EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 145 AM EST MON NOV 06 2017 VALID 12Z THU NOV 09 2017 - 12Z MON NOV 13 2017 ...PATTERN OVERVIEW/MODEL GUIDANCE/UNCERTAINTIES/PREFERENCES... AN ACTIVE PATTERN AHEAD WITH A SPLIT FLOW REGIME IN PLACE. SOUTHERN STREAM SHORTWAVE ENERGY WILL BE TRAVERSING THROUGH THE MID-SOUTH ON THURSDAY MORNING WITH AN ACCOMPANYING WEAK AREA OF LOW PRESSURE EVENTUALLY EXITING THE SOUTHEASTERN U.S. COAST. TO THE NORTH...AN IMPRESSIVE LONGWAVE TROUGH DESCENDS FROM HUDSON BAY TOWARD THE GREAT LAKES/NORTHEASTERN U.S. ON THURSDAY/FRIDAY. THIS WILL DRAW THE FIRST SIGNIFICANT SHOT OF MODIFIED ARCTIC AIR TO THE CENTRAL/EASTERN STATES. A RATHER DEEP SURFACE CYCLONE WILL FORM AS AN EMBEDDED SHORTWAVE ATTAINS A NEGATIVE TILT OVER SOUTHERN QUEBEC ON THURSDAY NIGHT. IN THE WAKE OF THIS SYSTEM...THE GUIDANCE SHOWS SURFACE PRESSURES NEARING 1040-MB FROM THE UPPER MIDWEST TO NEW ENGLAND. LOOKING BACK TOWARD THE WESTERN U.S...A DEEP UPPER LOW SHOULD BE APPROACHING THE NORTHERN CA/PACIFIC NORTHWEST COAST EARLY THURSDAY. THIS WILL QUICKLY EVOLVE INTO AN OPEN WAVE WHICH SLIDES DOWNSTREAM TOWARD THE GREAT PLAINS/MS RIVER VALLEY BY SATURDAY. DEPENDING ON THE DEGREE OF AMPLIFICATION...A COASTAL LOW MAY EMERGE OVER THE COASTAL ATLANTIC BY THE DAY 6/7...NOVEMBER 12/13 TIME PERIOD. EVENTUALLY ANOTHER SYSTEM MOVES ONTO THE MAP LATE IN THE FORECAST AS HEIGHTS BEGIN TO LOWER OVER THE EASTERN PACIFIC BY LATE NEXT WEEKEND. WHILE MUCH OF THE COUNTRY SEES AN ACTIVE PERIOD AHEAD...UPPER RIDGING SHOULD DOMINATE THE RIO GRANDE RIVER VALLEY DOWN INTO MX. REGARDING THE AVAILABLE MODEL GUIDANCE...THERE HAS BEEN A FASTER TREND NOTED IN THE 00Z CMC/UKMET WITH THE SHORTWAVE TRACKING THROUGH THE MID-SOUTH ON THURSDAY. THIS SHIFT IN THE 00Z UKMET IS AN ADJUSTMENT TO ITS PREVIOUSLY OUTLYING MORE WESTWARD DISPLACED SOLUTION. SOME WEAK WAVES ARE EXPECTED TO DEVELOP ALONG THE WAVY FRONT AS IT SLIDES SOUTH AND EAST. TO THE NORTH...THE EXPECTED AMPLIFICATION STILL REMAINS THE MOST CONTENTIOUS AREA ON THE MAP. ENSEMBLE SPAGHETTI PLOTS CONTINUE TO SHOW A GREAT DEAL OF MOVEMENT BETWEEN MODEL RUNS AND AMONG THE GLOBAL GUIDANCE. MULTI-DAY COMPARISONS INDICATE THE ECMWF/ECMWF ENSEMBLE MEAN HAVE BEEN THE MOST STABLE SOLUTIONS. THE 00Z/12Z CMC HAVE BEEN SHOWING SUPPORT FOR THIS ECMWF SCENARIO WHICH IS MORE AGGRESSIVE WITH THE ARCTIC PUSH. TRENDS IN THE GFS HAVE ALSO BEEN MOVING IN THAT DIRECTION