EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 103 AM EST SUN NOV 05 2017 VALID 12Z WED NOV 08 2017 - 12Z SUN NOV 12 2017 ...PATTERN OVERVIEW/GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES... A SPLIT FLOW REGIME IS NOTED DURING THE PERIOD WITH A FEW PRIMARY FEATURES TO NOTE. ON WEDNESDAY MORNING...A STRETCHED OUT STRIP OF VORTICITY SLIDING THROUGH THE CENTER OF THE COUNTRY WILL CONTINUE TO ACCELERATE EASTWARD REACHING THE MID-SOUTH BY THE FOLLOWING MORNING. DEPENDING ON THE DEPTH OF THIS SHORTWAVE...SOME WAVES OF LOW PRESSURE SHOULD DEVELOP ALONG THE EXITING WAVY FRONTAL BOUNDARY. THE PRIMARY SURFACE CYCLONE MAY STRENGTHEN OVER THE WESTERN ATLANTIC BUT SHOULD REMAIN FAR ENOUGH OFF THE EASTERN SEABOARD. LOOKING BACK TOWARD THE WEST COAST...A POWERFUL UPPER LOW WILL DROP DOWN FROM THE GULF OF ALASKA WITH FORECAST 500-MB HEIGHT ANOMALIES ON THE ORDER OF 2.5 TO POSSIBLY 3 STANDARD DEVIATIONS BELOW AVERAGE VIA THE 00Z GFS. THIS CLOSED CIRCULATION SHOULD REACH THE NORTHERN CA/PACIFIC NORTHWEST COAST BY LATE WEDNESDAY WHILE GRADUALLY EVOLVING INTO MORE OF AN OPEN WAVE THEREAFTER. A LIKELY PROGRESSIVE STACK OF SHORTWAVES SHOULD ACCELERATE TOWARD THE GREAT PLAINS/MISSISSIPPI RIVER VALLEY BY NEXT WEEKEND. LASTLY...AMPLIFICATION IN THE NORTHERN STREAM WILL POTENTIALLY DRAW A SIGNIFICANT SHOT OF ARCTIC AIR TO SECTIONS OF THE CENTRAL/EASTERN U.S. FROM THURSDAY ONWARD. MODELS EXHIBIT EXTREME DIFFERENCES WITH THIS FEATURE WHICH WILL BE NOTED IN THE FOLLOWING SECTION. LASTLY...AMPLIFIED FLOW RETURNS TO THE EASTERN PACIFIC TOWARD THE LATTER HALF OF NEXT WEEKEND. REGARDING THE INITIAL STRUNG OUT SHORTWAVES MOVING TOWARD THE LOWER TENNESSEE VALLEY...MODELS HAVE BEEN TRENDING TOWARD A SLIGHTLY HIGHER AMPLITUDE WAVE. THIS WOULD INDUCE A MODEST REGION OF LOW PRESSURE ALONG THE FRONT WHICH COULD DEEPEN WHILE MOVING OUT OVER THE WESTERN ATLANTIC. SHIFTING FOCUS TO THE WEST COAST...MULTI-MODEL CYCLE TRENDS SHOW AN EASTWARD JOG WITH THE POSITION OF THE EASTERN PACIFIC UPPER LOW. IT APPEARS THAT THE LARGEST DIFFERENCES ARE WITH REGARD TO THE STRENGTH OF THE DOWNSTREAM RIDGE WITH MANY OF THE 12Z ECMWF ENSEMBLE MEMBERS FAVORING A HIGHER DEGREE OF AMPLITUDE. AS THE PROGRESSIVE SHORTWAVE CONTINUES ITS MARCH TOWARD THE CENTER OF THE COUNTRY...THE SURFACE PATTERN BECOMES COMPLICATED BY EFFECTS FROM THE AMPLIFICATION IN THE NORTHERN STREAM. WITH THE PAST FEW RUNS OF THE ECMWF SHOWING MUCH GREATER SOUTHWARD ELONGATION OF THIS TROUGH...A ROBUST SURFACE ANTICYCLONE SETS UP OVER THE UPPER MIDWEST BY FRIDAY MORNING. MEAN SEA LEVEL PRESSURES WOULD BE AROUND 1036-MB ON 10/1200Z BASED ON THESE ECMWF RUNS. MEANWHILE...THE 00Z/18Z/12Z GFS WOULD SUPPORT A SURFACE CYCLONE IN THIS EXACT POSITION WHICH LEADS TO QUITE A FORECAST CHALLENGE. THE 00Z GFS FROM YESTERDAY WAS THE LAST