000 AXNT20 KNHC 161800 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 205 PM EDT SAT SEP 16 2006 TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL AMERICA...THE GULF OF MEXICO...THE CARIBBEAN SEA...NORTHEASTERN 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...WEATHER OBSERVATIONS...RADAR... AND METEOROLOGICAL ANALYSIS. BASED ON 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 1715 UTC. ...SPECIAL FEATURES... HURRICANE GORDON IS CENTERED NEAR 31.3N 53.7W OR ABOUT 655 MILES...1055 KM...EAST OF BERMUDA AT 16/1500 UTC AND IS NEAR STATIONARY. ESTIMATED MINIMUM CENTRAL PRESSURE IS 987 MB. MAXIMUM SUSTAINED WIND SPEED IS 65 KT WITH GUSTS TO 80 KT. SEE THE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT2/WTNT22 KNHC FOR MORE DETAILS. GORDON REMAINS IN A WEAK STEERING ENVIRONMENT AND HAS CHANGED LITTLE TODAY. THE CYCLONE IS FAIRLY SYMMETRIC WITH A RAGGED EYE ABOUT 30-40 NM IN DIAMETER. CONVECTION IS NOT VERY DEEP AROUND THE SYSTEM...THE COLDEST OF THE CLOUD TOPS ARE NEAR -70 C IN A SMALL AREA IN THE EASTERN EYEWALL. GORDON HAS BEEN A COMPACT SYSTEM THROUGHOUT ITS LIFE WITH ITS RAIN FIELD COVERING AN AREA OF LESS THAN 300 NM ACROSS FROM 30N-33N BETWEEN 52W-56W . HELENE HAS BEEN UPGRADED TO A HURRICANE. HELENE IS CENTERED NEAR 18.8N 45.6W OR ABOUT 1145 MILES...1845 KM...EAST OF THE NORTHERN LEEWARD ISLANDS 16/1500 MOVING WEST-NORTHWEST AT 12 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 987 MB. MAXIMUM SUSTAINED WIND SPEED IS 65 KT WITH GUSTS TO 80 KT. SEE THE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT3/WTNT23 KNHC FOR MORE DETAILS. UNLIKE GORDON...HELENE IS A LARGE SYSTEM WITH SCATTERED MODERATE CONVECTION COVERING AN AREA FROM 16N-24N BETWEEN 42W-49W. THE STRONGEST CONVECTION EXISTS TO THE N AND W OF THE CENTER. LARGE BANDING FEATURES ARE IMPROVING WITH A POSSIBLE RAGGED EYE FEATURE ATTEMPTING TO FORM. ...TROPICAL WAVES... A 1011 MB LOW PRES AREA IS IN THE E ATLC CENTERED NEAR 16N21W...ABOUT 150 NM E OF THE CAPE VERDE ISLANDS. THIS LOW HAS BEEN RELOCATED TO THE E OF THE PREVIOUS POSITION. THE LOW LEVEL CENTER IS FAIRLY EASY TO SEE ON VISIBLE IMAGES AS IT IS PARTIALLY EXPOSED AND LIES NEAR THE SOUTHERN EDGE OF THE CONVECTION. BASED UPON THIS STRUCTURE...THIS LOW PRES AREA IS FEELING EFFECTS OF S-SW VERTICAL WIND SHEAR. WEAKENING SCATTERED MODERATE IS FROM 17N-22N BETWEEN 19W-23W. DEVELOPMENT...IF ANY...WOULD BE SLOW TO OCCUR AS IT TRACKS NW NEAR 10 KT. TROPICAL WAVE IS ALONG 78W S OF 17N MOVING W NEAR 15 KT. SOME LOW-MID LEVEL CYCLONIC TURNING IS NOTED ON VIS IMAGES ABOUT THE AXIS. A FAIRLY LARGE AREA OF DEEP CONVECTION IS IN THE SW CARIBBEAN AHEAD OF THE ANALYZED AXIS FROM 9N-18N BETWEEN 79W-86W...LIKELY ENHANCED BY UPPER LEVEL DIFFLUENT FLOW. TROPICAL WAVE IS ALONG 86W S OF 16N MOVING W NEAR 15 KT. THE WAVE IS ENHANCING THE ITCZ TO LIFT NWD. SCATTERED MODERATE CONVECTION LIES MAINLY TO THE E OF THE WAVE ENHANCED BY THE WAVE JUST TO ITS E. WELL-DEFINED LOW/MID LEVEL INVERTED-V SATELLITE SIGNATURE IS OBSERVED IN THE E PACIFIC REGION AS THE N PORTION IS INLAND OVER CENTRAL AMERICA. WAVE IS BENEATH THE WESTERN EDGE OF AN UPPER RIDGE WITH SCATTERED MODERATE/ISOLATED STRONG CONVECTION IS FROM 13N-16N BETWEEN 82W-84W. ...THE ITCZ... ITCZ AXIS IS CENTERED ALONG 6N10W 10N32W 15N40W 12N43W TO THE COAST OF SOUTH AMERICA NEAR 9N61W. BESIDES FOR THE CONVECTION ASSOCIATED WITH HELENE