Tropical Storm ZETA

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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
 
 
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