Flash Flood Warning 1, NWS New Orleans, LA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS/BATON ROUGE LA
320 PM CST THU JAN 18 2001

WILL FIRST ADDRESS FORECAST CONCERNS 12-24 HOURS WITH ONE OF THESE
CONCERNS BEING THE SEVERE WEATHER THREAT. WILL GO WITH THE 12Z ETA
MODEL FORECAST WHICH INITIALIZED WELL AND VERIFIED WELL WITH THE 18Z
DEWPOINT DEPICTION ACROSS THE CWA. POSITIVE CLUES FOR THE FORMATION
OF SEVERE THUNDERSTORM DEVELOPMENT LATE TONIGHT SHOW THE RIGHT REAR
QUADRANT OF A JET MAX PASSING ACROSS THE CWA FROM 06 TO 12Z WHICH
CREATES AN AREA OF MARGINAL 200 MB DIFFLUENCE AS A FRONTAL BOUNDARY
MOVES ACROSS THE CWA FROM 8Z TO 15Z FRIDAY. SURFACE DEWPOINTS ARE
GOING TO CLIMB INTO THE MID 60S AHEAD OF THE FRONT WITH A 1000-700
MB TEMPERATURE LAPSE RATE FORECAST FROM 14 TO 16 DEGREES FROM 06 TO
12Z. THE LIMITING FACTORS FOR THE DEVELOPMENT OF SEVERE THUNDERSTORMS
TONIGHT APPEAR TO BE SURFACE INFLOW/CONVERGENCE AS THE ETA BACKS OFF
ON THE DEVELOPMENT OF A SURFACE LOW IN THE GULF AND INSTEAD
GRADUALLY DEEPENS A SURFACE LOW ALONG THE MISSISSIPPI COAST FRI
MORNING. THE OTHER LIMITING FACTOR IS THE LACK OF AN UPPER LEVEL
COLD POOL WHICH IS ASSOCIATED WITH THE UPPER LEVEL TROUGH FROM
700-200 MB LAGGING THE SURFACE FRONT BY 18 TO 24 HOURS. BELIEVE JET
DYNAMICS AND LAPSE RATES WILL BE ENOUGH TO WARRANT THE SWODY1
OUTLOOK OF SLIGHT RISK WITH MAIN THREAT FROM A RAPIDLY MOVING SQUALL
LINE BEING HIGH WINDS AND TORRENTIAL RAINFALL WHICH MAY CREATE
LOCALIZED FLOODING DUE TO PREVIOUS RAINFALL.

SECOND ISSUE FOR FORECAST IS THE HEAVY RAIN THREAT. WITH THE LI
FIELDS OF THE NGM AND ETA HINTING AT A RAPIDLY MOVING SQUALL LINE
THE MAIN THREAT LOOKS TO BE LOCALIZED HEAVY RAINFALL WITHIN A 3 HOUR
PERIOD. THIS CAN LIKELY BE HANDLED WITH URBAN SMALL STREAM FLOOD
ADVISORIES BUT WILL LEAVE FLASH FLOOD WATCH AREA AS IS. WILL ISSUE
REFRESHER STATEMENT AND LEAVE TO EVENING SHIFT FOR ANY AREAL CHANGES
SINCE THEY WILL HAVE A LOOK AT 00Z SOUNDING AND SPECIAL 6Z SOUNDING
FOR UPDATE.

THIRD ISSUE IS WHAT CHANGES IF ANY TO MAKE TO FORECAST PACKAGE FOR
THE FIRST THREE PERIODS. IT APPEARS PACKAGE IS ON TRACK AND LITTLE
CHANGES PLANNED EXCEPT TO ADD SMALL CRAFT ADVISORIES FOR COASTAL
WATERS AND LAKES FROM THE MARINE DESK.

