EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST THU DEC 28 2017 VALID 12Z SUN DEC 31 2017 - 12Z THU JAN 04 2018 ...ARCTIC AIR WILL CONTINUE ITS DOMINANCE EAST OF THE ROCKIES WELL INTO THE NEW YEAR... ...PATTERN OVERVIEW/GUIDANCE/PREDICTABILITY ASSESSMENT/PREFERENCES... LITTLE CHANGE IN THE MID/UPPER PATTERN IS EXPECTED MOVING INTO THE NEW YEAR AS AN EXPANSIVE NEGATIVE HEIGHT ANOMALY REMAINS A FIXTURE IN THE FORECAST FROM THE GREAT PLAINS EASTWARD. GENERALLY SPEAKING...MULTIPLE SHORTWAVES WILL PEEL OFF A REGION OF BROAD CYCLONIC FLOW ACROSS CENTRAL/EASTERN U.S. THIS SHOULD ALLOW FOR SEVERAL REINFORCING SURGES OF ARCTIC AIR WHICH WILL MAINTAIN WELL BELOW NORMAL TEMPERATURES FROM THE ROCKIES ALL THE WAY TO THE EASTERN SEABOARD. THE INITIAL SYSTEM WITHIN THE WAVE TRAIN WILL BE IN THE PROCESS OF EXITING NEW ENGLAND ON SUNDAY WHILE THE NEXT ROUND OF HEIGHT FALLS PUSH THROUGH THE NORTHERN ROCKIES EARLY NEW YEARS DAY. THIS PARTICULAR FEATURE IS OF INTEREST AS IT ROUNDS THE BASE OF THE DOWNSTREAM TROUGH AND POTENTIALLY STRENGTHENS BY ADDITIONAL AMPLIFICATION WITHIN THE NORTHERN STREAM. A COASTAL WAVE MAY DEVELOP ON WEDNESDAY MORNING OFFSHORE OF GA/SC WITH A NORTHWARD TRACK UP THE GULF STREAM INDICATED IN THE 00Z ENSEMBLES. PREVIOUS MODEL CYCLES DID NOT SHOW THIS SCENARIO SO WILL BE INTERESTING TO SEE WHAT FUTURE RUNS SHOW. ELSEWHERE...A MEAN RIDGE IS LIKELY TO STAY IN PLACE INTO EARLY NEXT WEEK ACROSS THE EASTERN PACIFIC/WEST COAST. SPLIT FLOW OVER THE OFFSHORE WATERS WILL ALLOW A SOUTHERN STREAM FEATURE TO SLOWLY PUSH TOWARD THE COASTLINE BY MID-WEEK BEFORE LIKELY WEAKENING/SHEARING THEREAFTER. IT DOES APPEAR THE UPSTREAM TROUGHING WILL RE-LOAD INTO DAY 7/JANUARY 4 WHILE HELPING REINFORCE THE PERSISTENT RIDGE OVER THE WESTERN U.S. WHILE GENERAL AGREEMENT EXISTS WITH THE INITIAL PATTERN ACROSS THE COUNTRY...THE GUIDANCE BREAKS DOWN WITH THE EVOLVING SHORTWAVE ENERGETICS MOVING THROUGH THE CENTRAL/EASTERN U.S. ON NEW YEARS DAY. THE 00Z UKMET WAS A SLOWER OUTLIER WITH THE MEAN TROUGH SETTLING OVER THE GREAT LAKES/NORTHEASTERN U.S. WHILE FURTHER DIFFERENCES ARE NOTED UPSTREAM WITH THE SHORTWAVE DIGGING INTO THE NORTHERN ROCKIES. ENSEMBLE SPAGHETTI PLOTS SHOW THIS SPREAD WELL AS 00Z GEFS MEMBERS ARE QUICKER TO ACCELERATE AND STRETCH THIS SYSTEM OUT ACROSS THE OZARKS/TN VALLEY ON TUESDAY. LIKEWISE...THIS SPEED DIFFERENCE IS EVIDENT WITHIN THE AMPLIFIED NORTHERN STREAM ACROSS THE GREAT LAKES. THE PAST COUPLE OF ECMWF RUNS AS WELL AS THE 00Z CMC FAVOR A SLOWER PROGRESSION OF BOTH ENTITIES WHICH LEADS TO SOMEWHAT SIMILAR FORECASTS MOVING INTO THE EARLY/MIDDLE PART OF NEXT WEEK. WHILE NOT UTILIZED...IT IS WORTH NOTING THE 00Z UKMET IS