808  
FXHW01 KWNH 161219  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
719 AM EST FRI FEB 16 2018  
 
VALID 00Z SAT FEB 17 2018 - 00Z SAT FEB 24 2018  
 
MODELS/ENSEMBLE SUGGEST A BLOCKY PATTERN DEVELOPING ACROSS THE  
NORTH/CENTRAL PACIFIC OVER THE NEXT SEVERAL DAYS, WITH AN RIDGE  
AMPLIFYING SOUTH OF ALASKA AND PERSISTENT NEGATIVE HEIGHT  
ANOMALIES IN THE VICINITY OF HAWAI'I IN THE FORM OF A COUPLE  
UPPER-LEVEL LOWS. THE FIRST UPPER LOW CLOSES OFF DIRECTLY OVER  
HAWAI'I BY SUN, WITH THE LOW WEAKENING AND A SECOND UPPER LOW  
CLOSING OFF WEST OF THE ISLANDS BY WED. LOW-LEVEL SOUTHERLY FLOW  
WILL TRANSPORT COPIOUS AMOUNTS OF MOISTURE NORTHWARD ACROSS  
HAWAI'I, WITH PWATS OF 1.50-2.0 INCHES BY EARLY NEXT WEEK,  
ESPECIALLY ACROSS THE EASTERN ISLANDS. MODELS SHOW SOME  
DISAGREEMENT ON WHAT HAPPENS BY MID-WEEK AS THE SECOND UPPER LOW  
FORMS, WITH THE ECMWF DEVELOPING A SURFACE WAVE OF LOW PRESSURE  
THAT MOVES EAST OF HAWAI'I, ALLOWING DRIER AIR TO MOVE IN IN ITS  
WAKE, WHILE THE GFS KEEPS THE MOIST AIR MASS IN PLACE FOR LONGER.  
THUS, THE ECMWF SHOWS A PERIOD OF POTENTIAL FOR VERY HEAVY  
RAINFALL AMOUNTS FROM TODAY THROUGH WED (ESPECIALLY ACROSS THE  
EASTERN ISLANDS), WHILE THE GFS KEEPS PRECIPITATION AROUND FOR  
LONGER (ALTHOUGH AMOUNTS ARE NOT AS HEAVY AS THE ECMWF). ECENS  
DATA SUGGEST THE POTENTIAL FOR 24-HOUR RAINFALL TOTALS EXCEEDING 2  
INCHES BECOMING MORE LIKELY ACROSS THE BIG ISLAND BY MON-TUE  
(PROBABILITIES REACHING 40 PERCENT), WITH MULTI-DAY TOTALS OF  
SEVERAL INCHES POSSIBLE. THESE HEAVY RAINS WILL SUPPORT THE  
POTENTIAL FOR SOME AREAS OF FLASH FLOODING ACROSS THE BIG ISLAND.  
ENSEMBLE DATA SHOW SOME POTENTIAL FOR AN ECMWF-LIKE SOLUTION WITH  
A SURFACE WAVE DEVELOPING, BUT THERE IS A RANGE OF SPREAD AS TO  
THE EXACT POSITION/TRACK OF THE SYSTEM. THIS, WHILE A SOLUTION  
RESEMBLING THE ECMWF IS POSSIBLE, CONFIDENCE IN THE SPECIFIC  
DETAILS IS RELATIVELY LOW AT THIS TIME.  
 
IN TERMS OF SURFACE WINDS, FLOW WILL GENERALLY BE  
EAST-SOUTHEASTERLY THROUGH THE WEEKEND, GRADUALLY INCREASING INTO  
EARLY NEXT WEEK. DEPENDING ON WHETHER A SURFACE LOW DEVELOPS BY  
MON NIGHT/TUE (AS THE ECMWF) SHOWS, THIS WILL PLAY A ROLE IN THE  
EVOLUTION OF LOW-LEVEL WINDS BY THAT TIME. IF THE ECMWF SOLUTION  
VERIFIES, WOULD EXPECT INCREASING EAST-SOUTHEASTERLY FLOW (20-30  
KT) BY MON NIGHT, WITH HIGHER GUSTS POSSIBLE AS THE SYSTEM PASSES  
ON TUE, BEFORE A WIND SHIFT TO WESTERLY OR NORTHWESTERLY IN THE  
WAKE OF THE SYSTEM. IF A SURFACE WAVE DOES NOT DEVELOP, EXPECT  
BROAD EAST-SOUTHEASTERLY FLOW TO PERSIST THROUGH MUCH OF THE  
PERIOD.  
 
RYAN  
 

 
 
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