EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1017 AM EST FRI MAR 10 2017 VALID 12Z MON MAR 13 2017 - 12Z FRI MAR 17 2017 ...WINTER AIN'T OVER TILL IT'S OVER... ...OVERVIEW... A POTENTIALLY SIGNIFICANT LATE-WINTER STORM IS FORECAST TO DEVELOP OFF THE MID-ATLANTIC COAST EARLY NEXT WEEK AND TRACK JUST OFF THE NEW ENGLAND COAST. VERY COLD AIR FOR MID-MARCH WILL ACCOMPANY THE NOR'EASTER AND WILL LINGER THROUGH THE WEEK. THE SOUTHWEST WILL BE MUCH QUIETER (AND WARM) WHILE THE PACIFIC NORTHWEST WILL SEE A COUPLE SYSTEMS PUSH THROUGH. ...GUIDANCE PREFERENCES AND EVALUATION... A BLEND OF THE 06Z GFS AND A COMBINATION OF THE 00Z ECMWF AND 00Z EC MEAN WAS USED. THE OVERALL TREND WITH THE NOR'EASTER WAS A SLIGHT NORTHWESTWARD SHIFT IN THE STORM TRACK. THE 00Z GFS AND TO A LESS EXTENT THE 06Z RUN WERE ON THE WESTERN PORTION OF THE ENSEMBLE SPREAD...TAKING THE LOW CENTER INTO NEW ENGLAND WITH A 990 MB LOW. THE ECMWF/CANADIAN MODELS WERE TRACKING JUST OFF THE NEW ENGLAND COAST NEAR THE BENCHMARK AND A DEEPER LOW. THE ECMWF WAS THE MOST AGGRESSIVE WITH A SUB 975 MB LOW. THE 500 MB PATTERN IS IN GOOD ENOUGH AGREEMENT OVERALL BUT DETAILS IN THE EVOLUTION AND INTERACTION OF SHORTWAVE ENERGY ROTATING AROUND THE EASTERN U.S. TROUGH LEAD TO MODEST DIFFERENCES IN THE SURFACE LOW FORECAST. CONFIDENCE IS RATHER HIGH FOR A MAJOR STORM SYSTEM TO DEVELOP OFF THE EAST COAST BUT DIFFERENCES IN THE SURFACE FEATURES MAKE THE SENSIBLE WEATHER IMPACTS MORE UNCERTAIN AT THIS TIME. FROM THE PREVIOUS DISCUSSION.. MAIN EVENT WILL BE IN THE NORTHEAST EARLY NEXT WEEK (TUE INTO WED... PI DAY AND THE IDES OF MARCH). DESPITE THE EARLY TASTE OF SPRING FOR MUCH OF THE EAST IN FEBRUARY AND EARLY MARCH, WINTER APPEARS TO BE READY FOR A COMEBACK. THE MODELS/ENSEMBLES HAVE MOSTLY CONVERGED ON A RAPIDLY DEEPENING STORM ON TUESDAY, THOUGH THE EVOLUTION REMAINS COMPLICATED. IN ADDITION, EARLIER RUNS WERE NOT AS ROBUST SO NOTHING IS SET IN STONE. THE SETUP IS NOT YOUR CLASSIC MILLER TYPE-A OR TYPE-B STORM BUT SEEMINGLY A COMBINATION OF THE TWO -- THERE IS FORECAST TO BE AN ALREADY-DEFINED SURFACE LOW OFF THE GA COAST LATE MONDAY (VERY MILLER TYPE-A) IN ADDITION TO AN UPSTREAM CLIPPER MOVING THROUGH THE OHIO VALLEY (LIKE A MILLER TYPE-B). HOW THE UPPER TROUGH SWINGS THROUGH THE MIDWEST WILL DETERMINE HOW THE DOMINANT LOW TRACKS -- IT COULD BE TUGGED FARTHER WEST BY A