EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 146 AM EST WED DEC 16 2015 VALID 12Z SAT DEC 19 2015 - 12Z WED DEC 23 2015 ...PATTERN OVERVIEW... THE UPPER PATTERN WILL BE ON THE AMPLIFIED SIDE TO START THE WEEKEND WITH A BROAD NEGATIVE HEIGHT ANOMALY ENCOMPASSING THE NORTHEASTERN U.S. THE ATTENDANT COOLER THICKNESSES AFFECTING THE REGION WILL GRADUALLY GIVE WAY TO MORE OF A WARM ADVECTION REGIME AS THE NEXT SYSTEM STRENGTHENS OVER THE CENTER OF THE COUNTRY. LOOKING UPSTREAM...MODEST HEIGHT FALLS MOVING INTO THE WEST COAST EARLY SATURDAY WILL PROPAGATE EASTWARD WITH THE SHORTWAVE ENERGY GENERALLY REMAINING OF LOWER AMPLITUDE RELATIVE TO PREVIOUS TROUGH PASSAGES. THERE ARE INDICATIONS THE SOUTHERN MOST IMPULSE SHOULD STRENGTHEN A BIT WHILE MOVING THROUGH THE GREAT PLAINS. THIS WILL AID IN SURFACE CYCLOGENESIS WITH AN AREA OF LOW PRESSURE MIGRATING TOWARD THE MIDDLE MS VALLEY/UPPER GREAT LAKES BY 21/0600Z. THE SYSTEM SHOULD CONTINUE LIFTING NORTHEASTWARD TOWARD THE ONTARIO/QUEBEC BORDER GIVEN THE PREVAILING UPPER RIDGE ANCHORING THE WESTERN ATLANTIC. MODELS INDICATE THE WESTERN U.S. TROUGH WILL RE-LOAD YET AGAIN WITH A SERIES OF VORTICITY MAXIMA SLIDING INTO THE WEST COAST...PARTICULARLY LATE IN THE MEDIUM RANGE PERIOD. THIS AMPLIFIED PATTERN WILL ULTIMATELY MAINTAIN POSITIVE HEIGHTS OVER THE EASTERN U.S. FAVORING MORE MILD CONDITIONS FOR MID/LATE DECEMBER. ...GUIDANCE EVALUATION/PREFERENCES... RECENT MODEL GUIDANCE IS IN DECENT AGREEMENT WITH THE DEPARTING UPPER TROUGH/STRENGTHENING SURFACE CYCLONE ACROSS THE NORTHEASTERN U.S./FAR EASTERN CANADA. THERE ARE VARIATIONS IN SURFACE LOW INTENSITY BUT THESE ISSUES OCCUR WELL NORTH OF THE MAP DOMAIN. MEANWHILE...OVER THE WESTERN U.S...THE INITIAL SHORTWAVE TRACKING TOWARD CA EARLY SATURDAY CONTAINS SOME TIMING DIFFERENCES. THE 18Z GFS SEEMED TO BE QUICKER THAN OTHER 12Z SOLUTIONS INCLUDING ALL RELEVANT ENSEMBLE SYSTEMS. THE 12Z GFS APPEARED TO BE MORE IN LINE WITH THE TIMING OF THIS FEATURE SO IT WAS VALUED MORE THAN THE 18Z CYCLE. A COMBINATION OF THE 12Z GFS/ECMWF/UKMET WAS FEASIBLE THROUGH ABOUT DAY 5/DEC 21 TRACKING THIS SYSTEM FROM THE DESERT SOUTHWEST THROUGH THE CENTRAL PLAINS AND INTO THE UPPER GREAT LAKES. DID NOT CONSIDER THE 12Z CMC AS IT WAS WELL NORTH/WEST OF THE CONSENSUS WITH THE SURFACE LOW. BY 21/1200Z...THE PAST THREE CYCLES OF THE ECMWF