WTNT44 KNHC 021452
TCDAT4
TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042010
1100 AM EDT MON AUG 02 2010

DATA FROM AN 1148 UTC ASCAT PASS SUGGEST THAT THE DISTURBANCE IN THE
TROPICAL ATLANTIC NOW HAS A WELL-DEFINED SURFACE CIRCULATION...AND
ADVISORIES ARE INITIATED ON TROPICAL DEPRESSION FOUR.  THE INITIAL
INTENSITY IS SET AT 30 KT IN AGREEMENT WITH THE ASCAT DATA AND
DVORAK DATA T-NUMBERS OF 2.0 FROM TAFB AND SAB.

THE LOW-LEVEL CENTER APPEARS TO BE LOCATED ON THE SOUTHEASTERN EDGE
OF THE DEEP CONVECTION...AND IS MOVING RATHER BRISKLY TOWARD THE
WEST-NORTHWEST AT ABOUT 15 KT.  MUCH OF THE GUIDANCE SUGGESTS AN
EVEN FASTER MOTION IS LIKELY DURING THE NEXT COUPLE OF DAYS AS THE
DEPRESSION IS STEERED BY A STRONG SUBTROPICAL RIDGE TO ITS NORTH.
IN FOUR OR FIVE DAYS...THE GLOBAL MODELS SHOW A WEAKNESS DEVELOPING
IN THE RIDGE...WHICH SHOULD RESULT IN A TURN TOWARD THE NORTHWEST
WITH A DECREASE IN FORWARD SPEED.  THERE IS QUITE A BIT OF SPREAD
IN THE MODELS BY THE END OF THE PERIOD...WITH THE GFS ON THE LEFT
SIDE OF THE GUIDANCE SUITE AND THE HWRF AND ECMWF FARTHER RIGHT
SHOWING MORE RECURVATURE.  THE OFFICIAL FORECAST IS CLOSE TO THE
MULTI-MODEL TVCN CONSENSUS AND IN BEST AGREEMENT WITH THE GFDL.  IT
IS WORTH REMINDING USERS THAT THE AVERAGE ERROR OF THE OFFICIAL
TRACK FORECAST AT DAYS 4 AND 5 IS 250 TO 300 MILES.

THE ENVIRONMENT APPEARS FAVORABLE FOR SOME STRENGTHENING OVER THE
NEXT COUPLE OF DAYS...BUT AFTER THAT TIME VERTICAL WIND SHEAR IS
EXPECTED TO INCREASE AS THE CYCLONE APPROACHES AN UPPER-LEVEL
TROUGH AROUND 65W.  THE SHIPS MODEL SHOWS THE SHEAR INCREASING TO
AROUND 30 KT BY 72 HOURS...AND THE OFFICIAL FORECAST SHOWS A PEAK
IN INTENSITY AROUND THAT TIME...IN BEST AGREEMENT WITH THE ICON
INTENSITY CONSENSUS.  IF THE CYCLONE DOES NOT INTENSIFY IN THE NEXT
DAY OR SO...IT COULD STRUGGLE TO MAINTAIN A CLOSED SURFACE
CIRCULATION WITH SUCH A FAST FORWARD SPEED...AND COULD OPEN BACK UP
INTO A TROPICAL WAVE.

FORECAST POSITIONS AND MAX WINDS

INITIAL      02/1500Z 12.6N  41.1W    30 KT
12HR VT     03/0000Z 13.5N  44.0W    35 KT
24HR VT     03/1200Z 14.8N  48.9W    40 KT
36HR VT     04/0000Z 16.4N  53.4W    45 KT
48HR VT     04/1200Z 18.4N  57.4W    50 KT
72HR VT     05/1200Z 22.0N  63.0W    50 KT
96HR VT     06/1200Z 26.0N  66.5W    45 KT
120HR VT     07/1200Z 29.0N  68.5W    45 KT

$$
FORECASTER BRENNAN




WTNT44 KNHC 022037
TCDAT4
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
 

WTNT44 KNHC 030252
TCDAT4
TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042010
1100 PM EDT MON AUG 02 2010
 
