000 AXNT20 KNHC 300003 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 705 PM EST FRI DEC 29 2007 TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL AMERICA...THE GULF OF MEXICO...THE CARIBBEAN SEA...NORTHERN SECTIONS OF SOUTH AMERICA...AND THE 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 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 2315 UTC ...SPECIAL FEATURE... THE WELL-DEFINED NON-BAROCLINIC CYCLONE CONTINUES IN THE NORTHEAST ATLANTIC NEAR 28N38W. THE SYSTEM HAS BEEN DRIFTING NORTH-NORTHEASTWARD DURING THE DAY AND HAS DEVELOPED PRONOUNCED ANTICYCLONIC OUTFLOW IN THE UPPER TROP. DESPITE WATER TEMPERATURES NEAR THE CYCLONE OF AROUND 22-23C...MODERATE CONVECTION HAS BEEN PERSISTENT ON THE NORTHEAST SEMICIRCLE. SATELLITE CLASSIFICATIONS SUGGEST THAT THE SYSTEM COULD BE A MINIMAL SUBTROPICAL STORM. QUIKSCAT WINDS...WHILE HAVING THE USUAL AMBIGUITY ISSUES IN RAINY CONDITIONS...INDICATE THAT GALE FORCE WINDS LIKELY ARE OCCURRING IN THE NORTHERN SEMICIRCLE. TROPOSPHERIC VERTICAL SHEAR IS ABOUT 20 KT AS THE STRONGEST UPPER TROP WESTERLIES ARE BYPASSING THE CYCLONE TO THE NORTH AND THE SOUTH. THE CYCLONE SHOULD MEANDER OVER THE NEXT 36 HOURS BEFORE BEING PUSHED OFF TOWARD THE WEST OR SOUTHWEST BY A DEVELOPING MID-OCEAN RIDGE ON MONDAY. GIVEN THAT THE VERTICAL SHEAR SHOULD INCREASE SUBSTANTIALLY TOMORROW...TONIGHT MAY BE THE LAST OPPORTUNITY FOR THIS SYSTEM TO GAIN A BIT MORE ORGANIZATION TO ITS CONVECTION AND MAKE THE TRANSITION TO A NAMED SUBTROPICAL STORM. ...THE ITCZ... ITCZ AXIS IS LOCATED ALONG 6N11W AT THE LIBERIA COAST 3N16W 3N30W INTO NE BRAZIL NEAR 2S45W. WIDELY SCATTERED MODERATE CONVECTION IS WITHIN 60 NM OF THE ITCZ AXIS BETWEEN 30W-38W. WIDELY SCATTERED SHOWERS ARE ALSO FOUND WITHIN 120 NMI OF THE ITCZ AXIS ELSEWHERE. ...DISCUSSION... THE GULF OF MEXICO... AS OF 2100 UTC...A FRONTAL BOUNDARY EXTENDS ACROSS THE GULF OF MEXICO FROM THE FLORIDA PANHANDLE TO THE BAY OF CAMPECHE. SOUTH OF 28N...THE FRONT HAS STALLED OUT AND IS WEAKENING. IN THE UPPER LEVELS...SOUTHWESTERLIES PREVAIL OVER THE GULF RANGING FROM 40 KT IN THE SE TO 90 KT IN THE NW GULF. NORTH OF 28N AND INTO THE FLORIDA PANHANDLE...THE FRONT IS SLIGHTLY PROGRESSIVE AND IS CONSIDERED A COLD FRONT. LIGHTNING...IR SATELLITE...AND RADAR IMAGERY SHOW THAT THE FRONT IS CONVECTIVE ACTIVE NORTH OF 26N. WINDS BEHIND THE FRONT ARE NORTHEAST TO NORTH 15 KT OR LESS. WINDS AHEAD OF THE FRONT ARE SOUTHEAST TO SOUTH ABOUT 10 KT OR LESS. THE CURRENT FRONTAL BOUNDARY SHOULD LIKELY DISSIPATE TONIGHT OR EARLY TOMORROW TO BE REPLACED BY A NEWLY EMERGING FRONT REACHING THE NORTHWEST GULF OF MEXICO TOMORROW. THIS NEW COOL AIR MASS SHOULD BE SHORT-LIVED AND NOT LIKELY TO PUSH ACROSS THE WHOLE GULF. A MUCH MORE VIGOROUS COLD FRONT WILL IMPACT THE GULF ON TUESDAY WITH THE ENTIRE REGION EXPERIENCING STRONG NORTHERLY WINDS WITH SOME GALES EXPECTED IN THE WESTERN GULF. THE CARIBBEAN SEA... THE CARIBBEAN IS EXPERIENCING NO LARGE-SCALE AREAS OF CONVECTION CURRENTLY...AS THE REGION IS BEING DOMINATED BY SUBSIDENCE FROM A UPPER LEVEL RIDGE CENTERED NEAR JAMAICA. THE RIDGE IS BECOMING MORE HIGH AMPLITUDE AS A STRONG SHORT-WAVE TROUGH IS IMPINGING ON THE REGION WITH THE TROUGH AXIS A COUPLE HUNDRED NM EAST OF THE LESSER ANTILLES. SURFACE WINDS ARE 15 TO 25 KT EASTERLY...WITH THE STRONGER WINDS IN THE CENTRAL CARIBBEAN NORTH OF THE COLOMBIAN COAST. SOME WEAKENING OF THE WINDS ARE EXPECTED AS THE PRESSURE GRADIENT RELAXES. THE AFOREMENTIONED VIGOROUS COLD FRONT WILL REACH THE NORTHWESTERN CARIBBEAN ON WEDNESDAY MORNING AND WILL CAUSE MODERATE NORTH TO NORTHEAST WINDS ON WEDNESDAY AND THURSDAY. THE ATLANTIC OCEAN... A 1006 MB LOW IS NEAR 28N38W. SEE SPECIAL FEATURE ABOVE FOR DETAILS. ELSEWHERE...SURFACE RIDGING IS IN PLACE THROUGHOUT THE REST OF THE REGION WITH EAST TO NORTHEAST WINDS 15 TO 20 KT GENERALLY. WIDELY SCATTERED MODERATE CONVECTION CONTINUES IN ASSOCIATION WITH A WEAK SURFACE TROUGH ALONG 40W BETWEEN 4N-11N. THE STRONG UPPER LEVEL TROUGH MENTIONED IN THE PREVIOUS SECTION IS HELPING TO PRODUCE WIDESPREAD BUT ISOLATED SHOWERS AND UPPER LEVEL CLOUDINESS FROM NEAR THE SURFACE TROUGH TO THE CAPE VERDE ISLANDS TO THE MAURITANIAN COAST. THE AFOREMENTIONED VIGOROUS COLD FRONT WILL REACH THE WESTERN ATLANTIC LATE ON TUESDAY AND NORTH WINDS WILL IMPACT THE BAHAMAS AND THE ATLANTIC EASTWARD TO ABOUT 65W ON WEDNESDAY. $$ CWL