[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 271450
TCDAT2
HURRICANE KATRINA DISCUSSION NUMBER  17
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
10 AM CDT SAT AUG 27 2005

REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE
THAT THE CENTRAL PRESSURE OF KATRINA DROPPED TO 940 MB AT 0932Z.
SINCE THEN...THE HURRICANE HAS STARTED A CONCENTRIC EYEWALL
CYCLE...WITH A FILLING OF THE EYE AND WARMING OF THE CLOUD TOPS IN
SATELLITE IMAGERY.  THE INITIAL INTENSITY REMAINS AT 100 KT BASED
MAINLY ON SATELLITE INTENSITY ESTIMATES OF 102 KT FROM TAFB AND
AFWA...AND 115 KT FROM SAB.  IT SHOULD BE NOTED THAT THE MAXIMUM
FLIGHT-LEVEL WINDS REPORTED BY AIRCRAFT SO FAR ARE 106 KT...WHICH
ARE LOWER THAN WOULD NORMALLY BE EXPECTED FOR A 940 MB HURRICANE.

THE INITIAL MOTION IS NOW 275/6.  KATRINA IS SOUTH OF A DEEP-LAYER
RIDGE OVER THE NORTHERN GULF COAST.  THIS RIDGE IS FORECAST TO
WEAKEN AS A STRONG DEEP-LAYER TROUGH DEVELOPS OVER THE CENTRAL
UNITED STATES...AND A NEW DEEP-LAYER RIDGE FORMS OVER THE FLORIDA
PENINSULA AND THE ADJACENT ATLANTIC.  THIS PATTERN CHANGE SHOULD
CAUSE KATRINA TO TURN NORTHWARD DURING THE NEXT 72 HR AND MAKE
LANDFALL OVER THE NORTHERN GULF COAST.  ALL TRACK GUIDANCE AGREES
ON THIS SCENARIO...ALTHOUGH THERE REMAINS SOME SPREAD...
PARTICULARLY FROM THE NOGAPS AND GFDN MODELS WHICH CALL FOR
LANDFALL NEAR MORGAN CITY AND INTRACOASTAL CITY LOUISIANA
RESPECTIVELY.  THE OTHER GUIDANCE IS CLUSTERED WITH LANDFALL
BETWEEN GRAND ISLE LOUISIANA AND PENSACOLA FLORIDA.  THE OFFICIAL
FORECAST REMAINS CLOSE TO THE MODEL CONSENSUS...CALLING FOR
LANDFALL IN SOUTHEASTERN LOUISIANA IN 48-60 HR.  THE NEW TRACK IS
BASICALLY AN UPDATE OF THE PREVIOUS PACKAGE.

KATRINA SHOULD STRENGTHEN SLOWLY FOR THE FIRST 12 HR OR SO AS THE
CONCENTRIC EYEWALL COMPLETES AND SOME RESIDUAL NORTHERLY SHEAR
AFFECTS THE STORM.  AFTER THAT...IT SHOULD STRENGTHEN IN A LIGHT
SHEAR ENVIRONMENT OVER VERY WARM WATER.  THE INTENSITY FORECAST
CALLS FOR THE HURRICANE TO REACH 125 KT IN 48 HR AS A COMPROMISE
BETWEEN THE 120 KT GFDL...THE 126 KT GFDN...THE 127 KT SHIPS...AND
THE 132 KT FSU SUPERENSEMBLE MODELS.  HOWEVER...IT IS NOT OUT OF
THE QUESTION THAT KATRINA COULD REACH CATEGORY 5 STATUS AT SOME
POINT BEFORE LANDFALL.  THERE IS A POSSIBILITY THAT SOUTHERLY OR
SOUTHWESTERLY SHEAR COULD AFFECT KATRINA STARTING AT 48 HR...AND AS
ALWAYS HAPPENS IN HURRICANE OF THIS INTENSITY ADDITIONAL CONCENTRIC
EYEWALL CYCLES COULD OCCUR.

THE NEW FORECAST TRACK AND WIND RADII REQUIRE A HURRICANE WATCH FOR
PORTIONS OF SOUTHEASTERN LOUISIANA AT THIS TIME...INCLUDING
METROPOLITAN NEW ORLEANS.  THIS WATCH WILL LIKELY NEED TO BE
EXTENDED ALONG THE COAST LATER TODAY OR TONIGHT.

FORECASTER BEVEN


FORECAST POSITIONS AND MAX WINDS

INITIAL      27/1500Z 24.5N  85.0W   100 KT
 12HR VT     28/0000Z 24.6N  86.0W   105 KT
 24HR VT     28/1200Z 25.3N  87.6W   115 KT
 36HR VT     29/0000Z 26.7N  89.0W   120 KT
 48HR VT     29/1200Z 28.6N  89.9W   125 KT
 72HR VT     30/1200Z 33.0N  89.5W    60 KT...INLAND
 96HR VT     31/1200Z 37.5N  86.0W    30 KT...INLAND
120HR VT     01/1200Z 41.5N  80.0W    25 KT...EXTRATROPICAL


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