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Tropical Depression FOUR


ZCZC MIATCDAT4 ALL
TTAA00 KNHC DDHHMM
TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042010
500 PM EDT MON AUG 02 2010
 
TROPICAL DEPRESSION FOUR IS GRADUALLY GETTING BETTER ORGANIZED AND
IS PROBABLY VERY NEAR TROPICAL STORM STRENGTH BASED ON THE RECENT
DEVELOPMENT OF SHARPLY CURVED CONVECTIVE BANDS IN THE NORTHERN
SEMICIRCLE. SATELLITE INTENSITY ESTIMATES WERE T2.0/30 KT FROM BOTH
TAFB AND SAB...AND THAT IS THE INITIAL INTENSITY ESTIMATE USED FOR
THIS ADVISORY. UPPER-LEVEL OUTFLOW IS SLIGHTLY RESTRICTED TO THE
EAST...BUT HAS BEEN IMPROVING AND BECOMING MORE SYMMETRICAL IN THE
REMAINING QUADRANTS DURING THE PAST 6 HOURS.
 
THE INITIAL MOTION ESTIMATE IS 290/14. DESPITE THEIR POOR
INITIALIZATION OF THE DEPRESSION AT 12Z...THE NHC MODEL GUIDANCE IS
TIGHTLY CLUSTERED AND IN GOOD AGREEMENT ON A WEST-NORTHWESTWARD
MOTION ACCOMPANIED BY A STEADY INCREASE IN FORWARD SPEED EXCEEDING
20 KT THROUGH 72 HOURS. AFTER THAT...HOWEVER...THE MODELS DIVERGE
SIGNIFICANTLY WITH THE UKMET...ECMWF...AND GFDL MODELS TAKING A
VERY WEAK CYCLONE QUICKLY NORTHWARD THROUGH A LARGE BREAK IN THE
SUBTROPICAL RIDGE...WHEREAS THE HWRF...GFS...AND NOGAPS MODELS
FAVOR A SLOWER AND MORE NORTHWESTWARD MOTION. THE MAIN DIFFERENCES
LIE IN HOW THE MODELS HANDLE A MID-LEVEL TROUGH ALONG THE U.S. EAST
COAST THAT IS FORECAST TO DIG SOUTHWARD OUT OF NORTHERN CANADA OVER
THE NEXT 5 DAYS. THE UKMET...ECMWF...AND GFDL HAVE MORE AMPLIFIED
TROUGH WHICH ERODES THE RIDGE OFF THE SOUTHEAST U.S. COAST...WHILE
THE OTHER MODELS HAVE LESS AMPLITUDE AND MORE ZONAL FLOW ACROSS THE
U.S. MID-ATLANTIC STATES. GIVEN THE CURRENT 5-WAVE PROGRESSIVE
PATTERN...MY FEELING IS THAT SUCH SIGNIFICANT EROSION OF THE
SUBTROPICAL RIDGE WOULD BE SOMEWHAT OVERDONE FOR THIS TIME OF THE
YEAR. AS A RESULT...THE OFFICIAL FORECAST TRACK IS A LITTLE TO THE
LEFT OF AND SLOWER THAN THE PREVIOUS ADVISORY...AND IS SIMILAR TO
THE CONSENSUS MODELS...TVCN AND TVCC...ESPECIALLY AT DAYS 4 AND 5.

THE UPPER-LEVEL ENVIRONMENT IS EXPECTED TO REMAIN FAVORABLE FOR SOME
STRENGTHENING OVER THE NEXT COUPLE OF DAYS...ESPECIALLY IN 36
HOURS WHEN ALL OF THE MODELS ARE FORECASTING A SMALL ANTICYCLONE TO
DEVELOP OVER THE CYCLONE. BY 48-60 HOURS...THE GFS-BASED SHIPS
INTENSITY MODEL IS FORECASTING THE SHEAR TO INCREASE TO MORE THAN
30 KT...WHICH WOULD NORMALLY RESULT IN WEAKENING OF A TROPICAL
CYCLONE. HOWEVER...THESE HIGH SHEAR VALUES MAY BE AN ARTIFACT OF
THE THE GFS HAVING MUCH STRONGER UPPER-LEVEL WINDS AS COMPARED TO
THE OTHER GLOBAL MODELS...AND ALSO DUE TO 500 KM RADIUS THAT THE
SHIPS USES IN COMPUTING SHEAR VALUES. IF THE DEPRESSION REMAINS
UNDERNEATH THE UPPER-LEVEL ANTICYCLONE RATHER THAN OUTRUNNING IT
...THEN THE CYCLONE WILL END UP STRONGER THAN THE OFFICIAL FORECAST
INDICATES.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      02/2100Z 13.0N  42.5W    30 KT
 12HR VT     03/0600Z 13.8N  45.2W    35 KT
 24HR VT     03/1800Z 15.2N  49.9W    40 KT
 36HR VT     04/0600Z 16.8N  54.4W    45 KT
 48HR VT     04/1800Z 18.5N  57.8W    55 KT
 72HR VT     05/1800Z 22.5N  64.0W    55 KT
 96HR VT     06/1800Z 26.0N  67.5W    50 KT
120HR VT     07/1800Z 29.0N  69.5W    50 KT
 
$$
FORECASTER STEWART
 
NNNN


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Page last modified: Friday, 15-Apr-2011 12:09:04 UTC