000 AXPZ20 KNHC 172153 TWDEP TROPICAL WEATHER DISCUSSION NWS NATIONAL HURRICANE CENTER MIAMI FL 2205 UTC WED AUG 17 2011 TROPICAL WEATHER DISCUSSION FOR THE EASTERN PACIFIC OCEAN FROM THE EQUATOR TO 32N...EAST OF 140W. THE FOLLOWING INFORMATION IS BASED ON SATELLITE IMAGERY...WEATHER OBSERVATIONS...RADAR...AND METEOROLOGICAL ANALYSIS. BASED ON 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 2100 UTC. ...SPECIAL FEATURES... TROPICAL STORM FERNANDA AT 12.1N 138.2W 1001 MB AT 2100 UTC AUG 17 MOVING W OR 290 DEG AT 7 KT. MAXIMUM SUSTAINED WIND 45 GUSTS 55 KT. SCATTERED MODERATE CONVECTION WITHIN 60 NM NW QUADRANT. FERNANDA EXPECTED TO MOVE W AND INTENSIFY SLIGHTLY OVERNIGHT...THEN BEGIN TO WEAKEN THU AS IT MOVES W OF 140W. REFER TO FORECAST ADVISORIES ISSUED UNDER WMO HEADER WTPZ21 KNHC AND AWIPS HEADER MIATCMEP1 FOR FURTHER DETAILS. TROPICAL STORM GREG AT 17.1N 106.5W 994 MB AT 2100 UTC AUG 17 MOVING W-NW OR 290 DEG AT 20 KT. MAXIMUM SUSTAINED WINDS 55 KT GUSTS 65 KT. NUMEROUS STRONG CONVECTION FROM 14N TO 18N BETWEEN 104W AND 106W. TROPICAL STORM GREG EXPECTED TO INTENSIFY OVER WARM WATERS S OF MEXICO AND BECOME OVERNIGHT. REFER TO FORECAST ADVISORIES ISSUED UNDER WMO HEADER WTPZ22 KNHC AND AWIPS HEADER MIATCMEP2 FOR FURTHER DETAILS. ...ITCZ/MONSOON TROUGH... MONSOON TROUGH FROM 10N85W TO 13N94W...THEN RESUMES FROM 16N112W TO LOW PRES 1008 MB AT 14N116W TO 10N124W TO 13N130W. SCATTERED MODERATE ISOLATED STRONG CONVECTION WITHIN 210 NM S OF AXIS FROM 117W TO 131W. ...DISCUSSION... MODERATE EAST FLOW DOMINATES THE MID AND UPPER LEVELS OF THE ATMOSPHERE MAINLY E OF 120W...TO THE SOUTH OF A LARGE DEEP LAYER RIDGE CENTERED OVER THE SOUTH CENTRAL UNITED STATES AND NORTHERN MEXICO. THIS IS PROVIDING THE MAIN STEERING FLOW FOR T.S. GREG AS WELL AS SOME EASTERLY SHEAR ON THE STORM. GREG WILL EXPERIENCE DECREASING SHEAR AS IT MOVES RAPIDLY WESTWARD...TOWARD THE WESTERN EDGE OF THE RIDGE. A 1007 MB LOW IS NOTED TO THE WEST OF GREG NEAR 14N114W WITH WINDS TO 20 KT OUT AS FAR AS 360 NM FROM THE CENTER. THIS SLOW WILL MOVE W AHEAD OF GREG THROUGH 24 HOURS ALONG 14N...THEN DISSIPATE THROUGH 48 HOURS. SOUTH OF GREG...A MAINLY MID LEVEL DISTURBANCE IS NOTED EVIDENT IN SATELLITE IMAGERY OFF COSTA RICA ALONG 90W S OF 13N...INTERACTING WITH THE MONSOON TROUGH. CONVECTION REMAINS FAIRLY LIMITED WITH THIS SYSTEM AND ALTHOUGH GFS AND ECMWF SHOW A WEAK CIRCULATION FORMING BY 24 HOURS AND DRIFTING W ALONG 12N...CONVECTION AND WINDS WILL REMAIN LIMITED. LAND BASED OBSERVATIONS AND DATA FROM A WINDSAT PASS FROM AROUND 1300 UTC SHOWED 20 TO 25 KT OF NORTHERLY FLOW THROUGH THE GULF OF TEHUANTEPEC. THIS IS THE RESULT OF THE GRADIENT BETWEEN 1019 MB HIGH PRES OVER THE GULF OF MEXICO AND TROUGHING OVER THE NW CARIBBEAN. MODERATE CONVECTION IS NOTED ALONG THE MONSOON TROUGH BETWEEN 115W AND 130W...DUE TO CONVERGENCE OF MODERATE SW FLOW. SHIP DATA AND AN ASCAT PASS FROM 1640 UTC SHOWED WINDS TO 20 KT AS FAR AS 300 NM N OF THE CENTER...BETWEEN THE TROPICAL STORM AND HIGH PRES N OF THE AREA. DEEP LAYER DRY AIR W OF 120W AT THE BASE OF AN UPPER TROUGH ALONG 135W N OF 23N IS NOT ONLY SUPPRESSING CONVECTION RELATED TO T.S. FERNANDA...BUT IS ALSO LIMITING IN LARGE SCALE CONVECTION THROUGHOUT THE AREA W OF 120W. MEANWHILE S SWELL TO 8 FT WITH PERIODS TO 14 SECONDS IS PUSHING ACROSS THE EQUATOR AS FAR N AS 14N BETWEEN 100W AND 114W. $$ CHRISTENSEN