AND THE E ATLC LOW...THE ONLY OTHER AREA IS MOVING OFF THE W COAST OF AFRICA WHERE THERE IS SCATTERED MODERATE E OF 15W FROM 7N-12N. ...DISCUSSION... GULF OF MEXICO... FAIRLY QUIET CONDITIONS EXIST IN THE GULF THIS AFTERNOON. AN UPPER LEVEL TROUGH STRETCHES FROM THE NE U.S. OFF THE U.S. EAST COAST AND ACROSS S FLORIDA. MID TO UPPER RIDGING EXTENDS FROM MEXICO NORTHEASTWARD TO THE SOUTH CENTRAL U.S. UPPER DIFFLUENCE BETWEEN THESE FEATURES IS TRIGGERING SCATTERED SHOWERS AND ISOLATED TSTMS IN THE CENTRAL GULF FROM 25N-28N BETWEEN 88W-90W...MOVING SW ON THE E PERIPHERY OF THE MID LEVEL RIDGE. OTHERWISE...FAIR WEATHER IS THE RULE MAINLY DUE TO VERY DRY STABLE UPPER LEVEL AIR IN PLACE. THE DRIEST AIR IS ACROSS THE NE GULF. AT THE SFC...HIGH PRES CENTERED ACROSS THE GREAT LAKES IS BUILDING INTO THE AREA. LIGHT E-SE WINDS COVER THE E GULF...WITH STRONGER 15 KT SLY RETURN FLOW IN THE W GULF. LITTLE SIGNIFICANT PATTERN CHANGE IS EXPECTED TOMORROW. A COLD FRONT WILL LIKELY ENTER THE NORTHERN GULF LATE MON BRINGING THE NEXT CHANCE OF ORGANIZED RAINFALL. CARIBBEAN SEA... THE MOST ACTIVE WEATHER IN THE REGION IS IN THE SW CARIBBEAN WHERE SCATTERED MODERATE CONVECTION EXISTS FROM 9N-13N BETWEEN 78W-83W. OVER THE LAST FEW HOURS...A LINE OF SIMILAR CONVECTION HAS BECOME MORE ACTIVE FROM 13N-18N BETWEEN 82W-85W. THIS ACTIVITY IS LIKELY ENHANCED BY UPPER LEVEL DIFFLUENCE ON THE S SIDE OF A SMALL UPPER HIGH AND SFC CONVERGENCE IN THE VICINITY OF A COUPLE TROPICAL WAVES. THIS ENTIRE CONVECTIVE MASS IS MOVING WESTWARD TOWARDS CENTRAL AMERICA. A WEAK UPPER LOW IN THE SE BAHAMAS IS ALSO HELPING TO PRODUCE A FAIR AMOUNT OF MID-HIGH LEVEL CLOUDINESS AND SCATTERED SHOWERS IN THE NW CARIBBEAN...BUT A LIMITED AMOUNT OF DEEP MOISTURE. A MID-LEVEL RIDGE IN THE CENTRAL ATLC IS ADVECTING DRY AIR INTO THE CARIBBEAN E OF 73W. THIS DRY AIR WILL CONTINUE TO PRESS WESTWARD ACROSS THE CARIBBEAN OVER THE NEXT COUPLE OF DAYS AND WILL LIKELY LIMIT WIDESPREAD DEVELOPMENT OF DEEP CONVECTION AS IT DOES SO. TRADE WINDS ARE GENERALLY IN THE 15-25 KT RANGE AND THESE WINDS ARE EXPECTED TO REMAIN THAT WAY THRU TOMORROW. ATLANTIC OCEAN... A DEEP LAYER TROUGH HAS MOVED OFF THE E COAST OF THE U.S WITH THE UPPER AXIS EXTENDING INTO THE BAHAMAS AND S FLORIDA. VERY DRY AIR LIES ALONG AND TO THE W OF THE AXIS W OF 76N N OF 27N. AN ASSOCIATED COLD FRONT EXTENDS FROM A 1011 MB LOW NEAR 35N72W ALONG 32N70W 28N75W THEN AS A DISSIPATING FRONT TO 27N79W. A PRE-FRONTAL SFC TROUGH ENTERS THE AREA NEAR 32N65W AND CONTINUES SW ALONG 27N71W TO THE W BAHAMAS. SCATTERED SHOWERS AND TSTMS ARE WITHIN 300 NM AHEAD OF THE SFC FRONT. A WEAK MID-UPPER LEVEL RIDGE COVERS THE CENTRAL ATLC WITH A HIGH CENTER NEAR 22N56W. DRY UPPER LEVEL AIR COVERS THE MAJORITY OF THE AREA BETWEEN 50W-70W. A WEAK 1019 MB HIGH IS LOCATED NEAR 24N61W HELPING TO KEEP THE WEATHER CONDITIONS TRANQUIL IN THIS ZONE. GORDON REMAINS N OF THIS UPPER RIDGE IN VERY WEAK STEERING CURRENTS KEEPING THE STORM NEARLY STATIONARY. RECENTLY UPGRADED HURRICANE HELENE IS MOVING TOWARDS THE NW INTO A WEAKNESS IN THE RIDGE PARTIALLY INDUCED BY HURRICANE GORDON. AN UPPER TROUGH LIES TO THE E OF THE HURRICANES N OF 22N FROM 25W-41W. AN UPPER LEVEL RIDGE IS INLAND OVER AFRICA. ALL OF THE SIGNIFICANT CONVECTION IN THE CENTRAL AND E ATLC IS ASSOCIATED WITH GORDON...HELENE AND THE E ATLC LOW PRES SYSTEM DISCUSSED ABOVE. $$ CANGIALOSI