000 AXPZ20 KNHC 071012 TWDEP TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 1005 UTC TUE APR 07 2009 ...HURRICANE FORCE WIND WARNING IN EFFECT FOR THE GULF OF TEHUANTEPEC THROUGH 07/1500 UTC... TROPICAL WEATHER DISCUSSION FOR THE EASTERN PACIFIC OCEAN FROM THE EQUATOR TO 32N...EAST OF 140W. BASED ON 0600 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 0900 UTC. ...ITCZ... THE INTERTROPICAL CONVERGENCE ZONE LIES ALONG A LINE FROM 04N77W TO 05N97W TO 00N113W...THEN FROM 01N129W TO 04N134W TO 04N140W. SCATTERED MODERATE CONVECTION IS FROM 03N TO 06N BETWEEN 96W AND 105W. SCATTERED MODERATE CONVECTION IS ALSO FROM 04N TO 07N BETWEEN 132W AND 136W. ...DISCUSSION... GAP WINDS... A HURRICANE FORCE WIND WARNING HAS BEEN POSTED FOR THE GULF OF TEHUANTEPEC...WHERE A LATE SEASON GAP WIND EVENT IS CURRENTLY UNDERWAY. THUS...THE STORM WARNING IS BEING UPGRADED TO A HURRICANE FORCE WIND WARNING FOR THE GULF OF TEHUANTEPEC WITHIN 30 NM EITHER SIDE OF A LINE FROM 15.5N95W TO 13.5N95.5W...NOW IN EFFECT FROM 07/0600 UTC UNTIL 07/1500 UTC. THE NAM APPEARS TO HANDLE THIS WIND EVENT THE BEST AMONG VARIOUS COMPUTER MODEL GUIDANCE. A DISCUSSION FOR THE RATIONALE BEHIND THIS WARNING IS PROVIDED AT THE END OF THIS PRODUCT. FOLLOWING THE NAM FORWARD IN TIME...WINDS WILL LIKELY FALL BELOW STORM FORCE BY 08/0000 UTC...AND THEN FALL BELOW GALE FORCE BY 08/1800 UTC. THE STRONG WINDS WILL BUILD SEAS UP TO 18 FT BASED ON WAVEWATCH GUIDANCE BY MID-DAY TODAY...WITH SEAS UP TO 9 FT IN NORTHEASTERLY SWELL PROPAGATING AS FAR DOWNSTREAM AS 05N102W BY LATE WEDNESDAY. INCREASING TRADE WINDS OVER THE CARIBBEAN REGION ARE FORECAST TO EXTEND INTO THE GULF OF PAPAGAYO AND PANAMA DURING THE NEXT FEW DAYS...WHEN SURFACE WINDS ARE EXPECTED TO OCCASIONALLY REACH 20 KT. ALSO...ACROSS THE GULF OF CALIFORNIA...SW TO W WINDS WILL LIKELY DOWNSLOPE OFF THE BAJA CALIFORNIA MOUNTAINS IN RESPONSE TO INCREASING PRESSURE FALLS OVER WESTERN MEXICO BY MIDWEEK. ELSEWHERE... A PAIR OF SURFACE TROUGHS ARE OVER THE NW DISCUSSION AREA...THE EASTERN TROUGH EXTENDING FROM 30N125W TO 25N130W AND THE WESTERN TROUGH EXTENDING FROM 30N130W TO 28N132W. THE ASCAT PASS AROUND 07/0400 UTC INDICATES W WINDS UP TO 20 KT NEAR THE WESTERN TROUGH. THIS SURFACE TROUGHS ARE CURRENTLY BEING SUPPORTED BY A CLOSED UPPER LOW CENTERED NEAR 35N130W. COMPUTER MODEL GUIDANCE SUGGESTS THAT THIS LOW WILL BE PUSHED EASTWARD DURING THE NEXT DAY AND WEAKEN AS AN UPPER LEVEL RIDGE TO ITS WEST MOVES EASTWARD AND AN UPPER LEVEL JET MAXIMUM PROPAGATES AROUND THE LOW. IN RESPONSE...THE FRONT WILL CONTINUE TO DISSIPATE...WITH THE CORRESPONDING STRONGER WIND FIELD CONTRACTING AND WEAKENING IN INTENSITY. ALSO...WAVEWATCH GUIDANCE SUGGESTS THAT NW SWELL CONTAINING SEAS UP TO 14 FT IS MOVING SOUTHEASTWARD ACROSS THE NW DISCUSSION AREA. AS THIS SWELL DEPARTS IT SOURCE REGION...MAXIMUM SEAS WILL SUBSIDE...THOUGH NORTHWESTERLY SWELL OF 8 TO 11 FT WILL OVERTAKE A LARGE PART OF THE DISCUSSION AREA THROUGH MIDWEEK. BY LATE WEEK...A SURFACE COLD FRONT WILL APPROACH THE NORTHERN PORTIONS OF THE DISCUSSION AREA...BRINGING WINDS UP TO 25 KT EITHER SIDE OF THE FRONT...WITH AN