EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1058 AM EST MON DEC 25 2017 VALID 12Z THU DEC 28 2017 - 12Z MON JAN 01 2018 ...OVERVIEW... THE LARGE SCALE PATTERN ACROSS NORTH AMERICA WILL REMAIN LARGELY PERSISTENT THROUGH THE MEDIUM RANGE, WITH UPPER RIDGING OFF THE U.S. WEST COAST, EXTENDING NORTH INTO THE ARCTIC AND TROUGHING ACROSS THE MIDWEST/GREAT LAKES/NORTHEAST. A SHORTWAVE MOVING INTO THE PAC NW LATE FRI/SAT WILL BRIEFLY WEAKEN THE RIDGE BEFORE IT RECOVERS BY EARLY NEXT WEEK. THIS PATTERN WILL SUPPORT CONTINUED COLD TEMPERATURES FOR MUCH OF THE CENTRAL AND EASTERN U.S., WITH EVEN FLORIDA SUBJECT TO POTENTIAL INTRUSIONS OF COLD AIR MASSES. A CLIPPER IS EXPECTED TO CROSS THE GREAT LAKES ON FRI AND THE EASTERN U.S. ON SAT. THE ENERGY ASSOCIATED WITH THIS SYSTEM IS EXPECTED TO PHASE OFFSHORE WITH AN ADDITIONAL FRONTAL WAVE, WITH THE POTENTIAL TO BRING SOME WINTER WEATHER TO PORTIONS OF THE EASTERN/NORTHEASTERN STATES. BY EARLY NEXT WEEK THE ARCTIC FRONT IS EXPECTED TO BE IN THE GULF OF MEXICO AND NEAR OR PAST SOUTH FLORIDA. GUIDANCE INDICATES SOME POTENTIAL FOR A FRONTAL WAVE TO DEVELOP EITHER ACROSS THE SOUTHEASTERN U.S. OR IN THE GULF OF MEXICO ALONG THIS FRONT, ALTHOUGH CONFIDENCE IN THAT SCENARIO IS LOW AT THIS TIME (MORE DETAILS BELOW). ...GUIDANCE/PREDICTABILITY ASSESSMENT... MODELS INITIALLY SHOW TIMING AND AMPLITUDE DIFFERENCES WITH A SHORTWAVE CROSSING THE NORTHERN PLAINS/MIDWEST THU NIGHT/FRI. THE GFS HAS CONSISTENTLY BEEN AMONG THE FASTEST SOLUTIONS WITH THIS WAVE, WITH THE ECMWF ON THE SLOW SIDE, BUT CLOSE TO THE ECENS/NAEFS MEANS. THE CMC APPEARED TO BE A COMPROMISE, ALBEIT A BIT CLOSER TO THE GFS THAN THE ECMWF. THESE DIFFERENCES TRANSLATE DOWNSTREAM AND AFFECT THE EVENTUAL PHASING OF THIS ENERGY WITH A FRONTAL WAVE OFF THE EAST COAST. IN GENERAL THE ECMWF HAS BEEN A BIT MORE CONSISTENT WITH THE HANDLING OF THIS ENERGY AND THE PHASING/NEW SURFACE CYCLONE OFF THE NEW ENGLAND COAST BY LATE SAT, EVENTUALLY GIVING THE UPPER TROUGH A NEGATIVE TILT AND WRAPPING THE SURFACE CYCLONE INTO NEW BRUNSWICK/NOVA SCOTIA. GIVEN THE RELATIVE CONSISTENCY OF THE ECMWF SOLUTION ALONG WITH THE SUPPORT OF THE ECENS/NAEFS MEANS, AM INCLUDED TO FAVOR SOMETHING IN THAT GENERAL DIRECTION FOR THE FORECAST. FARTHER WEST, MODELS ALSO SHOW A RANGE OF TIMING FOR THE SHORTWAVE AND ASSOCIATED SURFACE FRONT REACHING THE NORTHWESTERN U.S. FRI-SAT, WITH THE GFS FASTEST/WEAKEST AND THE ECMWF/CMC SLOWER. ACROSS THIS AREA, A BLENDED APPROACH SEEMS BEST. AS A RESULT, THE WPC FORECAST DURING DAYS 3-5 WAS BASED LARGELY ON A MULTI-MODEL BLEND INCLUDING THE 00Z ECMWF/00Z CMC/06Z GFS (WITH MORE WEIGHT PLACE ON THE ECMWF). DIFFERENCES WITH HOW VARIOUS MODELS HANDLE THE CYCLONE HAVE EFFECTS ON THE POTENTIAL OR LACK THEREOF FOR ANOTHER