Tropical Depression THIRTEEN-E

ZCZC MIATCDEP3 ALL
TTAA00 KNHC DDHHMM
 
TROPICAL DEPRESSION THIRTEEN-E DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132011
100 AM PST SUN NOV 20 2011
 
CONVENTIONAL AND MICROWAVE SATELLITE IMAGERY INDICATE THE DEPRESSION
HAS CHANGED LITTLE DURING THE PAST 6 HOURS. CONVECTION HAS BEEN
WAXING AND WANING...AND THE LOW-LEVEL CENTER NOTED IN MICROWAVE
IMAGERY IS LOCATED NEAR THE SOUTHWESTERN EDGE OF THE MAIN
CONVECTIVE CLOUD MASS AND NOT IN THE COLD CLOUD SHIELD. ALSO...A
20/0350Z ASCAT PASS REVEALED THAT WINDS IN THE NORTHERN SEMICIRCLE
HAD WEAKENED AND DECREASED IN AREAL COVERAGE COMPARED THE PREVIOUS
OVERPASS JUST 12 HOURS EARLIER. BASED ON THIS INFORMATION AND A
CONSENSUS T2.0/30 KT CLASSIFICATION FROM TAFB AND SAB...THE INITIAL
INTENSITY IS BEING HELD AT 30 KT FOR THIS ADVISORY.

THE INITIAL MOTION ESTIMATE IS 280/12 KT BASED ON MICROWAVE FIXES
OVER THE PAST 12 HOURS. THE DEPRESSION IS MOVING ALONG THE SOUTHERN
PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE LOCATED FROM CENTRAL
MEXICO AND EXTENDING WESTWARD ALONG 20N LATITUDE AND INTO THE
CENTRAL PACIFIC OCEAN. THE EAST-WEST ORIENTATION AND STRENGTH OF
THE RIDGE IS EXPECTED TO CHANGE LITTLE THROUGHOUT THE FORECAST
PERIOD DESPITE A SERIES OF WEAK SHORTWAVE TROUGHS PASSING ACROSS
BAJA CALIFORNIA...WHICH MAY ACT TO BRIEFLY WEAKEN THE RIDGE AND
CREATE SOME MINOR STAIR-STEP MOTION IN THE TRACK. THE LATEST NHC
MODEL GUIDANCE IS IN EXCELLENT AGREEMENT ON THIS GENERAL
SCENARIO...AND THE OFFICIAL FORECAST TRACK IS A LITTLE SOUTH OF AND
FASTER THAN THE PREVIOUS ADVISORY...AND IS NEAR THE SOUTHERN EDGE
OF THE GUIDANCE ENVELOPE.
 
VERTICAL SHEAR IS EXPECTED TO REMAIN LESS THAN 10 KT FOR THE NEXT
12-18 HOURS...WITH A GRADUAL INCREASE AROUND 15 KT FORECAST AFTER
THAT. HOWEVER...THE SHEAR DIRECTION IS EXPECTED TO BE FROM THE
EAST...WHICH IS A FAVORABLE DIRECTION FOR STRENGTHENING TO OCCUR.
BY 48-72 HOURS...THE CYCLONE WILL BE MOVING INTO A REGION OF DRIER
MID-LEVEL AIR AND ALSO OVER COOLER SSTS. THE RESULT IS THAT SLOW
BUT STEADY STRENGTHENING IS FORECAST...ALTHOUGH THERE MAY BE SHARP
FLUCTUATIONS IN THE INTENSIFICATION PROCESS AS THE VERTICAL SHEAR
PULSES UP AND DOWN. AFTER 96 HOURS...THE CYCLONE WILL ENCOUNTER
SUB-26C SEA SURFACE TEMPERATURES...WHICH SHOULD INDUCE GRADUAL
WEAKENING. THE INTENSITY FORECAST IS IDENTICAL TO THE PREVIOUS
ADVISORY FORECAST...AND IS SIMILAR TO A BLEND OF THE LGEM AND GFS
MODELS...AND REMAINS WELL BELOW THE HWRF AND GFDL INTENSITY
FORECASTS...ESPECIALLY AT 96 AND 120 HOURS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  20/0900Z 10.4N 103.7W   30 KT  35 MPH
 12H  20/1800Z 10.9N 105.5W   35 KT  40 MPH
 24H  21/0600Z 11.4N 107.9W   40 KT  45 MPH
 36H  21/1800Z 11.6N 110.1W   50 KT  60 MPH
 48H  22/0600Z 11.8N 112.3W   55 KT  65 MPH
 72H  23/0600Z 11.9N 116.4W   65 KT  75 MPH
 96H  24/0600Z 12.6N 120.7W   70 KT  80 MPH
120H  25/0600Z 13.5N 124.6W   55 KT  65 MPH
 
$$
FORECASTER STEWART
 
NNNN