Tropical Storm LAN Advisory Tue Oct 17

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 25W (LAN)
WARNING NR 04//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 25W (LAN), LOCATED APPROXIMATELY 220 NM WEST-
NORTHWEST OF YAP, FSM, HAS TRACKED WEST-NORTHWESTWARD AT 11 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) SHOWS DEEP CONVECTIVE BANDS FROM THE SOUTHERN AND EASTERN
QUADRANTS HAVE MOSTLY MAINTAINED DEPTH AND DEGREE OF WRAP INTO A MORE
DEFINED LOW-LEVEL CIRCULATION CENTER. THE INITIAL POSITION IS PLACED
WITH FAIR CONFIDENCE BASED ON THE MSI LOOP AND ON A LOW REFLECTIVITY
CIRCULATION FEATURE IN THE 160403Z NPP ATMS MICROWAVE PASS. THE
INITIAL INTENSITY OF 35 KNOTS IS SUPPORTED BY A DVORAK ESTIMATE OF
T2.5 FROM PGTW, KNES, AND RJTD. UPPER-LEVEL ANALYSIS INDICATES THE
SYSTEM IS IN AN AREA OF MODERATE VERTICAL WIND SHEAR (20-25 KNOTS)
THAT IS MOSTLY OFFSET BY GOOD RADIAL OUTFLOW. ADDITIONALLY, SEA
SURFACE TEMPERATURES (SSTS) REMAIN CONDUCIVE AT 30 DEG CELSIUS AS THE
SYSTEM TRACKS POLEWARD IN THE PERENNIALLY WARM PHILIPPINE SEA ALONG
THE WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) TO THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TS 25W WILL CONTINUE TO TRACK NORTHWESTWARD UP TO TAU 24.
AFTERWARD, THE SYSTEM IS EXPECTED TO TURN MORE NORTHWARD AS THE STR
RECEDES IN RESPONSE TO A MID-LATITUDE TROUGH DIGGING THROUGH JAPAN.
FAVORABLE ENVIRONMENTAL CONDITIONS WILL PERSIST AND ALLOW FOR STEADY
INTENSIFICATION. BY TAU 72, TS 25W IS EXPECTED TO STRENGTHEN TO A
MODERATE TYPHOON AT 90 KNOTS.
   C. AFTER TAU 72, TS LAN WILL CONTINUE ITS NORTHWARD TRACK UNDER
THE STEERING INFLUENCE OF THE STR, WITH A SLIGHT NUDGE TOWARD THE
NORTH-NORTHEAST AT LATER TAUS. GIVEN THE WARM SSTS AND FAVORABLE
UPPER-LEVEL CONDITIONS, A PERIOD OF RAPID INTENSIFICATION REMAINS A
POSSIBILITY AS THE SYSTEM GAINS LATITUDE AND UPPER-LEVEL OUTFLOW IS
ENHANCED BY STRONG WINDS AHEAD OF THE MID-LATITUDE TROUGH. TS 25W IS
FORECAST TO REACH 110 KNOTS BY TAU 120. GLOBAL MODELS ARE GENERALLY
IN GOOD AGREEMENT WITH NAVGEM AS THE ERRATIC OUTLIER TO THE LEFT OF
THE MODEL ENVELOPE, A POSSIBLE BINARY INTERACTION WITH A CYCLONE
FORECAST TO DEVELOP TO THE SOUTHWEST. FURTHERMORE, THE GFS AND
NAVGEM MODEL FIELDS SUGGEST ANOTHER CIRCULATION COULD FORM ON THE
EASTERN PERIPHERY OF TS 25W, WHICH WOULD FURTHER COMPLICATE THE
MOTION DYNAMICS. OVERALL, THERE IS GOOD CONFIDENCE IN THE EARLY TO
MID PORTION OF THE JTWC TRACK FORECAST, BUT LOW CONFIDENCE
AFTERWARD.//
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