952  
FXHW01 KWNH 081226  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
726 AM EST FRI DEC 08 2017  
 
VALID 00Z SAT DEC 09 2017 - 00Z SAT DEC 16 2017  
 
A PERSISTENT AREA OF NEGATIVE 500 HPA HEIGHT ANOMALIES (-3 TO -4  
STANDARD DEVIATIONS) NORTH OF HAWAI'I WILL SUPPORT THE PASSAGE OF  
A COUPLE COLD FRONTS THROUGH THE ISLANDS OVER THE NEXT WEEK. FIRST  
SHORTWAVE AMPLIFIES NORTH OF HAWAI'I ON SATURDAY. MODELS/ENSEMBLES  
SHOW GENERALLY GOOD AGREEMENT SURROUNDING THIS SYSTEM. THE  
ASSOCIATED SURFACE FRONT SHOULD REACH THE NORTHERN ISLANDS ON  
SATURDAY AND THE GFS AND ECMWF NOW BOTH SHOW THE FRONT STALLING  
AND DISSIPATING ACROSS HAWAII SATURDAY NIGHT INTO SUNDAY. A  
RELATIVELY DRY AIR MASS WILL BE IN PLACE ACROSS THE STATE AHEAD OF  
THE FRONT (PWATS NEAR OR < 1 INCH), WITH MODEST MOISTURE (PWATS  
NEAR 1.25 INCH) POOLING ALONG THE FRONT. THUS, EXPECT SHOWER  
ACTIVITY TO BE GENERALLY LIMITED TO NORTHWARD FACING TERRAIN GIVEN  
THE NORTHERLY LOW-LEVEL WINDS ALONG THE FRONT. A SECOND SHORTWAVE  
AMPLIFIES NORTH OF THE STATE ON TUESDAY, WITH GUIDANCE ONCE AGAIN  
SHOWING RELATIVELY MINOR DIFFERENCES WITH RESPECT TO THE  
TIMING/AMPLITUDE. THE ASSOCIATED SURFACE FRONT REACHES HAWAI'I  
EARLY TUESDAY MORNING AND CROSSES THE STATE ON TUESDAY.  
SIGNIFICANT UNCERTAINTY REMAINS ON WHETHER THE FRONT WILL CLEAR  
THE STATE OR POTENTIALLY STALL AS A WAVE DEVELOPS NEAR THE STATE.  
ENSEMBLE SUPPORT IS GROWING FOR A 500-HPA LOW TO CLOSE OFF JUST  
NORTH OF HAWAI'I BY LATE IN THE WEEK, WHICH WOULD SUPPORT SOME  
SORT OF STALLING/WAVE DEVELOPMENT ALONG THE FRONT, PERHAPS JUST  
SOUTH/EAST OF HAWAI'I. THE ECMWF HAS BACKED OFF THE IDEA OF  
STALLING THE FRONT DIRECTLY OVER HAWAI'I WED-THU AND NOW RESEMBLES  
THE GFS ON PUSHING THE FRONT THROUGH THE ISLANDS ON THURSDAY. THE  
ECMWF DIFFERS FROM THE GFS AFTER THAT, HOWEVER, DEVELOPING A WAVE  
ALONG THE FRONT EAST OF THE ISLANDS THAT RETROGRADES BACK ACROSS  
THE STATE ON FRIDAY, WHILE THE GFS CONTINUES TO PUSH THE FRONT  
QUICKLY THROUGH HAWAI'I ALLOWING MUCH DRIER AIR TO MOVE IN.  
ENSEMBLE SPREAD SURROUNDING THIS WAVE EVOLUTION IS QUITE LARGE,  
ALTHOUGH MANY ECENS MEMBERS DO SHOW A FRONTAL WAVE DEVELOPING IN  
THE VICINITY. NONETHELESS, HIGHER PWATS ASSOCIATED WITH THIS  
SECOND FRONT (1.50-1.80 INCH) WILL RESULT IN GREATER PRECIP  
COVERAGE AND THE POTENTIAL FOR LOCALLY HEAVY RAINFALL. ECMWF  
ENSEMBLE PROBABILITIES CONTINUE TO SHOW 10-20 PERCENT  
PROBABILITIES OF RAINFALL TOTALS EXCEEDING 1-2 INCHES ACROSS THE  
EASTERN ISLANDS ASSOCIATED WITH THIS SYSTEM. AT THIS TIME,  
SIGNIFICANT UNCERTAINTY REMAINS SURROUNDING THE EXACT EVOLUTION OF  
THIS FRONTAL BOUNDARY AS IT REACHES/CROSSES HAWAI'I NEXT WEEK, AND  
THE POTENTIAL FOR SOME HEAVY RAINFALL ACROSS THE EASTERN ISLANDS  
REMAINS.  
 
LOW-LEVEL EASTERLIES WILL BE SUPPRESSED SOUTH OF HAWAI'I THROUGH  
THE NEXT WEEK, THE RESULT OF THE NEGATIVE HEIGHT ANOMALIES ACROSS  
THE CENTRAL PACIFIC AND THE FREQUENT FRONTAL INCURSIONS INTO THE  
REGION. SURFACE FLOW ACROSS HAWAI'I WILL SWITCH BETWEEN  
NORTHERLY/SOUTHERLY COMPONENTS AHEAD OF/BEHIND EACH FRONT AS IT  
PASSES THROUGH THE STATE.  
 
RYAN  
 

 
 
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