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HURRICANE OLGA DISCUSSION NUMBER  13
NATIONAL WEATHER SERVICE MIAMI FL
4 PM EST TUE NOV 27 2001

OLGA REMAINS QUITE IMPRESSIVE IN SATELLITE IMAGERY AND THE 
EYE...PLUS THE EYEWALL CONVECTION...HAVE BECOME MORE DISTINCT
DURING THE PAST 3 HOURS. THE INITIAL INTENSITY WAS BUMPED UP 
SLIGHTLY TO 80 KT BASED ON CONSENSUS SATELLITE INTENSITY ESTIMATES
OF T4.5...77 KT...FROM ALL THREE AGENCIES...AND THE IMPROVED INNER- 
CORE CONVECTIVE PATTERN.  UPPER-LEVEL OUTFLOW IS CONFINED PRIMARILY 
TO THE NORTH...BUT HAS BEEN IMPROVING TO THE SOUTH.
 
THE INITIAL MOTION ESTIMATE IS 010/10.  HOWEVER...OLGA HAS BEEN
MAKING A RATHER LARGE CYCLONIC LOOP DURING THE PAST 18 HOURS.  THE
CYCLONE REMAINS EMBEDDED IN THE LARGER-SCALE MID- TO UPPER-LEVEL
LOW/TROUGH PATTERN.  A SERIES OF SHARP VORTICITY CENTERS HAVE BEEN
TRACKING AROUND THE SOUTHERN AND EASTERN PERIPHERY OF OLGA...WHICH
HAS RESULTED IN BINARY INTERACTION MOTION.  THE LATEST AND POSSIBLY
LAST STRONG VORTICITY MAXIMUM IS LOCATED ABOUT 400 NM EAST OF OLGA
AND HAS BEEN HELPING TO LIFT THE SYSTEM NORTHWARD DURING THE PAST 6
HOURS.  OLGA AND THE VORTICITY CENTER SHOULD SEPARATE DURING THE
NEXT 6 TO 12 HOURS...WHICH SHOULD ALLOW FOR OLGA TO SLOW DOWN AND
GRADUALLY TURN BACK TO THE WEST AND EVENTUALLY TO THE SOUTHWEST.
THE STRONG DEVELOPING WINTER STORM OVER THE SOUTHERN PLAINS IS
EXPECTED TO LIFT OUT TO THE NORTHEAST DURING THE NEXT 72 HOURS WHICH
SHOULD HELP TO BUILD THE MID- AND UPPER-LEVEL RIDGE TO THE NORTH OF
THE OLGA.  ALL OF THE GLOBAL MODELS AGREE ON THE SAME GENERAL
SYNOPTIC PATTERN DEVELOPING THROUGHOUT THE FORECAST PERIOD AND TAKE
THE CYCLONE TO THE SOUTHWEST...ALBEIT AT SIGNIFICANTLY DIFFERENT
FORWARD SPEEDS.  NOGAPS AND THE UKMET ARE THE FASTEST WHILE THE GFDL
AND AVN ARE THE SLOWEST OF THE MODELS.  THE AVN-GFDL SOLUTION USES A
DEEPER LAYER STEERING FLOW...WHEREAS THE WEAKER UKMET-NOGAPS
SOLUTIONS ALLOW FOR A SHALLOW SYSTEM TO BE STEERED RAPIDLY WESTWARD
BY STRONG LOW-LEVEL EASTERLY STEERING FLOW.  THE OFFICIAL FORECAST
IS CLOSE TO THE SLOWER AVN-GFDL CONSENSUS SINCE THOSE TWO MODELS DO
NOT WEAKEN OLGA AS QUICKLY AS THE NOGAPS AND UKMET MODELS.

THE INTENSITY FORECAST IS A LITTLE TRICKY GIVEN THE COLD SSTS AND 
THE COLD MID- AND UPPER-LEVEL ENVIRONMENT SURROUNDING OLGA. THERE IS 
NOT MUCH INTENSITY GUIDANCE AVAILABLE FOR THESE "COLD" TROPICAL 
CYCLONES.  THE SHIPS INTENSITY MODEL WEAKENS OLGA STEADILY THROUGH 
THE PERIOD AND BRINGS THE SYSTEM BELOW HURRICANE STRENGTH IN 24 TO 
36 HOURS AND DOWN TO 50 KT IN 72 HOURS.  HOWEVER...THE MAIN NEGATIVE 
CONTRIBUTING FACTOR IN THE MODEL IS THE COOLER THAN NORMAL SSTS.  
ALTHOUGH OLGA IS CURRENTLY OVER SUB-24C SSTS...THE SURROUNDING 
ENVIRONMENT IS ALSO MUCH COLDER THAN THE TYPICAL TROPICAL 
ATMOSPHERE.  SINCE THERE IS AVAILABLE INSTABILITY AND LOW 
SHEAR...LESS THAN 15 KT...FORECAST THROUGHOUT THE PERIOD...OLGA 
SHOULD REMAIN A SIGNIFICANT TROPICAL CYCLONE FOR AT LEAST THE NEXT 
FEW DAYS. THE OFFICIAL INTENSITY CALLS FOR ONLY SLOW WEAKENING TO 
BEGIN BY 36 HOURS...AFTER WHICH SOME MID-LEVEL DRY AIR FROM THE 
NORTH MAY GET ENTRAINED INTO THE SYSTEM.
 
LARGE SWELLS WILL CONTINUE TO AFFECT MOST OF THE MAJOR LAND AREAS 
AND ISLANDS FROM THE U.S. EAST COAST TO THE CARIBBEAN ISLANDS.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     27/2100Z 32.3N  55.9W    80 KTS
12HR VT     28/0600Z 33.1N  55.9W    80 KTS
24HR VT     28/1800Z 32.0N  56.4W    80 KTS
36HR VT     29/0600Z 30.9N  56.9W    75 KTS
48HR VT     29/1800Z 30.0N  57.4W    70 KTS
72HR VT     30/1800Z 28.5N  59.0W    65 KTS
 
 
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