731
WTPZ44 KNHC 241428
TCDEP4
TROPICAL STORM NORMA DISCUSSION NUMBER 7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 AM PDT SAT SEP 24 2005
SEVERAL MICROWAVE SATELLITE IMAGES DURING THE PAST 9 HOURS INDICATE
NORMA HAS MOVED SLOWLY NORTHWEST AND THE CENTER IS LOCATED IN THE
NORTHEASTERN PORTION OF THE DEEP CONVECTIVE CLOUD MASS AND NOT IN
THE CENTER NEAR THE COLDER CLOUD TOPS. THE INITIAL INTENSITY OF 50
KT IS BASED ON A BLEND OF DVORAK CURRENT INTENSITY ESTIMATES OF 55
KT FROM TAFB... 45 KT FROM SAB AND AFWA... AND A 24/1029Z UW-CIMSS
AMSU INTENSITY ESTIMATE OF 994 MB AND 50 KT.
BASED ON THE MICROWAVE POSITIONS... THE INITIAL MOTION IS 340/05.
THE MID-LEVEL RIDGE TO THE NORTH AND EAST OF NORMA IS FORECAST TO
REMAIN WEAK... SO THE RESULTANT WEAK STEERING FLOW MAY CAUSE SOME
ERRATIC MOTION FOR THE NEXT 36 HOURS OR SO. AFTERWARDS... STEERING
CURRENTS ARE EXPECTED TO BECOME BETTER ESTABLISHED AS A STRONG
UPPER-LEVEL TROUGH/LOW DIGS SOUTHWARD OFF THE CALIFORNIA COAST...
WITH ASSOCIATED DOWNSTREAM RIDGING EXPECTED TO BUILD HIGH PRESSURE
ACROSS THE SOUTHWESTERN U.S. AND MEXICO THROUGH 72 HOURS. BY DAYS 4
AND 5... THE FLOW IS ONCE AGAIN FORECAST TO WEAKEN AS THE FUTURE
UPPER-LEVEL LOW OFF CALIFORNIA LIFTS OUT TO THE NORTH... WHICH
ALLOWS WEAK RIDGING TO EXTEND WESTWARD TO THE NORTH OF NORMA. THE
LATTER STEERING FLOW IS EXPECTED TO TURN THE CYCLONE SLOQLY
WESTWARD AND EVENTUALLY WEST-SOUTHWESTWARD.
CURRENT MODERATE EAST TO SOUTHEASTERLY VERTICAL SHEAR IS EXPECTED TO
CONTINUE TO AFFECT NORMA THROUGHOUT THE FORECAST PERIOD. HOWEVER...
THE SHEAR IS NOT FORECAST TO INCREASE ABOVE THE CURRENT 20 KT
VALUE. THEREFORE... THE INTENSITY IS FORECAST TO REMAIN STEADY
UNTIL NORMA REACHES COOLER WATER IN 48-72 HOURS... AFTER WHICH SLOW
WEAKENING IS EXPECTED TO OCCUR. THE OFFICIAL INTENSITY FORECAST IS
SIMILAR TO THE GFDL MODEL... BUT HIGHER THAN THE SHIPS INTENSITY
MODEL... ESPECIALLY AT 96 AND 120 HOURS... WHICH IS WHEN THE SHIPS
MODEL FORECASTS NORMA TO DISSIPATE.
FORECASTER STEWART
FORECAST POSITIONS AND MAX WINDS
INITIAL 24/1500Z 16.4N 110.7W 50 KT
12HR VT 25/0000Z 16.9N 111.0W 50 KT
24HR VT 25/1200Z 17.6N 111.5W 50 KT
36HR VT 26/0000Z 18.5N 112.3W 50 KT
48HR VT 26/1200Z 19.5N 113.5W 45 KT
72HR VT 27/1200Z 20.5N 115.5W 45 KT
96HR VT 28/1200Z 21.0N 117.0W 40 KT
120HR VT 29/1200Z 20.0N 119.5W 35 KT
$$
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