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Tropical Storm MIRIAM Forecast Discussion Number 5

Issued at 800 PM PDT SAT SEP 22 2012

000
WTPZ43 KNHC 230232
TCDEP3
 
TROPICAL STORM MIRIAM DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132012
800 PM PDT SAT SEP 22 2012
 
MIRIAM HAS BECOME BETTER ORGANIZED OVER THE PAST SEVERAL HOURS.
BANDING FEATURES ARE MORE DISTINCT TO THE NORTHEAST AND SOUTHWEST
OF THE CENTER...AND CLOUD TOPS HAVE COOLED IN THE CENTRAL DENSE
OVERCAST. DVORAK CI-NUMBERS WERE 2.5/35 KT FROM TAFB...3.0/45 KT
FROM SAB...AND 3.3/51 KT FROM UW-CIMSS. SINCE EARLIER SCATTEROMETER
DATA INDICATED THAT THE STORM WAS WEAKER THAN ITS SATELLITE
PRESENTATION SUGGESTED...THE INITIAL WIND SPEED IS ONLY INCREASED
TO 40 KT.
 
THE ATMOSPHERIC AND OCEANIC CONDITIONS ARE EXPECTED TO BE QUITE
CONDUCIVE FOR MIRIAM TO GAIN MORE STRENGTH DURING THE NEXT COUPLE
OF DAYS. THE SHIPS RAPID INTENSIFICATION INDEX SHOWS MORE THAN A 50
PERCENT CHANCE OF A 30-KT INTENSITY INCREASE DURING THE NEXT 24
HOURS. THE NHC INTENSITY FORECAST LIES BETWEEN THE LOWER
STATISTICAL GUIDANCE AND THE HIGHER DYNAMICAL MODELS...LEANING
TOWARDS THE HIGHER END OF THE MODEL GUIDANCE FOR THE NEXT 48 HOURS.
HOWEVER...IT WOULD NOT BE SUPRISING IF MIRIAM STRENGTHENS MORE THAN
FORECAST. IN ABOUT 3 DAYS...MIRIAM IS EXPECTED TO BE MOVING OVER
WATERS OF ABOUT 26C AND INTO MODERATE SOUTHWESTERLY WIND SHEAR.
THESE LESS FAVORABLE CONDITIONS SHOULD END THE STRENGTHENING
TREND...AND CAUSE THE CYCLONE TO WEAKEN.
 
AFTER MOVING ERRATICALLY EARLIER TODAY...IT APPEARS THAT MIRIAM IS
MOVING MORE STEADILY NOW. THE LATEST INITIAL MOTION ESTIMATE IS
300/7. THE STORM SHOULD CONTINUE MOVING WEST-NORTHWESTWARD TO
NORTHWESTWARD FOR THE NEXT COUPLE OF DAYS AS IT REMAINS STEERED BY
A MID-LEVEL RIDGE TO ITS NORTHEAST. AFTER THAT TIME...THE TRACK
BECOMES MUCH MORE UNCERTAIN. THE GFS...GFS ENSEMBLE MEAN...AND SOME
OF THE HFIP GUIDANCE PREDICT MIRIAM TO TURN TO THE NORTH OR
NORTH-NORTHEAST. CONVERSELY...THE ECMWF...HWRF...AND GFDL MODELS
SHOW A TURN TO THE WEST BEYOND A COUPLE OF DAYS. IN ADDITION...THE
LATEST SPREAD IN THE GFS ENSEMBLE MEMBERS IS ABOUT 1000 N MI BY DAY
5. THE MAIN CAUSE OF THE LARGE MODEL DIVERGENCE APPEARS TO BE DUE
TO DIFFERENCES IN THE POSITION OF A SHORTWAVE TROUGH OFF THE
CALIFORNIA COAST AND THE VERTICAL DEPTH OF MIRIAM. OVERALL...THE
MODEL GUIDANCE ENVELOPE HAS SHIFTED WESTWARD...AND THE OFFICIAL
FORECAST HAS BEEN NUDGED IN THAT DIRECTION.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/0300Z 14.9N 108.5W   40 KT  45 MPH
 12H  23/1200Z 15.6N 109.6W   50 KT  60 MPH
 24H  24/0000Z 16.8N 111.3W   60 KT  70 MPH
 36H  24/1200Z 17.7N 112.9W   75 KT  85 MPH
 48H  25/0000Z 18.5N 114.3W   80 KT  90 MPH
 72H  26/0000Z 19.4N 116.0W   75 KT  85 MPH
 96H  27/0000Z 20.3N 117.0W   60 KT  70 MPH
120H  28/0000Z 21.5N 118.0W   45 KT  50 MPH
 
$$
FORECASTER CANGIALOSI
 

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