EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1158 AM EDT THU APR 12 2018 VALID 12Z SUN APR 15 2018 - 12Z THU APR 19 2018 ...A POWERFUL AND HIGH-IMPACT SPRING STORM STRENGTHENING OVER THE GREAT PLAINS ON FRIDAY WILL SHIFT INTO THE EASTERN U.S. ON SUNDAY... ...GUIDANCE/PREDICTABILITY ASSESSMENT... MODEL GUIDANCE REMAINED IN VERY GOOD AGREEMENT WITH THE PROGRESSION OF A LARGE SCALE UPPER TROUGH/CLOSED LOW EXITING THE MISSISSIPPI VALLEY ON SUNDAY-MONDAY AND AN UPSTREAM TROUGH BEGINNING TO DIG INTO THE NORTHWESTERN U.S.. THERE EVEN APPEARS TO BE SOME PREDICTABILITY WITH THE SMALLER SCALE DETAILS OF THE FORECAST, INCLUDING THE TRACK OF OF A DEEP SURFACE LOW LIFTING FROM THE OHIO VALLEY TO THE GREAT LAKES AND THE TIMING OF A STRONG COLD FRONT SWEEPING THROUGH THE EASTERN U.S. EARLY NEXT WEEK. CONFIDENCE IN THE DETAILS OF THE FORECAST STARTS TO DECREASE TOWARDS THE MIDDLE OF NEXT WEEK AS THE LARGE SCALE UPPER TROUGH/CLOSED LOW PIVOTS NORTHEASTWARD INTO THE GREAT LAKES/NORTHEAST/SOUTHEASTERN CANADA, AND ENERGY FROM THE TROUGH DIGGING INTO THE NORTHWEST EARLY IN THE PERIOD EJECTS OUT INTO THE NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY. HOWEVER, DESPITE THE DIFFERENCES IN THE DETAILS, THERE WAS A GOOD ENOUGH CONSENSUS WITH THE LARGE SCALE PATTERN TO USE A BLEND OF THE GFS/ECMWF/GEFS MEAN/ECMWF ENSEMBLE MEAN ALL THE WAY THROUGH DAY 7. USING DETERMINISTIC GUIDANCE LATE IN THE FORECAST PERIOD ALSO BEST CAPTURES THE GOOD AGREEMENT WITH THE TIMING OF AN UPPER TROUGH/CLOSED LOW REACHING CALIFORNIA NEXT WEDNESDAY/THURSDAY. ...WEATHER/THREAT HIGHLIGHTS... A PRONOUNCED PERIOD WITH SNOW/ICE WILL OCCUR ACROSS THE COOLED GREAT LAKES AND SPREAD INTO THE COLD AIR DAMMED INTERIOR NORTHEAST SUN INTO TUE AS THE SUPPORTING UPPER LEVEL LOW AND WELL ORGANIZED SURFACE LOW/FRONTAL SYSTEM TRACK THROUGH/UNDERNEATH AT MODEST SPEED. FARTHER SOUTH...DEEP SOUTHERLY FLOW IN ADVANCE OF A TRAILING WAVY COLD FRONT WILL FEED COPIOUS AMOUNTS OF MOISTURE FROM THE GULF OF MEXICO AND WRN ATLANTIC. WITH STRONG DYNAMIC LIFT AND INSTABILITY...CONVECTION IS EXPECTED TO FLOURISH AND MAY OFFER A SEVERE WEATHER THREAT AS MONITORED BY THE STORM PREDICTION CENTER. A WIDESPREAD SWATH OF HEAVY CONVECTIVE RAINS OFFER TRAINING POTENTIAL...BUT THE FRONT SWEEPS THROUGH TO LIMIT EVEN HEAVIER TOTALS. THE WEST COAST WILL HAVE ANOTHER INCOMING SYSTEM BEGIN TO PUSH THROUGH BY THE END OF THE WEEKEND AND INTO EARLY NEXT WEEK. THE MID-UPPER LEVEL TROUGH WILL SWING OVER THE WEST COAST TO THE INTERMOUNTAIN WEST/ROCKIES MON/TUE AND THEN EJECT EASTWARD OVER THE CENTRAL AND ERN U.S...PRODUCING MODEST PRECIPITATION. BY NEXT WED/THU...YET ANOTHER AMPLIFIED SYSTEM WILL REACH THE WEST COAST. PRECIPITATION WILL AGAIN BEGIN TO INCREASE ACROSS THE PACIFIC NORTHWEST AND N-CENTRAL CALIFORNIA AS THE MID-UPPER LEVEL TROUGH/HEIGHT FALLS APPROACH. RAINFALL ALONG WITH HIGHER ELEVATION SNOW IS THEN EXPECTED TO SPREAD INLAND INTO THE INTERMOUNTAIN WEST BY AROUND NEXT THU. WPC MEDIUM RANGE 500MB PROGS...SURFACE/FRONTS PROGS...SENSIBLE WEATHER GRIDS...QPF GUIDANCE...AND WINTER WEATHER OUTLOOK SNOW/SLEET PROBABILITY FORECASTS CAN BE FOUND AT... http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 SCHICHTEL/GERHARDT