WTNT42 KNHC 140240
TCDAT2
 
TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER   1
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072011
1100 PM EDT SAT AUG 13 2011
 
SATELLITE IMAGERY INDICATES THAT THE LOW PRESSURE AREA
SOUTH-SOUTHEAST OF BERMUDA HAS DEVELOPED PERSISTENT CENTRAL
CONVECTION AROUND A TIGHTLY-WOUND LOW-LEVEL CENTER.  SATELLITE
INTENSITY ESTIMATES FROM SAB AND TAFB ARE 30 KT...AND BASED ON THESE
ADVISORIES ARE INITIATED ON THE SYSTEM AS TROPICAL DEPRESSION
SEVEN.

THE INITIAL MOTION IS 290/9.  THE DEPRESSION IS CURRENTLY ON THE
SOUTHWEST SIDE OF A LARGE LOW-/MID-LEVEL RIDGE OVER THE CENTRAL
NORTH ATLANTIC.  A DEEP-LAYER TROUGH ADVANCING INTO THE THE EASTERN
UNITED STATES AND THE NORTHWESTERN ATLANTIC WILL BREAK DOWN THE
WESTERN END OF THE RIDGE...CAUSING THE DEPRESSION TO TURN NORTHWARD
BY 36 HR AND RECURVE NORTHEASTWARD INTO THE WESTERLIES BY 72 HR. 
THE FORECAST TRACK FOLLOWS THIS SCENARIO...WITH THE FORECAST BEING
A BLEND OF THE ECMWF AND GFDL MODEL FORECASTS.  A POTENTIAL
COMPLICATION TO THE TRACK FORECAST IS SHOWN BY THE UKMET MODEL...
WHICH FORECASTS THE DEPRESSION TO EXPERIENCE ERRATIC MOTION DURING
THE NEXT 48 HR AS IT INTERACTS WITH A DISTURBANCE CURRENTLY ABOUT
400 N MI TO THE SOUTHEAST.   IF SUCH INTERACTION OCCURS...IT WOULD
DELAY THE APPROACH OF THE DEPRESSION TO BERMUDA.

WATER VAPOR IMAGERY SHOWS SMALL UPPER-LEVEL LOWS TO THE SOUTHWEST
AND NORTHEAST OF THE DEPRESSION.  THESE SYSTEMS ARE HELPING TO
PRODUCE LIGHT NORTHWESTERLY VERTICAL WIND SHEAR...AS WELL AS
ADVECTING UPPER-LEVEL DRY AIR INTO THE DEPRESSION.  IN ADDITION...
A LARGE PLUME OF AFRICAN DUST AND ASSOCIATED DRY AIR ARE
APPROACHING THE DEPRESSION FROM THE EAST.  WHILE VERTICAL SHEAR
SHOULD BE LIGHT ENOUGH FOR THE NEXT 48 HR TO ALLOW DEVELOPMENT...
THE ABUNDANT DRY AIR AROUND THE SYSTEM SHOULD AT LEAST SLOW
INTENSIFICATION.  THE INTENSITY FORECAST CALLS FOR GRADUAL
STRENGTHENING THROUGH 48 HR WHILE THE CYCLONE REMAINS OVER WARM
WATER.  AFTER 72 HR...THE CYCLONE IS EXPECTED TO BE ABSORBED INTO A
FRONTAL SYSTEM OVER THE FAR NORTH ATLANTIC.

THE FORECAST TRACK AND WIND RADII REQUIRE A TROPICAL STORM WATCH FOR
BERMUDA AT THIS TIME.  A TROPICAL STORM WARNING WILL LIKELY BE
REQUIRED SOMETIME ON SUNDAY.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/0300Z 27.9N  61.6W   30 KT  35 MPH
 12H  14/1200Z 28.6N  62.6W   35 KT  40 MPH
 24H  15/0000Z 30.1N  63.7W   40 KT  45 MPH
 36H  15/1200Z 32.0N  64.0W   45 KT  50 MPH
 48H  16/0000Z 35.0N  63.0W   50 KT  60 MPH
 72H  17/0000Z 43.0N  55.0W   50 KT  60 MPH
 96H  18/0000Z...ABSORBED BY FRONTAL SYSTEM
 
$$
FORECASTER BEVEN
 

WTNT42 KNHC 140831
TCDAT2
 
TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072011
500 AM EDT SUN AUG 14 2011
 
