761  
FXHW01 KWNH 041222  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
722 AM EST MON DEC 04 2017  
 
VALID 00Z TUE DEC 05 2017 - 00Z TUE DEC 12 2017  
 
WITH A MEAN CLOSED UPPER RIDGE CENTERED WEST OF THE INTERNATIONAL  
DATELINE, PERSISTENT NORTHWESTERLY FLOW ALOFT WILL RULE THE  
PATTERN. A SERIES OF SHORTWAVES ARE FORECAST TO TRACK TO THE EAST  
OF THE HAWAI`IAN ISLAND CHAIN WHICH SHOULD AFFORD A COUPLE OF  
FRONTAL PASSAGES THROUGHOUT THE FORECAST PERIOD. ONE SUCH SYSTEM  
WILL PEEL OFF FROM THE PREVAILING WESTERLIES WITH SHORTWAVE ENERGY  
SWINGING SOUTH OF 30N, THUS ALLOWING WEAK HEIGHT FALLS TO GRAZE  
THE REGION. THE NORTHERN EXTENT OF THE UPSTREAM RIDGE SHOULD  
BUCKLE SOMEWHAT WHICH ALLOWS THE MID/UPPER-LEVEL WINDS TO SHIFT TO  
A MORE WEST-NORTHWESTERLY DIRECTION. RENEWED UPPER TROUGHING  
RETURNS TO THE PICTURE EARLY IN THE WEEKEND WHICH AFFORDS THE NEXT  
COLD FRONTAL PASSAGE BEFORE THE FLOW BEGINS TO FLATTEN OUT INTO  
EARLY NEXT WEEK. OVERALL MODEL AGREEMENT IS ABOVE AVERAGE GIVEN  
ONLY MODEST SPREAD AS NOTED IN THE ENSEMBLE SPAGHETTI PLOT. THE  
BIGGEST DISAGREEMENT IS THE DEPTH OF THE TROUGH THIS COMING  
WEEKEND WITH THE 00Z GEFS MEMBERS BEING MORE AMPLIFIED THAN THE  
00Z ECMWF ENSEMBLES. A SPLIT BETWEEN THESE TWO CAMPS APPEARS  
APPROPRIATE HERE.  
 
AT THE SURFACE, AN ABRUPT SHIFT TO NORTH-NORTHEASTERLY FLOW IS  
EXPECTED TODAY WITH FURTHER INTENSIFICATION OF THESE WINDS LATER  
TONIGHT AND INTO TUESDAY. SOME SOLUTIONS FAVOR UP TO 30 TO 35 KNOT  
WINDS AT TIMES WITH EVEN HIGHER SPEEDS OVER THE MOUNTAINOUS  
TERRAIN. THIS IS IN RESPONSE TO THE APPROACHING BAROCLINIC ZONE  
WHICH WILL FORCE BRISK MOVING SHOWERS THROUGH THE ISLAND CHAIN  
THROUGH MIDDAY TUESDAY. IT SHOULD BE FAIRLY DRY THE FOLLOWING  
COUPLE OF DAYS AS A SUB-0.50 INCH PRECIPITABLE WATER AIR MASS  
INFILTRATES THE REGION. LOW-LEVEL WINDS SHOULD DIMINISH  
CONSIDERABLY INTO THE MIDDLE OF THE WEEK AS PRESSURE GRADIENTS  
WEAKEN WITH GENERALLY ONLY 5 TO 10 KNOT NORTHERLY WINDS REMAINING.  
A RETURN OF A HIGHER PRECIPITABLE WATER ENVIRONMENT RETURNS BY  
THURSDAY THROUGH SATURDAY AHEAD OF THE NEXT UPPER TROUGH.  
EVENTUALLY THIS WILL BE SHUNTED SOUTHWARD AS NORTH-NORTHEASTERLY  
WINDS RETURN BY SATURDAY IN RESPONSE TO THE NEXT FRONTAL PASSAGE.  
THIS ALSO AFFORDS THE NEXT RAINFALL CHANCES ALTHOUGH THIS THREAT  
WILL BE SHORT-LIVED AS WELL. LIGHT SOUTHEASTERLY FLOW IS EXPECTED  
BY NEXT WEEK AS NOTED BY THE 00Z GFS/ECMWF.  
 
RUBIN-OSTER  
 

 
 
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