Severe Thunderstorm Warning 165, NWS New Orleans, LA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
454 AM CST THU DEC 20 2007

.SHORT TERM...
SEVERE WEATHER POTENTIAL AND HEAVY RAINFALL ARE THE MAIN CONCERNS
TODAY. THIS WEATHER HAS BEEN WELL ADVERTISED BY THE NUMERICAL
MODELS AND SPC/HPC GUIDANCE OVER THE LAST COUPLE DAYS. DEEPENING
POSITIVELY TILTED SHORTWAVE TROUGH AND ASSOCIATED SURFACE LOW ARE
MOVING QUICKLY EAST ACROSS THE SOUTHERN PLAINS. LARGE SCALE
ASCENT FROM WARM AIR ADVECTION...PVA...AND MID/UPPER LEVEL
DIVERGENCE IS CAUSING THUNDERSTORMS TO INCREASE AND SPREAD EAST
ACROSS AR AND NORTHERN AND WESTERN LA EARLY THIS MORNING.
CONDITIONS ARE BECOMING MORE FAVORABLE FOR SEVERE THUNDERSTORMS
ALONG THE SABINE RIVER AND ACROSS PORTIONS OF CENTRAL AND
SOUTHWEST LA AND A WATCH BOX MAY BE ISSUED SHORTLY.

FOR OUR FORECAST AREA...SEVERE THUNDERSTORM AND HEAVY RAIN
POTENTIAL WILL INCREASE FROM NORTHWEST TO SOUTHEAST THROUGH THE
EARLY AFTERNOON HOURS TODAY. LOW TO MID LEVEL SHEAR AND
INSTABILITY WILL BE SUFFICIENT FOR SEVERE THUNDERSTORMS PRODUCING
LARGE HAIL...DAMAGING WINDS...AND ISOLATED TORNADOES. HOWEVER...
THE POSITIVELY TILTED NATURE OF THE MID/UPPER SHORTWAVE TROUGH MAY
WORK AGAINST THE OTHER FAVORABLE LOW LEVEL CONVERGENCE/MOISTURE
AND UPPER DIVERGENCE PARAMETERS TO MAKE THIS A LOW END SLIGHT
RISK.

THE HEAVIEST RAINFALL...LIKELY EXCEEDING ONE INCH WITH LOCALIZED
AMOUNTS OF 2 TO 3 INCHES...IS EXPECTED WEST AND NORTH OF LAKE
PONTCHARTRAIN AND ACROSS SOUTH MISSISSIPPI. DUE TO THE UNUSUALLY
DRY CONDITIONS...THE HEAVY RAIN SHOULD NOT CAUSE ANY MAJOR FLOOD
ISSUES. THERE COULD STILL BE SOME LOCALIZED MINOR FLOODING OF
STREETS...SMALL STREAMS...AND POORLY DRAINED LOW LYING AREAS.

THE SHOWERS AND THUNDERSTORMS WILL TAPER OFF FROM WEST TO EAST
DURING THE AFTERNOON IN ADVANCE OF A WEAK COLD FRONT. THE WEAK
FRONT AND SLIGHT DRIER AIR WILL CAUSE COOLER TEMPERATURES TONIGHT
WITH A HEFTY CONTRIBUTION FROM RADIATIONAL COOLING. WITH A WET
GROUND EXPECTED AND CLEARING OF MOST CLOUDS...AREAS OF DENSE FOG
ARE EXPECTED AFTER MIDNIGHT TONIGHT WHICH SHOULD LINGER INTO THE
MOD MORNING HOURS ON FRIDAY. AFTER FOG BURNS OFF...A MOSTLY SUNNY
AND MILD DAY WILL PREVAIL.

.LONG TERM...
THE NEXT UPPER LEVEL DISTURBANCE AND ASSOCIATED STRONGER COLD FRONT
WILL MOVE ACROSS THE LOWER MS VALLEY SATURDAY INTO SATURDAY
NIGHT. THIS SYSTEM IS A BIT SLOWER THAN PREVIOUS RUNS...AND MORE
OF THE RAIN MAY BE DELAYED UNTIL THE LATER AFTERNOON AND EVENING
HOURS. THERMODYNAMICS ARE NOT AS IMPRESSIVE AS PREVIOUS RUNS AND
THE BETTER MOISTURE APPEARS REMOVED FROM THE STRONGER DYNAMICS
THAT LIFT MOSTLY NORTH OF THE FORECAST AREA. THE THREAT OF SEVERE
THUNDERSTORMS ON SATURDAY AND SATURDAY NIGHT IS 5 PERCENT OR LESS.
HAVE STILL MAINTAINED A GOOD CHANCE OF SHOWERS AND THUNDERSTORMS
WITH THIS SYSTEM...BUT SEVERITY AND RAINFALL APPEAR LESS THAN
TODAY/S SYSTEM. THERE WILL BE COLDER AIR BEHIND THIS COLD
FRONT...BUT LOWS SHOULD STAY AT LEAST SEVERAL DEGREES ABOVE
FREEZING IN THE COLDER LOCATIONS MONDAY MORNING.

