WTPZ43 KNHC 232042 RRA

TCDEP3



TROPICAL DEPRESSION EIGHTEEN-E DISCUSSION NUMBER   1

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP182017

400 PM CDT SAT SEP 23 2017



ASCAT-A/-B SCATTEROMETER DATA FROM AROUND 16-17Z INDICATED THAT THE

LOW PRESSURE SYSTEM LOCATED JUST OFFSHORE OF THE SOUTHWESTERN COAST

OF MEXICO THAT THE NHC HAS BEEN TRACKING THE PAST SEVERAL DAYS HAS

BECOME MUCH BETTER DEFINED, AND IT ALSO POSSESSED SURFACE WINDS OF

30-33 KT. AS A RESULT, THE SYSTEM HAS BEEN UPGRADED TO A TROPICAL

DEPRESSION, THE EIGHTEENTH OF THE EASTERN PACIFIC HURRICANE SEASON.



THE INITIAL MOTION ESTIMATE IS 335/04, BASED PRIMARILY ON MICROWAVE

SATELLITE FIXES. THE LATEST NHC MODEL GUIDANCE IS IN FAIRLY GOOD

AGREEMENT ON THE CYCLONE MOVING SLOWLY IN A GENERAL

NORTH-NORTHWESTWARD DIRECTION AROUND THE WESTERN PERIPHERY OF A

DEEP-LAYER RIDGE FOR THE NEXT 5 DAYS. SOME OF THE MODELS LIKE THE

GFS, CANADIAN, AND HCCA TAKE THE SYSTEM JUST INLAND NEAR CABO

CORRIENTES IN ABOUT 24 HOURS, WHEREAS THE REMAINDER OF THE GUIDANCE,

ESPECIALLY THE UKMET AND ECMWF, KEEP THE CYCLONE JUST OFFSHORE OF

THE SOUTHWESTERN COAST OF MEXICO. THE FORECAST MOTION FOR THE NEXT

FEW DAYS IS EXPECTED TO BE 5 KT OR LESS, AN INDICATION THAT STEERING

CURRENTS WILL BE WEAK. SINCE THERE IS NO STRONG FORCING THAT WOULD

WANT TO DRIVE THE DEPRESSION INLAND OVER THE MOUNTAINOUS TERRAIN OF

SOUTHWESTERN MEXICO, THE OFFICIAL FORECAST CALLS FOR THE CENTER OF

THE CYCLONE TO REMAIN JUST OFFSHORE OF THE COAST THROUGHOUT THE

FORECAST PERIOD, SIMILAR TO THE ECMWF AND UKMET SOLUTIONS.



GIVEN THE WELL-DEFINED CIRCULATION NOTED IN THE AFOREMENTIONED

SCATTEROMETER DATA, ALONG WITH VERTICAL WIND SHEAR DECREASING TO