WTPZ43 KNHC 241433 RRA

TCDEP3



TROPICAL STORM PILAR DISCUSSION NUMBER   4

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP182017

1000 AM CDT SUN SEP 24 2017



PILAR HAS BEEN A DIFFICULT TROPICAL STORM TO ANALYZE AND FORECAST

THIS MORNING.  THE OVERNIGHT SHORT-WAVE AND CLEAN INFRARED CHANNELS

FROM GOES-16 WERE SUGGESTIVE THAT THE SYSTEM HAD ACCELERATED

NORTH-NORTHWESTWARD UNEXPECTEDLY.  HOWEVER, THE AVAILABLE MICROWAVE

IMAGERY WAS AMBIGUOUS AND THE SPREAD BETWEEN THE SAB AND TAFB DVORAK

FIXES WAS A DEGREE.  I'VE RE-POSITIONED PILAR NORTH-NORTHWESTWARD

SIGNIFICANTLY, BUT NOT AS MUCH AS MAY BE NEEDED LATER ONCE

ADDITIONAL VISIBLE IMAGERY BECOMES AVAILABLE.  THE INITIAL MOTION,

THEN, IS A VERY UNCERTAIN 335 DEGREES AT 7 KT, AS PILAR IS BEING

ADVECTED AROUND A DEEP-LAYER RIDGE LOCATED OVER CENTRAL MEXICO.

DESPITE THE INITIAL SPEED UP IN MOVEMENT, THE GUIDANCE INSISTS THAT

PILAR SHOULD SLOW DOWN SHORTLY.  THE OFFICIAL FORECAST TRACK IS

QUITE SIMILAR TO THE PREVIOUS ONE, WITH A COURSE ALONG OR JUST WEST

OF THE COAST OF SOUTHWESTERN MEXICO.  THIS FORECAST IS BASED UPON A

BLEND OF THE GFS, HWRF, AND UKMET MODELS. HOWEVER, THE GUIDANCE HAS

A LARGE SPREAD (AND SOME OF THE MODELS COULD NOT EXPLICITLY TRACK

PILAR), AND A PLAUSIBLE ALTERNATIVE SCENARIO IS THAT THE CENTER OF

PILAR MAKES LANDFALL AND DISSIPATES SHORTLY THEREAFTER. THIS

ALTERNATIVE IS SUPPORTED BY THE ECMWF, COAMPS, AND HMON MODELS.

HOPEFULLY, THE 12Z MODEL RUNS WILL BE MORE CONSISTENT.



THE INITIAL INTENSITY OF PILAR IS UNCHANGED AT 40 KT, AS THE SAB AND

TAFB DVORAK CURRENT INTENSITY CLASSIFICATIONS REMAINED THE SAME.

WHILE THE SSTS ARE WARM AND THE VERTICAL SHEAR IS ONLY MODERATE FOR