EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST MON NOV 06 2017 VALID 12Z THU NOV 09 2017 - 12Z MON NOV 13 2017 ...PATTERN OVERVIEW/MODEL GUIDANCE/UNCERTAINTIES/PREFERENCES... THE MEDIUM RANGE PERIOD FEATURES A FAIRLY ACTIVE WEATHER PATTERN WITH A SPLIT FLOW REGIME IN PLACE. TO BEGIN...ON THURSDAY/DAY 3...SHORTWAVE ENERGY IN THE SOUTHERN STREAM WILL BE TRAVERSING THROUGH THE SOUTHEASTERN STATES WHILE AN IMPRESSIVE LONGWAVE TROUGH DESCENDS FROM HUDSON BAY TOWARD THE GREAT LAKES/NORTHEASTERN U.S.. EXPECT THIS TROUGH TO ATTAIN A NEGATIVE TILT OVER SOUTHERN QUEBEC THURSDAY NIGHT WHICH WOULD ALLOW FOR THE FORMATION OF A RATHER DEEP CYCLONE AT THE SURFACE WHICH LIFTS FARTHER INTO EASTERN CANADA BY DAY 5/SATURDAY. OUT WEST...AN INITIAL DEEP UPPER LOW SHOULD BE APPROACHING THE NORTHERN CA/PACIFIC NORTHWEST COAST EARLY THURSDAY WHICH QUICKLY EVOLVES INTO AN OPEN WAVE AND SLIDES DOWNSTREAM TOWARDS THE GREAT PLAINS/MISSISSIPPI RIVER VALLEY BY THE WEEKEND. BEHIND THIS...ANOTHER SHORTWAVE EVENTUALLY MOVES INTO THE WESTERN EDGE OF THE MAP LATE IN THE FORECAST PERIOD WHICH SHOULD KEEP THE PACIFIC NORTHWEST IN AN ACTIVE/WET PATTERN INTO EARLY NEXT WEEK. WHILE MUCH OF THE CONTIGUOUS U.S. SEES AN ACTIVE MEDIUM RANGE PERIOD...UPPER RIDGING SHOULD DOMINATE THE RIO GRANDE VALLEY AND DOWN INTO MEXICO. REGARDING THE AVAILABLE MODEL GUIDANCE...THE 00Z CMC/UKMET CONTINUE TO BE ON THE FASTER SIDE OF THE GUIDANCE WITH THE SHORTWAVE TRAVERSING THE DEEP SOUTH EARLY ON. THE 06 GFS/00Z ECMWF AND RESPECTIVE ENSEMBLE MEANS ARE NOTABLY SLOWER TO DRAG BOTH THE SHORTWAVE AND WEAK WAVE OF LOW PRESSURE OFF THE COAST BY THE WEEKEND. TO THE NORTH...MODELS SEEM TO HAVE COME INTO MUCH BETTER AGREEMENT WITH THE DIGGING LONGWAVE TROUGH AND DESCENDING ARCTIC AIR MASS INTO THE NORTHEAST BY THE END OF THE WEEK WITH SOME MINOR TIMING DIFFERENCES ARISING AS THE ASSOCIATED DEEP SURFACE CYCLONE LIFTS INTO CANADA DAY 4 AND BEYOND. TO THE WEST...DIFFERENCES BEGIN TO ARISE AMONGST THE OPERATIONAL MODELS AS THE SHORTWAVE MOVES INTO THE GREAT PLAINS ON DAY 5 WITH THE 00Z CMC ON THE FASTER SIDE...AND THE ECMWF SHOWING THE SLOWEST SOLUTION. THE ENSEMBLE MEANS SHOW GOOD SUPPORT FOR A SOLUTION SOMEWHERE IN THE MIDDLE...OR CLOSEST TO THAT OF THE 06 GFS...ALBEIT SLIGHTLY WEAKER AND FLATTER. DEPENDING ON THE AMPLIFICATION OF THIS SHORTWAVE...SOME MODELS/ENSEMBLES CONTINUE HINT AT A POSSIBLE COASTAL LOW OFF THE NORTHEAST COAST BY DAY 6/7 BUT THERE REMAINS SIGNIFICANT SPREAD AND UNCERTAINTY ON THIS...BOTH BETWEEN THE OPERATIONAL MODELS AND THE INDIVIDUAL ENSEMBLE MEMBERS. AND FINALLY...OPERATIONAL MODELS AND ENSEMBLES SHOW FAIRLY GOOD AGREEMENT ON BOTH TIMING AND AMPLIFICATION OF RENEWED