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WTNT44 KNHC 202317
TCDAT4
TROPICAL STORM IRENE SPECIAL DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011
700 PM AST SAT AUG 20 2011
AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INVESTIGATING THE
TROPICAL WAVE EAST OF THE LESSER ANTILLES FOUND A SMALL LOW-LEVEL
CIRCULATION CENTER JUST SOUTHWEST OF A LARGE CONVECTIVE BURST AND A
MINIMUM PRESSURE OF ABOUT 1006 MB. THE PLANE ALSO MEASURED A
MAXIMUM WIND OF 53 KT AT 1400 FT AND BELIEVABLE WINDS OF ABOUT
45 KT FROM THE SFMR. THUS ADVISORIES ARE BEING INITIATED ON
TROPICAL STORM IRENE WITH AN INITIAL INTENSITY OF 45 KT.
A ROUGH ESTIMATE OF INITIAL MOTION IS ABOUT 280/19. A LARGE
MID-LEVEL RIDGE OVER THE CENTRAL AND WESTERN TROPICAL ATLANTIC IS
EXPECTED TO STEER IRENE TOWARD THE WEST-NORTHWEST OVER THE NEXT TWO
OR THREE DAYS. THIS RIDGE SHOULD WEAKEN DURING THAT TIME AND CAUSE
IRENE TO GRADUALLY DECELERATE. THE GUIDANCE IS IN GOOD AGREEMENT
ON A TRACK GENERALLY IN THE DIRECTION OF HISPANIOLA TO THE SOUTH OF
PUERTO RICO. THEREAFTER...A MID-LATITUDE TROUGH MOVING OFFSHORE OF
THE EASTERN UNITED STATES IS EXPECTED TO CAUSE A WEAKNESS IN THE
RIDGE. THIS COULD ALLOW IRENE TO GAIN SOME LATITUDE...ALTHOUGH THE
MODELS BEGIN TO SPREAD OUT LATE IN THE PERIOD. THE GFS AND ECMWF
SHOW ENOUGH OF A WEAKNESS TO BRING THE STORM NORTH OF CUBA...WHILE
THE UKMET AND THE CANADIAN HAVE ENOUGH RIDGING IN PLACE TO KEEP THE
STORM IN THE CARIBBEAN SEA. THE NHC FORECAST WILL SPLIT THOSE TWO
IDEAS AND LIES JUST TO THE WEST OF THE MODEL CONSENSUS.
THE INITIAL VORTEX IS STILL IN ITS FORMATIVE STAGES AND IT WILL
PROBABLY TAKE SOME TIME TO STRENGTHEN. HOWEVER...IRENE IS MOVING
OVER RATHER WARM WATERS WITH LIGHT SHEAR EXPECTED. MOST OF THE
RELIABLE GUIDANCE BRINGS THE STORM TO A HURRICANE BEFORE REACHING
HISPANIOLA...AND THE OFFICIAL FORECAST IS IN AGREEMENT WITH THIS
SCENARIO. THE INTENSITY FORECAST IN AT DAY 3 AND BEYOND IS RATHER
CHALLENGING DUE TO THE POTENTIAL FOR LAND INTERACTION. ALTHOUGH
THE NHC FORECAST WILL SHOW ONLY A SMALL AMOUNT OF
RE-INTENSIFICATION...ENVIRONMENTAL CONDITIONS COULD BE QUITE
FAVORABLE FOR STRENGTHENING IF IRENE AVOIDS SIGNIFICANT LAND
INTERACTION.
BASED ON THE OFFICIAL FORECAST...TROPICAL STORM WARNINGS HAVE BEEN
ISSUED FOR MANY OF THE LEEWARD ISLANDS...THE VIRGIN ISLANDS AND
PUERTO RICO.
NOTE...THIS SPECIAL ADVISORY TAKES THE PLACE OF THE STANDARD 800 PM
AST/0000 UTC INTERMEDIATE PUBLIC ADVISORY. THE NEXT ADVISORY WILL
BE THE FULL ADVISORY PACKAGE ISSUED AT 1100 PM AST/0300 UTC.