STRONGLY DIGGING TROUGH OR BE LEFT TO SWING FARTHER EAST. THIS IS ONE OF THOSE SITUATIONS WHERE THE SPREAD OF POSSIBILITIES IS NECESSARILY LARGE, ENCOMPASSING NEARLY ALL SCENARIOS FROM A LIGHT/MINOR EVENT TO AN OUTRIGHT BLIZZARD, DEPENDING ON YOUR LOCATION. ECMWF/CANADIAN LIED ON THE MORE EXTREME SIDE OF THE ENSEMBLE SPREAD (BUT WITHIN THE ENVELOPE) AS THEY MAINTAINED TWO SEPARATE UPPER LOWS DUMBBELLING THROUGH THE NORTHEAST. THE GFS RUNS WERE MORE COHERENT BUT STILL SHOWED SEPARATED VORTICITY SWINGING NORTHEASTWARD AHEAD OF THE UPPER LOW. SUFFICE IT TO SAY, THE MODELS WILL NOT GET THE DETAILS RIGHT AT THIS TIME RANGE, SO DETERMINISTIC SUCCESS IS FUTILE. RATHER, THE RANGE OF SCENARIOS SHOULD BE KEPT IN MIND. ONCE THIS SYSTEM LIFTS INTO ATLANTIC CANADA, MODELS DIVERGE IN HOW TO HANDLE THE UPSTREAM RIDGE OVER THE SOUTHWEST (STRONGER VS FLATTER). THIS HAD BIGGER IMPLICATIONS FOR TEMPERATURES OVER THE SOUTHERN PLAINS BUT ALSO FOR PRECIPITATION (GFS/GEFS WETTER THAN THE ECMWF/ECMWF ENSEMBLES). MUCH FARTHER UPSTREAM, THE NORTHEAST PACIFIC WILL BE DIFFICULT TO ACCURATELY FORECAST WITH SEVERAL IMPULSES MOVING TOWARD SW CANADA AND THE NW CONUS. ...WEATHER HIGHLIGHTS... WARM WEATHER WILL HOLD IN THE WEST AS UPPER RIDGING HELPS TEMPERATURES TO ABOVE AVERAGE VALUES (BY ABOUT 10-20F WHICH WILL TAKE DESERT AREAS INTO THE 90S AND NEAR RECORD HIGHS). THAT WILL MOVE SLIGHTLY EASTWARD BY LATER NEXT WEEK INTO THE CENTRAL PLAINS. THE EAST WILL BE WELL BELOW AVERAGE (15-25F BELOW CLIMO) WITH TEMPERATURES IN THE 30S IN THE MID-ATLANTIC AND TEENS/LOW 20S INTO W NC AND SW VA. SHOULD TAKE UNTIL NEXT WEEKEND TO GET BACK TOWARD MORE TYPICAL VALUES FOR MID-MARCH. ASSUMING A STORM TRACK AS CURRENTLY FORECAST, ACCUMULATIONS MAY EXCEED A FOOT OF SNOW OVER A MODEST AREA. HOWEVER, DETERMINING THE RAIN/SNOW LINE COUPLED WITH WHEN THE PRECIPITATION OCCURS (OVERNIGHT VS DAYTIME) WILL MAKE SNOWFALL FORECASTS MORE DIFFICULT EVEN IN THE SHORT RANGE. THE PAC NW WILL SEE PERIODS OF VALLEY RAIN AND HIGH ELEVATION SNOW THAT SHOULD EXTEND EASTWARD INTO THE BITTERROOTS AND SAWTOOTH RANGE AND TO YELLOWSTONE. SNOW LEVELS SHOULD REMAIN RATHER HIGH GIVEN THE WARM PACIFIC AIR MASS MOVING IN. DOWNSTREAM PRECIPITATION IN THE CENTRAL AND SOUTHERN PLAINS AND TN VALLEY IS LESS CERTAIN, BUT AT LEAST SOME RAIN SHOULD BREAK OUT AHEAD OF THE NEXT NORTHERN STREAM SYSTEM. FRACASSO/KREKELER