EVEN SLOWER HERE. BY DAY 6/7...JANUARY 3/4...THERE IS A GROWING CONSENSUS OF COASTAL WAVE DEVELOPMENT DURING THE PERIOD ALTHOUGH IT IS HEAVILY DRIVEN BY THE 00Z CMC/ECMWF ENSEMBLES. IT IS A WORTHWHILE TREND THOUGH WITH THE LATEST 00Z ECMWF ENSEMBLE MEAN SHOWING A 1004-MB ISOBAR NEAR THE 40N/70W BENCHMARK BY 04/1200Z WHILE PRECEDING ECMWF ENSEMBLES SHOWED LITTLE TO NOTHING THERE. THE OPERATIONAL 00Z ECMWF IS A MORE EXTREME OUTLIER AS ENSEMBLE SPAGHETTI PLOTS SHOW BUT THERE IS A LOT OF SUPPORT FOR THIS MORE AMPLIFIED SCENARIO. IT IS DIFFICULT TO COMMIT TO THIS JUST YET BUT IT IS WORTH WATCHING AS FUTURE MODEL RUNS ARRIVE. ACROSS THE WEST...THE BIGGEST TREND IN THE GUIDANCE IS TO BE SLOWER BY A CONSIDERABLE AMOUNT RELATIVE TO MODELS A DAY OR TWO AGO. THIS ENSURES THE SOUTHERN STREAM FEATURE WOULD NOT REACH THE CA/OR COAST UNTIL THE MIDDLE OF NEXT WEEK. HOWEVER...THERE ARE A LOT OF ENSEMBLE SOLUTIONS WHICH ARE QUICKER THAN THE OPERATIONAL RUNS SO THERE COULD BE EASTWARD MOVEMENT IN THE NEXT MODEL RUNS. INITIALLY BASED ON THE FORECAST ON A COMBINATION OF THE 00Z GFS/ECMWF/CMC THROUGH DAY 4/MONDAY WITH A SMALL AMOUNT OF ENSEMBLE GUIDANCE INCLUDED IN THE MIX. BY DAY 5/TUESDAY ONWARD...STARTED TO MOVE TOWARD A 00Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF ENSEMBLE MEAN BLEND BEFORE MOVING TO AN 80 PERCENT ENSEMBLE MEAN BASED SOLUTION BY DAY 7/JANUARY 4 AS UNCERTAINTY GROWS ACROSS THE MAP. CONFIDENCE RESIDES AROUND AVERAGE ALTHOUGH WOULD LIKE TO SEE MORE GEFS MEMBERS SUPPORTING THE COASTAL LOW DEVELOPMENT OFF THE EAST COAST. ...WEATHER HIGHLIGHTS/THREATS... MULTIPLE ARCTIC INTRUSIONS WILL ENSURE WELL BELOW NORMAL TEMPERATURES TO SPILL FROM THE ROCKIES EASTWARD WITH THE MOST ANOMALOUS READINGS EXPECTED TO REACH THE 30 TO 35 DEGREE BELOW AVERAGE RANGE. THIS WILL LIKELY BREAK A NUMBER OF DAILY TEMPERATURE RECORDS FOR COLD AFTERNOON HIGHS. HIGHS ACROSS SOUTH DAKOTA/NEBRASKA ON SUNDAY WILL STAY IN THE NEGATIVE SINGLE DIGITS WHILE THE FOLLOWING DAY NUMBERS IN THE SINGLE DIGITS AND TEENS ARE POSSIBLE ALONG THE I-95 CORRIDOR DOWN TO THE UPPER MID-ATLANTIC. EVEN SOME TEMPERATURES MAY STRUGGLE TO GET ABOVE FREEZING AS FAR SOUTH AS NORTHERN LOUISIANA ON MONDAY. THE TRUE COLD SPOT WILL BE ACROSS THE NORTHERN ROCKIES EASTWARD THROUGH THE NORTHERN PLAINS/UPPER MIDWEST AS LOW TEMPERATURES ON SUNDAY/MONDAY MAY STAY IN THE -20S TO POSSIBLY -30S IN SOME LOCATIONS. MEANWHILE...PERSISTENT RIDGING ALONG THE WEST COAST SHOULD KEEP TEMPERATURES AROUND 5 TO 10 DEGREES ABOVE CLIMATOLOGY DURING MUCH OF THE FORECAST PERIOD. REGARDING PRECIPITATION...CYCLONIC FLOW ACROSS THE GREAT LAKES WILL LEAD TO SOME LAKE EFFECT SNOWS DURING THE PERIOD. RUBIN-OSTER