784  
FXHW01 KWNH 221205  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
704 AM EST WED NOV 22 2017  
 
VALID 00Z THU NOV 23 2017 - 00Z THU NOV 30 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 10-20 KT WILL GRADUALLY VEER TO A  
MORE EASTERLY DIRECTION AND STRENGTHEN TO 15-25 KT OVER THE NEXT  
FEW DAYS 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 (NEAR 2 INCHES) NORTHWARD ACROSS  
THE EASTERN ISLANDS. MODELS DIFFER A BIT WITH RESPECT TO THIS  
MOISTURE RETURN, AND IN CONTRAST TO YESTERDAY, THE GFS IS NOW MORE  
AGGRESSIVE THAN THE ECMWF WITH INCREASING PWATS EARLY NEXT WEEK,  
AND THUS SHOWS GREATER POTENTIAL FOR HEAVY RAINFALL BY THAT TIME.  
DESPITE THE WAVERING SHOWN BY THE DETERMINISTIC ECMWF WITH RESPECT  
TO MOISTURE RETURN EARLY NEXT WEEK, EC ENSEMBLE PROBABILITIES  
SUGGEST SOMEWHAT INCREASED CONFIDENCE FOR HEAVY RAINFALL, WITH  
48-HOUR PROBABILITIES (ENDING 00Z WED) OF EXCEEDING 2 INCHES OF  
RAIN HAVING INCREASED TO NEAR 30 PERCENT ACROSS THE WINDWARD  
TERRAIN OF THE BIG ISLAND. THE GFS AND ECMWF ARE IN AGREEMENT THAT  
ANOTHER PUSH OF DRY AIR WILL MAKE ITS WAY ACROSS HAWAI'I BY  
WEDNESDAY, BRINGING AN END TO ANY THREAT FOR HEAVY RAINFALL.  
 
RYAN  
 

 
 
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