EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 144 AM EST WED DEC 07 2016 VALID 12Z SAT DEC 10 2016 - 12Z WED DEC 14 2016 ...OVERVIEW AND PREFERENCES... FOR DAYS 3/4 SAT 10 DEC-SUN 11 DEC THE MANUAL PRODUCT SUITE USED A BLEND OF THE 18 UTC GFS/GEFS ENSEMBLE MEAN AND 12 UTC ECMWF/ECMWF ENSEMBLE MEAN/12Z CANADIAN ENSEMBLE MEAN. MORE WEIGHTING IS GIVEN TO THE ENSEMBLE MEANS ON DAYS 6 AND 7 WHERE THE 12Z ECMWF/18Z GFS MODELS HAVE DIFFERENT SOLUTIONS...AND RUN TO RUN CHANGES THAT INCREASE THE UNCERTAINTY. ON DAYS 3-4 AGREEMENT IS GOOD THAT ONE AREA OF LOW PRESSURE REMAINS JUST OFF THE PACIFIC NORTHWEST COAST WITH A STATIONARY FRONT EXTENDING INLAND ACROSS THE NORTHERN ROCKIES TO A FRONTAL SYSTEM DEVELOPING ON THE IN CENTRAL PLAINS THAT MOVES NORTHEAST TOWARDS THE UPPER MS VALLEY AND UPPER GREAT LAKES SUN 11 DEC. THE DIFFERENCES START TO GROW SUN INTO MON AS THE LOW MOVES INTO SOUTHEAST CANADA WITH A TRAILING COLD FRONT MOVING ACROSS THE NORTHEAST TO THE OH VALLEY AND APPALACHIANS. THE 12Z ECMWF HAS A MORE AMPLIFIED 500 MB WAVE THAN MOST ECMWF ENSEMBLE MEMBERS WITH THE MEAN INDICATING A LOWER AMPLITUDE/LONGER WAVELENGTH SYSTEM AND THUS A LOW PROBABILITY OF A SECOND CYCLONE DEVELOPING ON THE FRONT. FOLLOWING THIS SYSTEM WILL BE A SURGE OF COLDER AIR ACROSS THE LAKES TO THE NORTHEAST...FOLLOWED BY ANOTHER COLDER SURGE SOUTH OF SOUTHWEST CANADA INTO THE NORTHERN PLAINS. THE COLD FRONT AT THE LEADING EDGE OF THE PLAINS ARCTIC SURGE HAS A FEW WAVES OF LOW PRESSURE POSSIBLE ALONG THE FRONT WITH THE 18Z GFS DEVELOPING A WAVE THAT ON 12Z TUE 13 DEC HAS SURFACE LOW PRESSURE NEAR THE MI/IN BORDER. THE 00Z GFS NO LONGER HAS THE WAVE DEVELOPING...WHILE THE ECMWF/CANADIAN GLOBAL LOW PRESSURE IS JUST NORTH OF LK SUPERIOR. THE ENSEMBLE MEANS LIKE THE LOCATION OF THE FURTHER NORTH LOCATION OF LOW PRESSURE ON 12Z TUE SO THE MAJORITY CLUSTER WAS USED. THIS SYSTEM IS FORECAST TO DEVELOP AS IT MOVES ACROSS SOUTHEAST CANADA...FOLLOWED BY THE SURGE OF ARCTIC AIR MOVING EAST FROM THE NORTHERN PLAINS ACROSS THE UPPER MS VALLEY AND GREAT LAKES. THE 12Z ECMWF DEVELOPED A WAVE ON THE ARCTIC FRONT SUN NIGHT INTO EARLY MON IN THE OH VALLEY...TRACKING LOW ACROSS THE NORTHEAST. THIS A DISTINCT MINORITY AMONG THE ECMWF ENSEMBLES...SO MORE WEIGHTING WAS