【午後對流全面爆發 熱帶低壓裸奔北上】
熱帶性低氣壓０３Ｗ持續北上，中心受到強垂直風切影響，高低層分離相當嚴重，可以發現有個旋轉中心快速北上，而對流受到高層東北風影響，無法順利捲入中心，也因此暖心結構無法更有效建立，難以有進一步發展。
　
另方面，太平洋高氣壓減弱加上熱帶低壓外圍水氣影響，台灣上空午後對流雲系全面爆發，許多地區皆出現瞬間強降雨。

FW
2報結束這回合

低層很糟，也沒啥深層對流
老J都編到03W了，小J還在1601 XD

聯合颱風警報中心（美軍ＪＴＷＣ）將熱帶性低氣壓升格為０３Ｗ，下午兩點位於日本琉球南方海域，沿著高壓邊緣朝西北進行，由於所處大氣環境垂直風切偏強，高低層分離相當嚴重，對流雲系多集中在中心西側，也因此未來要有更進一步發展的機會並不高。
　
以目前資料研判，０３Ｗ將朝日本那霸以南近海前進，對台灣不會有直接影響，但其外圍水氣會讓台灣附近午後對流雲系發展更旺盛，雨勢更明顯，東半部也有零星降雨。

JTWC發佈TCFA

WTPN21 PGTW 170400
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/TROPICAL CYCLONE FORMATION ALERT//
RMKS/
1. FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE WITHIN
120 NM EITHER SIDE OF A LINE FROM 19.1N 130.3E TO 23.5N 127.3E
WITHIN THE NEXT 12 TO 24 HOURS. AVAILABLE DATA DOES NOT JUSTIFY
ISSUANCE OF NUMBERED TROPICAL CYCLONE WARNINGS AT THIS TIME.
WINDS IN THE AREA ARE ESTIMATED TO BE 20 TO 25 KNOTS. METSAT
IMAGERY AT 170000Z INDICATES THAT A CIRCULATION CENTER IS LOCATED
NEAR 19.4N 130.0E. THE SYSTEM IS MOVING NORTHWESTWARD AT 10
KNOTS.
2. REMARKS: THE AREA OF CONVECTION PREVIOUSLY LOCATED NEAR 18.3N
131.8E IS NOW LOCATED NEAR 19.4N 130.0E, APPROXIMATELY 435 NM SOUTH-
SOUTHEAST OF KADENA AIR BASE, OKINAWA. RECENT ANIMATED MULTISPECTRAL
SATELLITE IMAGERY SHOWS DEEP CONVECTION PERSISTING OVER THE WESTERN
PORTION OF A CONSOLIDATING, PARTIALLY-EXPOSED LOW LEVEL CIRCULATION
CENTER (LLCC). A 170030Z METOP-B ASCAT IMAGE INDICATES 20 KNOT WINDS
ON THE SOUTHERN AND EASTERN PERIPHERIES OF THE CIRCULATION WRAPPING
INTO THE CENTER. THE UPPER-LEVEL ENVIRONMENT IS MARGINAL, WITH GOOD
WESTWARD DIFFLUENCE AND MODERATE EASTERLY VERTICAL WIND SHEAR THAT
IS EXPECTED TO INCREASE OVER THE NEXT DAY OR TWO. GIVEN INCREASING
ORGANIZATION DURING THE PAST 12 HOURS, IT IS LIKELY THE DISTURBANCE
WILL REACH THE MINIMUM WARNING THRESHOLD INTENSITY OF 25 KNOTS AND
REMAIN A WEAK TROPICAL DEPRESSION FOR A SHORT PERIOD BEFORE
INCREASING VERTICAL SHEAR ULTIMATELY RESULTS IN DISSIPATION. MAXIMUM
SUSTAINED SURFACE WINDS ARE ESTIMATED AT 20 TO 25 KNOTS. MINIMUM SEA
LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1007 MB. BASED ON THE
CONSOLIDATING LOW LEVEL SIGNATURE AND PERSISTENT CONVECTIVE
SIGNATURE, THE POTENTIAL FOR FORMATION FOR A SIGNIFICANT TROPICAL
CYCLONE WITHIN THE NEXT 24 HOURS IS HIGH.
3. THIS ALERT WILL BE REISSUED, UPGRADED TO WARNING OR CANCELLED BY
180400Z.//
NNNN

JMA於17日00Z再升為TD

TROPICAL DEPRESSION 1008 HPA
AT 19.5N 130.1E SEA EAST OF PHILIPPINES MOVING NORTHNORTHWEST 10
KNOTS.

昨天入夜後對流改善不少


可惜受東側較強風切影響，深層對流不斷被颳去西側，讓對流始終無法完全覆蓋低層環流

JMA再度將其升格為T.D

JTWC再度評級，這次直接評Medium

(1) AN AREA OF CONVECTION HAS PERSISTED NEAR 18.3N 131.8E,
APPROXIMATELY 535 NM SOUTH-SOUTHEAST OF KADENA AIR BASE, OKINAWA.
RECENT ANIMATED INFRARED SATELLITE IMAGERY SHOWS DEEP CONVECTION
FLARING OVER THE WESTERN PERIPHERY OF A LOW LEVEL CIRCULATION CENTER
(LLCC) EVIDENT IN A 161158Z ASCAT PASS. THIS DISTURBANCE LIES IN AN
AREA OF MODERATE EASTERLY VERTICAL WIND SHEAR AND WESTWARD
DIFFLUENCE ALOFT. SEA SURFACE TEMPERATURES IN EXCESS OF 30 DEGREES
CELSIUS ARE FAVORABLE FOR FURTHER DEVELOPMENT. ALTHOUGH VERTICAL
WIND SHEAR IS EXPECTED TO INCREASE AS THE LLCC TRACKS POLEWARD OVER
THE NEXT DAY OR TWO, INCREASING ORGANIZATION DRIVEN BY CYCLICAL
CONVECTIVE DEVELOPMENT MAY BE SUFFICIENT TO INCREASE SURFACE WIND
SPEEDS TO THE WARNING THRESHOLD OF 25 KNOTS. MAXIMUM SUSTAINED
SURFACE WINDS ARE ESTIMATED AT 15 TO 20 KNOTS. MINIMUM SEA LEVEL
PRESSURE IS ESTIMATED TO BE NEAR 1009 MB. BASED AN IMPROVING LOW
LEVEL SIGNATURE AND RECENT CONVECTIVE DEVELOPMENT, THE POTENTIAL FOR
FORMATION FOR A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24
HOURS IS MEDIUM.