TROPICAL DEPRESSION SEVEN HAS NOT BECOME ANY BETTER ORGANIZED SINCE
THE LAST ADVISORY.  IN FACT...THE LOW-LEVEL CENTER HAS MOVED FARTHER
AWAY FROM THE MAIN AREA OF DEEP CONVECTION IN THE PAST FEW HOURS. 
DVORAK CLASSIFICATIONS FROM TAFB AND SAB AT 0600 UTC WERE 30
KT...AND AN ASCAT PASS AT 0100 UTC SHOWED MAXIMUM WINDS IN THE 25
TO 30 KT RANGE.  BASED ON THESE DATA...THE INITIAL WIND SPEED IS
HELD AT 30 KT.  SOME STRENGTHENING OF THE DEPRESSION IS FORECAST
DURING THE NEXT 36 TO 48 HOURS AS IT REMAINS OVER RELATIVELY WARM
WATER AND IN A LOW TO MODERATE WIND SHEAR ENVIRONMENT.  HOWEVER...
GIVEN THE CURRENT LACK OF ORGANIZATION OF THE SYSTEM AND DRY AIR
AROUND IT...SIGNIFICANT STRENGTHENING APPEARS UNLIKELY.  THE
OFFICIAL INTENSITY FORECAST FOLLOWS THE INTENSITY MODEL CONSENSUS
AND IS A LITTLE LOWER THAN THE PREVIOUS ONE.  EXTRATROPICAL
TRANSITION IS NOW FORECAST TO OCCUR WITHIN 72 HOURS AS THE CYCLONE
CROSSES A VERY SHARP SEA SURFACE TEMPERATURE GRADIENT AND INTERACTS
WITH A DEEP-LAYER TROUGH.  BY DAY 4...THE POST-TROPICAL CYCLONE IS
EXPECTED TO BE ABSORBED BY A FRONT. 

THE DEPRESSION HAS TURNED A LITTLE TO THE RIGHT...AS EXPECTED...AND
THE INITIAL MOTION ESTIMATE IS 300/09.  A GRADUAL TURN TO THE NORTH
IS FORECAST DURING THE NEXT 24 TO 36 HOURS AS THE CYCLONE MOVES
AROUND THE PERIPHERY OF A SUBTROPICAL HIGH LOCATED OVER THE CENTRAL
ATLANTIC.  BEYOND THAT TIME...THE SYSTEM IS EXPECTED TO ACCELERATE
TO THE NORTH-NORTHEAST AS IT BECOMES EMBEDDED IN STRONGER FLOW
BETWEEN THE AFOREMENTIONED HIGH AND A TROUGH MOVING OFFSHORE OF THE
EASTERN UNITED STATES.  THE NEW OFFICIAL FORECAST IS SIMILAR TO THE
PREVIOUS ONE AND IS NEAR THE MULTI-MODEL CONSENSUS.  ONE SOURCE OF
UNCERTAINTY IN THE TRACK FORECAST IS THAT BOTH THE ECMWF AND UKMET
MODELS SHOW THE DEPRESSION INTERACTING WITH...OR ABSORBING...A
TROUGH OF LOW PRESSURE TO ITS SOUTHEAST.  IF THIS OCCURS...THE
DEPRESSION COULD MOVE SLOWER THAN CURRENTLY FORECAST.  
 
A TROPICAL STORM WARNING MAY BE REQUIRED FOR BERMUDA LATER TODAY.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/0900Z 28.5N  62.4W   30 KT  35 MPH
 12H  14/1800Z 29.4N  63.2W   35 KT  40 MPH
 24H  15/0600Z 31.2N  63.8W   40 KT  45 MPH
 36H  15/1800Z 33.7N  63.2W   45 KT  50 MPH
 48H  16/0600Z 37.3N  61.0W   45 KT  50 MPH
 72H  17/0600Z 45.5N  52.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  18/0600Z...ABSORBED BY FRONT
 
$$
FORECASTER CANGIALOSI/BRENNAN
 

WTNT42 KNHC 141444
TCDAT2
 
TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072011
1100 AM AST SUN AUG 14 2011
 
DURING THE PAST 6 HOURS...CONVECTION HAS BEGUN TO REDEVELOP OVER AND
SOUTHEAST OF THE LOW-LEVEL CENTER OF THE DEPRESSION IN RESPONSE TO
DECREASING VERTICAL WIND SHEAR AND INCREASING MID-LEVEL MOISTURE.
ALTHOUGH DATA T-NUMBERS HAVE DECREASED TO T1.5 AND T1.0 FROM TAFB
AND SAB...RESPECTIVELY...THE INITIAL INTENSITY REMAINS AT 30 KT
BASED ON SATELLITE CURRENT INTENSITY ESTIMATES OF T2.0/30 KT AND
THE IMPROVED CONVECTIVE ORGANIZATION SINCE THE 12Z SATELLITE FIXES.

