Flash Flood in Mississippi, February 21, 2003

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS/BATON ROUGE LA
310 PM CST FRI FEB 21 2003

LATEST MSAS SHOWS A DISTINCT EQUIVALENT POTENTIAL TEMPERATURE
GRADIENT WITH THE STABLE AIR AT THE SURFACE OVER SOUTH MISSISSIPPI
AND UNSTABLE AIR OVER EXTREME SOUTHEAST....SOUTH CENTRAL AND
SOUTHWEST LOUISIANA. THE BOUNDARY WAS LOCATED FROM MOBILE BAY TO
JUST SOUTH OF THE NEW ORLEANS METRO AREA LAFAYETTE. DEWPOINT
READINGS WERE IN THE 55 TO 60 WITH EAST WINDS NORTH OF THE BOUNDARY
WHILE DEWPOINT READINGS WERE AROUND 70 WITH GUSTY SOUTH WINDS SOUTH
OF BOUNDARY NEAR PATTERSON. IR SATELLITE IMAGERY CONTINUED TO SHOW
THE COLDER CLOUD TOPS OVER SOUTH MISSISSIPPI AND RADAR SHOWS THE
SHOWERS AND THUNDERSTORMS DRIFTING NORTH INTO MISSISSIPPI. IR
SATELLITE IMAGERY ALSO SHOWED THE UPPER LEVEL LOW JUST EAST OF THE
BIG BEND OF TEXAS.

ALL OF THE SHORT RANGE MODELS ARE HAVING PROBLEMS WITH THE SURFACE
MOISTURE FIELDS AND WIND FIELDS. THE PROBLEM IS TURNING THE WINDS TO
THE SOUTHEAST TOO FAST THIS AFTERNOON. THE OUTFLOW BOUNDARY FROM THE
LINE OF STORMS THIS MORNING MAY HAVE HAMPERED THE WARM AIR
ADVECTION...ERGO LOW AFTERNOON TONIGHT. AS THE UPPER LEVEL LOW AND
SURFACE LOW PUSHES EAST NORTHEAST AND PUSH A PRE-FRONTAL TROUGH
ACROSS THE FORECAST AREA TONIGHT. WITH THIS BOUNDARY SO FAR
SOUTH...EAST TO SOUTHEAST WINDS NEAR THIS BOUNDARY MAY IMPROVE
DIRECTIONAL SHEAR WITH THE SOUTHWEST UPPER LEVELS WINDS OF 100
KNOTS. THE BOUNDARY POSITION IS UNCERTAIN AT THIS TIME. DEFINITELY
SOUTH OF BOUNDARY IN THE WARM SECTOR...ROTATING SUPERCELL ARE
POSSIBLE AND PROBABLE. ERGO...TORNADO WATCHES OVER THE FORECAST AREA
ARE LIKELY.

18Z SOUNDING AT LIX INDICATED A STABLE BUT MOIST SOUNDING. PW VALUES
WERE AROUND 1.7 INCHES WHEREAS THE LIFTED INDEX WAS PLUS 9. MOST
AREAS HAVE RECEIVED A GENERAL 2 TO 3 INCHES WITH SOME ISOLATED
AMOUNTS NEAR 5 INCHES. ADDITIONAL RAINFALL OF 2 TO 4 INCHES WILL
OCCUR WITH THE SCATTERED TO NUMEROUS THUNDERSTORMS AHEAD OF THE
SQUALL LINE/PRE-FRONTAL TROUGH TONIGHT AND IN THE SQUALL LINE.
WESTERLY WINDS AFTER THE PRE-FRONTAL PASSAGE SLIGHTLY AFTER
MIDNIGHT.

WITH THE PASSAGE OF THE UPPER LEVEL LOW SO FAR SOUTH...WRAP-AROUND
IS POSSIBLE SATURDAY MORNING AND WILL MAINTAIN POPS. ZONAL FLOW WILL
TAKE OVER. HIGH PRESSURE WILL MOVE WEST TO EAST ACROSS THE NORTH
GULF SATURDAY THROUGH SUNDAY NIGHT. MOISTURE RETURN WILL BE LIMITED
AND SEE NO REAL METEOROLOGICAL REASONING TO REINSERT RAIN CHANCES
MONDAY. A MASSIVE SURFACE HIGH IS EXPECTED TO BUILD OVER THE CENTRAL
CONUS MONDAY THROUGH WEDNESDAY. A DEVELOPING UPPER LEVEL TROUGH OVER
THE ROCKIES WILL ADVECT MOISTURE OVER THE AREA WEDNESDAY. THIS
TROUGH WILL TRACK EAST. THE ASSOCIATED SURFACE LOW WILL TRACK AND
DRAG A COLD FRONT ACROSS THE AREA THURSDAY. WILL INSERT RAIN CHANCES
THURSDAY.

HERE ARE SOME NUMBERS TO PONDER...

