485  
FXHW01 KWNH 201230  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
729 AM EST WED DEC 20 2017  
 
VALID 00Z THU DEC 21 2017 - 00Z THU DEC 28 2017  
   
..HEAVY RAIN THREAT THROUGH THURSDAY
 
 
A DEEP UPPER TROUGH OVER THE CENTRAL PACIFIC WILL KEEP CONDITIONS  
WET OVER THE HAWAI`IAN ISLAND CHAIN THROUGH THURSDAY. ABUNDANT  
MOISTURE POOLING ALONG THE PARENT SURFACE BOUNDARY CONTINUES TO BE  
A CONDUIT TO ACTIVE CONVECTION, PARTICULARLY OFFSHORE OF MAUI  
WHERE THUNDERSTORMS WERE OBSERVED JUST EAST OF THE ISLAND. THE  
FRONTAL ZONE WILL BE IN THE PROCESS OF SWEEPING SOUTHWARD AS  
ATTENDANT MID-LEVEL HEIGHT FALLS PROJECT EASTWARD. CONSEQUENTLY, A  
COLD ADVECTION REGIME IS ANTICIPATED INTO THE EARLY PART OF THE  
WEEKEND WITH NORTHERLY LOW-LEVEL PREVAILING. AT ITS PEAK ON  
THURSDAY/FRIDAY, WIND SPEEDS MAXIMIZE AROUND 20 TO POSSIBLY 25  
KNOTS BEFORE SLACKENING IN RESPONSE TO A WEAKENING PRESSURE  
GRADIENT. OVER THE WEEKEND, THE MID/UPPER-LEVEL PATTERN IS  
FORECAST TO SHIFT TO A MORE WEST-NORTHWESTERLY DIRECTION AS THE  
STRONGER JET ENERGY RESIDES UP OVER THE MID-LATITUDES. IT SHOULD  
BE A QUIET PERIOD IN TERMS OF PRECIPITATION GIVEN THE  
SEMI-PERSISTENT LOW-LEVEL NORTHERLY TO NORTHEASTERLY FLOW.  
 
MOVING INTO NEXT WEEK, IN ADVANCE OF UPSTREAM AMPLIFICATION IN THE  
VICINITY OF THE INTERNATIONAL DATELINE, LOW-LEVEL WINDS TURN  
EASTERLY LEADING TO AN INCREASE IN WINDWARD SHOWERS ACROSS  
HAWAI`I. AT THIS POINT, MODELS HAVE BEEN IN REASONABLE AGREEMENT  
UNTIL THEY DIVERGE WITH THE APPROACH OF THE NEXT POTENT TROUGH.  
ENSEMBLE SPAGHETTI PLOTS AGREE ON AN EVEN MORE AMPLIFIED SYSTEM  
THAN THE ONE CURRENTLY IN PLACE. WHILE THE 06Z GFS/00Z ECMWF/00Z  
CMC ALL SHOW THIS SCENARIO UNFOLDING, THE 00Z ECMWF AS WELL AS ITS  
PREVIOUS RUN FAVOR AN UPPER LOW CUTTING OFF JUST WEST OF THE STATE  
BY DECEMBER 26-27. THIS LATTER EVOLUTION WOULD FAVOR AN IMPRESSIVE  
PERIOD OF HEAVY RAINFALL OVER THE NORTHERN MOST ISLANDS. HOWEVER,  
OTHER SOLUTIONS ARE MORE PROGRESSIVE AND WOULD KEEP THIS AXIS OF  
PRECIPITATION TO THE NORTH. REGARDLESS, OF SOLUTION, WARM  
ADVECTION SHOULD PICK UP CONSIDERABLY AS BRISK SOUTHEASTERLY FLOW  
ENSUES. AN EVENTUAL FRONTAL PASSAGE IS LIKELY BUT THE TIMING IS  
HIGHLY MODEL DEPENDENT. WHILE THE 00Z ECMWF IS PLAUSIBLE, IT DOES  
NOT HAVE A WHOLE LOT OF ENSEMBLE SUPPORT EXCEPT FROM ITS OWN  
MEMBERS. IT DEFINITELY BARES WATCHING GIVEN THE NOTED HEAVY  
RAINFALL SIGNATURE.  
 
RUBIN-OSTER  
 

 
 
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