Tropical Depression ALEX

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TROPICAL DEPRESSION ALEX DISCUSSION NUMBER   8
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL012010
1000 AM CDT SUN JUN 27 2010
 
RECENT SATELLITE IMAGERY INDICATES THAT THE CONVECTIVE TOPS NEAR THE
CENTER OF ALEX HAVE WARMED AND DECREASED IN COVERAGE OVER THE PAST
FEW HOURS.  BASED ON THESE TRENDS...AND A LACK OF ANY OBSERVATIONS
OF STRONGER WINDS...THE INITIAL INTENSITY IS LOWERED TO 30 KT FOR
THIS ADVISORY.
 
THE INTENSITY FORECAST REASONING REMAINS UNCHANGED.  AFTER ALEX
EMERGES INTO THE GULF...WARM WATERS AND LOW SHEAR WILL OFFER THE
OPPORTUNITY FOR RESTRENGTHENING. GIVEN THAT THE LOW-LEVEL
CIRCULATION OF ALEX REMAINS WELL DEFINED...THE FORECAST CALLS FOR
ALEX TO REGAIN TROPICAL STORM STATUS BY TONIGHT...AND FOR STEADY
INTENSIFICATION THROUGH 72 HOURS.  THE OFFICIAL FORECAST IS ABOVE
ALL OF THE INTENSITY GUIDANCE THROUGH 48 HOURS...AND IS IN GOOD
AGREEMENT WITH THE DECAY SHIPS MODEL BEYOND THAT TIME. NEEDLESS TO
SAY...THE AMOUNT OF TIME THAT ALEX SPENDS OVER THE GULF WATERS WILL
BE THE PIVOTAL FACTOR IN HOW MUCH INTENSIFICATION OCCURS.
 
WHILE THE SHORT-TERM MOTION OF ALEX APPEARS ALMOST DUE WEST...A
LONGER-TERM INITIAL MOTION ESTIMATE IS TOWARD THE WEST-NORTHWEST...
295 DEGREES AT 10 KNOTS....A LITTLE TO THE LEFT OF THE PREVIOUS
FORECAST TRACK. DURING THE FORECAST PERIOD...THE GLOBAL MODELS ARE
IN GENERAL AGREEMENT WITH THE EVOLUTION OF THE LARGE-SCALE FLOW
OVER NORTH AMERICA...WITH A LONGWAVE TROUGH DEVELOPING OVER THE
EASTERN UNITED STATES IN TWO TO THREE DAYS...AND A MID- TO
UPPER-LEVEL RIDGE DEVELOPING UPSTREAM OF THE TROUGH OVER CENTRAL
NORTH AMERICA. HOWEVER...SUBTLE DIFFERENCES BETWEEN THE MODELS
RESULT IN LARGE DIFFERENCES IN THE ULTIMATE TRACK OF ALEX. THE
GFS...CANADIAN AND GFS ENSEMBLE MEAN ARE WEAKER WITH THE CENTRAL
U.S. RIDGE IN 4 OR 5 DAYS. AS A RESULT...THESE MODELS HAVE BEEN
PERSISTENTLY TAKING ALEX MORE POLEWARD. THE REMAINING MODELS
SUGGEST THAT A STRONGER RIDGE NORTH OF ALEX WILL RESULT IN A MORE
WESTWARD TURN LATE IN THE FORECAST PERIOD. THE ECMWF ENSEMBLE MEAN
AND THE HWRF ARE ON THE NORTH SIDE OF THIS GROUP OF MODELS...BUT DO
NOT SHOW A SHARP NORTHWARD TURN. ALL THAT BEING SAID...THE NEW
OFFICIAL FORECAST IS ADJUSTED A LITTLE TO THE LEFT OF THE PREVIOUS
ONE THROUGH 36 HOURS DUE TO THE INITIAL MOTION...AND BEYOND THAT
TIME IS A LITTLE TO THE RIGHT OF THE PREVIOUS PACKAGE AND CLOSE TO
THE TVCN MULTI-MODEL CONSENSUS.
 
ALL OF THE MODEL UNCERTAINTY WITH THE TRACK OF ALEX OFFERS A
REMINDER TO NOT TO FOCUS ON THE EXACT FORECAST TRACK...PARTICULARLY
IN THE EXTENDED PART OF THE FORECAST PERIOD...WHERE AVERAGE
FORECAST ERRORS CAN BE 200 TO 300 MILES.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      27/1500Z 18.7N  90.6W    30 KT
 12HR VT     28/0000Z 19.6N  91.6W    35 KT...OVER WATER
 24HR VT     28/1200Z 20.4N  92.6W    45 KT
 36HR VT     29/0000Z 21.1N  93.4W    55 KT
 48HR VT     29/1200Z 21.8N  94.2W    65 KT
 72HR VT     30/1200Z 23.0N  96.0W    75 KT
 96HR VT     01/1200Z 23.5N  98.0W    65 KT...INLAND
120HR VT     02/1200Z 24.0N 100.5W    30 KT...INLAND
 
$$
FORECASTER BRENNAN
 
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