GIVEN TO THE ENSEMBLE MEAN. ADDITIONAL SPREAD DEVELOPS OVER THE EASTERN PACIFIC AND ADJACENT POSITIONS OF THE PACIFIC NORTHWEST AND NORTHERN CA AS THE ECMWF/GFS/CANADIAN GLOBAL ALL SHOW LOW PRESSURE DEVELOPING ON A WAVY SURFACE FRONT WITH EACH MODEL INDICATING A DIFFERING CYCLONE TRACK/SPEED/INTENSITY. THE STRONGER 12Z ECMWF SOLUTION WITH A CYCLONE OFF THE OR/WA BORDER 00Z TUE 13 DEC AND THEN OFF THE WA COAST 12Z TUE 13 DEC LEADS TO STRONG ONSHORE FLOW AND A POTENTIAL HEAVY PRECIP EVENT IN SOUTHWEST OR/NORTHWEST CA. THE ECMWF ENSEMBLE MEMBERS ARE SPREAD WIDE WITH THE LACK OF A DEFINED CLUSTER OF SOLUTIONS. THE WIDE SPREAD LEADS TO LOW CONFIDENCE IN ANY PARTICULAR SOLN...AND GREATER WEIGHTING ON THE ENSEMBLE MEAN FORECASTS. ...WEATHER HIGHLIGHTS/THREATS... OVER THE WEST THE LARGE SCALE PATTERN WILL LIKELY PROMOTE PERSISTENT VALLEY RAIN AND INLAND/HIGHER ELEV SNOW ACROSS THE PACIFIC NORTHWEST/NRN CA/NORTHERN GREAT BASIN/NORTHERN ROCKIES WITH A PRONOUNCED COASTAL/TERRAIN FOCUS FOR HIGHEST AMTS. THE POTENTIAL EXISTS FOR ONE OR MORE EPISODES OF LOCALLY HVY PCPN...MOST LIKELY OVER NORTHWEST CALIFORNIA AND/OR WRN OREGON. ABOVE NORMAL TEMPS PERSISTS IN THE DESERT SOUTHWEST THROUGH THE WEEKEND AND EARLY NEXT WEEK BUT BECOME BELOW NORMAL IN THE CENTRAL TO NORTHERN GREAT BASIN/CENTRAL TO NORTHERN ROCKIES/PACIFIC NORTHWEST. WITH THE FORECAST ARCTIC SURGE COMING OUT OF SOUTHWEST CANADA INTO THE NORTHERN PLAINS...AN IMPRESSIVE AREA OF TEMPERATURES 20-30 DEGREES BELOW NORMAL DEVELOPS IN THE NORTHERN HIGH PLAINS AND SPREADS EAST...WITH HIGH TEMPERATURES POSSIBLY STAYING A FEW DEGREES BELOW ZERO F IN NORTH DAKOTA DAYS 5 THROUGH 7. EXPECT TEMPS TO BE 10-20F BELOW NORMAL OVER MUCH OF THE UPPER MS VALLEY DAY 5 SPREADING INTO THE GREAT LAKES AND OH VALLEY/MID MS VALLEY AND CENTRAL PLAINS DAYS 6-7. THE SYSTEM SAT TRAVERSING FROM THE CENTRAL PLAINS TO UPPER MS VALLEY AND UPPER LAKES HAS POTENTIAL TO PRODUCE SEVERAL INCHES OF SNOW ALONG THE STORM TRACK...WITH LIGHTER AMOUNTS IN THE DOWNSTREAM WARM ADVECTION ACROSS THE LOWER LAKES AND NORTHEAST. HEAVY SNOWS ARE EXPECTED AGAIN IN THE WA CASCADES OVER THE WEEKEND WITH LIGHT TO MODERATE SNOW IN RANGES FURTHER INLAND IN ID/WY/CO/NORTHERN UT THAT CONTINUE INTO THE EARLY AND MIDDLE PARTS OF NEXT WEEK. PETERSEN