Flash Flood Warning 1, NWS New Orleans, LA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
647 PM CST SUN FEB 21 2010

.UPDATE...
...SOUNDING DISCUSSION...

NO PROBLEMS WITH THE FLIGHT THIS EVENING. INCREASINGLY UNSTABLE
THIS EVENING WITH AN LI DECREASING TO -0.7. A FAIR AMOUNT OF
MOISTURE WITH A PW OF 1.12 INCHES. HOWEVER...STILL A LARGE CAP OF
DRY AIR ABOVE 750 MILLIBARS...LIMITING THE DEVELOP OF SHOWER AND
THUNDERSTORM ACTIVITY THIS EVENING. THERE IS A STRONG AMOUNT OF
DIRECTIONAL SHEAR IN THE LOWEST LEVELS AND A HELOCITY VALUE OF
219. STILL A DAMP MARINE LAYER AROUND WHICH HAS LIMITED SURFACE
HEATING AND ALLOWED FOR THE SEVERE WEATHER THREAT TO STAY TO A
MINIMUM.

&&

.PREV DISCUSSION... /ISSUED 329 PM CST SUN FEB 21 2010/

SHORT TERM...

A FAST MOVING VORT MAX AND AN ASSOCIATED SURFACE LOW OVER THE
SOUTHERN PLAINS WILL CONTINUE TO PULL TO THE NORTHEAST INTO THE
TENNESSEE VALLEY TONIGHT. SURFACE REPORTS IN TEXAS SHOW THAT A
DRY LINE HAS FORMED IN CENTRAL TEXAS TODAY. IN ADDITION...A COLD
FRONT IS CURRENTLY MOVING THROUGH NORTHWEST TEXAS. THE FRONT SHOULD
CATCH UP TO THE DRY LINE OVERNIGHT AS THE ENTIRE SYSTEM MOVES
THROUGH LOUISIANA. FORTUNATELY...IT APPEARS THAT THE THREAT FOR
SEVERE WEATHER WITH THIS SYSTEM WILL BE FAIRLY LIMITED. MODEL
SOUNDINGS SHOW DECENT LAPSE RATES IN THE MID-LEVELS AND SOME JET
SUPPORT ALOFT...BUT THE THREAT FOR DEEP CONVECTION WILL BE LOW DUE
TO A MORE STABLE LOW LEVEL AIR MASS. AMPLE CLOUD COVER
TODAY...COMBINED WITH A MARINE LAYER ADVECTING IN FROM THE GULF
HAVE LIMITED THE AMOUNT OF SURFACE HEATING ACROSS THE CWA. THIS
HAS REDUCED THE OVERALL INSTABILITY OF THE AIR MASS PRECEDING THE
FRONTAL BOUNDARY. EXPECT SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS TO INITIALLY DEVELOP THIS EVENING OVER THE
REGION...WITH A MORE SOLID LINE OF CONVECTION ALONG THE BOUNDARY
ITSELF. THIS BOUNDARY LOOKS TO PASS THROUGH IN THE LATE EVENING
AND EARLY MORNING HOURS. THE MAIN THREAT FROM ANY STRONG
CONVECTION LOOKS TO BE HAIL.

STRONG SUBSIDENCE AND NEGATIVE VORTICITY ADVECTION IN THE WAKE OF
THE EXITING UPPER LEVEL SYSTEM WILL LEAD TO RAPIDLY CLEARING SKIES
AND LOWER HUMIDITIES FOR TOMORROW. HOWEVER...COLD AIR ADVECTION
WILL BE LIMITED...WITH TEMPERATURES ONLY A FEW DEGREES COOLER THAN
TODAY. THESE CONDITIONS SHOULD PERSIST INTO TOMORROW NIGHT...AS
DEEP NORTHWEST FLOW ASSOCIATED WITH A STRONG LONG WAVE TROUGH
DIGGING INTO THE EASTERN HALF OF THE NATION TAKES HOLD.

LONG TERM...