WATER VAPOR IMAGERY INDICATES THE DEPRESSION IS MOVING INTO A COL
REGION ON THE NORTH SIDE OF AN UPPER-LEVEL LOW THAT IS LOCATED
ABOUT 150 NMI SOUTHWEST OF THE CYCLONE. AS A RESULT...THE PREVIOUS
WESTERLY VERTICAL SHEAR REGIME IS BEING REPLACED BY A MORE
FAVORABLE SOUTHEASTERLY LOW-SHEAR ENVIRONMENT...WHICH SHOULD FAVOR
THE CONTINUED DEVELOPMENT OF DEEP CONVECTION FOR THE NEXT 12-24
HOURS. THE CYCLONE IS EXPECTED TO REMAIN WITHIN A FAVORABLE
UPPER-LEVEL FLOW ENVIRONMENT FOR THE NEXT 48-60 HOURS...WHICH WOULD
NORMALLY FAVOR SIGNIFICANT INTENSIFICATION. HOWEVER...THERMODYNAMIC
CONDITIONS ARE FORECAST TO BE LESS FAVORABLE AS THE CYCLONE MOVES
OVER SUB-26C SSTS BY 48 HOURS. THESE OUT-OF-PHASE CONDITIONS ARE
EXPECTED TO RESULT IN ONLY MODEST STRENGTHENING. BY DAY 4...THE
POST-TROPICAL CYCLONE IS FORECAST TO BE ABSORBED BY A FRONT. THE
OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND
CLOSELY FOLLOWS A BLEND OF THE SHIPS AND LGEM INTENSITY MODELS.
 
THE DEPRESSION HAS SLOWED AND MADE AN UNEXPECTED JOG TO THE WEST...
AND THE INITIAL MOTION ESTIMATE IS NOW 285/09. THE TRACK FORECAST
IS LESS STRAIGHTFORWARD THAN THE INTENSITY FORECAST. THE RECENT
WESTWARD JOG IS PROBABLY DUE IN PART TO THE SYSTEM BECOMING MORE
VERTICALLY SHALLOW AFTER THE DEEP CONVECTION WEAKENED EARLIER THIS
MORNING. NOW THAT CONVECTION HAS REDEVELOPED...THE CYCLONE SHOULD
TURN NORTHWESTWARD WITHIN THE NEXT 12 HOURS AS IT COMES UNDER THE
INFLUENCE OF DEEP-LAYER SOUTHEASTERLY STEERING FLOW. BEYOND 12
HOURS...THE TRACK FORECAST BECOMES MORE COMPLICATED. THE 12Z
UPPER-AIR DATA FROM BERMUDA THIS MORNING INDICATES THE MID-LEVEL
FLOW FROM 700-400 MB HAS BACKED AROUND TO THE SOUTH-SOUTHWEST AND
HEIGHTS HAVE FALLEN 40-60 METERS WITHIN THIS LAYER. THIS SUGGESTS
THAT THE SUBTROPICAL RIDGE TO THE NORTH OF THE DEPRESSION HAS
WEAKENED...WHICH SHOULD ALLOW THE SYSTEM TO TURN NORTHWESTWARD OR
EVEN NORTHWARD. HOWEVER...WATER VAPOR IMAGERY INDICATES THAT A
SHORTWAVE TROUGH APPROACHING BERMUDA RAPIDLY FROM THE WEST IS THE
LIKELY CAUSE OF THE EROSION OF THE RIDGE. WITH THE MAIN UPPER-LEVEL
TROUGH STILL OVER THE U.S. OHIO AND TENNESSEE VALLEYS...THE RIDGE
COULD TEMPORARILY BUILD BACK WESTWARD BETWEEN BERMUDA AND THE
DEPRESSION...RESULTING IN A STAIR-STEP TYPE OF MOTION TO THE
NORTHWEST DURING THE NEXT 24 HOURS OR SO. THE MAIN UPPER-LEVEL
TROUGH IS EXPECTED TO LIFT THE CYCLONE QUICKLY TO THE NORTHEAST BY
36-48 HOURS AND TAKE IT ACROSS MUCH COLDER WATER IN 60-72
HOURS...WHICH WILL INDUCE EXTRATROPICAL TRANSITION. THE OFFICIAL
FORECAST TRACK HAS BEEN NUDGED A LITTLE TO THE WEST OF THE PREVIOUS
FORECAST TRACK THROUGH 24 HOURS BASED ON THE MORE WESTWARD INITIAL
POSITION...AND LIES JUST TO THE LEFT OF THE GLOBAL MODEL
CONSENSUS...BUT WELL TO THE EAST OF THE TIGHTLY PACKED BAM MODELS.
 
AS A RESULT OF THE WESTWARD SHIFT IN THE FORECAST TRACK...WHICH
BRINGS THE CYCLONE CLOSER TO BERMUDA...THE BERMUDA WEATHER SERVICE
HAS ISSUED A TROPICAL STORM WARNING FOR BERMUDA.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/1500Z 28.2N  63.2W   30 KT  35 MPH
 12H  15/0000Z 29.6N  63.7W   35 KT  40 MPH
 24H  15/1200Z 31.8N  63.8W   40 KT  45 MPH
 36H  16/0000Z 34.9N  62.4W   45 KT  50 MPH
 48H  16/1200Z 38.6N  58.9W   50 KT  60 MPH
 72H  17/1200Z 47.0N  49.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  18/1200Z...DISSIPATED
 
$$
FORECASTER STEWART
 

