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Tropical Storm MIRIAM Forecast Discussion Number 4

Issued at 200 PM PDT SAT SEP 22 2012

000
WTPZ43 KNHC 222034
TCDEP3
 
TROPICAL STORM MIRIAM DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132012
200 PM PDT SAT SEP 22 2012
 
THE SATELLITE PRESETNATION OF MIRIAM HAS IMPROVED OVER THE PAST FEW
HOURS. THERE ARE NOW SEVERAL CURVED CONVECTIVE BANDS FEEDING INTO
AN AREA OF COOLING CLOUD TOPS NEAR THE CENTER. HOWEVER...AN ASCAT
PASS FROM AROUND 1730 UTC SUGGESTED THAT MIRIAM WAS NOT AS STRONG
AS EARLIER SATELLITE ESTIMATES INDICATED...SHOWING ONLY A COUPLE OF
30 KT WIND VECTORS SOUTHEAST OF THE CENTER. BASED ON THE IMPROVING
SATELLITE PRESENTATION...THE INITIAL INTENSITY IS HELD AT 35 KT.
MIRIAM APPEARS POISED FOR STRENGTHENING...WITH SOME SIGNS OF A
DEVELOPING INNER CORE AND A TRACK OVER VERY WARM WATERS IN A LOW
SHEAR ENVIRONMENT FOR THE NEXT 24 TO 36 HOURS. IN FACT...THE SHIPS
RAPID INTENSIFICATION INDEX SUGGESTS THAN A 25-KT INCREASE IN THE
NEXT 24 HOURS IS LIKELY AND A 30-KT INCREASE IS ABOUT A 50/50
PROPOSITION. THE OFFICIAL INTENSITY FORECAST HAS BEEN INCREASED IN
THE SHORT RANGE AND IS A LITTLE HIGHER THAN THE SHIPS MODEL AND
CLOSE TO THE HWRF THROUGH 48 HOURS. AFTER THAT TIME...MIRIAM WILL
BE MOVING INTO COOLER WATERS AND EXPERIENCE AN INCREASE IN
SOUTHWESTERLY SHEAR...WHICH SHOULD LEAD TO STEADY WEAKENING BY THE
END OF THE PERIOD.
 
THE AFOREMENTIONED ASCAT PASS AND VISIBLE IMAGERY SUGGEST THAT THE
LOW-LEVEL CENTER HAS REFORMED TO THE NORTHEAST OF THE POSITION FROM
THE LAST ADVISORY. DUE TO THE RELOCATION...THE INITIAL MOTION
ESTIMATE IS A HIGHLY-UNCERTAIN 300/08. OVER THE NEXT COUPLE OF DAYS
MIRIAM SHOULD MOVE NORTHWESTWARD ON THE SOUTHWEST SIDE OF A
SUBTROPICAL RIDGE THAT EXTENDS FROM NORTHERN MEXICO INTO THE
EASTERN PACIFIC. THROUGH 48 HOURS THE NEW NHC FORECAST IS ABOUT
40-50 N MI TO THE RIGHT OF THE OLD ONE DUE TO THE REPOSITIONING OF
THE CENTER AND AN ADJUSTMENT HALFWAY TOWARD THE NEW TVCE MULTI-
MODEL CONSENSUS. AT DAYS 3 THROUGH 5 AN AMPLIFYING MID/UPPER-LEVEL
TROUGH MOVING INTO THE WESTERN UNITED STATES WILL WEAKEN THE
RIDGE...BUT THE TRACK MODEL GUIDANCE BECOMES HIGHLY DIVERGENT AS TO
HOW MUCH LATITUDE MIRIAM WILL GAIN. AT ONE EXTREME...THE GFDL AND
GFS SHOW A MOTION EAST OF DUE NORTH AT THE END OF THE PERIOD AS
MIRIAM INTERACTS WITH A SHORTWAVE TROUGH NEAR 30N/130W BY DAY 5. ON
THE OTHER EXTREME THE ECMWF AND HWRF SHOW MIRIAM TURNING WESTWARD
AND BYPASSING THE SHORTWAVE. SINCE MIRIAM IS EXPECTED TO BE A DEEP
CYCLONE IT SHOULD GAIN AT LEAST SOME LATITUDE IN RESPONSE TO THE
BREAK IN THE RIDGE. GIVEN THIS REASONING...AND A DESIRE TO MAINTAIN
CONTINUITY DUE TO THE INCREASING MODEL SPREAD...THE NHC TRACK AT
DAYS 3 THROUGH 5 SHOWS A SLOW NORTHWESTWARD MOTION. 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  22/2100Z 14.3N 107.8W   35 KT  40 MPH
 12H  23/0600Z 14.8N 108.5W   45 KT  50 MPH
 24H  23/1800Z 16.1N 110.2W   60 KT  70 MPH
 36H  24/0600Z 17.2N 111.9W   75 KT  85 MPH
 48H  24/1800Z 18.2N 113.3W   80 KT  90 MPH
 72H  25/1800Z 19.3N 115.2W   75 KT  85 MPH
 96H  26/1800Z 20.2N 116.1W   60 KT  70 MPH
120H  27/1800Z 21.5N 117.0W   50 KT  60 MPH
 
$$
FORECASTER BRENNAN
 

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