A SHORT-WAVE TROUGH AXIS ROTATING AROUND THE MAIN PARENT VORTEX
WILL SWEEP DOWN FROM THE NORTHERN ROCKIES INTO THE GULF COAST
STATES TUESDAY AND WEDNESDAY. A STRONG COLD FRONT WILL ACCOMPANY
THIS UPPER LEVEL FEATURE AS COLDER AIR ACROSS THE NORTHERN PLAINS
AND ROCKIES IS DRIVEN SOUTHWARD ACROSS THE LOWER MISSISSIPPI
VALLEY. MID TO UPPER LEVEL MOISTURE ASSOCIATED WITH THIS SYSTEM
WILL ALSO STREAM IN...ALLOWING FOR A GRADUAL SATURATION OF AIR MASS
HEADING INTO TUESDAY NIGHT. STRONG FORCING ASSOCIATED WITH THIS
SYSTEM SHOULD ALLOW FOR SOME PRECIPITATION LATE TUESDAY INTO
TUESDAY NIGHT. THE SYSTEM WILL QUICKLY PULL OFFSHORE WEDNESDAY
MORNING WITH STRONG SUBSIDENCE AND COLD AIR ADVECTION IN THE WAKE
OF THE TROUGH TAKING HOLD. RAPIDLY CLEARING SKIES AND MUCH COLDER
TEMPERATURES CAN BE EXPECTED WEDNESDAY INTO WEDNESDAY NIGHT.
OVERALL...READINGS WILL FALL TO A GOOD 10 TO 15 DEGREES BELOW
NORMAL WEDNESDAY AND THURSDAY.

A TRANSITION IN THE PATTERN WILL TAKE PLACE THURSDAY NIGHT INTO
FRIDAY. THE DEEP LONG WAVE TROUGH WILL BEGIN TO LIFT TO THE NORTH
AND EAST THURSDAY NIGHT...ALLOWING A WEAK UPPER LEVEL RIDGE AXIS
TO SLIDE OVER THE REGION. IN THE LOW LEVELS...HIGH PRESSURE WILL
ALSO DOMINATE...WITH WIND TURNING MORE EASTERLY THROUGH TIME.
HOWEVER...TEMPERATURES WILL REMAIN WELL BELOW NORMAL THURSDAY
NIGHT INTO FRIDAY AS A DEEP COLD POOL CONTINUES TO DOMINATE THE
EASTERN THIRD OF THE NATION. UNFORTUNATELY...THE DRY CONDITIONS
WILL QUICKLY COME TO AN END BY LATE FRIDAY AND FRIDAY NIGHT...AS
ANOTHER SHORT WAVE TROUGH QUICKLY MOVES FROM THE SOUTHERN ROCKIES
INTO THE GULF COAST STATES. AN ENHANCED THERMAL GRADIENT OVER THE
NORTHERN GULF OF MEXICO...COMBINED WITH A REGION OF ENHANCED LIFT
ON THE LEE SIDE OF THE TROUGH AXIS WILL ALLOW FOR A SURFACE LOW TO
FORM OVER THE NORTHERN GULF OF MEXICO ON FRIDAY. THIS LOW WILL
BRING PRECIPITATION TO THE FORECAST AREA FRIDAY NIGHT INTO
SATURDAY MORNING. IT IS FAR TOO EARLY TO SAY WITH ANY CERTAINTY
THAT A WINTRY MIX COULD DEVELOP OVER NORTHERN ZONES...BUT GIVEN
THE COLD POOL IN PLACE...THIS COULD BE A POSSIBILITY AND WILL
CONTINUE TO BE MONITORED. UPPER LEVEL RIDGING IS EXPECTED TO BUILD
BACK OVER THE AREA ON SUNDAY...WITH SLIGHTLY WARMER TEMPERATURES
AND DRY WEATHER IN PLACE.

AVIATION...
CONVECTION WILL BE STEADILY INCREASING THROUGH THE EVENING AND
TONIGHT AS AIR MASS DESTABILIZES IN ADVANCE OF COLD FRONT. THIS WILL
CAUSE VARIATIONS IN CEILINGS AND VSBYS AT TERMINALS FROM VFR TO
MAINLY MVFR...THOUGH HEAVIER STORMS MAY LOWER VSBYS BRIEFLY TO IFR
LEVELS TONIGHT PRIMARILY BEFORE 06Z. IFR CONDITIONS TO PREVAIL
POST-FRONTAL THROUGH 18Z AS COLD AIR ADVECTION MAINTAINS LOW
CEILINGS. 24/RR

MARINE...

