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| #63927 (Received by flhurricane at: 4:17 PM 01.Jan.2006) TCDAT5 TROPICAL STORM ZETA DISCUSSION NUMBER 10 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 4 PM EST SUN JAN 01 2006 ALTHOUGH THE CONVECTION IS NOT VERY DEEP...ZETA'S CLOUD PATTERN HAS BECOME MORE SYMMETRIC OVER THE PAST FEW HOURS. THIS SUGGESTS THAT THE VERTICAL SHEAR MAY HAVE...AT LEAST TEMPORARILY...RELAXED SLIGHTLY. DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB...ALONG WITH AMSU ESTIMATES FROM THE UNIVERSITY OF WISCONSIN CIMSS... CONTINUE TO SUPPORT A CURRENT WIND SPEED OF 45 KT. GLOBAL MODEL GUIDANCE INDICATES THAT WESTERLY SHEAR SHOULD INCREASE SOON AND THIS WILL LIKELY CAUSE WEAKENING. THE SHIPS MODEL...USING THE GFS OUTPUT...SHOWS VERTICAL SHEAR OF 50 KT ON ZETA WITHIN 12 HOURS. THIS IS LIKELY AN EXAGGERATION CAUSED BY AVERAGING OVER TOO LARGE OF A HORIZONTAL AREA AROUND THE STORM AND...POSSIBLY...TOO DEEP OF A VERTICAL LAYER. THE SHIPS MODEL CALCULATES THE SHEAR FROM THE 850 TO THE 200 MB LEVEL. PHASE SPACE ANALYSES FROM FLORIDA STATE UNIVERSITY SHOW THAT ZETA HAS A RATHER SHALLOW WARM CORE SO IT MAY NOT HAVE AS DEEP A VERTICAL EXTENT AS MOST TROPICAL CYCLONES...AND IT MAY NOT BE AS RESPONSIVE TO SHEAR PRODUCED BY UPPER-TROPOSPHERIC...I.E. NEAR 200 MB...WINDS AS A TYPICAL TROPICAL STORM. THE OFFICIAL FORECAST DOES NOT WEAKEN THE CYCLONE AS RAPIDLY AS SHIPS. ZETA HAS BEEN MEANDERING IN A PARTIAL COUNTERCLOCKWISE LOOP TODAY...PERHAPS ASSOCIATED WITH A BROADER-SCALE MID-TROPOSPHERIC CYCLONIC CIRCULATION. ASSUMING THAT THE STORM WEAKENS AS FORECAST...THE SYSTEM SHOULD BEGIN TO RESPOND MORE TO THE EAST-NORTHEASTERLY FLOW ON THE SOUTH SIDE OF A WEAK AND NARROW LOW- TO MID-LEVEL RIDGE. THE CONSENSUS OF THE TRACK MODEL GUIDANCE GOES ALONG WITH THAT IDEA...AS DOES THE OFFICIAL FORECAST. FORECASTER PASCH FORECAST POSITIONS AND MAX WINDS INITIAL 01/2100Z 25.0N 38.2W 45 KT 12HR VT 02/0600Z 24.7N 39.2W 40 KT 24HR VT 02/1800Z 24.5N 40.3W 35 KT 36HR VT 03/0600Z 24.4N 41.4W 30 KT 48HR VT 03/1800Z 24.4N 42.4W 25 KT...REMNANT LOW 72HR VT 04/1800Z...DISSIPATED |
