000
AXNT20 KNHC 171803
TWDAT 

TROPICAL WEATHER DISCUSSION 
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT FRI APR 17 2009

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

BASED ON 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY
THROUGH 1745 UTC. 

...THE ITCZ...
THE ATLC ITCZ EXTENDS FROM THE AFRICAN COAST AT 06N09W TO ALONG 
THE EQUATOR FROM 22W TO S AMERICA AT 49W. SCATTERED SHOWERS AND 
TSTMS ARE OBSERVED WITHIN 120 NM EITHER SIDE OF LINE FROM 01S06W 
TO 04N14W AND WITHIN 60 NM OF THE ITCZ AXIS W OF 15W.

...DISCUSSION...

THE GULF OF MEXICO...
THE UPPER LEVEL FLOW HAS BECOME ZONAL...W TO E...OVER THE GULF. 
A MID LEVEL ANTICYCLONE IS NOTED NEAR 26N91W. REMNANTS OF A 
FRONTAL TROUGH ARE CLEARLY EVIDENT FROM NW CUBA TO NE TEXAS WITH 
THE PORTION OVER THE CENTRAL GULF MOVING NE AND ANALYZED AS A 
WARM FRONT. MID LEVEL DIFFLUENCE OVER THE E SEMICIRCLE OF THE 
MOSTLY MID LEVEL ANTICYCLONE ENHANCED CLUSTERS OF TSTMS NEAR 
28N92W AND NEAR 25.5N27.5W EARLIER THIS MORNING...BUT THE 
CONVECTION IS CURRENTLY DECREASING IN BOTH INTENSITY AND AREAL 
COVERAGE. THE RESULTANT DEBRIS MOISTURE IS NOW ADVECTED E IN THE 
UPPER FLOW OVER THE GULF N OF 25N E OF 90W...CONTINUING ACROSS 
THE ENTIRE STATE OF FLORIDA AND THE NW BAHAMAS. ELSEWHERE AT THE 
SURFACE A WEAK LOW PRES IS NOTED NEAR 21N97W WITH WEAK SHOWERS 
WITHIN 120 NM OF CENTER. UPPER DRY AIR AND SUBSIDENCE IS 
SUPPRESSING CONVECTION ELSEWHERE ACROSS THE GULF OF MEXICO.

UPSTREAM OVER CENTRAL TEXAS...AN UPPER SHORT WAVE IS SWINGING E 
AND ENHANCING IMPRESSIVE CONVECTION. SOME OF THE DEBRIS MOISTURE 
MAY SPREAD E OVER THE NW GULF OF MEXICO LATER TODAY WITH THE 
ASSOCIATED SURFACE COLD FRONT MOVING OFF THE TEXAS COAST SAT.

THE CARIBBEAN SEA...
AN UPPER TROUGH EXTENDS S OVER THE N CENTRAL CARIBBEAN ACROSS 
HISPANIOLA TO A BASE NEAR 12N82W...AND IS PROGRESSING EASTWARD 
WITH TIME. IN THE WAKE OF THE TROUGH A MID LEVEL RIDGE HAS BUILT 
SE FROM THE CENTRAL GULF OF MEXICO TO NEAR JAMAICA AND IS 
ACCOMPANIED BY DRY UPPER AIR SUPPRESSING CONVECTION OVER MOST OF 
THE W CARIBBEAN. THE EXCEPTION IS THAT AN ASSOCIATED...BUT  
WEAKENING SURFACE COLD FRONT HAS MOVED OVER CUBA...WITH A 
PORTION OF THE FRONT EXTENDING ACROSS THE S-CENTRAL CUBAN 
COASTAL WATERS AND CURRENTLY ACCOMPANIED BY TOWERING CUMULUS IN 
THE VICINITY OF 21N81W. 

