[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 060838
TCDAT4
TROPICAL STORM COLIN DISCUSSION NUMBER  10
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042010
500 AM AST FRI AUG 06 2010

VERTICAL WIND SHEAR...ANALYZED AT 25 KT BY THE SHIPS MODEL...HAS
AGAIN TAKEN A TOLL ON COLIN...WITH THE LOW-LEVEL CENTER NOW
COMPLETELY EXPOSED TO THE WEST OF THE PULSING DEEP CONVECTION.  AN
ASCAT PASS AROUND 0118 UTC SHOWED SEVERAL 35-40 KT RETRIEVALS EAST
OF THE CENTER...AND THE INITIAL INTENSITY IS LOWERED TO 40 KT FOR
THIS ADVISORY.  THE SHIPS MODEL SHOWS THE SHEAR DECREASING TO LESS
THAN 10 KT IN 24 HOURS...WHICH SHOULD GIVE COLIN AN OPPORTUNITY TO
RE-INTENSIFY.  HOWEVER...THE SHEAR INCREASES RAPIDLY AGAIN BY 48
HOURS...WHICH SHOULD LIMIT INTENSIFICATION BEYOND THAT TIME. THE
OFFICIAL INTENSITY FORECAST IS ADJUSTED DOWNWARD TOWARD THE ICON
INTENSITY CONSENSUS THROUGH 48 HOURS AND IS SIMILAR TO THE PREVIOUS
PACKAGE AFTER THAT.

THE 12-HOUR INITIAL MOTION ESTIMATE IS 325/12...TO THE LEFT OF THE
PREVIOUS TRACK FORECAST. COLIN IS SITUATED TO THE SOUTHEAST OF AN
UPPER-LEVEL SHEAR AXIS ANCHORED BY ONE LOW CENTER NEAR 27N 75W AND
ANOTHER NEAR BERMUDA.  THE GLOBAL MODELS SUGGEST THAT THESE LOWS
WILL MOVE AWAY AND ALLOW COLIN TO MOVE NORTHWARD AT A SLOWER SPEED
INTO A BREAK IN THE SUBTROPICAL RIDGE OVER THE NEXT COUPLE OF DAYS.
BEYOND THAT TIME...THE GLOBAL MODELS SHOW A LONGWAVE TROUGH
ADVANCING EASTWARD ACROSS THE NORTHEASTERN U.S. AND EASTERN
CANADA.  HOWEVER...THE TIMING OF THE INTERACTION BETWEEN COLIN AND
INDIVIDUAL SHORTWAVES IN THE TROUGH VARIES WIDELY IN THE
GUIDANCE...AND SO DOES THE FORWARD SPEED AND TRACK OF THE CYCLONE.
THE GFS...ECMWF...AND GFDL MOVE COLIN MORE SLOWLY AND DELAY
INTERACTION WITH THE TROUGH UNTIL AROUND DAY 4...WHILE THE UKMET...
NOGAPS...AND HWRF TAKE COLIN MORE QUICKLY NORTHWARD AND SHOW
INTERACTION WITH THE TROUGH ABOUT A DAY EARLIER. GIVEN THAT
THE 72-HOUR FORECAST POSITIONS FROM THE GFS AND NOGAPS ARE ABOUT
1000 MILES APART...THIS TRACK FORECAST HAS LOW CONFIDENCE...
ESPECIALLY IN THE LATER PERIODS.  THE OFFICIAL FORECAST HAS BEEN
ADJUSTED TO THE LEFT OF THE PREVIOUS NHC TRACK THROUGH 48 HOURS DUE
TO THE MORE WESTERLY INITIAL POSITION AND MOTION...AND LIES ALONG
THE LEFT SIDE OF THE GUIDANCE ENVELOPE. BEYOND THAT TIME...THE
BIGGEST CHANGE TO THE OFFICIAL FORECAST WAS TO SLIGHTLY DECREASE THE
FORWARD SPEED DUE TO THE LARGE SPREAD IN THE GUIDANCE. ONE
ALTERNATIVE SCENARIO IS THAT IF COLIN REMAINS A SHALLOW SYSTEM
WITHOUT MUCH DEEP CONVECTION...IT COULD MOVE FARTHER TO THE LEFT
AND MORE SLOWLY IN THE SHORT TERM ALONG A TRACK SIMILAR TO THE
SHALLOW BAM.

FORECAST POSITIONS AND MAX WINDS

INITIAL      06/0900Z 26.8N  67.4W    40 KT
 12HR VT     06/1800Z 27.9N  67.7W    40 KT
 24HR VT     07/0600Z 29.8N  67.4W    45 KT
 36HR VT     07/1800Z 31.5N  66.8W    50 KT
 48HR VT     08/0600Z 33.6N  65.7W    55 KT
 72HR VT     09/0600Z 38.5N  62.5W    60 KT
 96HR VT     10/0600Z 44.0N  56.0W    55 KT
120HR VT     11/0600Z 50.5N  45.0W    50 KT...POST-TROP/EXTRATROP

$$
FORECASTER BRENNAN



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