776  
FXHW01 KWNH 191202  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
702 AM EST MON FEB 19 2018  
 
VALID 00Z TUE FEB 20 2018 - 00Z TUE FEB 27 2018  
 
A WET PATTERN APPEARS SET TO CONTINUE ACROSS HAWAI'I THROUGH MUCH  
OF THE WEEK. A SURFACE BOUNDARY CONTINUES TO LINGER ACROSS THE  
STATE EARLY THIS MORNING. AN UPPER LOW THAT DEVELOPED OVER THE  
PAST 24 HOURS ESSENTIALLY OVER HAWAI'I HAS, FOR THE MOMENT, PUSHED  
THE DEEPEST MOISTURE PLUME JUST EAST OF THE ISLANDS. THE LARGE  
SCALE PATTERN ACROSS THE CENTRAL PACIFIC IS NOT EXPECTED TO CHANGE  
APPRECIABLY OVER THE NEXT WEEK. WELL TO THE NORTH OF HAWAI'I, A  
BLOCKING RIDGE IS EXPECTED TO WEAKEN SLIGHTLY, BUT WILL REMAIN  
PREVALENT. SOUTH OF THE RIDGE, THE PATTERN WILL CONTINUE TO FAVOR  
THE DEVELOPMENT AND PERSISTENCE OF UPPER-LEVEL LOWS. MODELS SHOW  
THE AFOREMENTIONED UPPER LOW ACROSS HAWAI'I AS RELATIVELY  
TRANSIENT, WITH MOST GUIDANCE SHOWING IT DISSIPATING BY MID-WEEK  
AS ANOTHER, DEEPER UPPER LOW DEVELOPS FARTHER TO THE WEST. MODELS  
SHOW A GENERAL CONSENSUS THAT THE SURFACE BOUNDARY OVER THE STATE  
THIS MORNING WILL CONTINUE TO DRIFT WESTWARD AND WEAKEN AS THE  
INITIAL UPPER LOW WEAKENS. AS THIS SECOND UPPER LOW DEVELOPS BY  
LATE IN THE WEEK, A RIDGE AXIS IS FORECAST TO DEVELOP JUST EAST OF  
HAWAI'I. THE COMBINATION OF THESE TO FEATURES WILL BRING A  
RESUMPTION OF DEEP SOUTHERLY FLOW, AND WILL SUPPLY THE REGION WITH  
ABUNDANT MOISTURE TO SUPPORT CONVECTION ONCE AGAIN. MEANWHILE,  
SURFACE FLOW REMAINS EAST-SOUTHEASTERLY DUE TO A STRONG SURFACE  
HIGH WELL NORTHEAST OF HAWAI'I. LOW-LEVEL ESE FLOW SHOULD  
GRADUALLY STRENGTHEN THROUGH THE WEEK AND INTO NEXT WEEKEND AS THE  
AFOREMENTIONED SURFACE HIGH DRIFTS SOUTHWARD, STRENGTHENING THE  
PRESSURE GRADIENT ACROSS THE CENTRAL PACIFIC. LOOKING OUT TO THE  
BEGINNING OF NEXT WEEK, A NUMBER OF MODEL SOLUTIONS SHOW THE UPPER  
LOW WEST OF HAWAI'I STARTING TO BREAK DOWN AS A LARGER UPPER  
TROUGH AMPLIFIES FARTHER TO THE WEST, BRINGING THE UPPER LOW UNDER  
THE INFLUENCE OF GRADUALLY INCREASING WESTERLIES. MODELS SHOW  
VARYING SOLUTIONS AS TO HOW QUICKLY THIS OCCURS, AND CONFIDENCE IN  
THE SPECIFICS AT THIS TIME RANGE IS LOW.  
 
BOTH THE ECMWF AND THE GFS SUGGEST THE BRIEF LULL IN CONVECTIVE  
ACTIVITY CONTINUING INTO TUE AS THE INITIAL UPPER LOW WEAKENS AND  
THE SURFACE BOUNDARY DRIFTS WEST OF THE STATE. BUT A RESUMPTION IN  
THE POTENTIAL FOR CONVECTION AND HEAVY RAINFALL IS SHOWN FROM THU  
ONWARD AS THE UPPER LOW TAKES HOLD FARTHER WEST, AND DEEPER  
MOISTURE ONCE AGAIN BEGINS TO STREAM ACROSS HAWAI'I EAST OF THE  
LOW.  
 
RYAN  
 

 
 
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