WTPZ45 KNHC 130848 RRA

TCDEP5



TROPICAL DEPRESSION FIFTEEN-E DISCUSSION NUMBER   7

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP152017

200 AM PDT WED SEP 13 2017



THE OVERALL CONVECTIVE PATTERN OF THE DEPRESSION HAS CHANGED LITTLE

SINCE THE PREVIOUS ADVISORY AND REMAINS SHEARED TO THE WEST OF THE

EXPOSED SURFACE CIRCULATION.  DISPLACED INTERMITTENT BURSTS OF DEEP

CONVECTION WITH CLOUD TOPS COLDER THAN -70C YIELD AN INITIAL SHEAR

PATTERN INTENSITY ESTIMATE OF 30 KT, AND THIS IS FURTHER SUPPORTED

BY A 0540 UTC ASCAT-B SCATTEROMETER PASS WHICH SUGGESTS THAT THE

DEPRESSION IS JUST BELOW TROPICAL STORM STRENGTH.  THE DECAY-SHIPS

INTENSITY MODEL AND THE LARGE-SCALE DETERMINISTIC GUIDANCE SHOW THE

SHEAR GRADUALLY DIMINISHING THROUGH THE ENTIRE FORECAST PERIOD.

ACCORDINGLY, THE OFFICIAL FORECAST REFLECTS SLOW STRENGTHENING

THROUGH 3 DAYS, THEN LITTLE CHANGE THEREAFTER, WHICH IS JUST BELOW

THE HCCA, DSHP, AND THE BETTER PERFORMING IVCN MULTI-MODEL

CONSENSUS.



A 0201 UTC WINDSAT IMAGE AND A 0247 UTC AMSU 89 GHZ COMPOSITE

OVERPASS WERE HELPFUL IN ESTIMATING THE POSITION AND THE INITIAL

MOTION WHICH IS WESTWARD, OR 270/8 KT.  THERE IS NO SIGNIFICANT

CHANGE TO THE PREVIOUS TRACK FORECAST OR REASONING.  A WEAKENING

LOW- TO MID-LEVEL SUBTROPICAL RIDGE TO THE NORTH SHOULD STEER THE

DEPRESSION GENERALLY WESTWARD THROUGH 48 HOURS, BUT WITH A SLIGHT

REDUCTION IN SPEED WITH TIME.  BEYOND 48 HOURS, THE CYCLONE BECOMES

TRAPPED IN A MUCH WEAKER STEERING FLOW INDUCED BY A MID-LEVEL RIDGE

BETWEEN THE DEPRESSION, A DEVELOPING SYSTEM TO THE EAST, AND A

BUILDING HIGH FROM THE NORTHWEST.  SUBSEQUENTLY, THE OFFICIAL

FORECAST INDICATES VERY LITTLE MOTION, OR A MEANDER THROUGH DAY 5.