735  
WTPZ45 KNHC 270854  
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  
WARNING FOR EL SALVADOR. THIS IS A LOW CONFIDENCE FORECAST DUE TO  
THE LARGE SPREAD IN THE GUIDANCE AND A TROPICAL STORM WATCH OR  
WARNING COULD BE REQUIRED FOR PORTIONS OF THE COAST OF GUATEMALA  
LATER TODAY.  
 
SELMA HAS A DAY OR SO OF VERY WARM WATERS AND LIGHT-TO-MODERATE  
SHEAR IN ITS PATH. THESE CONDITIONS SHOULD PROMOTE STRENGTHENING,  
AND THE NHC INTENSITY FORECAST IS CONSISTENT WITH THE BULK OF THE  
MODELS. IT SHOULD BE EMPHASIZED, HOWEVER, THAT THE MOST SIGNIFICANT  
HAZARD EXPECTED WITH SELMA IS HEAVY RAINFALL. UP TO 10 INCHES OF  
RAIN ARE POSSIBLE OVER EL SALVADOR AND SOUTHERN GUATEMALA, WHICH  
COULD CAUSE LIFE-THREATENING FLASH FLOODS AND MUDSLIDES.  
 
FORECAST POSITIONS AND MAX WINDS  
 
INIT 27/0900Z 10.7N 89.5W 35 KT 40 MPH  
12H 27/1800Z 11.3N 89.9W 40 KT 45 MPH  
24H 28/0600Z 12.5N 90.0W 45 KT 50 MPH  
36H 28/1800Z 13.7N 89.7W 35 KT 40 MPH...INLAND  
48H 29/0600Z...DISSIPATED  
 

 
FORECASTER BLAKE  
 
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