THERE HAS BEEN LITTLE CHANGE TO THE ORGANIZATION OF THE DEPRESSION
THIS EVENING.  VERY LITTLE BANDING FEATURES ARE SEEN AND THE
CENTER IS NEAR THE EASTERN EDGE OF A SMALL AREA OF DEEP
CONVECTION.  THE INITIAL INTENSITY WILL REMAIN 30 KT...BASED ON
DVORAK CLASSIFICATIONS OF T2.0 FROM SAB AND TAFB.  NOAA BUOY 41041
HAS RECENTLY REPORTED A PEAK 1-MINUTE SUSTAINED WIND OF 25 KT ABOUT
70 N MI NORTHWEST OF THE CENTER.
 
A SERIES OF MICROWAVE IMAGES THIS EVENING WERE HELPFUL IN LOCATING
THE CENTER OF THE DEPRESSION.  THE CYCLONE HAS ACCELERATED DURING
THE PAST 6-12 HOURS...WITH AN INITIAL MOTION OF 285/20.  THE TRACK
GUIDANCE IS IN RELATIVELY GOOD AGREEMENT THROUGH 48 HOURS.  DURING
THIS TIME THE DEPRESSION IS EXPECTED TO CONTINUE ON A
WEST-NORTHWESTWARD HEADING TO THE SOUTH OF A MID-LEVEL RIDGE OVER
THE CENTRAL ATLANTIC.  BEYOND 48 HOURS...THE CYCLONE WILL BE
APPROACHING A BREAK IN THE RIDGE OVER THE WESTERN ATLANTIC...WHICH
SHOULD ALLOW THE DEPRESSION TO TURN MORE NORTHWESTWARD. THE TRACK
GUIDANCE IN THE LONGER RANGE IS IN POOR AGREEMENT.  THE GFDL AND
HWRF MODELS TURN THE CYCLONE NORTHWARD MUCH EARLIER...WHILE THE GFS
MAINTAINS A MORE WEST-NORTHWESTWARD MOTION AND LIES ALONG THE
WESTERN EDGE OF THE GUIDANCE ENVELOPE.  ALTHOUGH THE TVCN CONSENSUS
SHIFTED CONSIDERABLY EASTWARD THIS CYCLE...WE PREFER TO KEEP THE
TRACK ON THE LEFT SIDE OF THE GUIDANCE SINCE THE SYSTEM IS EXPECTED
TO BE WEAK AT THAT TIME.
 
THE DEPRESSION IS FORECAST TO BE IN A FAVORABLE UPPER-LEVEL
ENVIRONMENT DURING THE NEXT DAY OR SO AND THE OFFICIAL FORECAST
WILL CONTINUE TO CALL FOR SLOW STRENGTHENING.  IN 36-48 HOURS...
SOUTHWESTERLY SHEAR IS FORECAST TO INCREASE AS THE CYCLONE
APPROACHES AN UPPER-LEVEL TROUGH OVER THE WESTERN ATLANTIC.  THIS
IS EXPECTED TO HALT ANY ADDITIONAL STRENGTHENING.  THE NEW FORECAST
IS A LITTLE BELOW THE PREVIOUS ADVISORY AND IS IN GOOD AGREEMENT
WITH THE LGEM AND GFDL MODELS.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      03/0300Z 13.6N  45.1W    30 KT
 12HR VT     03/1200Z 14.6N  48.1W    35 KT
 24HR VT     04/0000Z 16.0N  52.2W    40 KT
 36HR VT     04/1200Z 17.7N  56.4W    45 KT
 48HR VT     05/0000Z 19.6N  59.9W    50 KT
 72HR VT     06/0000Z 23.5N  65.5W    45 KT
 96HR VT     07/0000Z 27.0N  68.5W    45 KT
120HR VT     08/0000Z 31.0N  69.5W    45 KT
 
$$
FORECASTER BROWN
 