AN UPPER ANTICYCLONE CENTERED WELL TO THE SE OF THE CARIBBEAN 
NEAR 03N43W RIDGES NW ACROSS VENEZUELA AND N COLOMBIA AND OVER 
THE CENTRAL CARIBBEAN...CRESTING ALONG POINTS FROM 10N80W TO 
BEYOND 18N64W. ALTHOUGH UPPER LEVEL THIN CIRRIFORM CLOUDS ARE 
OBSERVED STREAMING NE ACROSS THE UPPER RIDGE E OF 70W...THE 
UPPER LEVELS ARE FOR THE MOST PART QUITE DRY OVER THE E 
CARIBBEAN. AT THE LOW LEVELS...LOCALLY BROKEN TRADE WIND 
CUMULUS IS RACING W ACROSS THE CARIBBEAN. CLUSTERS OF SHOWERS 
ARE NOTED ALONG 17N BETWEEN 66W AND 74W AND ALSO JUST S OF 
JAMAICA. 
        
 
THE ATLANTIC OCEAN...
AN UPPER LEVEL TROUGH IS SWINGING E THROUGH THE SUBTROPICAL ATLC 
WITH MEAN ITS AXIS ALONG 70W AT THE MOMENT. A SERIES OF SURFACE 
LOW PRES CENTERS ARE NOTED OVER THE CENTRAL N ATLC TO THE E OF 
THIS UPPER TROUGH...WITH THE SOUTHERNMOST LOW PRES NEAR 32N64W. 
AN ASSOCIATED SURFACE FRONT EXTENDS IN THE AREA ALONG 32N56W TO 
THE SE BAHAMAS WHERE THE FRONT BECOMES DIFFUSE...BUT IS STILL 
ENHANCING A WELL DEFINED LINE OF SHOWERS AS THE FRONT DIPS 
ACROSS CUBA. UPPER DIFFLUENCE IS ENHANCING CONVECTION ALONG THE 
COLD FRONT TO THE N OF 27N...BUT LIGHTNING DATA INDICATES THE 
CONVECTION IS COMPOSED OF MOSTLY HEAVY SHOWERS THIS MORNING. A 
COUPLE MORE SURFACE BOUNDARIES ARE SWINGING CYCLONICALLY AROUND 
THE AFOREMENTIONED SURFACE LOW...AND DEFINED BY LOCALLY BROKEN 
STRATOCUMULUS CLOUDS SPREADING S OVER THE SUBTROPICS TO THE W OF 
65W.     

AN UPPER ANTICYCLONE CENTERED NEAR 03N43W RIDGES NE ACROSS THE 
CENTRAL ATLC TO BEYOND 32N45W CRESTING NEAR 50N35W. DEBRIS 
MOISTURE FROM THE FRONTAL CONVECTION IS ADVECTED ACROSS THIS 
UPPER RIDGE COVERING THE UPPER LEVELS OF THE ATLC TO THE N OF 
30N E OF 50W...WITH SOME MOISTURE DIVING BACK SE ACROSS THE 
RIDGE OVER N AFRICA. AN UPPER TROUGH EXTENDS SW FROM SPAIN 
ALONG THE EXTREME NW COAST OF AFRICA TO NEAR 18N37W...AND IS 
ACCOMPANIED BY A UPPER DRY AIR WITHIN 180 NM OF ITS AXIS. A 
LARGE UPPER ANTICYCLONE OVER AFRICA NEAR 12N13E RIDGES W ALONG 
10N INTO THE AREA NEAR 07N30W TAPPING DEBRIS MOISTURE FROM THE 
ITCZ CONVECTION...DESCRIBED ABOVE...AND ADVECTING THE MOISTURE 
ACROSS N AFRICA TO THE S OF THE UPPER TROUGH. AT THE LOW LEVELS 
A SURFACE FRONT IS LOSING IDENTITY ALONG 32N17W TO 27N33W. HIGH 
PRES NO THE FRONT NEAR 32N28W RIDGES SW TO 22N55W. LOCALLY OVER 
STRATOCUMULUS CLOUDS ARE OBSERVED FROM 05S TO 23N BETWEEN 25W 
AND 58W MOVING WSW IN THE ASSOCIATED LOW LEVEL ANTICYCLONIC 
FLOW.      

$$
NELSON