ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 300 PM EDT FRI AUG 05 2016 VALID 12Z TUE AUG 09 2016 - 12Z SAT AUG 13 2016 CHALLENGING MEDIUM RANGE FORECAST --- AS 'OMAIS' BECOMES ENTRAINED IN THE MID-LATITUDE WESTERLIES DURING THE LATTER PORTION OF THE FORECAST PERIOD (DAYS 7-8). NO PROBLEM FOLLOWING CONTINUITY THROUGH THE END OF DAY 5 (11/00Z) BEFORE THE ENSEMBLE MEANS THEMSELVES BREAKDOWN AND BECOME COMPLETELY OUT-OF-PHASE ALONG 145W LONGITUDE AFTER DAY 7 (13/00Z)...IE THE 5/00Z ECENS HAS A FLAT TROUGH AXIS AND THE NAEFS/GEFS MEANS --- A MODERATE AMPLITUDE RIDGE AXIS. THROUGH DAY 5, A BLEND OF THE ECENS/GEFS AND DETERMINISTIC ECMWF WILL SUFFICE AND ALTHOUGH TODAY'S PACKAGE LEANS THE ECENS (60-70 PCT) AND ONLY 20 PCT GEFS MEAN --- THESE RATIOS WILL BE CHANGING IN THE NEXT FEW DAYS AS THE SHORT-TERM TRAJECTORIES AND INTENSITY FORECASTS FOR 'OMAIS' BECOME EVEN BETTER ALIGNED. SEVERAL DAYS AGO, BEFORE 'OMAIS' WAS DEEMED A 'SYSTEM' --- PREVIOUS MEDIUM RANGE TRACK FORECASTS HAD THE ENTITY MOVING INTO THE SEA OF OKHOTSK (WEST SIDE OF KAMCHATKA). AND FORTUNATELY, THIS DOES NOT NOW APPEAR TO BE A PLAUSIBLE SOLUTION. A SHORTWAVE MIGRATING ACROSS NORTHEAST SIBERIA AND CENTRAL BERING SEA WILL EJECT WELL AHEAD OF 'OMAIS' AND REACH THE WEST COAST OF ALASKA AROUND DAY 6. WHAT BECOMES OF THIS DAY-6 SHORTWAVE REMAINS UNCLEAR AND THE END RESULT IS THE 'OUT-OF-PHASE' WAVE PATTERN DOWNSTREAM IN THE GULF OF ALASKA. AS FAR AS 'OMAIS' IS CONCERNED...THE DETERMINISTIC 5/00Z ECMWF GUIDANCE CONTINUES TO OUTPACE THE 5/00Z AND 5/12Z GFS BY ABOUT 3 DEGREES OF LONGITUDE INVOF THE WESTERN TIP OF THE ALEUTIANS (165E-170E LONGITUDE). THE ECMWF SOLUTION LIES IN-BETWEEN THE SLOWER GEFS AND FASTER ECENS ENVELOPE. ON A POSITIVE NOTE...THE ENTIRE ENSEMBLE PACKAGE SHOULD REASONABLY DEPICT THE MEAN 'TRACK' --- A WEST-TO-EAST PROGRESSION --- AND LATER CYCLES SHOULD HELP RE-DEFINE THE LATITUDE OF THE TRACK ACROSS THE NORTH PACIFIC BASIN. VOJTESAK