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| #38760 (Received by flhurricane at: 4:43 PM 10.Aug.2005) 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 |
