EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 127 AM EST TUE JAN 16 2018 VALID 12Z FRI JAN 19 2018 - 12Z TUE JAN 23 2018 ...ACTIVE PATTERN FOR MUCH OF THE WEST WHILE HEAVY SNOW LOOMS FOR THE UPPER MIDWEST/GREAT LAKES BY SUNDAY/MONDAY... ...PATTERN OVERVIEW... THE MID/UPPER-LEVEL PATTERN WILL FEATURE A SPLIT FLOW REGIME WITH A SOUTHERN STREAM SHORTWAVE TRACKING FROM EXTREME SOUTH TX OUT TOWARD THE NORTHERN GULF OF MX OVER THE WEEKEND. LARGE-SCALE AMPLIFICATION BEGINS TO TAKE SHAPE WITHIN THE MID-LATITUDES AS SHARPENING HEIGHT FALLS DIG INTO THE FOUR CORNERS REGION ON SATURDAY. ADDITIONAL STRENGTHENING IS EXPECTED AS THE SYSTEM ATTAINS A NEGATIVE TILT OVER THE MO RIVER VALLEY ON SUNDAY EVENING. CONSEQUENTLY...A SIGNIFICANT WAVE OF LOW PRESSURE SWINGS UP TOWARD THE UPPER GREAT LAKES ON MONDAY WITH A TRACK INTO SOUTHERN QUEBEC BY THE FOLLOWING DAY. MEANWHILE...AS THE INITIAL SYSTEM MOVES FROM THE WEST COAST TOWARD THE SOUTHERN HIGH PLAINS THIS WEEKEND...ADDITIONAL UPPER TROUGHING IS LIKELY TO MOVE INTO THE PACIFIC NORTHWEST/NORTHERN CA BY SUNDAY WITH EASTWARD PROGRESSION THEREAFTER. BY DAY 7/JANUARY 23...A PAIR OF TROUGHS SHOULD INHABIT THE COUNTRY...ONE ACROSS THE WESTERN/CENTRAL STATES WHILE THE OTHER EXITS NEW ENGLAND. ...GUIDANCE ASSESSMENT/UNCERTAINTIES/PREFERENCES... TO BEGIN THE PERIOD ON FRIDAY...LOW-AMPLITUDE SHORTWAVES WILL BE IN THE PROCESS OF EXITING LOWER NEW ENGLAND WHICH GIVES WAY TO MARKED HEIGHT RISES ACROSS THE EASTERN U.S. THIS WEEKEND. WITHIN THE SOUTHERN STREAM...THERE HAVE BEEN TIMING DIFFERENCES EJECTING A COMPACT CLOSED LOW FROM SOUTH TX EASTWARD INTO THE ADJACENT GULF WATERS. THE GFS/ECMWF HAVE GENERALLY REMAINED ON THE SLOWER SIDE OF THE ENVELOPE WITH THE EXCEPTION BEING THE 12Z GFS WHICH APPEARS TO BE AN OUTLIER. CONSIDERING THE 570-DM ENSEMBLE SPAGHETTI PLOTS...A MAJORITY OF THE 12Z ECMWF ENSEMBLE MEMBERS SUPPORT THIS SLOWER PROGRESSION. EVENTUALLY THIS SYSTEM SHOULD FIZZLE OUT WHILE MOVING INTO THE NORTHEASTERN GULF OF MX LATER IN THE WEEKEND. ON THE HEELS OF THIS FEATURE...LARGE-SCALE HEIGHT FALLS TAKE SHAPE ACROSS THE SOUTHWESTERN U.S. WITH EVENTUALLY EJECTION INTO THE SOUTHERN HIGH PLAINS BY SUNDAY MORNING. MULTI-DAY ENSEMBLE SPAGHETTI TRENDS SHOW ONLY MINIMAL CHANGES WITH AGREEMENT IN DEVELOPING A STRONG AREA OF LOW PRESSURE INTO THE MIDDLE MS VALLEY BY 22/0000Z. A SUB-990 MB CYCLONE SHOULD BE CROSSING INTO THE UPPER GREAT LAKES ON MONDAY MORNING WITH THE 12Z ECMWF ENSEMBLE MEAN BEING THE STRONGEST AMONG THE ENSEMBLE SOLUTIONS. EVENTUALLY A COLD FRONTAL PASSAGE WILL OCCUR ALONG THE I-95 CORRIDOR BY LATE MONDAY WITH LOWER HEIGHTS LINGERING ACROSS THE NEW ENGLAND INTO THE END OF THE FORECAST