ALONG WITH THE 12Z UKMET FAVORED A DEEPER SOLUTION OVER THE GREAT LAKES VERSUS THE WEAKER GFS. SKEWED THE PREFERENCE SLIGHTLY MORE IN THE DIRECTION OF THE 12Z ECMWF/UKMET GIVEN THIS INTENSITY DIFFERENCE BEFORE UPSTREAM UNCERTAINTIES MADE IT DIFFICULT TO USE ANY DETERMINISTIC SOLUTIONS. AGREEMENT EXISTS IN THE FLOW RE-AMPLIFYING ACROSS THE ROCKIES/GREAT PLAINS BY DAY 6/7...DEC 22/23. HOWEVER...EACH ENSEMBLE SYSTEM VARIES IN THE FORECAST EVOLUTION WITH THE 12Z ECMWF/CMC ENSEMBLE MEANS BEING MORE AGGRESSIVE WITH THE ADVANCE OF COLDER AIR OVER THE WESTERN U.S. ADDITIONALLY...A SURFACE LOW SHOULD BE MOVING THROUGH THE GREAT PLAINS/MS RIVER VALLEY AT SOME POINT LATE IN THE MEDIUM RANGE BUT WHERE IT IS LOCATED AND HOW INTENSE IT WILL BE IS UNKNOWN. ENDED UP GOING 70/30 12Z ECMWF ENSEMBLE/18Z GEFS MEANS...RESPECTIVELY...FOR DAYS 6/7. GAVE MORE CREDIT TO THE FORMER AS THE 12Z CMC ENSEMBLE MEAN MORE CLOSELY FAVORED ITS SOLUTION. ...WEATHER HIGHLIGHTS/THREATS... THE DEPARTING UPPER TROUGH OVER THE NORTHEASTERN U.S. SHOULD CARRY A THREAT OF SCATTERED RAIN/SNOW SHOWERS OVER SECTIONS OF NEW ENGLAND ON SATURDAY. OTHERWISE...THE POST FRONTAL AIR SPREAD ACROSS THE EAST COAST SHOULD FAVOR SLIGHTLY BELOW NORMAL TEMPERATURES TO BEGIN THE WEEKEND. EVENTUALLY THE DE-AMPLIFICATION OF THE PATTERN AND THE RETURN OF WARM ADVECTION TO THE CENTRAL/EASTERN U.S. WILL FAVOR MORE ABOVE NORMAL READINGS...PARTICULARLY INTO NEXT WEEK. FORECAST TEMPERATURE ANOMALIES SHOULD REACH THE 15 TO 25 DEGREE RANGE FROM THE GREAT PLAINS EASTWARD WHICH WILL CONTINUE THE MILD DECEMBER FOR MANY LOCATIONS. ON THE OTHER HAND...TEMPERATURES SHOULD BE COOLER FROM THE WEST COAST TOWARD THE ROCKIES GIVEN THE COOL/UNSETTLED WEATHER EXPECTED. IT SHOULD BE QUITE THE WET PERIOD OVER MUCH OF THIS REGION...PARTICULARLY ACROSS THE PACIFIC NORTHWEST/UPPER INTERMOUNTAIN WEST. GIVEN A LACK OF ARCTIC INTRUSIONS...THE DOMINANCE OF PACIFIC AIR SHOULD FAVOR SNOW IN THE HIGHER ELEVATIONS WITH RAIN MORE COMMONPLACE ALONG THE COASTAL PLAINS AND VALLEY LOCALES. AS THE HEIGHT FALLS ADVANCE TOWARD THE CENTRAL U.S. BY EARLY NEXT WEEK...THERE ARE DECENT INDICATIONS OF HEAVY RAINFALL AFFECTING A SIGNIFICANT PORTION OF THE LOWER MS VALLEY/OZARKS AND EXTENDING INTO THE TN VALLEY. HOWEVER...THESE DETAILS WILL BE DIFFICULT TO PINPOINT GIVEN THIS IS IN THE DAY 6/7 RANGE. RUBIN-OSTER