[Tropical] Tropical Weather Discussion

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AXNT20 KNHC 121801
TWDAT

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT SUN 12 SEP 2004

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL
AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHEASTERN
SECTIONS OF SOUTH AMERICA...AND ATLC OCEAN TO THE AFRICAN
COAST FROM THE EQUATOR TO 32N. THE FOLLOWING INFORMATION IS
BASED ON SATELLITE IMAGERY...WEATHER OBSERVATIONS...RADAR...
AND METEOROLOGICAL ANALYSIS.

BASED ON 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
1715 UTC.

...SPECIAL FEATURE...
HURRICANE IVAN IS CENTERED NEAR 19.2N 82.1W OR 60 MILES W OF
GRAND CAYMAN AT 12/1800 UTC MOVING WNW AT 9 KT. MAXIMUM
SUSTAINED WIND SPEED HAS DECREASED TO 130 KT WITH GUSTS TO 160
KT. MINIMUM CENTRAL PRESSURE REPORTED BY AIR FORCE RESERVE UNIT
HURRICANE HUNTER AIRCRAFT HAS RISEN SLIGHTLY TO 922 MB. SEE
LATEST NHC PUBLIC ADVISORY OR NHC FORECAST/ADVISORY UNDER
AWIPS/WMO HEADERS MIATCPAT4/WTNT34 KNHC OR MIATCMAT4/WTNT24 KNHC
FOR MORE DETAILS. IVAN CONTINUES TO DEMONSTRATE A VERY
SYMMETRICAL APPEARANCE IN SATELLITE IMAGERY WITH A DISTINCT BUT
SOMEWHAT CLOUDED OVER EYE. NUMEROUS STRONG CONVECTION EXTENDS
OUTWARD UP TO 120 NM FROM THE CENTER IN ALL QUADRANTS ALTHOUGH
CLOUD TOP TEMPERATURES HAVE BEEN WARMING OVER THE N SEMICIRCLE.
OUTER RAINBANDS EXTEND AS FAR SOUTH AS 15N AND AS FAR NORTH AS
22N. THE NORTHERNMOST RAINBAND IS QUICKLY APPROACHING THE S
COAST OF CUBA. GUSTY WINDS TO TROPICAL STORM FORCE AND VERY
HEAVY RAIN CAN BE EXPECTED WITHIN THESE RAINBANDS. IN THE UPPER
LEVELS...IVAN IS FLANKED BY A WEAK UPPER LEVEL TROUGH TO THE
WEST AND STRONG MIDDLE TO UPPER LEVEL RIDGING TO THE EAST.  THIS
PATTERN CONTINUES TO PROVIDE EXCELLENT OUTFLOW IN ALL BUT THE NW
QUADRANT WHERE WATER VAPOR IMAGERY SHOWS MIDDLE TO UPPER LEVEL
DRY AIR EXTENDING FROM THE FLORIDA STRAITS SW OVER THE YUCATAN
PENINSULA.  NEARBY 1200 UTC UPPER AIR OBSERVATIONS CONFIRM THE
PRESENCE OF THIS DRY AIR AND ACTUALLY INDICATE THE DRY AIR
REACHES ALL THE WAY TO THE 900-800 MB LEVELS. OTHERWISE...THE
ATMOSPHERE LOOKS GENERALLY FAVORABLE AND IVAN SHOULD REMAIN A
MAJOR HURRICANE AS IT MOVES TOWARD W CUBA.

...TROPICAL WAVES...
TROPICAL WAVE OVER THE E ATLC ALONG 34W S OF 21N MOVING W 15-20
KT. THE WAVE CONTINUES TO DEMONSTRATE A CLASSIC SIGNATURE IN
SATELLITE IMAGERY. HOWEVER...THE SYSTEM APPEARS ELONGATED AND
ASSOCIATED CONVECTION IS POORLY ORGANIZED. WIDELY SCATTERED
MODERATE ACTIVITY FROM 5N-17N BETWEEN 25W-35W.

