000
WTPZ42 KNHC 202028
TCDEP2
HURRICANE IVO DISCUSSION NUMBER  10
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP122007
200 PM PDT THU SEP 20 2007
 
EARLIER MICROWAVE IMAGERY SHOWS A WELL-DEFINED EYE FEATURE EVEN
THOUGH IT IS ONLY INTERMITTENTLY VISIBLE IN CONVENTIONAL IMAGERY. 
THE CONSENSUS OF THE VARIOUS SATELLITE INTENSITY ESTIMATES IS
AROUND 70 KT AND THIS WILL BE THE INITIAL INTENSITY ESTIMATE. 
 
IVO IS ESSENTIALLY MOVING ALONG THE PREVIOUS FORECAST TRACK WITH AN
INITIAL MOTION ESTIMATE OF 345/7.  IN THE SHORT-TERM...IVO IS
EXPECTED TO TURN NORTHWARD AROUND THE WESTERN PERIPHERY OF A
MID-LEVEL RIDGE EXTENDING WESTWARD FROM THE SOUTHERN BAJA
PENINSULA.  THIS RIDGE WILL WEAKEN OVER THE NEXT COUPLE OF DAYS AS
A LARGE DEEP-LAYER CUTOFF LOW FORMS OFF THE CALIFORNIA COAST.  THIS
EVOLUTION IS EXPECTED TO CAUSE VERY WEAK STEERING CURRENTS
RESULTING IN A HIGHLY UNCERTAIN FORECAST BEYOND 48 HOURS.  NOT
SURPRISINGLY...DYNAMICAL MODELS OFFER A LARGE SPECTRUM OF SOLUTIONS
RANGING FROM THE ECMWF...WHICH SHOWS A RAPIDLY WEAKENING CYCLONE
ABRUPTLY STOPPING AND TURNING BACK TO THE SOUTH...TO THE NOGAPS AND
GFDN MODELS WHICH MAINTAIN A DEEP SYSTEM AND ACCELERATING IT
NORTHWARD WITHIN THE SOUTHERLY FLOW EAST OF THE AFOREMENTIONED
CUTOFF LOW.   THE GFDL IS SOMEWHERE IN BETWEEN AS IT SHOWS THE
CYCLONE REMAINING DEEP BUT MEANDERING WITHIN THE WEAK STEERING
ENVIRONMENT.  AT THIS TIME IT IS DIFFICULT TO DISTINGUISH WHICH OF
THESE SOLUTIONS IS MOST LIKELY AND THE OFFICIAL FORECAST IS CLOSE
TO THE DYNAMICAL MODEL CONSENSUS.  THE NEW FORECAST REPRESENTS A
SMALL NORTHWARD SHIFT PRIMARILY AT DAYS 4 AND 5. 
 
THE OVERALL ENVIRONMENT LOOKS MARGINAL FOR STRENGTHENING.  ON ONE
HAND THE OCEAN IS SUFFICIENTLY WARM ALONG THE FORECAST TRACK AND
THE SHEAR IS EXPECTED TO REMAIN LOW FOR ABOUT 48 HOURS. 
HOWEVER...AN UPPER-LEVEL SHEAR AXIS TO THE NORTHWEST OF CYCLONE IS
RESTRICTING OUTFLOW AND PRODUCING A LARGE AREA OF SUBSIDENCE AND
DRY AIR.  IVO COULD ALSO BECOME THERMODYNAMICALLY CHALLENGED GIVEN
THE PRESENCE OF RELATIVELY STABLE AIR TO THE NORTH.  THE OFFICIAL
INTENSITY FORECAST IS LOWERED SLIGHTLY WHICH IS ALSO CONSISTENT
WITH THE TREND IN THE MODELS. 
 
GIVEN THE EXTREME UNCERTAINTY PRESENT IN THE OFFICIAL FORECAST AT
THIS TIME...INTERESTS IN SOUTHERN BAJA CALIFORNIA SHOULD CLOSELY
MONITOR THE PROGRESS OF IVO.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      20/2100Z 17.8N 113.1W    70 KT
 12HR VT     21/0600Z 18.5N 113.3W    70 KT
 24HR VT     21/1800Z 19.5N 113.2W    70 KT
 36HR VT     22/0600Z 20.4N 112.9W    70 KT
 48HR VT     22/1800Z 21.2N 112.5W    65 KT
 72HR VT     23/1800Z 22.9N 111.8W    60 KT
 96HR VT     24/1800Z 24.5N 111.5W    50 KT...INLAND OVER BAJA
120HR VT     25/1800Z 26.5N 111.5W    30 KT...INLAND OVER BAJA
 
$$
FORECASTER RHOME