660  
FXHW01 KWNH 301224  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
723 AM EST TUE JAN 30 2018  
 
VALID 00Z WED JAN 31 2018 - 00Z WED FEB 07 2018  
 
A REX BLOCK PATTERN WILL DOMINATE THE NORTH CENTRAL PACIFIC INTO  
THE WEEKEND, BEFORE BEGINNING TO BREAK DOWN TO SOME DEGREE BY  
EARLY NEXT WEEK. WHILE THIS PATTERN REMAINS IN PLACE, AN UPPER LOW  
WILL PERSIST SOUTH OF THE ALEUTIANS, WITH A BROAD TROUGH AXIS  
EXTENDING INTO THE CENTRAL PACIFIC, AND GREATEST NEGATIVE 500 HPA  
HEIGHT ANOMALIES CENTERED ROUGHLY 1000 MILES WEST OF HAWAI'I.  
MODELS AND ENSEMBLES SHOW GENERAL AGREEMENT ON THIS LARGE SCALE  
PATTERN, WHICH SHOULD KEEP A SERIES OF SHORTWAVES TRAVERSING THE  
SOUTHWESTERLY UPPER FLOW NEAR OR JUST WEST OF HAWAI'I. AN  
INITIALLY DRY AIR MASS IN PLACE ACROSS HAWAI'I (PWATS UNDER 1  
INCH) SHOULD MOISTEN QUICKLY BY THE WEEKEND OWING TO DEEP  
SOUTHERLY FLOW EAST OF THE TROUGH, AND PWATS SHOULD RISE INTO THE  
1.50-1.80 INCH RANGE BY SATURDAY. THUS, EXPECT A GRADUAL INCREASE  
IN SHOWER ACTIVITY OVER THE NEXT SEVERAL DAYS, PARTICULARLY FOR  
THE WESTERN ISLANDS WHICH WILL BE IN CLOSEST PROXIMITY TO THE  
STREAM OF UPPER SHORTWAVE ENERGY AND THE BEST UPPER DIFFLUENCE.  
 
AS THE BLOCKING PATTERN BEGINS TO WEAKEN BY SAT NIGHT INTO SUN,  
THERE IS GENERAL AGREEMENT AMONG THE GUIDANCE THAT THE UPPER  
TROUGH SHOULD BEGIN TO PROPAGATE EASTWARD TOWARD HAWAI'I, PERHAPS  
BRINGING A SURFACE COLD FRONT EAST TO THE ISLANDS AS WELL. THE  
ECMWF IS QUICKER TO BRING THE SURFACE FRONT EAST TO HAWAI'I  
RELATIVE TO THE GFS/CMC, AND THE ECENS MEAN IS ALSO SLOWER,  
SUGGESTING THE DETERMINISTIC ECMWF MAY BE A BIT TOO QUICK WITH THE  
FRONT. THESE DIFFERENCES MANIFEST IN THE MODEL QPF FIELDS AS WELL,  
WITH THE ECMWF A BIT QUICKER TO BRING HEAVIER QPF AMOUNTS EAST  
ACROSS THE ISLANDS ON SUN, BUT MOST GUIDANCE DOES SPREAD RAINFALL  
ACROSS MUCH OF HAWAI'I DURING THE SUN-MON TIME FRAME. GIVEN DEEP  
SOUTHERLY FLOW, HIGH PWAT VALUES, AND INCREASINGLY DIFFLUENT UPPER  
FLOW AHEAD OF THE APPROACHING TROUGH BY LATE IN THE WEEKEND, THE  
ENVIRONMENT DOES APPEAR SUPPORTIVE OF POTENTIALLY HEAVY RAINFALL.  
 
MODEL CONSENSUS SUGGESTS THAT THE UPPER TROUGH MAY FURTHER AMPLIFY  
AS IT CROSSES HAWAI'I MON-TUE, WITH THE GFS A BIT STRONGER IN THIS  
RESPECT RELATIVE TO THE ECMWF. BY THIS TIME THE BEST MOISTURE AXIS  
SHOULD SHIFT EITHER TO THE BIG ISLAND OR PERHAPS EVEN EAST OF  
HAWAI'I IF THE GFS SOLUTION WERE TO VERIFY. THUS, THERE IS SOME  
INCREASED FORECAST UNCERTAINTY BY THE MON-TUE TIME FRAME,  
CENTERING PRIMARILY AROUND HOW MUCH UPPER TROUGH AMPLIFICATION CAN  
OCCUR OVER HAWAI'I AND ITS RESULTANT IMPACTS ON PUSHING THE  
SURFACE FRONT/MOISTURE AXIS EASTWARD.  
 
RYAN  
 

 
 
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