SURFACE WAVE TO AFFECT THE EASTERN U.S. BY LATE IN THE FORECAST PERIOD. AS THE GFS MOVES THE EASTERN CYCLONE NORTHWARD INTO CANADA MUCH MORE QUICKLY DAYS 5-6, THIS SEEMS TO RESULT IN A WEAKENING OF COLD ADVECTION AND RISING HEIGHTS/THICKNESSES TO SOME DEGREE ALONG THE EAST COAST. THIS WOULD MAKE THE ENVIRONMENT MORE FAVORABLE FOR A FRONTAL WAVE THAT MOVES NORTH OFF THE MID-ATLANTIC/NORTHEAST COAST BY DAY 7, AS THE GFS SHOWS. GIVEN THAT A SLOWER DAY 5-6 CYCLONE, AS SHOWN BY THE ECENS/NAEFS MEANS AND THE ECMWF IS FAVORED, THIS SHOULD HELP TO KEEP THE HEIGHT/THICKNESS FIELDS ACROSS THE EAST, AND ANY POTENTIAL FRONTAL WAVE BY DAY 7, MORE SUPPRESSED SOUTHWARD. THE GFS SOLUTION BY DAY 7 HAS VERY LITTLE IN TERMS OF ENSEMBLE SUPPORT AS WELL. AS A RESULT, THE WPC FORECAST BY DAYS 6-7 IS CONSISTENT WITH CONTINUITY IN SHOWING AN ARCTIC FRONT SOUTH OF FLORIDA AND INTO THE GULF OF MEXICO, WITH NO SIGNIFICANT FRONTAL WAVE ALONG THE EAST COAST. FARTHER WEST, MODELS ALSO SEEM TO SHOW THEIR TYPICAL BIASES WITH ANOTHER SHORTWAVE/FRONTAL SYSTEM REACHING THE PAC NW BY NEXT MON, WITH THE GFS ON THE FAST SIDE OF THE SPREAD AND THE ECMWF ON THE SLOW SIDE (EVEN SLOWER THAN THE ECENS MEAN). GIVEN THE INCREASED SPREAD AND UNCERTAINTY DURING DAYS 6-7, THE WPC FORECAST WAS BASED MORE ON A 00Z ECENS/NAEFS BLEND, WITH SMALLER COMPONENTS OF THE DETERMINISTIC SOLUTIONS USED DURING DAY 3-5. THE GFS WAS EXCLUDED FROM THE FORECAST BY DAY 7 GIVEN THE PREFERENCE AGAINST ITS EAST COAST LOW SOLUTION BY THAT TIME. ...WEATHER/THREATS HIGHLIGHTS... AS MENTIONED ABOVE, THE PATTERN WILL CONTINUE TO SUPPORT THE INCURSION OF ARCTIC AIR INTO MUCH OF THE CENTRAL AND EASTERN U.S., WITH HIGH TEMPERATURES 15 TO 25 DEG F BELOW AVERAGE THROUGH MUCH OF THE PERIOD. WHILE A FEW RECORD LOWS ARE POSSIBLE, THE TYPE OF RECORD MOST IN JEOPARDY APPEARS TO BE RECORD LOW MAXIMUM TEMPERATURES. THE POTENTIAL FOR AT LEAST SOME WINTER WEATHER WILL EXIST ACROSS PORTIONS OF THE SOUTHEAST THU-THU NIGHT AS A WAVE OF LOW PRESSURE BEGINS TO DEVELOP ALONG THE FRONT IN THE GULF/WESTERN ATLANTIC. AS THIS SYSTEM MOVES NORTH AND EVENTUALLY PHASES/DEVELOPS INTO A MATURE CYCLONE ACROSS SOUTHEASTERN CANADA BY SUN MORNING, THE POTENTIAL FOR SNOW/WIND WILL INCREASE, ESPECIALLY ACROSS NORTHERN/EASTERN NEW ENGLAND. CONFIDENCE IS LOW IN ANY ADDITIONAL WINTER THREATS FOR THE EASTERN/SOUTHEASTERN U.S. BY EARLY NEXT WEEK, BUT ANY POTENTIAL WEAK WAVE OF LOW PRESSURE ALONG THE FRONT IN THE GULF COULD RESULT IN POTENTIAL AREAS OF WINTER WEATHER. MEANWHILE, THE PACIFIC NORTHWEST WILL SEE A COUPLE ROUNDS OF POTENTIAL PRECIPITATION. THE FIRST WITH SHORTWAVE/FRONT ARRIVING FRI NIGHT/SAT, AND ANOTHER BY SUN-MON. BOTH OF THESE WILL PRODUCE RAIN AND MOUNTAIN SNOWS FOR THE NORTHWEST AND THE NORTHERN ROCKIES. RYAN