[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 262059
TCDAT2
HURRICANE KATRINA DISCUSSION NUMBER  14
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT FRI AUG 26 2005

MOST RECENT REPORTS FROM AN AIR FORCE HURRICANE HUNTER AIRCRAFT
INDICATE THE CENTRAL PRESSURE HAS DROPPED TO 965 MB...BUT THE
FLIGHT-LEVEL WINDS HAVE ONLY INCREASED TO 94 KT AT 700 MB...WHICH
IS ABOUT AN 85-KT SURFACE WIND. A 1701Z DROPSONDE IN THE NORTHEAST
QUADRANT REPORTED 85 KT SURFACE WINDS. THEREFORE...THE INITIAL
INTENSITY IS HELD AT 85 KT FOR THIS ADVISORY...EVEN THOUGH THE
CENTRAL PRESSURE SUPPORTS ABOUT 95-KT SURFACE WINDS. THE EYEWALL IN
THE NORTHWEST QUADRANT HAS REMAINED OPEN...PROBABLY DUE TO DRY AIR
ENTRAINMENT...AND THIS MAY PARTLY EXPLAIN THE DIFFERENCE BETWEEN
THE OBSERVED WINDS AND WHAT THE CENTRAL PRESSURE TYPICALLY
SUPPORTS.

THE INITIAL MOTION IS WEST-SOUTHWESTWARD...OR 255 DEGRESS...AT 07
KT. KATRINA REMAINS CAUGHT BETWEEN THE NORTHEASTERLY FLOW ON THE
WEST SIDE OF AN INVERTED TROUGH OVER THE WESTERN CARIBBEAN SEA AND
THE NORTHEASTERLY FLOW IN THE SOUTHEAST QUADRANT OF THE SUBTROPICAL
RIDGE LOCATED TO THE NORTH AND NORTHWEST OF KATRINA. BOTH THE RIDGE
AND TROUGH ARE FORECAST TO SLOWLY WEAKEN OVER THE NEXT 12-24 HOURS
...WHICH ALLOW THE HURRICANE TO TURN MORE WESTWARD...AND MOST OF
THE NHC MODEL GUIDANCE AGREES ON THAT SCENARIO. AFTER 24 HOURS...
THE MODELS ARE IN GENERAL AGREEMENT ON A SHORTWAVE TROUGH CURRENTLY
OVER THE NORTHERN AND CENTRAL PLAINS STATES TO GRADUALLY DIG
SOUTHEASTWARD TOWARD THE CENTRAL AND WESTERN GULF OF MEXICO AND
ERODE THE RIDGE...WHICH ALLOWS KATRINA TO MOVE NORTHWARD BY 72
HOURS. AS A RESULT...THE MODELS HAVE SHIFTED SIGNIFICANTLY WESTWARD
AND ARE NOW IN BETTER AGREEMENT. THIS HAS RESULTED IN THE OFFICIAL
FORECAST TRACK BEING SHIFTED ABOUT 150 NMI WEST OF THE PREVIOUS
TRACK...ON THE EAST SIDE OF THE GUIDANCE ENVELOPE. HOWEVER...
PROJECTED LANDFALL IS STILL ABOUT 72 HOURS AWAY...SO FURTHER
MODIFICATIONS IN THE FORECAST TRACK ARE POSSIBLE.

KATRINA IS EXPECTED TO BE MOVING OVER THE GULF LOOP CURRENT AFTER 36
HOURS...WHICH WHEN COMBINED WITH DECREASING VERTICAL SHEAR...SHOULD
ALLOW THE HURRICANE TO REACH CATEGORY FOUR STATUS BEFORE LANDFALL
OCCURS. THIS IS CONSISTENT WITH THE SHIPS AND GFDL MODELS...WHICH
BRING KATRINA UP TO 118 KT. THE FSU SUPERENSEMBLE MODEL IS MORE
ROBUST AND BRINGS KATRINA UP TO 129 KT JUST BEFORE LANDFALL.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      26/2100Z 24.8N  82.9W    85 KT
 12HR VT     27/0600Z 24.9N  83.9W    90 KT
 24HR VT     27/1800Z 25.2N  85.1W    95 KT
 36HR VT     28/0600Z 25.8N  86.4W   100 KT
 48HR VT     28/1800Z 26.9N  87.7W   105 KT
 72HR VT     29/1800Z 30.0N  88.5W   115 KT
 96HR VT     30/1800Z 35.0N  86.5W    35 KT...INLAND
120HR VT     31/1800Z 40.5N  79.0W    25 KT...DISSIPATING INLAND


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