EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST WED DEC 27 2017 VALID 12Z SAT DEC 30 2017 - 12Z WED JAN 03 2018 ...ARCTIC AIR WILL CONTINUE ITS DOMINANCE EAST OF THE ROCKIES INTO THE NEW YEAR... ...GUIDANCE/PREDICTABILITY ASSESSMENT..... THE MAIN FEATURE OF THE MEDIUM RANGE PERIOD IS MULTIPLE WAVES OF SHORTWAVE ENERGY DROPPING THROUGH THE WESTERN U.S. WHICH SHOULD ACT TO MAINTAIN TROUGHING ACROSS THE EASTERN U.S. AS WE MOVE INTO THE NEW YEAR. THE 00Z ECMWF RUN SHOWS MUCH STRONGER RIDGING OVER THE WEST COAST THAN ITS PREVIOUS FEW RUNS WHICH ALLOWS FOR MORE AMPLIFICATION OF TROUGHING ACROSS THE EAST. THOUGH STILL NOT AS AMPLIFIED/DEEP AS THE GFS...THE LASTEST RUN OF THE ECMWF AT LEAST SUGGESTS A POSSIBLE TREND IN THIS DIRECTION. AMPLIFIED TROUGHING IS ALSO SHOWN BY THE ENSEMBLE MEANS BUT WITH WEAKER RIDGING OUT WEST DUE TO SIGNIFICANT UNCERTAINTY WITH A SHORTWAVE PUSHING EASTWARD THROUGH THE EAST PAC AND POSSIBLY IMPACTING THE SOUTHWEST U.S. FOR THE EASTERN U.S. TROUGH...THE WPC PROGS FAVOR AN EQUAL BLEND OF THE 00Z ECMWF/06Z GFS WITH THEIR RESPECTIVE ENSEMBLE MEANS. AS MENTIONED PREVIOUSLY...ANOTHER HIGHLY UNCERTAIN FEATURE OF THE MEADIUM RANGE PERIOD IS THE POSITIONING OF A SHORTWAVE TROUGH/CLOSED LOW ACROSS THE EAST PAC AND WHETHER OR NOT THIS TROUGH MOVES INLAND ACROSS THE SOUTHWEST U.S. BY DAYS 6 AND 7. BIG SHIFT IN CONTINUITY WITH THE 00Z ECMWF WHICH NOW SUGGESTS THE LOW STAYS WELL OFFSHORE...WHICH IS IN MUCH CLOSER AGREEMENT TO THE PREVIOUS FEW RUNS OF THE GFS. HOWEVER...THERE ARE STILL MANY EC AND GEFS ENSEMBLE MEMBERS SUGGESTING SOUTHWEST U.S. TROUGHING AND THEREFORE THE MEANS ALSO SHOW WEAK TROUGHING MOVING ONSHORE. GIVEN THE SPREAD...ITS HARD TO LATCH ONTO ON SOLUTION OVER THE OTHER AND THUS THE WPC PROGS FAVORED AN EQUAL BLEND OF THE OPERATIONAL AND ENSEMBLE MEANS. COMPARED TO THE PREVIOUS SHIFT...THIS WOULD MEAN STRONGER RIDGING ACROSS THE WESTERN U.S. WITH A MUCH WEAKER SEMBLANCE OF TROUGHING ACROSS THE SOUTHWEST...WITH THE STRONGER TROUGHING/LOW REMAINING OFFSHORE. AGAIN...THIS PARTICULAR SOLUTION IS WITH VERY LOW CONFIDENCE GIVEN THE CONTINUED SPREAD. ...POTENTIAL WEATHER/THREATS HIGHLIGHTS... OVERALL...IT SEEMS QUITE LIKELY THAT MULTIPLE SHOTS OF REINFORCING ARCTIC AIR WILL ENSURE FRIGID TEMPERATURES OVER THE NATION FROM THE ROCKIES TO THE SOUTHERN AND EASTERN U.S. OVER THE NEXT WEEK AS COLD HIGH PRESSURE SETTLES TO THE EAST OF A SHARP QUASI-STATIONARY FRONT BANKING UP AGAINST THE ROCKIES. DOWNSTREAM WEEKEND WRN ATLANTIC COASTAL LOW DEVELOPMENT WOULD EFFECT MARITIME INTERESTS AND OFFER SOME IMPACT FOR THE EASTERN SEABOARD...BUT THE FORECAST TREND IS OFFSHORE. PERIODIC SURGES OF COLD CYCLONIC FLOW OVER THE GREAT LAKES FAVORS ADDITIONAL LAKE EFFECT SNOWS...PROLONGING AN ACTIVE RECENT PATTERN. ACROSS THE GULF COAST...ANY MOISTURE RETURN WITH IMPULSE PASSAGES COULD BRING A WINTERY OVERRUNNING PRECIPITATION THREAT. LOCATIONS OUT WEST WILL GENERALLY STAY ON THE MILD SIDE...BUT A WEEKEND SHORTWAVE TROUGH PASSAGE WOULD FAVOR A SWATH OF COOLED PRECIPITATION FROM THE NORTHWESTERN U.S. INTO THE NORTHERN ROCKIES AND NORTH-CENTRAL STATES. PRECIPITATION IN THE SOUTHWEST TO START THE NEW YEAR IS HIGHLY DEPENDENT ON HOW THE TROUGHING ACROSS THE EAST PAC PLAYS OUT. OF COURSE...A MORE OFFSHORE SOLUTION WOULD FAVOR DRYER CONDITIONS (LIKE THAT OF THE GFS AND TO SOME EXTENT THE 00Z ECMWF)...BUT ANY ENERGY WHICH DOES MAKE IT ONSHORE MAY MEAN ORGANIZED PRECIPITATION ACROSS PORTIONS OF CALIFORNIA AND THE SOUTHWEST. SANTORELLI