PERIOD. WHILE THIS LATTER SYSTEM IS OF HIGHER CONFIDENCE...THE TRAILING EVOLUTION OF THE PATTERN IS LESS CERTAIN. THE GFS/CMC SOLUTIONS APPEAR TO BE ON ONE SIDE WHICH FAVORS A FLATTER/QUICKER PROGRESSION FROM THE PACIFIC NORTHWEST TOWARD THE NORTH-CENTRAL U.S. ON THE OTHER END...THE 12Z ECMWF AND A MAJORITY OF ITS ENSEMBLE MEMBERS FAVOR A SOLUTION THAT DIGS A TROUGH BACK INTO THE SOUTHWESTERN U.S. WHICH APPEARS TO BE ITS THEME THE PAST COUPLE OF RUNS. GIVEN SOLID MODEL AGREEMENT BETWEEN THE 18Z GFS/12Z ECMWF...FAVORED AN EQUAL BLEND OF THESE SOLUTIONS THROUGH DAY 4/SATURDAY BEFORE INCORPORATING SOME ENSEMBLE INFLUENCES FROM DAY 5/SUNDAY ONWARD. DECIDED TO FAVOR THE 12Z ECMWF ENSEMBLE MEAN MORE STRONGLY DEEPER INTO THE PERIOD AS IT WAS THE ONLY ENSEMBLE SOLUTION WITH A DEEP GREAT LAKES LOW. THE 18Z/12Z GEFS MEAN WERE SEVERAL MB WEAKER WHICH IS NOT REFLECTIVE OF THE STRONG NATURE THE OPERATIONAL GUIDANCE HAS BEEN SHOWING. ...WEATHER HIGHLIGHTS/THREATS... FOR THE MOST PART...ABOVE AVERAGE TEMPERATURES SHOULD DOMINATE A MAJORITY OF THE FORECAST AS STRONG WARM ADVECTION ENSUES IN ADVANCE OF THE DEEP UPPER TROUGH. ON FRIDAY...READINGS APPROXIMATELY 15 TO 20 DEGREES ABOVE CLIMATOLOGY SHOULD BE COMMONPLACE FROM THE SOUTHERN/CENTRAL HIGH PLAINS UP INTO THE UPPER MIDWEST. SUCH DEPARTURES FROM AVERAGE WILL GRADUALLY SLIDE EASTWARD REACHING THE EASTERN U.S. ON SUNDAY/MONDAY. A FAIRLY SHARP TEMPERATURE GRADIENT WILL SET UP ALONG THE BOUNDARY WITH HIGHS ON SUNDAY IN THE 50S ACROSS CENTRAL IL/IN WHILE TEENS LOOM BACK ACROSS THE UPPER MIDWEST. ACROSS THE WESTERN U.S...OVERALL TEMPERATURES SHOULD BE SLIGHTLY BELOW NORMAL FOR MUCH OF THE PERIOD GIVEN ACTIVE PRECIPITATION. REGARDING PRECIPITATION...SOME CONVECTION COULD HUG THE TX COAST ON FRIDAY IN ADVANCE OF THE SOUTHERN STREAM CLOSED LOW. ACROSS THE WESTERN U.S...WIDESPREAD LOWER ELEVATION RAIN/MOUNTAIN SNOW IS LIKELY THROUGH SATURDAY WITH A MUCH HEAVIER BATCH OF RAIN/SNOW HITTING NORTHERN CA UP INTO THE PACIFIC NORTHWEST ON SUNDAY/MONDAY. SOME OF THE GUIDANCE HAS 24-HOUR AMOUNTS APPROACHING THE 2 TO 3 INCH RANGE AS AN ATMOSPHERIC RIVER EVENT SETS UP. CONSIDERING THE IMPACTS FROM THE DEEP UPPER TROUGH LIFTING TOWARD THE UPPER MIDWEST...HEAVY SNOW WILL INITIALLY BE POSSIBLE OVER THE CENTRAL ROCKIES. AS THE SURFACE LOW BEGINS TO WIND UP ACROSS THE MIDDLE MS VALLEY/UPPER GREAT LAKES...THE POTENTIAL FOR MODERATE/HEAVY SNOW IS LIKELY ANYWHERE FROM THE CENTRAL PLAINS INTO THE UPPER GREAT LAKES. EVEN SOME BLIZZARD CONDITIONS MAY BE A POSSIBILITY IF THE SURFACE LOW DEEPENS AS MUCH AS SUGGESTED. FARTHER SOUTH ALONG THE FRONT...LIGHT/MODERATE RAINFALL SHOULD SPREAD ALONG AND AHEAD OF THE BOUNDARY. RUBIN-OSTER