134  
FXHW01 KWNH 071218  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
718 AM EST THU DEC 07 2017  
 
VALID 00Z FRI DEC 08 2017 - 00Z FRI DEC 15 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, WITH THE  
EXCEPTION OF THE CMC WHICH IS LESS AMPLIFIED AND QUICKER TO  
PROGRESS THE WAVE EAST. THE ASSOCIATED SURFACE FRONT SHOULD REACH  
THE NORTHERN ISLANDS ON SATURDAY AND THEN SWEEP ACROSS THE  
REMAINING ISLANDS SATURDAY NIGHT. 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 NORTH-NORTHWESTERLY LOW-LEVEL  
WINDS ALONG THE FRONT. IN THE WAKE OF THE FIRST FRONT, ANOTHER  
ROUND OF VERY DRY AIR SHOULD MOVE ACROSS THE STATE, WITH PWATS  
DECREASING TO WELL BELOW 1 INCH SUNDAY INTO MONDAY. A SECOND  
SHORTWAVE AMPLIFIES NORTH OF THE STATE ON TUESDAY, WITH GUIDANCE  
ONCE AGAIN SHOWING DECENT CONFIDENCE IN THE TIMING/AMPLITUDE. THE  
ASSOCIATED SURFACE FRONT REACHES HAWAI'I STARTING MONDAY NIGHT AND  
CROSSING THE ISLANDS ON TUESDAY. MODELS SOLUTIONS BEGIN TO DIFFER  
BY LATE NEXT WEEK, WITH THE ECMWF DEVELOPING A WAVE ALONG THE  
FRONT WHICH CAUSES THE FRONT TO STALL FOR A TIME ACROSS THE  
EASTERN ISLANDS. HIGHER PWATS ASSOCIATED WITH THIS SECOND FRONT  
(1.50-1.80 INCH) WILL RESULT IN GREATER PRECIP COVERAGE, SO A  
POTENTIALLY STALLING FRONT AS SHOWN BY THE ECMWF WOULD RESULT IN  
THE POTENTIAL FOR HEAVY RAINFALL ACROSS THE EASTERN ISLANDS ON  
WEDNESDAY. THE GFS DIFFERS FROM THE ECMWF AND PUSHES THE FRONT  
THROUGH THE STATE QUICKLY, WITH DRY AIR ONCE AGAIN INVADING THE  
STATE BY NEXT THURSDAY. ECMWF ENSEMBLE PROBABILITIES SHOW 10-20  
PERCENT PROBABILITIES OF RAINFALL TOTALS EXCEEDING 1 INCH ACROSS  
THE EASTERN ISLANDS ASSOCIATED WITH THIS SYSTEM.  
 
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  
NORTH-NORTHWESTERLY AND SOUTH-SOUTHWESTERLY IN AHEAD OF/BEHIND  
EACH FRONT AS IT PASSES THROUGH THE STATE.  
 
RYAN  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page