
VALLEY. SEAS WILL ALSO INCREASE FROM 1 TO 3 FEET TO 3 TO 6 FEET
DURING THIS PERIOD. THE WINDS WILL QUICKLY DIMINISH SUNDAY
NIGHT...AND EXPECT TO SEE A LIGHT OFFSHORE FLOW OF 10 TO 15 KNOTS
IN PLACE BY MONDAY IN THE WAKE OF THE PASSING LOW AND WEAK FRONTAL
BOUNDARY. THIS OFFSHORE FLOW WILL LINGER INTO TUESDAY AS A COOLER
AND DRIER RIDGE OF HIGH PRESSURE SETTLES IN FOR THE MIDDLE OF THE
WEEK. SEAS WILL ALSO FALL BACK TO AROUND 2 TO 4 FEET BY MIDWEEK.

DECISION SUPPORT...

DSS CODE...BLUE.
DEPLOYED...NONE.
ACTIVATION...NONE.
ACTIVITIES...MONITORING HEAVY RAINFALL/FLOODING POTENTIAL
             ASSESSING SEVERE WEATHER THREAT THRU WEEKEND.

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 TONIGHT RISK SVR
         AND/OR DIRECT TROPICAL THREATS; EVENTS OF NATIONAL
         SIGNIFICANCE

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  79  66  75  64 /  60  70  90  70
BTR  81  68  77  65 /  60  70  90  70
ASD  79  68  77  68 /  80  70  90  70
MSY  80  70  77  69 /  80  70  90  60
GPT  77  68  75  70 /  80  60  80  70
PQL  78  68  77  69 /  80  60  80  70

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...FLASH FLOOD WATCH THROUGH SATURDAY EVENING FOR LAZ040-046>050-
     056>070-072.

GM...NONE.
MS...FLASH FLOOD WATCH THROUGH SATURDAY EVENING FOR MSZ077-080>082.

GM...NONE.
&&

$$



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