FOR THE FRI NIGHT/SAT MORNING PERIOD THE 1000-500 MB THICKNESSES OF
531-534 DM IN SW MISSISSIPPI TO 534-540 DM ACROSS THE REMAINDER OF
THE CWS SUPPORT MID 20S BUT BOUNDARY LAYER AND SURFACE WINDS OF
20-25 KT SHOW A LOT OF MIXING AND BOUNDARY LAYER AND 925 MB TEMPS
AROUND FREEZING SHOW COLD AIR STRATUS SHOULD LINGER THROUGH
SATURDAY. SO WILL KEEP TEMPS IN THE LOWER TO MID 30 RANGE WITH A
SLIGHT WARMUP TO THE MID 40S DUE TO THE MORNING CLOUDINESS. WILL GO
ABOVE AVN TEMPERATURES FOR SATURDAY NIGHT/SUN MORNING AS MRF/AVN
1000-500 MB THICKNESS VALUES HIGHER THAN PREVIOUS RUNS.

FOR EXTENDED WILL DEFER TO MRF SOLUTION OF ANOTHER SYSTEM COMING
ACROSS THE SOUTHERN STREAM FOR WEDNESDAY AND THURSDAY WITH
INCREASING CHANCES OF RAIN AND COLDER TEMPERATURES.

MCB 51/51/31/45 ++20  BTR 51/51/33/45 ++20
MSY 59/59/36/45 ++20  GPT 61/63/33/46 ++30

.NEW...SCA FROM KPQL TO THE ATCHAFALAYA RIVER INCLUDING LAKES
PONTCHARTRAIN AND MAUREPAS
LA...FFA TONIGHT LAZ034>037-046>050-056.
MS...FFA TONIGHT MSZ068>070.

13

LAZ034>040-046>050-056>070-MSZ068>071-077-080>082-191200-

THUNDERSTORM OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE NEW ORLEANS LA
900 PM CST THU JAN 18 2001

SCATTERED TO NUMEROUS THUNDERSTORMS ARE EXPECTED OVERNIGHT ACROSS
SOUTHEAST LOUISIANA AND SOUTH MISSISSIPPI. THE STRONGER
THUNDERSTORMS WILL BE CONFINED TO EAST OF LAKE PONTCHARTRAIN AND
MISSISSIPPI COUNTIES EAST OF INTERSTATE 55 INCLUDING THE MISSISSIPPI
COASTAL COUNTIES THROUGH FRIDAY MORNING.

A SURFACE LOW OVER SOUTH CENTRAL LOUISIANA WILL TRACK NORTHEAST
ALONG AN ASSOCIATED COLD FRONT. THE COLD FRONT...EXTENDED FROM SOUTH
CENTRAL LOUISIANA TO EAST CENTRAL MISSISSIPPI TO NORTH
ALABAMA...WILL TRACK EAST ACROSS SOUTHEAST LOUISIANA AND SOUTH
MISSISSIPPI TONIGHT. MOIST...MILD AND UNSTABLE AIR AHEAD OF THE
FRONT WILL INTERACT WITH A VIGOROUS UPPER LEVEL STORM SYSTEM
APPROACHING LOUISIANA FROM THE WEST. DUE TO THE TIMING AND POSITION
OF THESE TWO FEATURES...THE STRONGER THUNDERSTORMS ARE EXPECTED TO
OCCUR EAST OF A MCCOMB TO HAMMOND TO HOUMA LINE.

THE MAIN THREAT FROM THESE STORMS WILL BE WIND GUSTS UP TO 40 MILES
AN HOUR. ANOTHER THREAT FROM THIS WEATHER SYSTEM IS HEAVY RAINFALL.
DUE TO THE FRONTAL POSITION AND THE STEERING WINDS IN THE UPPER
LEVELS OF THE ATMOSPHERE...SHOWERS AND THUNDERSTORMS MAY PRODUCE
WIDESPREAD RAINFALL AMOUNTS OF 1 TO 2 INCHES AND ISOLATED RAINFALL
AMOUNTS UP TO 5 INCHES.

A FLASH FLOOD WATCH REMAINS IN EFFECT THROUGH FRIDAY MORNING FOR
AREAS WEST OF A MCCOMB TO HAMMOND TO THIBODAUX LINE.

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