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Tropical Storm ALVIN Forecast Discussion Number 7

Issued at 800 PM PDT THU MAY 16 2013


000
WTPZ41 KNHC 170241
TCDEP1

TROPICAL STORM ALVIN DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP012013
800 PM PDT THU MAY 16 2013

ALVIN CONTINUES TO PRODUCE DISORGANIZED DEEP CONVECTION IN BROKEN
BANDS TO THE SOUTHEAST AND WEST OF THE CENTER. THE CENTER OF
CIRCULATION REMAINS EMBEDDED IN THE ITCZ AND COULD BE IN THE
PROCESS OF DEGENERATING INTO AN OPEN TROUGH. SATELLITE INTENSITY
ESTIMATES ARE 1.5 AND 2.5 FROM SAB AND TAFB...RESPECTIVELY. DESPITE
THE RAGGED APPEARANCE OF THE CYCLONE IN SATELLITE IMAGERY...THE
INITIAL INTENSITY ESTIMATE IS CONSERVATIVELY REDUCED TO 35 KT UNTIL
THERE IS MORE DEFINITIVE PROOF THAT ADDITIONAL WEAKENING HAS
OCCURRED.

ALVIN IS LOCATED ON THE SOUTHERN EDGE OF A BELT OF STRONG WESTERLY
FLOW ASSOCIATED WITH AN UPPER-LEVEL TROUGH LOCATED SEVERAL HUNDRED
MILES WEST OF THE BAJA CALIFORNIA PENINSULA. AS A RESULT...UW-CIMSS
SHEAR ANALYSES CONTINUE TO DIAGNOSE 20-25 KT OF WEST-SOUTHWESTERLY
SHEAR OVER THE CYCLONE. IT IS UNLIKELY THAT THE SHEAR WILL DECREASE
DURING THE FORECAST PERIOD...ACCORDING TO THE ECMWF/UKMET MODELS
WHICH HAVE MORE ACCURATELY FORECAST THE UPPER-AIR PATTERN NEAR
ALVIN THAN HAS THE GFS. IT IS THEREFORE DIFFICULT TO IMAGINE ALVIN
STAGING A COMEBACK IN WHAT IS FORECAST TO BE AN INCREASINGLY HOSTILE
ENVIRONMENT OVER THE NEXT FEW DAYS. THE NEW NHC INTENSITY FORECAST
IS SIMILAR TO OR SLIGHTLY LOWER THAN THE PREVIOUS ONE AND A BIT
LOWER THAN THE LATEST INTENSITY GUIDANCE. ALVIN IS EXPECTED TO
DEGENERATE INTO A REMNANT LOW OR AN OPEN TROUGH IN THREE DAYS...IF
NOT SOONER.

THE INITIAL MOTION ESTIMATE IS 285/12. ALVIN SHOULD CONTINUE MOVING
WEST-NORTHWESTWARD FOR THE NEXT DAY OR TWO AROUND THE SOUTHWESTERN
PERIPHERY OF A MID-LEVEL RIDGE CENTERED OVER SOUTHERN MEXICO. THE
GLOBAL MODELS THEN SHOW THE MID-LEVEL CENTER OF ALVIN BEING CARRIED
NORTHWESTWARD AND NORTHWARD AHEAD OF A LARGE UPPER-LEVEL TROUGH
NEAR 120W...WITH THE LOW-LEVEL REMNANT VORTICITY CENTER LIKELY
SEPARATING FROM IT AND MOVING ON A MORE WESTERLY COURSE IN THE
TRADE WIND FLOW. THE TRACK GUIDANCE AGREES WELL ON THIS SCENARIO...
AND THE NHC TRACK FORECAST IS NEAR OR JUST LEFT OF THE PREVIOUS
ONE AND THE MULTI-MODEL CONSENSUS TVCE.

FORECAST POSITIONS AND MAX WINDS

INIT  17/0300Z  9.8N 110.8W   35 KT  40 MPH
 12H  17/1200Z 10.2N 111.8W   35 KT  40 MPH
 24H  18/0000Z 11.0N 113.2W   35 KT  40 MPH
 36H  18/1200Z 11.7N 114.5W   30 KT  35 MPH
 48H  19/0000Z 12.1N 115.9W   30 KT  35 MPH
 72H  20/0000Z 12.5N 119.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  21/0000Z 12.3N 121.3W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  22/0000Z 12.0N 123.5W   15 KT  15 MPH...POST-TROP/REMNT LOW

$$
FORECASTER KIMBERLAIN

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