Tropical Storm Isaac Less Than 24 Hours from the Leeward Islands

Storm Isaac Advisory Number 4
Nws National Hurricane Center Miami Fl
1100 Pm Ast Tue Aug 21 2012

Summary Of 1100 Pm Ast...0300 Utc...Information
-----------------------------------------------
Location...15.6n 55.6w
About 390 Mi...625 Km E Of Guadeloupe
Maximum Sustained Winds...40 Mph...65 Km/H
Present Movement...W Or 275 Degrees At 18 Mph...30 Km/H
Minimum Central Pressure...1006 Mb...29.71 Inches

Watches And Warnings
--------------------
Changes With This Advisory...

The Government Of Antigua And Barbuda Has Changed The Tropical Storm Watch For The British Virgin Islands To A Tropical Storm Warning.

The Tropical Storm Watch For Puerto Rico...Vieques...Culebra...And
The U.S. Virgin Islands Has Been Changed To A Tropical Storm
Warning.

A Hurricane Watch Has Been Issued For Puerto Rico...Vieques...
Culebra...And The U.S. Virgin Islands.

Summary Of Watches And Warnings In Effect...

A Tropical Storm Warning Is In Effect For...
* Martinique
* Dominica
* Guadeloupe And The Surrounding Islands...And St. Martin
* St. Kitts...Nevis...Antigua...Barbuda...Montserrat...And Anguilla
* Saba...St. Eustatius...And St. Maarten
* British Virgin Islands
* Puerto Rico...Vieques...Culebra...And The U.S. Virgin Islands

A Hurricane Watch Is In Effect For...
* Puerto Rico...Vieques...Culebra...And The U.S. Virgin Islands

A Hurricane Watch Means That Hurricane Conditions Are Possible
Within The Watch Area.

A Watch Is Typically Issued 48 Hours Before The Anticipated First Occurrence Of Tropical-Storm-Force Winds...Conditions That Make Outside Preparations Difficult Or Dangerous.

A Tropical Storm Warning Means That Tropical Storm Conditions Are
Expected Somewhere Within The Warning Area Within 36 Hours.

Interests In Hispaniola And Cuba Should Monitor The Progress Of
Isaac.

Discussion And 48-Hour Outlook
------------------------------
At 1100 Pm Ast...0300 Utc...The Center Of Tropical Storm Isaac Was
Located Near Latitude 15.6 North...Longitude 55.6 West.

Isaac Is Moving Toward The West Near 18 Mph...30 Km/H...And This General Motion Is Expected To Continue For The Next Couple Of Days.

On The Forecast Track...The Center Of Isaac Should Move Through The Leeward Islands Wednesday Evening...And Move Over The Northeastern Caribbean Sea On Thursday.

Maximum Sustained Winds Are Near 40 Mph...65 Km/H...With Higher
Gusts.

Strengthening Is Forecast During The Next 48 Hours...And Isaac Could Become A Hurricane By Thursday.

Tropical Storm Force Winds Extend Outward Up To 45 Miles...75 Km
From The Center.

Estimated Minimum Central Pressure Is 1006 Mb...29.71 Inches.

Hazards Affecting Land
----------------------
Wind...Tropical Storm Conditions Are Expected To Reach The Leeward Islands By Wednesday Afternoon Or Evening...Making Outside Preparations Difficult Or Dangerous.

Tropical Storm Conditions Are Expected Over Puerto Rico And The U.S. And British Virgin Islands On Thursday.

Hurricane Conditions Are Possible Over Puerto Rico And The U.S. Virgin Islands On Thursday.

Rainfall...Total Rain Accumulations Of 4 To 8 Inches Are Possible
Over The Northern Windward Islands And The Leeward Islands. Total
Rain Accumulations Of 1 To 3 Inches With Maximum Amounts Of 6
Inches Are Possible Over Puerto Rico And The Virgin Islands.

Storm Surge...A Storm Surge Will Raise Water Levels By As Much As 1 To 3 Feet Above Normal Tide Levels In The Northern Leeward
Islands...Puerto Rico...And The U.S. And British Virgin Islands.

Near The Coast...The Surge Will Be Accompanied By Dangerous Waves.

Surf...Dangerous Surf And Rip Current Conditions Will Affect The
Windward And Leeward Islands During The Next Couple Of Days.

Next Advisory
-------------
Next Intermediate Advisory...200 Am Ast.
Next Complete Advisory...500 Am Ast.

Forecaster Pasch
______________________________________________________
Tropical Storm Isaac Discussion Number 4
Nws National Hurricane Center Miami Fl
1100 Pm Ast Tue Aug 21 2012

There Has Been No Aerial Reconnaissance Into Isaac Recently...But
The Cloud Pattern Of The Storm Has Not Become Significantly Better
Organized Over The Past Few Hours.

The Intensity Is Held At 35 Kt Which Is In Agreement With A Dvorak Intensity Estimate From Tafb.

Another Hurricane Hunter Aircraft Is Scheduled To Investigate Isaac
At Around 0600 Utc.

There Is Some Spread In Center Fixes...But Based On Geostationary
And Microwave Imagery The Initial Motion Remains About The
Same...275/16.

A Well-Defined Mid-Tropospheric Ridge To The North Of The Tropical Cyclone Should Maintain The Westward Track For The Next Couple Of Days.

Later In The Forecast Period...Global Models Depict Isaac Nearing A Break In The Ridge Near Florida.

Consequently The Forward Speed Is Likely To Slow...And The Heading To Gradually Turn Toward The Right...In The Latter Half Of The Forecast Period.

Although The Gfs Has Shifted Just A Bit To The North Of Its Previous Forecast...Overall The Track Guidance Has Not Changed Much Since The Previous Cycle. The Official Forecast Is Very Close To The Latest Hwrf Track And Not Far From The Dynamical Consensus.

Water Vapor Imagery Suggests That East-Northeasterly Shear Is
Currently Affecting Isaac...But The Dynamical Guidance Predicts A
Decrease In Shear With Upper-Level Anticyclonic Flow Over The
Tropical Cyclone During The Next Several Days.

Therefore Strengthening Is Anticipated...And Isaac Could Become A Hurricane Within The Next Couple Of Days.

Later On...The Intensity Forecast Is Complicated By How Much The Core Of The System Interacts With The Mountainous Land Masses Of Hispaniola And Eastern Cuba.

The Official Wind Speed Forecast Is Fairly Close To The Statistical-
Dynamical Guidance For The First 48 Hours And Then Tries To Take
Into Account The Land Interaction.

Needless To Say...There Is Considerable Uncertainty In The Intensity Of Isaac In The 3 To 5 Day Time Frame.

Forecast Positions And Max Winds

Init 22/0300z 15.6n 55.6w 35 Kt 40 Mph
12h 22/1200z 15.8n 58.1w 40 Kt 45 Mph
24h 23/0000z 16.2n 61.2w 45 Kt 50 Mph
36h 23/1200z 16.7n 64.3w 55 Kt 65 Mph
48h 24/0000z 17.1n 67.1w 65 Kt 75 Mph
72h 25/0000z 18.3n 72.0w 65 Kt 75 Mph...Inland
96h 26/0000z 20.5n 76.0w 70 Kt 80 Mph...Inland
120h 27/0000z 23.0n 79.0w 70 Kt 80 Mph...Over Water

Forecaster Pasch


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