HAWAII EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD 740 AM EDT FRI SEP 28 2012 VALID 00Z SAT SEP 29 2012 - 00Z SAT OCT 06 2012 A BROAD UPPER RIDGE CURRENTLY SPANNING THE CENTRAL PACIFIC AND HAWAIIAN CHAIN WILL SPLIT AS A SERIES OF UPPER LEVEL TROUGHS DIG SOUTHWARD ACROSS THE NORTH CENTRAL PACIFIC OVER THE NEXT SEVERAL DAYS. A TRADE WIND REGIME MAY BE DISRUPTED BY THE STRONG TROUGH CURRENTLY SOUTH OF THE ALEUTIANS DIGGING SOUTHWARD TO NORTH OF THE ISLANDS BY LATE IN THE WEEKEND AND EARLY NEXT WEEK. THERE APPEARS TO BE TWO SEPARATE SYSTEMS ASSOCIATED WITH THIS TROUGH...AN INITIAL IMPULSE THAT APPROACHES THE ISLANDS ON SUNDAY...AND A SECOND REINFORCING SHORTWAVE ON TUESDAY. A STRONG COLD FRONT ASSOCIATED WITH THE FIRST SHORT WAVE WILL REMAIN NORTHWEST OF THE ISLANDS AND WILL LIKELY REMAIN NORTH OF THE AREA THROUGH THE FORECAST PERIOD. THE MAIN AXIS OF THE UPPER RIDGE WILL RETROGRADE TO A POSITION BETWEEN JAPAN AND HAWAII. SEVERAL TROPICAL CYCLONES OVER THE WESTERN PACIFIC EARLY IN THE FORECAST WILL BECOME IMPORTANT FACTORS IN ANY WEATHER SYSTEM THAT CROSSES THE TOP OF THE WESTERN PACIFIC RIDGE ALTHOUGH AT PRESENT THE MAIN EFFECTS WILL PROBABLY REMAIN WELL NORTH OF HAWAII. BOTH THE GFS AND ECMWF HAVE SIMILAR FORECASTS THROUGH NEXT WEEK WITH HAWAII REMAINING WITHIN A WEAKNESS IN THE UPPER LEVEL FLOW AS THE UPPER RIDGE REFORMS TO THE WEST AND THE SIGNIFICANT TROUGH REMAINS NORTH OF THE ISLANDS. BY THE END OF NEXT WEEK...THE EUROPEAN AND CANADIAN ENSEMBLES ARE MORE AMPLIFIED THAN THE GEFS SOLUTIONS IN DRIVING ANOTHER TROUGH AGAIN NORTH OF THE ISLANDS...THOUGH THIS FORECAST IS STILL QUITE UNCERTAIN A WEEK OUT. THE NET EFFECT OF THE SHORTWAVE SYSTEMS IS TO DISRUPT THE EASTERLY TRADE WIND REGIME AND RESULTING IN A WEAK SOUTHEASTERLY FLOW. ANY WEAKENING OF THE FLOW MAY ALLOW A MORE LAND/SEA CONVECTIVE REGIME TO BECOME ESTABLISHED DURING THE DAY RATHER THAN THE USUAL EASTERLY TRADE FLOW. KOCIN