RUN IN ITS SUITE WHICH DEPICTED THE DEGREE OF AMPLIFICATION SHOWN BY THE ECMWF. UNTIL THE 00Z CMC ARRIVED LOOKING MORE LIKE THE 12Z ECMWF...THE CMC/UKMET HAD NOT OFFERED MUCH RECENT SUPPORT FOR THIS AMPLIFIED SCENARIO. COMPARISON OF MULTI-DAY ENSEMBLE SPAGHETTI PLOTS...WITH THE EXCEPTION OF THE ECMWF ENSEMBLE MEMBERS...THE CMC/GEFS HAVE BEEN WAFFLING AROUND ALL OVER THE PLACE. SUFFICE TO SAY...A LOW CONFIDENCE FORECAST IS AT HAND HERE. FINALLY...ALL GLOBAL ENSEMBLES AGREE ON A RETURN OF LONGWAVE TROUGHING TO THE EASTERN PACIFIC BY NEXT WEEKEND....A GENERAL REPEAT OF WHAT IS IN PLACE ON DAY 3/WEDNESDAY. GIVEN THE RUN-TO-RUN CONSISTENCY EXHIBITED BY THE ECMWF AND ITS ENSEMBLE MEAN...DECIDED TO LEAN MORE TOWARD ITS SOLUTION THROUGHOUT THE PERIOD. AMONG THE GFS CYCLES...FAVORED THE PREVIOUS 00Z RUN AS IT HAD THE MOST IN COMMON WITH THE ECMWF WHILE ALSO STAYING SLIGHTLY CLOSER TO CONTINUITY. GENERALLY TOOK AN EVEN SPLIT OF THESE TWO SOLUTIONS THROUGHOUT THE PERIOD WHILE QUICKLY INCORPORATING A MORE ENSEMBLE BASED APPROACH AFTER FRIDAY. DID THROW A TOKEN AMOUNT OF THE 18Z GFS IN THE MIX TO ACCOUNT FOR ANY SHIFT TOWARD A MORE DE-AMPLIFIED SCENARIO. ...SENSIBLE WEATHER/THREATS... EXCEPT FOR THE SOUTHEASTERN U.S. AS WELL AS THE DESERT SOUTHWEST/FOUR CORNERS REGION...MUCH OF THE COUNTRY WILL BE ENTRENCHED IN BELOW AVERAGE TEMPERATURES THROUGHOUT THE FORECAST. OF COURSE THIS IS HIGHLY DEPENDENT ON THE MODEL CHOICE WITH THE GFS/ECMWF DIVERGING BY ALMOST 30 DEGREES BY DAY 5/FRIDAY. GIVEN A COLD SOLUTION WAS FAVORED...EXPECTED DEPARTURES FROM NORMAL WILL BE AROUND 10 TO 20 DEGREES BELOW AVERAGE AS THE MODIFIED ARCTIC AIR WORKS ITS WAY INTO THE CENTRAL/EASTERN U.S. THE COLD SPOT SHOULD BE FROM NORTHEASTERN MONTANA THROUGH NORTH DAKOTA AND INTO NORTHERN MINNESOTA WHERE LOWS WILL STRUGGLE TO ESCAPE THE SINGLE DIGITS UNTIL FRIDAY. AT THIS TIME...IT DOES NOT APPEAR DAILY TEMPERATURE RECORDS WILL BE BROKEN. THE SLOW-MOVING FRONT STRETCHING FROM THE ARKLATEX INTO THE CAROLINAS WILL BE AN INITIAL FOCUS FOR WIDESPREAD RAINFALL. ALL THIS ACTIVITY WILL SHIFT SOUTHWARD WITH CONDITIONS GRADUALLY IMPROVED TOWARD THE END OF THE WORK WEEK. ACROSS THE WEST COAST...HEAVY PRECIPITATION ACCOMPANIED THE APPROACHING CLOSED LOW WILL IMPACT NORTHERN CA UP ALONG THE PACIFIC NORTHWEST COAST. THE ACTIVITY IS FORECAST TO SHIFT TOWARD THE UPPER INTERMOUNTAIN WEST WITH ENOUGH COLD AIR TO ALLOW SNOWFALL OVER THE HIGHER TERRAIN INCLUDING THE CASCADES...BITTERROOTS...SAWTOOTH...AND TETONS. FARTHER DOWNSTREAM...SOME WARM ADVECTION SHOWERS ARE POSSIBLE OVER THE CENTRAL U.S. ALTHOUGH IT IS UNKNOWN HOW MUCH MOISTURE RETURN THERE WILL BE. AND ACROSS INTERIOR NEW ENGLAND...WHILE MOISTURE WILL LIKELY BE MEAGER...SOME ACCUMULATING SNOWS ARE POSSIBLE ACCOMPANYING THE POTENTIAL ARCTIC INTRUSION. RUBIN-OSTER