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ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM TROPICAL STORM IRWIN DISCUSSION NUMBER 2 NATIONAL WEATHER SERVICE MIAMI FL 8 PM PDT FRI OCT 08 1999 RADAR DATA FROM CUYUTLAN MEXICO CLEARLY REVEAL A TIGHT CENTER WITH BANDING FEATURES. THERE IS A CLUSTER OF DEEP CONVECTION NEAR THE CENTER AND T-NUMBERS ARE 3.0 AND 2.5 ON THE DVORAK SCALE. BASED ON THIS INFORMATION...THE TROPICAL DEPRESSION IS BEING UPGRADED TO TROPICAL STORM IRWIN AT THIS TIME. THERE IS SOME EASTERLY SHEAR OVER THE SYSTEM...SO ONLY SLIGHT STRENGTHENING IS INDICATED BEFORE IRWIN REACHES COOL WATERS IN 48 HOURS OR SO. IRWIN HAS BEEN MOVING NORTHWARD BETWEEN 5 AND 8 KNOTS TOWARD THE COAST OF MEXICO DURING THE PAST FEW HOURS. HOWEVER...GLOBAL MODELS DEVELOP A STRONG RIDGE OVER THE SOUTHWESTERN UNITED STATES AND NORTHERN MEXICO. THIS PATTERN WOULD FORCE THE TROPICAL STORM ON A MORE NORTHWEST MOTION SHORTLY AND THEN A WESTERLY TRACK DURING THE NEXT FEW DAYS AS INDICATED BY TRACK GUIDANCE. BECAUSE THIS TURN IS EXPECTED TO OCCUR VERY CLOSE TO THE SOUTHWEST COAST OF MEXICO...A TROPICAL STORM WARNING HAS BEEN ISSUED FOR A PORTION OF THE COAST. AVILA FORECAST POSITIONS AND MAX WINDS INITIAL 09/0300Z 18.5N 105.2W 40 KTS 12HR VT 09/1200Z 19.3N 105.4W 45 KTS 24HR VT 10/0000Z 20.0N 106.5W 50 KTS 36HR VT 10/1200Z 20.5N 108.0W 50 KTS 48HR VT 11/0000Z 20.5N 110.0W 40 KTS 72HR VT 12/0000Z 20.5N 114.5W 30 KTS NNNN