SOUTHEAST WINDS OF 10 TO 15 KNOTS AND CHOPPY SEAS WILL CONTINUE
THROUGH THE EVENING HOURS. HOWEVER...A COLD FRONT WILL SWEEP
THROUGH AFTER MIDNIGHT WITH WINDS QUICKLY SHIFTING OUT OF THE
NORTHWEST AND INCREASING TO A SOLID 15 KNOTS. NORTHERLY FLOW WILL
THEN CONTINUE THROUGH THURSDAY AS A STRONG SURFACE HIGH SITS
PARKED OVER THE MIDWEST. HOWEVER...WINDS ARE EXPECTED TO INCREASE
TO SMALL CRAFT ADVISORY CRITERIA TUESDAY NIGHT AND REMAIN THAT WAY
THROUGH EARLY THURSDAY NIGHT...AS A MUCH STRONGER FRONT AND AREA
OF LOW PRESSURE MOVE THROUGH THE GULF OF MEXICO TUESDAY NIGHT INTO
WEDNESDAY MORNING. WINDS OF 20 TO 25 KNOTS AND ROUGH SEAS OF UP TO
8 FEET WILL BE POSSIBLE. A BRIEF BREAK IN THE STRONG WINDS IS
EXPECTED LATE THURSDAY NIGHT INTO FRIDAY MORNING...BUT ANOTHER
GULF LOW AND INCREASING WINDS ARE EXPECTED TO AFFECT THE COASTAL
WATERS FRIDAY NIGHT.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  53  60  35  57 /  70  10   0  30
BTR  54  61  37  58 /  70  10   0  30
ASD  58  66  37  58 /  70  20   0  30
MSY  60  67  43  58 /  70  20   0  30
GPT  58  67  37  60 /  70  40   0  10
PQL  59  67  37  60 /  70  40   0  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...NONE.
GM...NONE.
MS...NONE.
GM...NONE.
&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE NEW ORLEANS LA
552 PM CST SUN FEB 21 2010

GMZ530-550-555-570-575-LAZ034>040-046>050-056>070-MSZ068>071-077-
080>082-230000-
LAKE PONTCHARTRAIN AND LAKE MAUREPAS-
COASTAL WATERS FROM THE SOUTHWEST PASS OF THE MISSISSIPPI RIVER
TO LOWER ATCHAFALAYA RIVER LA OUT 20 NM-
COASTAL WATERS FROM PASCAGOULA MS TO THE SOUTHWEST PASS OF THE
MISSISSIPPI RIVER OUT 20 NM-
COASTAL WATERS FROM THE SOUTHWEST PASS OF THE MISSISSIPPI RIVER
TO LOWER ATCHAFALAYA RIVER LA FROM 20 TO 60 NM-
COASTAL WATERS FROM PASCAGOULA MS TO THE SOUTHWEST PASS OF THE
MISSISSIPPI RIVER FROM 20 TO 60 NM-POINTE COUPEE-WEST FELICIANA-
EAST FELICIANA-ST. HELENA-TANGIPAHOA-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-WILKINSON-AMITE-PIKE-WALTHALL-PEARL RIVER-
HANCOCK-HARRISON-JACKSON-
552 PM CST SUN FEB 21 2010

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

.DAY ONE...TONIGHT
SCATTERED THUNDERSTORMS ARE EXPECTED TO ACCOMPANY A COLD FRONT
THAT IS FORECAST TO MOVE THROUGH THE AREA TONIGHT. MOST OF THE
THUNDERSTORMS WILL REMAIN WEAK. HOWEVER...A FEW STORMS MAY BECOME
STRONG TO SEVERE. THE MAIN THREATS FROM THE STRONGER THUNDERSTORMS
WILL BE NICKEL TO QUARTER SIZED HAIL AND A FEW WIND GUSTS OF UP
TO 50 MPH.

THE MISSISSIPPI RIVER NEAR RED RIVER LANDING...THE PEARL RIVER
NEAR BOGALUSA AND PEARL RIVER...AND THE ATCHAFALAYA RIVER NEAR
MORGAN CITY REMAIN ABOVE FLOOD STAGE. PLEASE REFER TO THE LATEST
RIVER FLOOD STATEMENTS FOR DETAILS.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY
A SURFACE LOW WILL DEVELOP OVER THE SOUTHERN GULF OF MEXICO EARLY
IN THE WEEK. THIS LOW IS FORECAST TO REMAIN SOUTH OF THE LOCAL
AREA...BUT WILL RESULT IN AN INCREASE IN WIND SPEEDS OVER THE GULF
WATERS. EXERCISE CAUTION HEADLINES OR SMALL CRAFT ADVISORIES WILL
LIKELY BE NECESSARY WEDNESDAY AND WEDNESDAY NIGHT.

RIVER FLOODING WILL CONTINUE TO BE A CONCERN ON THE MISSISSIPPI
RIVER NEAR RED RIVER LANDING...ON THE PEARL RIVER NEAR BOGALUSA
AND PEARL RIVER...AND ON THE ATCHAFALAYA RIVER NEAR MORGAN CITY.
PLEASE REFER TO THE LATEST RIVER FLOOD STATEMENTS FOR DETAILS
CONCERNING WHEN THESE RIVERS ARE FORECAST TO DROP BELOW FLOOD
STAGE.

.SPOTTER INFORMATION STATEMENT...
SPOTTER ACTIVATION IS NOT ANTICIPATED. HOWEVER...ANY REPORTS OF
STRONG WINDS OR HAIL SHOULD BE RELAYED TO THE NATIONAL WEATHER
SERVICE OFFICE IN SLIDELL.

95/DM

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