TROPICAL STORM ISAAC DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 AM AST WED AUG 22 2012

SATELLITE IMAGERY INDICATES A LARGE CENTRAL DENSE OVERCAST FEATURE
HAS DEVELOPED OVER THE LOW-LEVEL CENTER…WHICH INCLUDES SOME COLD
CLOUD TOPS OF -82 TO -86C VERY NEAR THE CENTER. HOWEVER…THE SHARP
INCREASE IN INNER-CORE CONVECTION HAS NOT YET TRANSLATED INTO ANY
SIGNIFICANT INTENSIFICATION. AN AIR FORCE RESERVE RECONNAISSANCE
AIRCRAFT INVESTIGATING ISAAC THIS MORNING HAS ONLY FOUND THE
CENTRAL PRESSURE TO BE 1003 MB AND 850 MB FLIGHT-LEVEL WINDS OF 47
KT…WHICH EQUATES TO ABOUT A 40-KT INTENSITY ESTIMATE. SFMR WINDS
IN VERY HEAVY RAIN HAVE BEEN AS HIGH AS 61 KT. THE BIAS-ADJUSTED
SFMR WINDS HAVE BEEN RUNNING AS HIGH AS 54 KT…BUT EVEN THOSE
VALUES APPEAR TO BE INFLATED. UNTIL THE AIRCRAFT COMPLETES ITS
MISSION… THE INITIAL INTENSITY WILL REMAIN AT 40 KT BASED ON THE
1003 MB PRESSURE-WIND RELATIONSHIP.

THE INITIAL MOTION ESTIMATE IS 270/15 KT. THE INITIAL POSITION IS A
BLEND OF THE RECON FIXES AND SATELLITE POSITIONS…AND CONTINUITY
WITH PREVIOUS FORECAST TRACK. THERE IS NO SIGNIFICANT CHANGE TO THE
PREVIOUS FORECAST TRACK OR REASONING. THE EAST-WEST ORIENTED
SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG 30N LATITUDE IS
FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN INTACT ACROSS
FLORIDA AND THE SOUTHEASTERN U.S. THROUGH 72 HOURS OR SO. AFTER
THAT…A BROAD MID- TO UPPER-LEVEL TROUGH CURRENTLY LOCATED OVER
THE CENTRAL U.S. FROM THE GREAT LAKES SOUTHWARD TO THE CENTRAL GULF
COAST IS EXPECTED TO DIG SOUTHWARD OVER THE NEXT 24 HOURS BEFORE
MOVING EASTWARD. THIS SHOULD RESULT IN AN EROSION OF THE RIDGE
ACROSS THE EASTERN GULF OF MEXICO AND THE SOUTHEASTERN U.S.
INCLUDING FLORIDA AND THE NORTHWESTERN BAHAMAS. THIS DEVELOPING
WEAKNESS IN THE RIDGE SHOULD ALLOW ISAAC TO TURN NORTHWESTWARD
ACROSS EASTERN AND CENTRAL CUBA BY DAYS 4 AND 5. THE NHC MODEL
GUIDANCE IS IN EXCELLENT AGREEMENT ON THIS EVOLVING SCENARIO WITH
THE ECMWF BEING THE LEFTMOST OF THE MODELS…AND THE GFDL AND HWRF
BEING THE RIGHTMOST OF THE MODELS. GIVEN THE RUN-TO-RUN CONSISTENCY
OF THE GFS AND UKMET MODELS…MORE WEIGHT HAS BEEN PLACED ON THOSE
SOLUTIONS…ESPECIALLY SINCE THEY HAVE A STRONGER AND VERTICALLY
DEEPER CYCLONE DEPICTED IN THE MODELS.

SHEAR ANALYSES FROM THE GFS-BASED SHIPS INTENSITY MODEL AND UW-CIMSS
INDICATE BOTH THE DEEP-LAYER AND MID-LEVEL SHEAR VALUES HAVE
DROPPED BELOW 10 KT…WHICH HAS FINALLY ALLOWED DEEP CONVECTION TO
DEVELOP AND PERSIST OVER THE LOW-LEVEL CENTER. THE SHEAR IS
FORECAST TO GRADUALLY DECREASE TO VALUES BELOW 5 KT BY 24 HOURS…
WHICH SHOULD ALLOW ISAAC TO STRENGTHEN TO A HURRICANE BY 48 HOURS.
AT 72 AND 96 HOURS…THE INTENSITY HAS BEEN DECREASED DUE TO
EXPECTED LAND INTERACTION WITH HAITI AND EASTERN CUBA. AFTER 96
HOURS…HOWEVER…ISAAC SHOULD BE MOVING BACK OVER THE VERY WARM
WATER BETWEEN CUBA AND FLORIDA. WITH SEA-SURFACE TEMPERATURES
EXPECTED TO BE NEAR 30C AND THE GFS AND UKMET MODELS DEPICTING A
EXTENSIVE UPPER-LEVEL OUTFLOW REGIME COMPLETE WITH DUAL OUTFLOW
CHANNELS TO THE NORTHWEST AND SOUTHEAST…STRENGTHENING APPEARS
LIKELY. THE OFFICIAL INTENSITY REMAINS ON THE CONSERVATIVE SIDE DUE
TO UNCERTAINTY IN HOW LONG AND HOW MUCH ISAAC WILL INTERACT WITH
HISPANIOLA AND CUBA…AND IS CLOSE TO A BLEND OF THE SHIPS…
DECAY-SHIPS…AND LGEM STATISTICAL-DYNAMICAL INTENSITY MODELS.