EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1130 AM EDT WED JUL 16 2014 VALID 12Z SAT JUL 19 2014 - 12Z WED JUL 23 2014 ...OVERVIEW... A WET PATTERN IS EXPECTED FROM THE LOWER MISSISSIPPI VALLEY EASTWARD THROUGH THE MEDIUM RANGE PERIOD AS UNSTABLE AND VERY MOIST CONDITIONS REMAIN IN VICINITY OF ONE OR MORE SLOW MOVING FRONTAL BOUNDARIES...WHILE A WARMING TREND DEVELOPS ACROSS THE INTERMOUNTAIN WEST/CENTRAL-SOUTHERN ROCKIES...A COOLING TREND FORMS ACROSS THE PACIFIC NORTHWEST...AND A QUASI-STATIONARY FRONTAL BOUNDARY DRAPES JUST SOUTH OF THE CANADIAN BORDER. ...MODEL PREFERENCES AND CONFIDENCE... THE FORECAST CHALLENGES ON DAY 3/SAT INCLUDE RESOLVING THE GROWING SPREAD ACROSS BRITISH COLUMBIA THAT IS A CULMINATION OF SEPARATE STREAMS AFFECTED BY THE BLOCKING PATTERN OVER ALASKA...AS WELL AS THE LONGWAVE TROUGH OVER THE MISSISSIPPI RIVER VALLEY. THE CONDITIONS ASSOCIATED WITH THE SHORTWAVE TROUGH REACHING BRITISH COLUMBIA WARRANT AN ENSEMBLE MEAN APPROACH UNTIL THE SYSTEMS CAN BE BETTER INITIALIZED BY GUIDANCE...WHILE ACROSS THE MISSISSIPPI RIVER VALLEY A SOLUTION SOMEWHERE BETWEEN THE WEAKER 00Z GFS AND STRONGER 00Z ECMWF IS PREFERRED... ALTHOUGH AN INTERMEDIATE SOLUTION INVOLVING THE ENSEMBLE MEANS WOULD WORK NEARLY EQUALLY WELL. THE NEXT FORECAST CHALLENGE DEVELOPS ACROSS THE PACIFIC NORTHWEST ON DAY 5/MON. THE 00Z ECMWF/CANADIAN LIE NEAR THE DEEP EDGE OF THE GUIDANCE AND THEREFORE ARE CONSIDERED LEAST PROBABLE AT THIS TIME...WHILE THE 00Z GFS SHOWS GOOD AGREEMENT WITH PHASE AND INTENSITY WITH THE ENSEMBLE MEANS...AND THEREFORE IS PREFERRED ALONG WITH THE MEANS FROM DAY 5 ONWARD. CONFIDENCE IS IN CONCERT WITH THE SOLUTION SPREAD...WHICH IS ABOUT AVERAGE FOR DAYS 3/4...AND BELOW AVERAGE FROM DAY 5 AND BEYOND AS THE SPREAD AND POTENTIAL ERROR INCREASE. ...SENSIBLE WEATHER HIGHLIGHTS... THE WEATHER HIGHLIGHTS INCLUDE THE LIKELIHOOD FOR PERSISTENT AND POSSIBLY HEAVY RAINFALL FROM THE LOWER MISSISSIPPI RIVER VALLEY EASTWARD THROUGH MUCH OF THE PERIOD...INCLUDING THE CENTRAL GULF COAST/SOUTHEAST/APPALACHIANS....WHILE HIGH TEMPERATURES STEADILY INCREASE ACROSS THE INTERMOUNTAIN WEST AND CENTRAL/SOUTHERN ROCKIES. JAMES