MCB 53/58/39/66 +300
BTR 54/60/40/68 +300
MSY 56/63/46/67 +200
GPT 58/61/44/64 +300

.NEW...SMALL CRAFT ADVZY...COASTAL WATERS KPQL TO INTRACOASTAL CITY
       INCLUDING LAKES PONTCHARTRAIN AND MAUREPAS.
LA...FLOOD WATCH LAZ034>040-046>050-056>070
MS...FLOOD WATCH MSZ068>071-077-080>082

18

LAZ034>040-046>050-056>070-MSZ068>071-077-080>082-221300-

HAZARDOUS WEATHER OUTLOOK...RETRANSMISSION...
NATIONAL WEATHER SERVICE NEW ORLEANS LA
100 PM CST FRI FEB 21 2003

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTHEAST LOUISIANA AND
EXTREME SOUTHWEST AND COASTAL MISSISSIPPI.

.DAY ONE...TODAY AND TONIGHT
A STRONG UPPER LEVEL DISTURBANCE ALONG WITH AN ASSOCIATED COLD FRONT
AT THE SURFACE IS EXPECTED TO MOVE THROUGH THE CENTRAL GULF COAST
REGION LATE THIS AFTERNOON AND CONTINUING THROUGH TONIGHT. NUMEROUS
SHOWERS AND THUNDERSTORMS WILL CONTINUE AHEAD OF THE APPROACHING
FRONT. AS THE SURFACE LOW PUSHES EAST...AN ASSOCIATED PRE-FRONTAL
TROUGH WILL SWEEP ACROSS THE AREA TONIGHT. A LINE OF STRONG
THUNDERSTORMS WITH VERY HEAVY RAINFALL IS EXPECTED ALONG THE
PRE-FRONTAL TROUGH TONIGHT.

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON
AND TONIGHT. SOME STORMS MAY REACH SEVERE THRESHOLDS LATER THIS
AFTERNOON AND TONIGHT. A SIGNIFICANT DAMAGING WIND EVENT WITH A FEW
STRONG TORNADOES ARE POSSIBLE. THE GREATEST SEVERE WEATHER THREAT
WILL OCCUR BETWEEN 5 PM LATE THIS AFTERNOON AND 3 AM SATURDAY
MORNING.

A FLOOD WATCH REMAINS IN EFFECT FOR SOUTHEAST LOUISIANA AND SOUTH
MISSISSIPPI THROUGH TONIGHT. SOME AREAS NORTH OF BATON ROUGE AND
EXTREME SOUTHWEST MISSISSIPPI HAVE ALREADY RECEIVED 3 TO 4 INCHES OF
RAIN. IN ADDITION...A SWATH OF 2 TO 3 INCHES OF RAINFALL OCCURRED
FROM SAINT CHARLES TO SAINT BERNARD PARISHES AND OVER PORTIONS OF
SAINT TAMMANY PARISH. A LINE OF THUNDERSTORMS WITH VERY HEAVY
RAINFALL IS EXPECTED ALONG THE PRE-FRONTAL TROUGH TONIGHT.
ADDITIONAL RAINFALL OF 3 TO 4 WILL CAUSE EXCESSIVE RUN-OFF WITH THE
POTENTIAL FOR FLASH FLOODING OVER RAIN SOAKED AREAS. ISOLATED STORM
TOTALS THROUGH SATURDAY MORNING COULD REACH 6 TO 8 INCHES.

MANY RIVERS ACROSS SOUTHEAST LOUISIANA AND SOUTH MISSISSIPPI ARE
ABOVE FLOOD STAGE OR ARE EXPECTED TO RISE ABOVE FLOOD STAGE. PLEASE
REFER TO RIVER FLOOD WARNINGS FOR EACH RIVER BASIN FOR ADDITIONAL
INFORMATION.

SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED TO DEVELOP ACROSS THE
TIDAL LAKES AND COASTAL WATERS IN ADVANCE OF THE FRONT TONIGHT AND
CONTINUE BEHIND THE FRONTAL BOUNDARY SATURDAY. WINDS AND SEAS ARE
EXPECTED TO DIMINISH ON SUNDAY.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY
SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED TO CONTINUE BEHIND THE
FRONTAL BOUNDARY SATURDAY. WINDS AND SEAS ARE EXPECTED TO DIMINISH
ON SUNDAY. OTHERWISE...NO SEVERE WEATHER IS EXPECTED SATURDAY
THROUGH THURSDAY.

.SPOTTER INFORMATION STATEMENT...

THE ACTIVATION OF STORM SPOTTERS AND HAM RADIO OPERATORS...IN
SUPPORT OF SEVERE WEATHER OPERATIONS...IS REQUESTED FOR THIS
AFTERNOON AND TONIGHT. LOCAL COUNTY AND PARISH EMERGENCY MANAGEMENT
SHOULD BE PREPARED AND STAFFED FOR SEVERE WEATHER POTENTIAL LATE
THIS AFTERNOON AND TONIGHT.