WTPZ45 KNHC 270854 RRA

TCDEP5



TROPICAL STORM SELMA DISCUSSION NUMBER   1

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP202017

400 AM CDT FRI OCT 27 2017



SATELLITE IMAGES INDICATE THAT DEEP CONVECTION HAS BECOME MORE

CONCENTRATED OVER THE PAST SEVERAL HOURS IN THE WESTERN SEMICIRCLE

OF THE LOW OVER THE FAR EASTERN PART OF THE EASTERN PACIFIC.

SCATTEROMETER DATA FROM OVERNIGHT ALSO INDICATE THAT THE LOW HAS

BECOME WELL DEFINED, WITH WINDS OF ABOUT 35 KT. THUS, ADVISORIES ARE

BEING INITIATED ON TROPICAL STORM SELMA.  THIS IS A RARE LOCATION

FOR A TROPICAL STORM TO FORM IN THE EASTERN PACIFIC, AND THIS IS THE

ONLY THE SECOND TROPICAL STORM TO FORM ON RECORD EAST OF 90W THAT

DIDN'T COME FROM AN ATLANTIC CYCLONE (THE FIRST WAS ALMA 2008).



A WEAK RIDGE OF HIGH PRESSURE OVER SOUTHEASTERN MEXICO IS CAUSING

THE STORM TO MOVE SLOWLY NORTHWESTWARD.  HOWEVER, THIS PATTERN IS

FORECAST TO CHANGE QUICKLY AS THE RIDGE BREAKS DOWN DUE TO A LARGE

MID-LATITUDE TROUGH DROPPING INTO THE GULF OF MEXICO DURING THE

NEXT 24 HOURS.  THIS PATTERN SHOULD STEER SELMA TOWARD THE NORTH

OVER THE WEEKEND, ALTHOUGH THE MODELS ARE IN RATHER POOR AGREEMENT

ON EXACTLY WHEN THAT TURN OCCURS.  TO COMPLICATE MATTERS, THERE IS

ALSO SOME CHANCE OF INTERACTION WITH THE CARIBBEAN DISTURBANCE AL93,

WITH THE MODELS SHOWING THE MOST INTERACTION, SUCH AS THE UKMET OR

ECMWF, HAVING TRACKS ON THE EASTERN SIDE OF THE GUIDANCE ENVELOPE.

THE GFS-BASED GUIDANCE SHOWS LITTLE INTERACTION AND SHOWS A TRACK

MORE TOWARD GUATEMALA.  THE NHC FORECAST SPLITS THE DIFFERENCE

BETWEEN THE MODEL CONSENSUS AND THE ECMWF, AND IS JUST EAST OF THE

LATEST NOAA-CORRECTED CONSENSUS, NECESSITATING A TROPICAL STORM