EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 259 AM EDT SAT AUG 16 2014 VALID 12Z TUE AUG 19 2014 - 12Z SAT AUG 23 2014 ...OVERVIEW... UPPER-LEVEL JET INTENSIFICATION ACROSS THE NORTHERN PACIFIC LEADS TO THE TRANSITION FROM HIGH-AMPLITUDE TROUGH OVER EASTERN CANADA AND THE NORTHEAST US--TO A MORE ZONAL AND FLATTER LOW AMPLITUDE/PROGRESSIVE PATTERN FOR THE THIS MEDIUM RANGE PERIOD. IN THE WEST AND NORTH CENTRAL US...THE PATTERN WILL BE DOMINATED BY A PROGRESSIVE LONGWAVE TROUGH TAKING SHAPE AS THE UPPER-LEVEL JET BEGINS TO AMPLIFY AND CARVE A LONGWAVE TROUGH ACROSS MUCH OF WESTERN CANADA AND THE NORTHWEST QUARTER OF THE LOWER 48 BEYOND DAY 5. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... AFTER SIFTING THROUGH THE POSSIBILITIES AND THE REALIZATION THAT A BLOCKY PATTERN DOWNSTREAM OVER ATLANTIC CANADA AND GREENLAND WILL CREATE A TEMPORARY SPLIT-FLOW PATTERN ACROSS THE LOWER 48 THIS FORECAST PERIOD...THOUGHT THE 15/12Z ECENS MEAN HAS BEEN DECIDEDLY--THE MORE CONSISTENT PIECES OF GUIDANCE. FOLLOWING THIS--ALONG WITH THE 15/12Z NAEFS--SEEMS TO BEST FIT THE DEVELOPING LOW-AMPLITUDE PATTERN AND WPC SFC/500MB GRAPHICS FROM WEST TO EAST THROUGH THE ENTIRE DAY 3-7 PERIOD. THOUGHT THIS FORECAST PACKAGE DISPLAYED A VERY SUBTLE AND INTRIGUING PATTERN SHIFT FOR THE LOWER 48. BEST WAY TO DESCRIBE THE CHANGES--THE PERSISTENT COOL PATTERN IN THE NORTHEASTERN US BECOMES CLOUDIER WITH HIGHER TEMPERATURES AND HUMIDITY. AND THE RELATIVELY DRY NORTHWEST STATES BECOME COOLER AND WETTER. A LOCALLY-ACTIVE MONSOON FLOW FOR PARTS OF THE SOUTHWEST (AZ NM CO) AND IN THE MID-SECTION OF THE COUNTRY...A PACIFIC COLD FRONT AND APPRECIABLY MOIST AND COOLER WEATHER PATTERN MIGRATES FROM THE NORTHERN DIVIDE EASTWARD INTO THE UPPER HALF OF THE MISSOURI RIVER VALLEY FOR DAYS 6-7. DOWNSTREAM OF THE ROCKIES...WHAT SIGNIFICANT DIFFERENCES EMERGING FROM THIS PATTERN TRANSITION DURING THE MEDIUM RANGE PERIOD...APPEARED TO FOLLOW THE INTENSITY AND LATITUDE OF THE DEVELOPING SPLIT-FLOW PATTERN AND 'WEAKER' SOUTHERN BRANCH OF THE JET. THE UNCERTAINTY AND EVENTUAL OUTCOME WILL DICTATE THE DEPTH...SPEED AND DOWNSTREAM TRAJECTORIES OF THE INDIVIDUAL LOW-AMPLITUDE WAVES MIGRATING ACROSS THE LOWER 48. WHAT SEEMS INTRIGUING TO ME IS HOW MUCH RUN-TO-RUN VARIABILITY EXISTS AND WHERE THIS NARROW 50KT-70KT 'SOUTHERN SPLIT' JET AXIS EXITS THE CONTINENTAL DIVIDE. THIS LEADS TO A VARIETY OF PERPLEXING RIDGE/TROUGH SOLUTIONS (LOW-AMPLITUDE WAVES MIND YOU)...ORIGINATING ANYWHERE FROM NORTH CENTRAL WYOMING TO SOUTH CENTRAL COLORADO. CONSEQUENTLY THIS LEADS TO A RATHER BROAD RANGE OF DAY 4-5 AND DAY 5-6 QPF POSSIBILITIES ALONG AND EAST OF 98W LONGITUDE BETWEEN SOUTHEAST DAKOTA AND NORTHEAST KANSAS STARTING AROUND THE DAY 5 MID-POINT (21/12Z). IN THE WEST...CAN LIVE WITH A BLEND OF THE 15/12Z DETERMINISTIC CANADIAN...GFS AND ECMWF HEADING INTO DAY 4--20/12Z--WITH A MID-LEVEL CYCLONIC CIRCULATION DRIFTING OFF THE CENTRAL CALIFORNIA COAST BETWEEN CAPE MENDOCINO...PT REYES AND THE BAY AREA. THEN TAKING A SOUTHWESTWARD TRACK AND HEADING WELL OUT TO SEA--WEST OF POINT CONCEPCION. AFTER DAY 4 AND INTO DAY 5...THAT LOOKED REASONABLE. WHAT DOESNT FIT THE ENSEMBLE MEANS--HAS TO BE THE 15/00Z AND 16/00Z DETERMINISTIC GFS RUNS AND THE 16/00Z CANADIAN. THE CONSISTENT AND 'TRACK-ABLE' SYNOPTIC-SCALE FEATURE FOR DAYS 5-7 CONTINUES TO BE THE SIGNIFICANT HEIGHT FALL MIGRATION FROM THE BC/WASHINGTON COAST (THE AMPLIFYING UPPER-LEVEL LONGWAVE TROUGH) TO THE HIGH PLAINS OF EASTERN MONTANA AND WYOMING. THESE 'OUTLIER' GFS/CANADIAN RUNS LEAVE ME WITH LITTLE CHOICE BUT TO RECOMMEND A GENERAL COMPROMISE APPROACH USING THE 15/12Z NAEFS-ECENS AND IF ITS TO YOUR LIKING...THE 18Z GEFS MEAN BLENDED EQUALLY TO COVER THE DIFFERENCES ALONG THE WEST COAST. ...SENSIBLE WEATHER HIGHLIGHTS... WET AND STORMY WEATHER WILL ACCOMPANY THE SURFACE WAVE TRACKING THROUGH THE OHIO VALLEY/MID-ATLANTIC STATES EARLY NEXT WEEK...WITH PERIODS OF HEAVY RAINS AND STRONG THUNDERSTORMS POSSIBLE. SHOWERS AND THUNDERSTORMS ARE ALSO EXPECTED WITH THE SHORTWAVE DROPPING INTO THE UPPER MIDWEST/GREAT LAKES AND AT THE LEADING EDGE OF HEIGHT FALLS DIGGING INTO THE NORTHWEST...WHICH SHOULD IMPACT THE NORTHERN PLAINS BY LATE NEXT WEEK. THE RELATIVELY FLAT AND PROGRESSIVE FLOW OVER THE CONUS SHOULD LIMIT ANY LARGE TEMPERATURE EXTREMES. IN GENERAL...TEMPERATURES WILL BE ABOVE NORMAL SURROUNDING THE CENTRAL ROCKIES/HIGH PLAINS...BENEATH MODEST RIDGING ALOFT. ALSO...WITH MEAN TROUGHING SETTING UP OVER THE WESTERN U.S. LATE NEXT WEEK...HIGHS AND LOWS SHOULD BE BELOW NORMAL. VOJTESAK/GERHARDT