000
WTPZ41 KNHC 170232
TCDEP1
TROPICAL STORM MARTY DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP162009
800 PM PDT WED SEP 16 2009

MARTY HAS NOT BECOME SIGNIFICANTLY BETTER ORGANIZED SINCE EARLIER
TODAY.  VISIBLE AND MICROWAVE IMAGES INDICATE THAT THE LOW-LEVEL
CENTER OF THE STORM REMAINS NEAR THE EASTERN EDGE OF THE MAIN MASS
OF DEEP CONVECTION.  DVORAK INTENSITY ESTIMATES REMAIN AT 35 KT AND
45 KT FROM SAB AND TAFB RESPECTIVELY AND THE ADVISORY WIND SPEED IS
HELD AT 40 KT.  VERTICAL SHEAR OVER MARTY IS EXPECTED TO CHANGE
FROM EASTERLY TO SOUTHWESTERLY...BUT REMAIN FAIRLY WEAK...DURING
THE NEXT 24 HOURS.  THUS THERE REMAINS A SMALL WINDOW OF
OPPORTUNITY FOR THE TROPICAL CYCLONE TO STRENGTHEN.  OCEANOGRAPHIC
ANALYSES SHOW THAT THE WATERS ALONG THE PROJECTED PATH OF MARTY ARE
SOMEWHAT WARMER THAN CLIMATOLOGY AND THIS COULD HELP THE CYCLONE
MAINTAIN ITS INTENSITY FOR A FEW DAYS.  BY AROUND 36 HOURS...
HOWEVER...GLOBAL MODELS INDICATE A SUBSTANTIAL INCREASE IN SHEAR
ASSOCIATED WITH STRONG UPPER-LEVEL WEST-SOUTHWESTERLIES TO THE
NORTH OF 20N LATITUDE.  THEREFORE WE EXPECT A WEAKENING TREND
WITHIN 1-2 DAYS AND MARTY SHOULD BE REDUCED TO A REMNANT LOW BY DAY
4 IF NOT EARLIER.  THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO
THE PREVIOUS ONE AND NOT FAR FROM THE MODEL CONSENSUS.

LATEST CENTER FIXES YIELD A MOTION ESTIMATE OF 340/3.  THERE IS A
WEAKNESS IN THE MID-LEVEL RIDGE THAT EXTENDS FROM JUST NORTH OF
THE CYCLONE TO THE BAJA CALIFORNIA PENINSULA.  THE GFDL AND HWRF
MODELS...ESPECIALLY THE LATTER...MAINTAIN MARTY AS A FAIRLY STRONG
TROPICAL CYCLONE AND THE HWRF TAKES THE SYSTEM OVER BAJA FOLLOWING
THE MID- TO UPPER-LEVEL FLOW.  HOWEVER SINCE THE STORM IS LIKELY TO
WEAKEN DUE TO STRONG SHEAR...ITS STEERING SHOULD BE DOMINATED BY
THE LOWER-LEVEL FLOW.  THIS IS THE SOLUTION GIVEN BY NEARLY ALL OF
THE OTHER AVAILABLE TRACK GUIDANCE THAT TURN MARTY GRADUALLY TO THE
LEFT IN RESPONSE TO A LOW-LEVEL RIDGE TO THE NORTHEAST AND NORTH OF
THE CYCLONE.  THE OFFICIAL FORECAST IS ADJUSTED JUST A TRIFLE TO
THE NORTH OF THAT FROM THE PREVIOUS ADVISORY AND IS CLOSE TO THE
DYNAMICAL CONSENSUS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      17/0300Z 19.2N 112.7W    40 KT
 12HR VT     17/1200Z 19.6N 112.9W    45 KT
 24HR VT     18/0000Z 20.4N 113.2W    50 KT
 36HR VT     18/1200Z 21.2N 113.7W    45 KT
 48HR VT     19/0000Z 22.1N 114.4W    35 KT
 72HR VT     20/0000Z 23.5N 116.0W    30 KT
 96HR VT     21/0000Z 24.0N 117.5W    25 KT...REMNANT LOW
120HR VT     22/0000Z 24.0N 120.0W    20 KT...REMNANT LOW
 
$$
FORECASTER PASCH