REINFORCEMENT OF HIGHER SEAS. FARTHER TO THE S...THE AXIS OF THE SUBTROPICAL JET STREAM STRETCHES ALONG A LINE FROM 19N140W TO 24N120W TO CENTRAL BAJA CALIFORNIA INTO N MEXICO AND TEXAS...WHERE A MAIN CORE OF 130 KT IS NOTED. THIS JET IS ADVECTING ABUNDANT MID TO UPPER LEVEL MOISTURE EASTWARD ACROSS THE DISCUSSION AREA. SE OF THIS JET...ATMOSPHERIC STABILITY AND DEEP LAYERED DRY AIR IN THE VICINITY OF AN UPPER HIGH NEAR 11N101W IS SUPPORTING FAIR WEATHER ACROSS MUCH OF THE SE PORTION OF THE DISCUSSION AREA. RATIONALE FOR HURRICANE FORCE WIND WARNING IN THE GULF OF TEHUANTEPEC... THE SYNOPTIC SET UP ACROSS THIS REGION IS PARTICULARLY CONDUCIVE FOR THIS HIGH-IMPACT EVENT...WITH A SURFACE RIDGE ALONG THE SIERRA MADRE OCCIDENTALS ANCHORED BY A STRONG 1030 MB SURFACE HIGH CENTERED NEAR 23N98W. THIS RIDGE...COMBINED WITH A SURFACE TROUGH ANALYZED S OF THE GULF OF TEHUANTEPEC...IS YIELDING A STRONG SURFACE PRESSURE GRADIENT OF ABOUT 12 MB IN 150 NM ACROSS THE ISTHMUS OF TEHUANTEPEC. THE SURFACE HIGH AND SURFACE RIDGE ARE IN THE WAKE OF A STRONG COLD FRONT NOW EXTENDING FROM THE SW NORTH ATLANTIC INTO THE NW CARIBBEAN...WHICH ARE ALSO PRECURSORS FOR THE GAP WIND EVENT. WHAT MAKES THIS EVENT PARTICULARLY UNIQUE IS THE STRONG N TO NE LOW LEVEL JET ASSOCIATED WITH THE FRONT. STABLE ATMOSPHERIC PROFILES AT RIDGE-TOP LEVEL ARE FORCING THESE WINDS TO DOWNSLOPE TO THE SURFACE INTO THE GULF OF TEHUANTEPEC...ENHANCING SURFACE WINDS. CORE WINDS AT 925 MB AS ARE REACHING THE 65 TO 70 KT RANGE AS PROJECTED BY THE NAM...AND ARE EXPECTED TO CONTINUE THROUGH 07/1500 UTC. HOWEVER...IT IS UNLIKELY THAT THE FULL INTENSITY OF THIS 925 MB MOMENTUM WILL BE MIXED TO THE SURFACE. HOWEVER...A LARGE PORTION OF THIS MOMENTUM WILL BE TRANSPORTED TO THE SURFACE...ESPECIALLY GIVEN THE STEEP LOW LEVEL LAPSE RATES IN THE GULF OF TEHUANTEPEC. THESE STEEP LAPSE RATES ARE BEING DRIVEN BY STRONG COLD AIR ADVECTION OVER WATERS WITH SEA SURFACE TEMPERATURES IN THE 26 TO 28 DEGREES CELSIUS RANGE. OVERALL...THE OPERATIONAL NAM APPEARS TO BE OUTPERFORMING OTHER COMPUTER MODEL GUIDANCE BASED ON REGIONAL OBSERVATIONS...BOTH IN REGARD TO RECENT MAXIMUM INTENSITY ESTIMATES OF THE WINDS...AS WELL AS THE GEOMETRY OF THE WIND FIELD. AT 07/0600 UTC...A SHIP OBSERVATION / WITH IDENTIFICATION DHEE / WAS REPORTING 55 KT WINDS AT 14.1N 95.7W...WHICH AGREES QUITE WELL WITH THE AVERAGE WINDS IN THE SURFACE TO 975 MB LAYER AS DEPICTED BY THE NAM. THE NAM ALSO DEPICTS THE CORE OF THE STRONGEST WINDS LOCATED 45 TO 60 NM NE OF THIS SHIP REPORT...WHERE MAXIMUM WINDS OF 60 KT ARE FORECAST THROUGH 07/1500 UTC. ADDITIONALLY...AN EARLIER HIGH RESOLUTION QUIKSCAT PASS INDICATED A LARGE AREA OF 50 TO 55 KT WINDS IN THE GULF OF TEHUANTEPEC...FURTHER VERIFYING THE INITIALIZATION OF THE NAM. ONE OF THE FACTORS THAT WILL INCREASE THE THREAT FOR MAXIMUM WINDS TO REACH 65 KT AS OPPOSED TO 60 KT IS ENHANCED NOCTURNAL DRAINAGE FLOW IN THE PRESENCE OF DEEP LAYER DRY AIR EMBEDDED WITHIN A BROAD UPPER LEVEL RIDGE OVER THE SE PORTION OF THE DISCUSSION AREA. THIS WILL INCREASE THE POTENTIAL FOR THE STRONGER 925 MB WINDS TO BE TRANSPORTED TO THE SURFACE AS WELL. $$ COHEN