TROPICAL WAVE APPROXIMATELY 300 NM E OF BARBADOS ALONG 55W S OF
19N MOVING W 20 KT. THE WAVE HAS BECOME QUITE CONVECTIVE DURING
THE LAST 6-12 HOURS WITH A LARGE AREA OF SCATTERED MODERATE
ACTIVITY CURRENTLY FROM 12N-20N BETWEEN 51W-59W. LATEST VISIBLE
SATELLITE IMAGERY SHOWS SOME LOW LEVEL CYCLONIC TURNING AND THE
CONVECTION DOES APPEAR A BIT BETTER ORGANIZED. HOWEVER...UPPER
LEVEL ENVIRONMENT ARE ONLY MARGINALLY FAVORABLE.
NONETHELESS...SHOWERS AND THUNDERSTORMS SHOULD SPREAD ACROSS THE
LESSER ANTILLES DURING THE NEXT DAY OR SO.

TROPICAL WAVE IN THE E CARIBBEAN ALONG 66W S OF 22N MOVING W
10-15 KT.  SATELLITE IMAGERY INDICATES THE TRANSLATION SPEED OF
THE WAVE HAS DECREASED DURING THE LAST 12 HOURS AS IT INTERACTS
WITH A MID-UPPER LEVEL TROUGH TO THE N.  ADDITIONALLY...THE
EARLIER SURFACE LOW ANALYZED ALONG THE WAVE HAS DISSIPATED AND
CONVECTION NEAR THE VIRGIN ISLANDS HAS WEAKENED.  ONLY A SMALL
AREA OF SCATTERED SHOWERS JUST SE OF PUERTO RICO ACCOMPANIES THE
WAVE.

...ITCZ...
ITCZ AXIS EXTENDS ALONG 15N1W 9N20W 9N40W 11N50W 9N70W 8N80W.
SCATTERED MODERATE CONVECTION FROM 3N-8N BETWEEN 10W-20W AND
FROM 6N-10N BETWEEN 25W-40W. SCATTERED MODERATE CONVECTION FROM
5N-7N BETWEEN 45W-51W.

...DISCUSSION...

GULF OF MEXICO...
DEEP LAYERED TROUGH COVERS THE EAST HALF OF THE UNITED STATES
WITH A STRONG UPPER LEVEL LOW OVER THE MISSISSIPPI VALLEY.
WATER VAPOR IMAGERY SHOWS THAT THIS TROUGH CONTINUES TO AMPLIFY
SOUTHWARD RESULTING IN A WEAKENING OF THE MID-UPPER LEVEL RIDGE
CURRENTLY IN PLACE ACROSS MUCH OF THE GLFMEX.  THIS EVOLUTION IS
EXPECTED TO ALLOW HURRICANE IVAN TO GRADUALLY TURN MORE
NORTHWARD TODAY WITH THE POWERFUL HURRICANE REACHING THE SE
GLFMEX IN 24-36 HOURS. CURRENTLY...GOOD UPPER LEVEL
DYNAMICS/LIFT AHEAD OF THE AMPLIFYING TROUGH IS PRODUCING A
LARGE AREA OF SCATTERED SHOWERS/THUNDERSTORMS ALONG A LOW LEVEL
TROUGH EXTENDING FROM SE LOUISIANA TO THE COAST OF MEXICO NEAR
VERACRUZ.  MOST OF THIS ACTIVITY IS FOCUSED WITHIN 300 NM E OF
THE LOW LEVEL TROUGH OWING TO STRONG WLY SHEAR.  AS SUCH...THIS
FEATURE SHOWS NO SIGNS OF TROPICAL DEVELOPMENT DESPITE BEING
RATHER CONVECTIVE. FURTHER E...MID-UPPER LEVEL SUBSIDENCE AND
VERY DRY AIR EXTENDS FROM THE YUCATAN PENINSULA NEWD OVER S
FLORIDA LIMITING CONVECTIVE ACTIVITY.  ADDITIONALLY...STRONG
SUBSIDENCE OVER THE CENTRAL UNITED STATES IS SLOWLY FILTERING
SWD OVER TEXAS INTO THE NW GLFMEX.