TROUGHING ALONG THE NORTHWEST COAST ON DAY 7. THIS CYCLE OF THE WPC MEDIUM RANGE PROGS FAVORS A BLEND OF THE LATEST OPERATIONAL GFS/ECMWF (WITH DECREASING AMOUNTS OF THE 00Z CMC) DAYS 3 AND 4. BY DAY 5...INCREASING AMOUNTS OF THE GEFS/EC MEAN WERE ADDED TO THE BLEND TO HELP MITIGATE SOME DIFFERENCES DISCUSSED ABOVE...BUT FAIRLY GOOD AGREEMENT IN THE OPERATIONAL GFS/ECMWF MEANT 40 PERCENT OPERATIONAL MODEL GUIDANCE WAS STILL USABLE EVEN ON DAY 7. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... WHILE AN IMPRESSIVE SHOT OF MODIFIED ARCTIC AIR ENTERS THE PICTURE DURING THE THURSDAY THROUGH SATURDAY TIME FRAME ACROSS THE NORTHEAST...VERY FEW DAILY TEMPERATURE RECORDS ARE EXPECTED TO BE BROKEN. DESPITE THIS...EXPECT DEPARTURES FROM NORMAL ON THE ORDER OF 10 TO 20 DEGREES BELOW AVERAGE TO BE THE NORM FROM THE NORTHERN PLAINS THROUGH THE UPPER MIDWEST AND INTO THE GREAT LAKES/OH VALLEY/NORTHEASTERN U.S.. SOME OVERNIGHT LOWS MAY PUSH BELOW ZERO DEGREES ON FRIDAY MORNING ACROSS FAR NORTHERN NORTH DAKOTA/MINNESOTA. THE AIR MASS WILL MODIFY IN TIME WHILE ADVANCING EASTWARD BUT MAY BRING THE FIRST FREEZE TO MANY LOCATIONS FARTHER EAST. GRADUALLY TEMPERATURES ACROSS THE COUNTRY WILL MODERATE SOME AS THE FLOW TURNS A BIT MORE ZONAL IN NATURE BY THE END OF THE WEEKEND. REGARDING PRECIPITATION...UNSETTLED CONDITIONS ARE LIKELY FROM NORTHERN CA UP INTO WESTERN OR/WA GIVEN STRONG AMPLE ONSHORE FLOW...AND FAVORABLE UPSLOPING ALONG THE TERRAIN. THROUGH EARLY FRIDAY...MODERATE TO LOCALLY HEAVY PRECIPITATION IS LIKELY FROM THE SHASTA/NORTHERN SIERRA NEVADA RANGES NORTHWARD TO THE OLYMPICS/CASCADES. SOME OF THIS WILL FALL IN THE FORM OF SNOWFALL ACROSS THE HIGHER ELEVATIONS AS WELL AS MORE INLAND LOCALES LIKE THE BITTERROOTS...SAWTOOTH...AND TETONS. THE WEST COAST WILL BRIEFLY DRY OUT OVER THE WEEKEND BEFORE THE NEXT TROUGH BRINGS A BOUT OF RAIN/SNOW BY LATE IN THE PERIOD. ACROSS THE EASTERN U.S...LINGERING SHOWERS ALONG A COLD FRONT ARE LIKELY ACROSS THE SOUTHEAST AND CENTRAL/EASTERN GULF COAST ON THURSDAY WHILE SOME ACTIVITY IS POSSIBLE OVER THE GREAT LAKES GIVEN STEEPENING LAPSE RATES ACCOMPANYING THE STRONG UPPER TROUGH. SOME SNOW IS ALSO POSSIBLE OVER THE INTERIOR NORTHEAST ALTHOUGH MOISTURE SHOULD BE SOMEWHAT SCANT. EVENTUALLY SOME WARM ADVECTION ACTIVITY IS POSSIBLE BY SATURDAY ACROSS THE GREAT PLAINS/MS RIVER VALLEY WITH THE PROGRESSIVE SHORTWAVE MOVING THROUGH. IF ENOUGH COLD AIR LINGERS ACROSS THE UPPER MIDWEST/GREAT LAKES...SOME PRECIPITATION MAY FALL AS SNOW OVER THE WEEKEND. EVENTUALLY ALL OF THE UNSETTLED WEATHER WILL MOVE TOWARD THE EASTERN SEABOARD BY NOVEMBER 12/13 WITH ANY DISCERNIBLE COASTAL WAVE AUGMENTING LOCAL AMOUNTS. SANTORELLI