HOME	ACTIVE CYCLONES	FORECASTS	IMAGERY	ABOUT TPC	RECONNAISSANCE

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TROPICAL DEPRESSION THIRTEEN-E DISCUSSION NUMBER   4
NATIONAL WEATHER SERVICE MIAMI FL
2 AM PDT SAT SEP 18 1999
 
PROBLEMS PROBLEMS! 06Z SATELLITE LOCATION FIXES FROM TAFB AND SAB
CAME IN SOUTH AND EAST OF THE PREVIOUS ADVISORY POSITION. A 0140Z
TRMM OVERPASS SUGGESTS A MORE NORTHERLY POSITION...BUT IS VERY
AMBIGUOUS ON THE LONGITUDE. THUS...THE INITIAL POSITION IS VERY
UNCERTAIN. WHILE IT IS A BAD IDEA TO RE-LOCATED A PARTLY EXPOSED
CENTER BASED ON INFRARED IMAGERY...AN EXTRAPOLATED POSITION FROM THE
PREVIOUS ADVISORY DOES NOT SEEM TO FIT THE CLOUD PATTERN THAT WELL.
THUS...SOME ADJUSTMENT IS NECESSARY. IT WOULD NOT BE SURPRISING IF
SOME RE-LOCATION IS NECESSARY WHEN VISIBLE IMAGERY BECOMES
AVAILABLE.

THE INITIAL MOTION IS EVEN MORE UNCERTAIN...WITH THE BEST ESTIMATE
295/3. WATER VAPOR IMAGERY INDICATES A RIDGE NORTH OF THE CYCLONE.
LARGE SCALE MODEL GUIDANCE INDICATES THIS RIDGE MAY WEAKEN AS AN
UPPER LEVEL TROUGH BUILDS SOUTHWEST FROM CALIFORNIA. INTERESTINGLY
ENOUGH...NHC HURRICANE GUIDANCE DOES NOT SHOW A SIGNIFICANT
NORTHWARD TURN IN RESPONSE TO THE DEVELOPING TROUGH. INSTEAD...IT
CALLS FOR A NORTHWEST MOTION THROUGH 72 HOURS. WITH ALL THE
UNCERTAINTIES...THE OFFICIAL FORECAST IS LITTLE CHANGED FROM THE
PREVIOUS PACKAGE...WITH ONLY SOME SOUTHWARD SHIFT FOR THE CURRENT
LOCATION.

ONE POSSIBLE PROBLEM WITH LOCATING THE CENTER IS THAT IT MAY BE
TRYING TO MOVE UNDER THE CONVECTIVE OVERCAST. A STRONG AND
PERSISTENT AREA OF CONVECTION...WITH TOPS TO -80C...IS WEST OR
SOUTHWEST OF THE CENTER. THIS SHEAR PATTERN INDICATES THE SYSTEM IS
NEAR TROPICAL STORM STRENGTH. THE SHEAR APPEARS TO BE DECREASING...
AND IF THIS TREND CONTINUES THE SYSTEM WILL LIKELY REACH TROPICAL
STORM STRENGTH LATER TODAY. THE INTENSITY FORECAST CALL FOR
STRENGTHENING SIMILAR TO THE SHIPS MODEL.
 
BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     18/0900Z 16.2N 110.0W    30 KTS
12HR VT     18/1800Z 16.3N 110.3W    35 KTS
24HR VT     19/0600Z 16.7N 111.2W    40 KTS
36HR VT     19/1800Z 17.2N 112.3W    45 KTS
48HR VT     20/0600Z 18.0N 113.5W    50 KTS
72HR VT     21/0600Z 19.0N 116.0W    50 KTS
 
 
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