CARIBBEAN...
THE MAIN FEATURE OVER THE CARIBBEAN IS HURRICANE IVAN WHICH IS
JUST SW OF GRAND CAYMAN ISLAND. IVAN CONTINUES TO MOVE SLOWLY
WNW OWING TO A PERSISTENT MID-LEVEL RIDGE WHICH EXTENDS ACROSS
FLORIDA. THIS RIDGE IS EXPECTED TO WEAKEN ALLOWING A MORE
NORTHWARD TURN TODAY TAKING IVAN TOWARD THE W TIP OF CUBA. OUTER
RAINBANDS ASSOCIATED WITH IVAN ARE ALREADY APPROACHING THE S
COAST OF CUBA W OF 75W AND INCREASING SHOWERS/THUNDERSTORMS WITH
GUSTY WINDS CAN BE EXPECTED ACROSS MUCH OF THE ISLAND TODAY AS
IVAN NEARS.  ELSEWHERE...STRONG MIDDLE TO UPPER LEVEL RIDGING
INDUCED BY OUTFLOW FROM IVAN DOMINATES THE E TWO-THIRDS OF THE
CARIBBEAN WITH ASSOCIATED SUBSIDENCE PRODUCING RATHER
TRANQUIL/DRY WEATHER. HOWEVER...A TROPICAL WAVE ABOUT 300 NM E
OF BARBADOS SHOULD BRING SHOWERS/THUNDERSTORMS TO THE LESSER
ANTILLES IN 12-24 HOURS.

SUBTROPICAL ATLANTIC...
A TROUGH-RIDGE-TROUGH PATTERN PREVAILS ACROSS MUCH OF THE
SUBTROPICAL ATLC.  A MID-UPPER LEVEL LOW WHICH SPLIT OFF THE
MID-OCEANIC TROUGH SEVERAL DAYS AGO REMAINS OVER THE W ATLC NEAR
BERMUDA. THE UPPER LOW EXTENDS A WEAK TROUGH SW OVER THE BAHAMAS
AND FLORIDA STRAITS. ASSOCIATED SUBSIDENCE AND DRY AIR COVERS
THE AREA W OF 65W LIMITING SIGNIFICANT CLOUDS/CONVECTION.  E OF
THE UPPER LOW...DIFFLUENT FLOW IN THE MID-UPPER LEVELS IS
PRODUCING A LARGE AREA OF CLOUDINESS AND SCATTERED SHOWERS/
THUNDERSTORMS FROM 20N-34N BETWEEN 55W-65W. THERE ARE NO SIGNS
OF ORGANIZATION OR DEVELOPMENT WITH THIS ACTIVITY.  FURTHER
E...MID-UPPER LEVEL RIDGE EXISTS OVER THE CENTRAL ATLC ALONG 55W
WITH THE MID-OCEANIC TROUGH LIMITED TO THE E ATLC ALONG 34N20W
TO 25N45W. SCATTERED SHOWERS/THUNDERSTORMS ARE OCCURRING WITH
THE MID-OCEANIC TROUGH FROM 20N-26N BETWEEN 30W-33W. OVER THE
FAR E ATLC...A WELL DEFINED MIDDLE TO UPPER LEVEL RIDGE EXTENDS
WESTWARD FROM AFRICA ALONG 17N/18N TO 30W.

TROPICAL ATLANTIC...
MOST OF THE TROPICAL ATLC IS UNDER THE INFLUENCE OF A MID/UPPER
RIDGE AXIS EXTENDING FROM AFRICA NEAR 17N WSW INTO THE E
CARIBBEAN ALONG 15N. A WEAK TROUGH SPLITS THE RIDGE NEAR 35W
RESULTING IN ENHANCED ITCZ CONVECTION WITHIN 5 DEG TO THE EAST
AND SUPPRESSED ITCZ CONVECTION WITHIN 5 DEG TO THE WEST.
ELSEWHERE...DEEP LAYERED ELY FLOW PREVAILS.

$$
RHOME




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