836 
 WTNT45 KNHC 040833
 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
  
  
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