EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 151 AM EDT FRI SEP 29 2017 VALID 12Z MON OCT 02 2017 - 12Z FRI OCT 06 2017 ...OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... SYNOPTIC-SCALE FLOW ACROSS THE COUNTRY TO COMMENCE THE MONTH OF OCTOBER WILL BE RATHER AMPLIFIED FEATURING SOME IMPRESSIVE MID-LEVEL HEIGHT ANOMALIES. BASED ON THE 12Z GFS...500-MB HEIGHTS SHOULD BE AROUND 2 TO 2.5 STANDARD DEVIATIONS BELOW/ABOVE AVERAGE RELATIVE TO CLIMATOLOGY FOR EACH KEY ANOMALY CENTER. BELOW AVERAGE HEIGHTS ARE FORECAST TO DOMINATE THE WESTERN U.S. DURING THE ENTIRE PERIOD AS AT LEAST A COUPLE OF UPPER LOWS INHABIT THE REGION. SOME EMBEDDED SHORTWAVES ARE EXPECTED TO LIFT NORTHEASTWARD AND BECOME ABSORBED INTO AN ACTIVE NORTHERN STREAM ACROSS CENTRAL/EASTERN CANADA. THE GENERAL IDEA WILL BE FOR THE INITIAL CLOSED LOW TO RETROGRADE FROM THE CENTRAL GREAT BASIN BACK TOWARD THE DESERT SOUTHWEST THROUGH AT LEAST MID-WEEK. THERE SHOULD BE GRADUAL FORWARD PROGRESSION BY LATER NEXT WEEK IN RESPONSE TO AN INTENSIFYING JET STREAM ACROSS THE NORTHEASTERN PACIFIC. THE OTHER PERSISTENT ANOMALY ON THE MAP WILL BE AN EXPANSIVE UPPER RIDGE WHICH WILL INITIALLY SETTLE EAST OF THE MS RIVER VALLEY. BY 03/1200Z...MANY OPERATIONAL MODEL RUNS...PARTICULARLY THE GFS...FAVOR A 594-DM RIDGE OVER A VAST MAJORITY OF THE EASTERN U.S. ON TUESDAY. A GRADUAL WESTWARD TRANSLATION OF THIS UPPER RIDGE IS NOTED BRINGING ITS POSITION CLOSER TO THE CENTRAL U.S. MODELS ARE NOT QUITE AS AGGRESSIVE WITH THE STRENGTH OF THIS RIDGE WITH THE 594-DM HEIGHT CONTOUR DISAPPEARING TOWARD THE END OF THE PERIOD. LOOKING TO THE ACTIVE NORTHERN STREAM...SOME OF THE AMPLIFIED FLOW MAY AFFECT THE GREAT LAKES AND UPPER NEW ENGLAND. HOWEVER...IT WILL LIKELY JUST BE A GRAZING BLOW AS THE TIGHTEST GRADIENT REMAINS NORTH OF THE U.S./CANADIAN BORDER. COMPLEX FLOW ACROSS THE EASTERN PACIFIC APPEARS TO HAVE FINALLY BEEN RESOLVED AFTER NOTING SIGNIFICANT CHANGES IN ENSEMBLE SPAGHETTI PLOTS THE PAST COUPLE OF DAYS. WHILE A STRONG UPPER RIDGE WAS EXPECTED TO INHABIT THE EASTERN PACIFIC ON MONDAY...SOLUTIONS OTHER THAN THE ECMWF CONTINUED TO SHOW SHORTWAVE ENERGY UNDERCUTTING THIS STOUT RIDGE. WHILE POSSIBLE...THE 12Z/00Z MODEL CYCLES SHOW MOST ENSEMBLE MEMBERS MOVING TOWARD THE CONSISTENT 00Z ECMWF ENSEMBLES. TOWARD MID-WEEK...A GENERAL POSITIVELY-TILTED TROUGH APPEARS TO BE WELL ESTABLISHED ACROSS THE SOUTHWESTERN U.S. INTO THE ADJACENT CENTRAL GREAT BASIN. MODELS SEEM TO STRUGGLE MOST WITH HOW MUCH SHORTWAVE ENERGY WILL ADJOIN THE NORTHERN STREAM. THE 12Z ECMWF WAS MOST EMPHATIC ABOUT THIS WHICH STOOD OUT AS AN OUTLIER RELATIVE TO THE OPERATIONAL/ENSEMBLE