076  
FXHW01 KWNH 091229  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
729 AM EST FRI FEB 09 2018  
 
VALID 00Z SAT FEB 10 2018 - 00Z SAT FEB 17 2018  
 
IN THE WAKE OF THE INITIAL LONGWAVE TROUGH EXITING EAST OF THE  
HAWAI`IAN ISLAND CHAIN, A COLD ADVECTION REGIME WILL ENSUE AS  
LOW-LEVEL NORTHERLY FLOW PREVAILS INTO THE WEEKEND. THIS SYSTEM IS  
FORECAST TO CLOSE OFF AND MEANDER A BIT ALONG 150W AS A REX BLOCK  
SETS UP WITH THE CLOSED HIGH BUILDING TO THE NORTH. LOCAL WINDS  
WEAKEN CONSIDERABLY THROUGHOUT THE WEEKEND AS THE PASSAGE OF A  
SURFACE WAVE ENCOURAGES A SHIFT TO THE MORE EASTERLY DIRECTION. A  
TONGUE OF HIGHER PRECIPITABLE WATER AIR WILL PEEL BACK TOWARD  
HAWAI`I RAISING THE CHANCE FOR SCATTERED SHOWERS BY AS EARLY AS  
SUNDAY.  
 
LOOKING INTO NEXT WEEK, THE CIRCULATION EAST OF THE STATE SHOULD  
SHEAR WHILE ALL EYES MOVE UPSTREAM WITH AN IMPRESSIVE LONGWAVE  
TROUGH LOOMING EAST OF THE INTERNATIONAL DATELINE. WHILE THE  
NORTHERN EXTENSION OF THESE HEIGHT FALLS SHOULD REMAIN  
PROGRESSIVE, OPERATIONAL AND ENSEMBLE GUIDANCE SUGGEST ANOTHER  
CLOSED LOW WILL PINCH OFF EAST OF 170W BY LATE MONDAY/EARLY  
TUESDAY. THIS SCENARIO IS AGREED UPON BY ALL GUIDANCE OUTSIDE OF  
THE 00Z UKMET. THE CHANGE IN THE PATTERN WILL LEAD TO A PERSISTENT  
WARM ADVECTION REGIME AS SOUTH-SOUTHEASTERLY LOW-LEVEL FLOW TAKES  
SHAPE LEADING TO MORE SUFFICIENT MOISTURE TRANSPORT OUT OF THE  
EQUATORIAL CENTRAL PACIFIC. BY WEDNESDAY MORNING, THE  
MID/UPPER-LEVEL CIRCULATION SHOULD SPIN JUST WEST OF KAUAI WITH  
THE STRONGEST 564-DM CLUSTERING NOTED BY THE 00Z ECMWF ENSEMBLE  
MEMBERS. WHILE THE OPERATIONAL MODELS AGREE ON THE DEVELOPMENT OF  
THIS SYSTEM, THEY DIVERGE WITH PLACEMENT WHICH WOULD HAVE  
RAMIFICATIONS ON WHERE HEAVY RAINFALL LIKELY WOULD SET UP. PREFER  
TO LEAN IN THE SLOWER DIRECTION GIVEN THE TENDENCY FOR MODELS TO  
BE TOO QUICK IN EJECTING CLOSED CIRCULATIONS. THIS SUPPORTS A  
COMBINATION OF THE 00Z ECMWF/ECMWF ENSEMBLE MEAN LATER IN THE  
PERIOD ALTHOUGH CONFIDENCE BECOMES RATHER LOW AT THIS POINT. THE  
CHOICE OF MODELS WILL OF COURSE ALSO INFLUENCE THE WIND FIELDS AS  
THE SLOWER 00Z ECMWF/ECMWF ENSEMBLE MEAN COMBINATION WOULD ENFORCE  
A LENGTHIER PERIOD OF SOUTHEASTERLY FLOW VERSUS A FRONTAL PASSAGE.  
WHERE AGREEMENT DOES AGREE IS WITH THE HEAVY RAINFALL SIGNAL OVER  
HAWAI`I WITH PRECIPITABLE WATER VALUES LIKELY REACHING THE 1.50 TO  
1.75 INCH RANGE. THE WETTER 00Z ECMWF WOULD SUGGEST SUCH VALUES  
EXCEEDING 2 INCHES WITH SEVERAL INCHES OF RAINFALL POSSIBLE.  
HOWEVER, DETAILS ARE HIGHLY UNCERTAIN, BUT IT IS DEFINITELY  
MONITORING AS FUTURE GUIDANCE ARRIVES.  
 
RUBIN-OSTER  
 

 
 
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