919  
FXHW01 KWNH 091228  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
728 AM EST SAT DEC 09 2017  
 
VALID 00Z SUN DEC 10 2017 - 00Z SUN DEC 17 2017  
 
AN ACTIVE WEATHER PATTERN WILL REMAIN IN PLACE ACROSS THE CENTRAL  
PACIFIC OVER THE NEXT WEEK, WITH A COUPLE SHORTWAVES AMPLIFYING  
NORTH OF HAWAI'I, AND COLD FRONTS AFFECTING THE STATE DURING THE  
PERIOD. THE FIRST SHORTWAVE WILL SKIRT NORTH OF THE STATE TODAY  
AND TONIGHT, PUSHING A WEAK COLD FRONT ACROSS THE ISLANDS. THE AIR  
MASS AHEAD OF THE FRONT CONTINUES TO BE RELATIVELY DRY, AND MODELS  
SHOW ONLY MODEST MOISTURE POOLING ALONG THE BOUNDARY (PWATS NEAR  
1.20"). AS A RESULT, ISOLATED TO SCATTERED SHOWER ACTIVITY WILL  
ACCOMPANY THE FRONTAL PASSAGE, PRIMARILY FOCUSED ON NORTH FACING  
TERRAIN GIVEN THE NORTHERLY WINDS ASSOCIATED WITH THE FRONTAL  
PASSAGE. A COUPLE DAYS OF RELATIVELY QUIET WEATHER WILL FOLLOW THE  
FIRST FRONT BEFORE A SECOND UPPER SHORTWAVE AMPLIFIES NORTH OF THE  
STATE MON-TUE. MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD  
AGREEMENT ON THE INITIAL AMPLIFICATION OF THIS SYSTEM. GUIDANCE  
HAS ALSO COME INTO BETTER AGREEMENT IN SHOWING THE WAVE RAPIDLY  
AMPLIFYING AND THEN AN UPPER LOW CUTTING OFF NEAR OR EVEN OVER  
HAWAI'I BY THU. AFTER CUTTING OFF, THERE IS SOME DISAGREEMENT  
AMONG THE GUIDANCE ON IF AND HOW QUICKLY THE FEATURE WOULD  
RETROGRADE WESTWARD, WITH THE 00Z ECMWF THE MOST AGGRESSIVE IN  
THIS, WHILE THE GFS/CMC ARE A BIT FARTHER EAST.  
ECENS/GEFS/NAEFS/CMC ENSEMBLE MEANS ARE ALL IN EXCELLENT AGREEMENT  
IN SHOWING A POSITION FAIRLY CLOSE TO WHAT IS SHOWN BY THE 06Z  
GFS, THUS WILL PREFER THE GFS HANDLING OF THIS FEATURE AT THIS  
TIME. THE DEGREE TO WHICH THE UPPER LOW RETROGRADES WILL HAVE  
MAJOR IMPLICATIONS FOR THE AMOUNT OF PRECIPITATION THAT OCCURS  
ACROSS HAWAI'I. EXPECT THE FRONTAL BOUNDARY TO STALL SOUTH/EAST OF  
HAWAI'I AS THE UPPER LOW CUTS OFF, WITH SOME POTENTIAL FOR DRIER  
AIR TO OVERSPREAD THE STATE. IF THE ECMWF SOLUTION WERE TO VERIFY  
AND THE UPPER LOW TO RETROGRADE MORE QUICKLY, THIS WOULD ALLOW  
HIGHER PWATS TO BE ADVECTED NORTH ACROSS THE STATE MORE QUICKLY.  
THIS IS SHOWN IN THE MODEL QPF, WITH THE ECMWF SOLUTION SHOWING  
MORE POTENTIAL FOR CONVECTION ACROSS HAWAI'I LATE NEXT WEEK  
RELATIVE TO THE GFS. ECENS PROBABILITIES CONTINUE TO SHOW ONLY A  
10-15 PERCENT CHANCE FOR QPF EXCEEDING 1 INCH OVER A 48-HOUR  
PERIOD ENDING AT 00Z SAT, SUGGESTING THAT THE DETERMINISTIC ECMWF  
SOLUTION IS NOT NECESSARILY THE MOST LIKELY ONE AT THIS TIME  
(ALTHOUGH IT STILL CANNOT BE RULED OUT). REGARDLESS, EXPECT  
ANOTHER PERIOD OF SHOWER ACTIVITY TUE NIGHT/WED AS THE FRONT  
CROSSES THE STATE, WITH A MORE UNCERTAIN FORECAST AFTER THAT TIME,  
LARGELY DEPENDING ON THE EXACT EVOLUTION AND TRACK OF THE UPPER  
LOW.  
 
RYAN  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page