[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 102042
TCDAT4
TROPICAL DEPRESSION IRENE DISCUSSION NUMBER  25
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT WED AUG 10 2005

IRENE'S CONVECTIVE CLOUD STRUCTURE AND OUTFLOW PATTERN CONTINUE TO
IMPROVE. HOWEVER...IT STILL REMAINS DIFFICULT TO PINPOINT AN EXACT
LOCATION OF THE LOW-LEVEL CENTER....BUT IT IS BELIEVED TO BE
SOUTHWEST OF THE MID-LEVEL CIRCULATION WHERE A FEW NEW CELLS HAVE
BEEN ROTATING CYCLONICALLY THE LAST COUPLE OF HOURS. SATELLITE
INTENSITY ESTIMATES ARE 30 KT FROM TAFB AND SAB...AND 35 KT FROM
AFWA AND THE MOST RECENT ODT VALUES...SUGGESTING THAT IRENE IS VERY
NEAR REGAINING TROPICAL STORM STATUS. HOWEVER...UNTIL SOME CENTRAL
CONVECTION PERSISTS...THE INTENSITY WILL BE HELD AT 30 KT.

THE INITIAL MOTION ESTIMATE IS 285/11. THE NHC MODEL GUIDANCE...
ESPECIALLY THE GLOBAL MODELS...HAVE MADE A SIGNIFICANT WESTWARD
SHIFT SINCE 06Z. IN FACT...THE GFS 10-MEMBER ENSEMBLE MODEL HAS
MADE A 450 NMI WESTWARD SHIFT AT 120 HOURS. THE LONE OUTLIER MODEL
IS THE UKMET ...WHICH INSISTS THAT IRENE WILL PLOW NORTHWESTWARD
THROUGH THE STRONG SUBTROPICAL RIDGE TO THE NORTH. THIS SOLUTION
WAS DISCOUNTED GIVEN THE 5950 METER 500 MB HEIGHTS AT BERMUDA AND A
WEST WIND OF 10 KT...WHICH SUGGESTS THE HIGH CENTER IS SOUTHWEST OF
BERMUDA AND DIRECTLY IN THE PATH OF THE UKMET SOLUTION. EXCLUDING
THE UKMET MODEL...THE REST OF THE NHC MODEL GUIDANCE IS TIGHTLY
CLUSTERED ABOUT THE PREVIOUS FORECAST TRACK THROUGH 72 HOURS WITH
LESS THAN A 60 NMI SPREAD. AFTER THAT...HOWEVER...THE MODELS
DIVERGE SIGNIFICANTLY BASED ON HOW THEY HANDLE THE DEVELOPMENT OF A
WEAKNESS IN THE SUBTROPICAL RIDGE OFF THE SOUTHEASTERN U.S. BY DAY
4. THE DYNAMICAL MODELS...ESPECIALLY THE CANADIAN...ARE MUCH LESS
ROBUST WITH DEVELOPING A SIGNIFICANT WEAKNESS IN THE RIDGE. THIS
SEEMS QUITE REASONABLE GIVEN THE CURRENT STRENGTH AND EAST-WEST
EXTENT OF THE RIDGE AXIS...AND THE FACT THAT EACH MODEL MAINTAINS
HIGHLY ZONAL FLOW ACROSS THE NORTHERN U.S. AND THE NORTHWESTERN
ATLANTIC. SUCH FAST MOVING WESTERLY MID- TO UPPER-LEVEL FLOW TENDS
NOT TO BE CONDUCIVE FOR SHORTWAVES TO DIG SOUTHWARD AND WEAKEN THE
RIDGE LIKE SOME OF THE MODELS ARE SUGGESTING. THE GFDN MODEL IS THE
FASTEST AND WESTERNMOST MODEL AND TAKES IRENE TO NEAR THE SOUTH
CAROLINA COAST BY 120H...WHILE THE NOGAPS MODEL BRINGS IRENE TO
ABOUT 90 NMI EAST OF THE OUTER BANKS OF NORTH CAROLINA. THE
OFFICIAL FORECAST TRACK REMAINS NEAR THE LEFT SIDE OF THE GUIDANCE
ENVELOPE AND IS JUST AND EXTENSION OF THE PREVIOUS FORECAST TRACK.

THE UPPER-LEVEL OUTFLOW PATTERN IS EXPECTED TO CONTINUE TO IMPROVE
THROUGHOUT THE FORECAST PERIOD WITH VERTICAL SHEAR REMAINING AROUND
10 KT OR LESS. HOWEVER...NORTHEASTERLY MID-LEVEL SHEAR IS EXPECTED
TO PERSIST FOR THE NEXT 36 HOURS...AND MID-LEVEL HUMIDITY IS
FORECAST TO DECREASE TO AS LOW AS 26 PERCENT BY DAYS 4 AND 5. SSTS
ARE EXPECTED TO REMAIN NEAR 29C ALONG THE TRACK...SO THE DRY
MID-LEVEL AIR WILL BE THE MAIN INHIBITING FACTOR. HOWEVER...
HURRICANES CAN STILL FORM IN SUCH DRY CONDITIONS...AND THAT IS
INDICATED BY THE SHIPS MODEL AND THE OFFICIAL FORECAST. ONE THING
THAT THE VERY DRY MID-LEVEL AIR SHOULD ENSURE IS THAT OUTER
CONVECTIVE BANDS WILL NOT EXTEND FAR FROM THE CENTER...WHICH SHOULD
HELP TO KEEP IRENE A RELATIVELY SMALL TROPICAL CYCLONE...AND SMALL
TROPICAL CYCLONES TEND NOT TO PLOW THROUGH STRONG RIDGES.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      10/2100Z 22.8N  58.8W    30 KT
 12HR VT     11/0600Z 23.2N  60.4W    35 KT
 24HR VT     11/1800Z 24.1N  62.8W    40 KT
 36HR VT     12/0600Z 25.1N  65.1W    45 KT
 48HR VT     12/1800Z 26.1N  67.3W    50 KT
 72HR VT     13/1800Z 27.4N  69.6W    55 KT
 96HR VT     14/1800Z 29.5N  72.5W    65 KT
120HR VT     15/1800Z 32.0N  75.5W    70 KT

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