ALTHOUGH IT WAS NOT TOO LONG AGO WHERE IT REMAINED MORE SUPPRESSED. THE 12Z UKMET APPEARED TO BE THE LARGER OUTLIER BEING SLOW BUT THE NEWER RUN HAS MADE SOME POSITIVE ADJUSTMENTS. ELSEWHERE...GENERAL AGREEMENT EXISTS WITH THE POSITION/STRENGTH OF THE UPPER LOW MOVING INTO THE WEST COAST ON THURSDAY. THEY DIFFER MORE GREATLY ON HOW QUICKLY TO EJECT THE STACK OF SHORTWAVES TOWARD THE CENTRAL U.S. OPERATIONAL RUNS OF THE GFS AS WELL AS MANY OF THE ENSEMBLE MEMBERS HAVE FAVORED A QUICKER/FLATTER EVOLUTION RELATIVE TO PRIOR RUNS OF THE ECMWF. AN AREA OF LOW PRESSURE IS LIKELY TO DEVELOP BY THE DAY 6/7...NOVEMBER 12/13 WINDOW ALTHOUGH ENSEMBLE LOW PLOTS SHOW A LOT OF SPREAD IF IT INDEED FORMS. BACK WEST...MODELS ARE VARIABLE WITH THE EVOLUTION OF THE RENEWED TROUGHING LATE IN THE PERIOD. HOWEVER...ENSEMBLE MEANS AGREE AS A WHOLE WITH THE 18Z/12Z GEFS MEAN BEING SLIGHTLY WEST OF THE 12Z ECMWF ENSEMBLE MEAN. AS GENERAL SYNOPTIC-SCALE AGREEMENT HAS IMPROVED...WAS ABLE TO UTILIZE A STEADY DOSE OF OPERATIONAL MODELS THROUGHOUT THE PERIOD. DID NOT INCORPORATE ANY ENSEMBLE MEANS UNTIL SATURDAY AS THE MODELS VARY WITH THE LONGWAVE TROUGH PUSHING INTO THE GREAT LAKES/NEW ENGLAND. BEFORE THIS...FAVORED AN EQUAL BLEND OF THE 18Z GFS/12Z ECMWF WITH SOME MINOR CONTRIBUTIONS FROM THE 12Z CMC. THEREAFTER...KEPT AN APPRECIABLE AMOUNT OF THE 18Z GFS/12Z ECMWF IN THE PICTURE WITH A MIX OF THE USUAL TRIO OF ENSEMBLE MEANS...GEFS/ECMWF/NAEFS. TOOK A SLIGHTLY HIGHER WEIGHT OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN GIVEN STRONGER MODEL CONSISTENCY. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... WHILE AN IMPRESSIVE SHOT OF MODIFIED ARCTIC AIR ENTERS THE PICTURE DURING THE THURSDAY THROUGH SATURDAY TIMEFRAME...NO DAILY TEMPERATURE RECORDS ARE LIKELY TO BE BROKEN. HOWEVER...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 ND/MN. THIS WILL BE THE NIGHT OF EFFICIENT RADIATIONAL COOLING AS A 1036-MB ANTICYCLONE SITS OVERHEAD. 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 DYNAMICAL LIFT...AMPLE ONSHORE FLOW...AND UPSLOPE TRAJECTORIES. 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 ARE LIKELY ACROSS THE CENTRAL/EASTERN GULF COAST ON THURSDAY WHILE SOME CONVECTIVELY FORCED ACTIVITY IS POSSIBLE OVER THE GREAT LAKES GIVEN STEEPENING LAPSE RATES ACCOMPANYING THE STRONG UPPER TROUGH. SOME SNOW IS 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. RUBIN-OSTER