196  
FXHW01 KWNH 211216  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
715 AM EST TUE NOV 21 2017  
 
VALID 00Z WED NOV 22 2017 - 00Z WED NOV 29 2017  
 
MODELS SHOW THE LARGE SCALE PATTERN ACROSS THE CENTRAL/NORTHERN  
PACIFIC CHANGING OVER THE NEXT WEEK. THIS WILL OCCUR AS A REX  
BLOCK WHICH HAS BEEN PERSISTENT FOR WEEKS WELL WEST OF HAWAI'I  
BREAKS DOWN. DETERMINISTIC MODEL SOLUTIONS AS WELL AS ENSEMBLES  
SHOW GOOD AGREEMENT ON THIS SCENARIO. AN ELONGATED AREA OF  
NEGATIVE 500 HPA HEIGHT ANOMALIES HAS BEEN PERSISTENT FROM HAWAI'I  
NORTHEASTWARD TOWARD THE GULF OF ALASKA OVER THE PAST WEEK  
DOWNSTREAM OF THE REX BLOCK. THIS ELONGATED TROUGH IS EXPECTED TO  
GRADUALLY WEAKEN OVER THE NEXT WEEK IN RESPONSE TO THE WEAKENING  
BLOCK.  
 
AT THE SURFACE, HIGH PRESSURE IS FORECAST BY ALL GUIDANCE TO  
REMAIN ANCHORED NORTH OF THE ISLANDS THROUGH THE WEEKEND BEFORE  
DRIFTING EASTWARD EARLY NEXT WEEK AS A WEAKNESS IN THE RIDGE  
DEVELOPS OVER THE CENTRAL PACIFIC, IN THE VICINITY OF HAWAI'I.  
NORTHEASTERLY SURFACE FLOW OF 10-15 KT WILL GRADUALLY VEER TO A  
MORE EAST-NORTHEASTERLY DIRECTION AND STRENGTHEN TO 15-25 KT  
THROUGH LATE THIS WEEK AND INTO THE WEEKEND AS THE SURFACE HIGH  
GRADUALLY SHIFTS EAST. AS LONG AS THE SURFACE RIDGE REMAINS NORTH  
OF THE ISLANDS IT WILL REMAIN A SOURCE OF RELATIVELY DRY AIR, AND  
PRECIPITABLE WATER VALUES SHOULD REMAIN NEAR OR JUST BELOW 1 INCH  
THROUGH SATURDAY. THIS SHOULD KEEP SHOWER ACTIVITY RELATIVELY  
SPARSE AND CONFINED TO WINDWARD TERRAIN. FROM SUNDAY ONWARD, PWAT  
VALUES SHOULD INCREASE AS LOW-LEVEL FLOW CHANGES IN RESPONSE TO  
THE DEVELOPING SURFACE WEAKNESS, PULLING HIGHER VALUES (>2 INCHES)  
NORTHWARD ACROSS THE EASTERN ISLANDS. MODELS DIFFER A BIT WITH  
RESPECT TO THIS MOISTURE RETURN, AND THE ECMWF IS MORE AGGRESSIVE  
WITH INCREASING PWATS EARLY NEXT WEEK (AND THUS SHOWS GREATER  
POTENTIAL FOR HEAVY RAINFALL BY THAT TIME. EC ENSEMBLE  
PROBABILITIES SHOW A SIGNIFICANT DEGREE OF UNCERTAINTY HERE AS  
WELL, BUT THE POTENTIAL EXISTS FOR HEAVY RAINFALL BY EARLY-MID  
NEXT WEEK ACROSS THE BIG ISLAND IF SUFFICIENT MOISTURE RETURN IS  
ABLE TO OCCUR.  
 
RYAN  
 

 
 
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