CONSENSUS. PREFERRED USE OF THE PRECEDING RUN OF THE 00Z ECMWF AS IT TUCKED MORE OF THE ENERGY BACK INTO THE MEAN WESTERN U.S. TROUGH. BY DAY 6/THURSDAY...MODELS VARY IN HOW QUICKLY TO PUSH THIS TROUGH AXIS EASTWARD WITH A TREND TOWARD THE EAST CONSIDERING MULTI-DAY ENSEMBLE SPAGHETTI PLOTS. HOWEVER...DID FEEL IT WAS PRUDENT TO STAY A BIT SLOWER GIVEN THE WELL KNOWN BIAS OF MODELS BEING TOO QUICK TO EJECT QUASI-STATIONARY UPPER LOWS. AS MENTIONED PREVIOUSLY...UTILIZED THE PREVIOUS 00Z ECMWF/ECMWF ENSEMBLE MEAN OVER THE 12Z VERSION GIVEN DIFFERENCES IN HANDLING OF THE WESTERN U.S. TROUGH. FELT THE 18Z GFS/GEFS MEAN IN CONJUNCTION WITH THE MENTIONED ECMWF SOLUTIONS WOULD PROVIDE A SOLID PATH TO FOLLOW. GENERALLY UTILIZED A 50/50 COMBINATION OF THIS DUO WHILE INCREASING THE WEIGHT OF ENSEMBLE MEANS DEEPER INTO THE FORECAST. BY DAY 7/OCTOBER 6...WENT SOLELY ENSEMBLE MEANS GIVEN DIMINISHING CONFIDENCE IN SYNOPTIC-SCALE DETAILS. ...SENSIBLE WEATHER/HIGHLIGHTS... UNDERNEATH THE CORRIDOR OF LOWER HEIGHTS ACROSS THE WEST...WELL BELOW NORMAL TEMPERATURES WILL BE THE NORM DURING MUCH OF NEXT WEEK. FORECAST HIGHS ARE EXPECTED TO BE AROUND 15 TO 20 DEGREES BELOW CLIMATOLOGY FROM THE CENTRAL GREAT BASIN INTO THE UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. THIS WILL ULTIMATELY KEEP HIGH TEMPERATURES IN THE 40S WITH EVEN A POCKET OF SOME MID/UPPER 30S OVER THE HIGHER ELEVATIONS OF WESTERN WY INTO WESTERN/CENTRAL MT. ON THE CONVERSE...MILD TEMPERATURES WILL BE FEATURED EAST OF THE ROCKIES AS HIGHS RISE WELL INTO THE 80S AS FAR NORTH AS SECTIONS OF THE GREAT LAKES. EXPECT LOCAL OVERNIGHT MINIMA INTO THE LOWER 60S ACROSS THE MIDDLE MS VALLEY/GREAT LAKES WHICH IS ROUGHLY 15 DEGREES ABOVE NORMAL FOR EARLY OCTOBER. ALONG A WAVY FRONTAL ZONE ACROSS THE NORTHERN PLAINS/UPPER MIDWEST....A FOCUS FOR MULTIPLE DAYS OF PRECIPITATION WILL BE IN PLACE. THE BOUNDARY SHOULD MEANDER THROUGHOUT THE PERIOD BUT WILL NOT APPRECIABLY MOVE EASTWARD UNTIL THE UPPER TROUGH IS ABLE TO EXIT TOWARD THE GREAT PLAINS. COLDER TEMPERATURES ALOFT TOWARD THE HIGHER TERRAIN OF WY/MT WILL BRING A THREAT FOR WINTRY PRECIPITATION. ELSEWHERE...SUFFICIENT RETURN FLOW FROM THE GULF OF MX SHOULD BRING DAILY CHANCES FOR SHOWERS AND THUNDERSTORMS ACROSS THE GREAT PLAINS. SOME MORE ORGANIZED COMPLEXES ARE POSSIBLE IN ADVANCE OF ANY MEANINGFUL SHORTWAVE ACROSS THE WEST. AND ACROSS FL...IT WILL LIKELY BE A WET PERIOD ACROSS THE SOUTHERN EXTENT OF THE STATE AS AN AREA OF DISTURBED WEATHER DRIFTS NORTHWARD FROM CUBA. THIS PARTICULAR SYSTEM IS BEING MONITORED FOR TROPICAL DEVELOPMENT BY THE NATIONAL HURRICANE CENTER WITH ODDS AROUND 50/50 AT THIS POINT. RUBIN-OSTER