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| #64525 (Received by flhurricane at: 4:17 AM 04.Jan.2006) TCDAT5 TROPICAL STORM ZETA DISCUSSION NUMBER 20 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 4 AM EST WED JAN 04 2006 ZETA CONTINUES TO GENERATE CONVECTIVE TOPS OF -55C TO -60C NEAR THE LOW-LEVEL CENTER....WITH RAGGED OUTER BANDING OVER THE NORTHEASTERN SEMICIRCLE. SOME ANTICYCLONIC OUTFLOW WAS APPARENT IN CIRRUS CLOUD MOTIONS EARLIER. HOWEVER...THIS HAS DECREASED OVER THE PAST FEW HR. SATELLITE INTENSITY ESTIMATES CONTINUE TO RANGE FROM 65 KT AT TAFB TO 45 KT AT AFWA AND ARE ESSENTIALLY UNCHANGED FROM 6 HR AGO. THUS...THE INITIAL INTENSITY REMAINS 55 KT. ZETA HAS MOVED MORE TOWARD THE SOUTH AND FASTER OVER THE PAST 6-12 HR...LIKELY DUE TO INTERACTION WITH AN UPPER-LEVEL LOW SEEN IN WATER VAPOR IMAGERY NEAR 21N38W. HOWEVER...THE LAST FEW IMAGES SUGGEST A MORE WESTWARD MOTION...AND THE INITIAL MOTION IS THUS A SOMEWHAT UNCERTAIN 220/6. ZETA IS SOUTH OF A MEAN DEEP-LAYER RIDGE OVER THE CENTRAL ATLANTIC...WHILE A MEAN DEEP-LAYER TROUGH IS OVER THE WESTERN ATLANTIC. A SHORTWAVE TROUGH AND SURFACE FRONT ARE CURRENTLY BETWEEN THE EASTERN UNITED STATES AND BERMUDA...WHILE LARGE-SCALE MODELS SUGGEST A SECOND STRONGER SHORTWAVE WILL MOVE INTO THE WESTERN ATLANTIC AFTER 48 HR. THE NEW TROUGH WILL CAUSE THE FIRST SHORTWAVE TROUGH TO MOVE NORTHEASTWARD NORTH OF ZETA... WITH THE RIDGE RE-BUILDING IN BETWEEN THE TROUGHS. THIS EVOLUTION SHOULD CAUSE THE STORM TO TURN NORTHWESTWARD DURING THE NEXT 48-72 HR...THEN TURN MORE WESTWARD THEREAFTER AS THE RIDGE RE-BUILDS. THE DYNAMICAL MODELS GENERALLY AGREE ON THIS...WITH THE EXCEPTION OF THE UKMET WHICH CALLS FOR RECURVATURE ALONG 48W IN 48-72 HR. THE OFFICIAL FORECAST WILL FOLLOW THE SCENARIO SUGGESTED BY THE OTHER DYNAMICAL MODELS...AND IS SIMILAR TO BUT WEST OF THE PREVIOUS FORECAST. THE NEW FORECAST IS IN BEST AGREEMENT WITH THE GFDL. ZETA IS WELL EMBEDDED IN UPPER-LEVEL WESTERLIES BETWEEN THE SUBTROPICAL AND POLAR JET STREAMS...WHICH OVERALL IS NOT A VERY FAVORABLE ENVIRONMENT. THE LARGE-SCALE MODELS SUGGEST A SERIES OF UPPER-LEVEL SHORT-WAVE FEATURES WILL PASS OVER ZETA DURING THE NEXT 48 HR...MOST NOTABLY THE SOUTHERN END OF THE CURRENT WESTERN ATLANTIC TROUGH WHICH SHOULD REACH THE STORM IN ABOUT 36 HR. THIS PATTERN SHOULD PRODUCE SOME SURGES AND LULLS IN VERTICAL SHEAR... AND BASED ON THIS THE INTENSITY FORCAST CALLS FOR SLOW WEAKENING FOR 48 HR. AFTER THAT...A BUILDING UPPER-LEVEL RIDGE WEST OF ZETA SHOULD CAUSE 40-50 KT UPPER-LEVEL NORTHWESTERLY FLOW OVER THE STORM...WHICH SHOULD CAUSE SIGNIFICANT WEAKENING. THEREFORE...THE INTENSITY FORECAST SHOWS ZETA WEAKENING TO A DEPRESSION BY 72 HR AND DISSIPATING BY 120 HR. IT SHOULD BE NOTED THAT THE GFDL STILL WANTS TO MAKE ZETA A HURRICANE...AND THERE IS A SMALL CHANCE THE THE APPROACHING SHORTWAVE TROUGH COULD BRIEFLY PROVIDE A MORE FAVORABLE ENVIRONMENT FOR STRENGTHENING IN ABOUT 36 HR. FORECASTER BEVEN FORECAST POSITIONS AND MAX WINDS INITIAL 04/0900Z 22.1N 43.1W 55 KT 12HR VT 04/1800Z 21.9N 43.8W 55 KT 24HR VT 05/0600Z 22.4N 45.2W 55 KT 36HR VT 05/1800Z 23.9N 46.3W 50 KT 48HR VT 06/0600Z 25.5N 46.9W 45 KT 72HR VT 07/0600Z 28.0N 49.0W 30 KT 96HR VT 08/0600Z 30.0N 53.0W 20 KT...REMNANT LOW 120HR VT 09/0600Z...DISSIPATED |
