000 AXNT20 KNHC 161734 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 105 PM EST TUE DEC 16 2008 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 1645 UTC. ...THE ITCZ... ITCZ IS CENTERED ALONG 6N8W 3N22W 3N43W 2N51W. SCATTERED MODERATE/ISOLATED STRONG CONVECTION IS WITHIN 90 NM S OF THE ITCZ FROM 26W-29W. LARGE CLUSTERS OF SIMILAR CONVECTION ARE WITHIN 75 NM OF LINE FROM 6N18W 5N30W TO 9N43W. ...DISCUSSION... GULF OF MEXICO... MODERATE/STRONG WLY MID/UPPER FLOW COVERS THE GULF WITH STRONGER WINDS OVER THE GULF COAST STATES FROM LOUISIANA TO GEORGIA ON THE N PERIPHERY OF AN UPPER RIDGE EXTENDING FROM THE W CARIBBEAN. THIS FLOW CONTINUES TO SPREAD PACIFIC HIGH LEVEL MOISTURE ACROSS THE N GULF WITH DRIER AIR PRESENT IN LIGHTER FLOW OVER THE S HALF. IN THE LOWER LEVELS...A SURFACE RIDGE OVER THE NW ATLC CONTINUES PRODUCING SELY FLOW OVER THE FAR E GULF AND LOW LEVEL ISOLATED SHOWERS IN THE FLORIDA STRAITS S OF THE FLORIDA KEYS AND ALONG THE E COAST OF FLORIDA. A COLD FRONT ENTERS THE GULF THROUGH A 1023 MB LOW OVER S MISSISSIPPI ACROSS THE MISSISSIPPI DELTA OF LOUISIANA SW TO 27N95W THEN S TO INLAND OVER MEXICO NEAR TAMPICO THEN N ALONG THE E SLOPES OF THE SIERRA MADRE MOUNTAINS. THIS FRONT IS BEGINNING TO LOSE ITS UPPER SUPPORT CAUSING THE FRONT TO STALL THEN PUSH NW AS THE SURFACE RIDGE OVER THE ATLC AGAIN GAINS CONTROL OF THE GULF. SCATTERED SHOWERS ARE N OF 27N FROM PANAMA CITY FLORIDA TO THE MISSISSIPPI DELTA WITH DENSE LOW LEVEL CLOUD COVER AND POSSIBLE ISOLATED SHOWERS W OF THE FRONTAL BOUNDARY. CARIBBEAN SEA... ABUNDANT STABLE AIR ALOFT DOMINATES MUCH OF THE CARIBBEAN WITH SOME POCKETS OF UPPER LEVEL MOISTURE OVER THE SW CARIBBEAN AND CENTRAL AMERICA. THE W CARIBBEAN IS DOMINATED BY AN UPPER RIDGE EXTENDS FROM PANAMA OVER CENTRAL AMERICA AND INTO THE GULF OF MEXICO. THE UPPER TROUGH OVER THE CENTRAL ATLC REMAINS OVER THE E CARIBBEAN ACROSS THE LESSER ANTILLES INTO THE W TROPICAL ATLC. THE PRESSURE GRADIENT BETWEEN STRONG SURFACE RIDGE OVER THE W ATLC AND RELATIVE LOWER PRESSURE OVER THE CARIBBEAN CONTINUES TO MAINTAIN MODERATE/STRONG ELY TRADE WINDS ACROSS THE AREA THAT ARE EXPECTED TO GRADUALLY DIMINISH STARTING TONIGHT INTO WED. TYPICAL PATCHES OF LOW LEVEL CLOUDS AND EMBEDDED QUICK MOVING ISOLATED SHOWERS ARE BEING DRIVEN BY THE TRADE WINDS...BUT THE LARGE SCALE SUBSIDENT ENVIRONMENT IS KEEPING THE MOISTURE SHALLOW. ATLANTIC OCEAN... THE UPPER RIDGE THAT COVERS THE W CARIBBEAN AND THE GULF OF MEXICO EXTENDS OVER THE W ATLC W OF 70W. THE SURFACE RIDGE OVER THE NW ATLC CONTINUES TO PRODUCE LOW LEVEL ISOLATED SHOWERS ACROSS THE W ATLC TO ALONG THE E COAST OF FLORIDA. A COMPLEX UPPER TROUGH REMAINS OVER THE CENTRAL ATLC WITH AN EMBEDDED UPPER LOW NEAR 23N59W AND CONTINUES TO PROVIDE LIMITED SUPPORT FOR A WEAKENING STATIONARY FRONT EXTENDING THROUGH 32N53W TO 23N59W. A WEAKENING 1012 MB LOW IS JUST TO THE S OF THE FRONT NEAR 29N56W WITH A SURFACE TROUGH EXTENDING THROUGH A DEVELOPING 1010 MB LOW NEAR 26N53W THEN INTO THE TROPICS ALONG 21N55W TO 16N56W. THE TIGHT PRESSURE GRADIENT BETWEEN THIS COMPLEX SURFACE SYSTEM AND THE SURFACE RIDGES OVER THE W AND E ATLC ARE PRODUCING NLY GALE FORCE WINDS N OF 27N W OF FRONT TO 60W AND 20-25 KT ELSEWHERE W OF THE FRONT. SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS WITHIN 90/120 NM OF SURFACE TROUGH N OF 23N AND WITHIN 75 NM OF THE FRONTAL BOUNDARY. SIMILAR ACTIVITY IS WITHIN 200 NM OF LINE FROM 23N45W TO BEYOND 32N51W IN AN UPPER DIFFLUENT ZONE. THE FRONT IS EXPECTED TO GRADUALLY WEAKEN AS THE 1010 MB SURFACE LOW AND SHOULD TRACK W THEN SW OVER THE NEXT FEW DAYS OPENING UP INTO A TROUGH LATE THU NEAR THEN SE GREATER ANTILLES. $$ WALLACE