A CONTINUED RAPID PROCESSION AND RELOADING OF SHORTWAVE TROUGHS
WILL CONTINUE MUCH OF NEXT WEEK WITH THE EARLY TO MID WEEK SYSTEMS
MOSTLY PASSING AND TAKING MORE OF THEIR ENERGY NORTH OF THE
FORECAST AREA. THE EXACT TIMING OF THESE SYSTEMS IS VERY DIFFICULT
TO DETERMINE...SO HAVE GENERALLY BROAD BRUSHED LOW POPS DURING THE
PERIOD.

&&

.AVIATION...
CONVECTION ASSOCIATED WITH THE APPROACHING SHORTWAVE TROUGH IS
EXPECTED TO AFFECT THE KBTR AND KMCB TAF SITES AFTER 11Z AND ALL
SITES BY LATE MORNING AS IT INCREASES IN COVERAGE.  PRECIPITATION
SHOULD END BY SUNSET...AND DENSE FOG FORMATION MAY BE POSSIBLE
OVERNIGHT.

&&

.MARINE...
A SMALL CRAFT ADVISORY HAS BEEN ISSUED VALID FROM 9AM THIS MORNING
THROUGH MIDNIGHT TONIGHT AS SOUTHERLY WINDS INCREASE AHEAD OF THE
APPROACHING WEAK FRONTAL BOUNDARY.  WINDS WILL DIMINISH FRIDAY AS
WEAK HIGH PRESSURE BRIEFLY BUILDS ACROSS THE AREA.  THE NEXT SYSTEM
WILL APPROACH AND MOVE THROUGH THE COASTAL WATERS SATURDAY NIGHT
INTO EARLY SUNDAY.  EXERCISE CAUTION OR SMALL CRAFT ADVISORY FLAGS
MAY BE NECESSARY. HIGH PRESSURE AND OFFSHORE FLOW WILL PREVAIL
SUNDAY IN THE WAKE OF THE COLD FRONT PASSAGE.

.PRELIMINARY POINT TEMPS/POPS...
MCB  71  49  74  55 / 100  20   0  10
BTR  72  51  75  57 / 100  10   0  10
MSY  72  56  74  60 /  90  10   0  10
GPT  69  51  72  56 / 100  30   0  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...SMALL CRAFT ADVISORY FROM 9 AM THIS MORNING TO MIDNIGHT CST
     TONIGHT FOR LAZ066.

GM...SMALL CRAFT ADVISORY FROM 9 AM THIS MORNING TO MIDNIGHT CST
     TONIGHT FOR GMZ530-550-555-570-575.

MS...NONE.
GM...SMALL CRAFT ADVISORY FROM 9 AM THIS MORNING TO MIDNIGHT CST
     TONIGHT FOR GMZ550-555-570-575.

&&

22/92

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MSZ068>071-201545-

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE NEW ORLEANS LA
846 AM CST THU DEC 20 2007

...A LINE OF THUNDERSTORMS WITH STRONG GUSTY WINDS AND HAIL MOVING
INTO EXTREME SOUTHWEST MISSISSIPPI...

AT 842 AM CST...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
LINE OF THUNDERSTORMS WITH STRONG GUSTY WINDS AND HAIL...ALONG A LINE
EXTENDING FROM MCCOMB TO LIBERTY TO JACKSON...MOVING EAST AT 25 MPH.

STRONG THUNDERSTORMS WILL BE NEAR...
SUMMIT AT 915 AM.
MCCOMB AIRPORT AND MCCOMB AT 930 AM.
GILLSBERG AT 945 AM.

ANY THUNDERSTORM ALONG THIS LINE OF STORMS HAS THE POTENTIAL TO
BECOME SEVERE AND PRODUCE LARGE HAIL AND/OR DAMAGING WINDS.

BE PREPARED TO TAKE QUICK ACTION IF A WARNING IS ISSUED. STAY TUNED
TO NOAA WEATHER RADIO OR YOUR FAVORITE LOCAL MEDIA OUTLETS FOR
FURTHER DETAILS OR UPDATES FROM THE NATIONAL WEATHER SERVICE.