834  
FXUS02 KWBC 110643  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
142 AM EST MON DEC 11 2017  
 
VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THERE WILL BE A SIGNIFICANT SHIFT IN THE SYNOPTIC-SCALE PATTERN  
DURING THE PERIOD WITH A MOVEMENT FROM HIGHLY AMPLIFIED TO MORE  
PROGRESSIVE WITH LOWER AMPLITUDE TRANSIENT FEATURES.  
INITIALLY...THERE WILL STILL BE ROBUST 500-MB HEIGHT  
ANOMALIES...GENERALLY IN THE 2 TO 2.5 STANDARD DEVIATION RANGE.  
THIS CONSISTS OF A BROAD LONGWAVE TROUGH EXTENDING OVER MUCH OF  
NORTHEASTERN NORTH AMERICA ALTHOUGH THE LOWEST HEIGHTS WILL BE  
CENTERED WITHIN A POWERFUL UPPER LOW OVER NORTHERN QUEBEC ON  
14/1200Z. ONE PRIMARY SHORTWAVE REMAINS IN THE WAVE TRAIN WITH  
SOME GAIN OF AMPLITUDE ON APPROACH TOWARD THE EAST COAST.  
HOWEVER...MANY SOLUTIONS DO NOT SHOW A WHOLE LOT OF SURFACE  
DEEPENING OFFSHORE AS THE WAVE EXITS THE EASTERN SEABOARD. THERE  
IS SOME HINT THAT A LOBE OF SHORTWAVE ENERGY WILL SPLIT OFF AND  
PUSH THROUGH THE SOUTHERN U.S. ON FRIDAY/SATURDAY ALTHOUGH THIS  
SCENARIO SEEMS TO BE A LOW PROBABILITY OUTLIER. LOOKING BACK TO  
THE WEST COAST...A STAGNANT UPPER RIDGE IS FORECAST TO FINALLY  
BUCKLE BY FRIDAY AS UPSTREAM HEIGHT FALLS SWING INTO THE PACIFIC  
NORTHWEST BY EARLY FRIDAY. THIS SYSTEM IN PARTICULAR APPEARS TO BE  
THE ONE WHICH FINALLY SHIFTS THE PATTERN TOWARD MORE PROGRESSIVE  
AND OF SOMEWHAT LOWER AMPLITUDE WAVES. THE SCENARIO OF SOME OF  
THIS ENERGY CUTTING OFF OVER THE DESERT SOUTHWEST APPEARS TO BE  
DISAPPEARING RELATIVE TO THE GUIDANCE YESTERDAY. GENERALLY  
SPEAKING...THIS SYSTEM SHOULD QUICKLY TRANSLATE EASTWARD WITH SOME  
POTENTIAL SURFACE DEEPENING OVER THE NORTHERN TIER STATES IF ANY  
NORTHERN INTERACTION TAKES PLACE. WHILE THIS PROGRESSIVE SHORTWAVE  
MOVES TOWARD THE EASTERN SEABOARD BY DAY 7/DECEMBER  
18...ADDITIONAL AMPLIFICATION IS LIKELY OVER THE NORTH-CENTRAL  
U.S. WHILE QUASI-ZONAL FLOW IS ESTABLISHED BACK TOWARD THE EASTERN  
PACIFIC.  
 
REGARDING THE SHORTWAVE MOVING THROUGH THE OH VALLEY LATE  
THURSDAY/EARLY FRIDAY...MODELS ARE STILL AT ODDS WITH ONE ANOTHER  
REGARDING SURFACE EVOLUTION. ENSEMBLE SCATTER LOW PLOTS SHOW A  
VAST MAJORITY OF THE 12Z ECMWF ENSEMBLES FAVORING A MORE SOUTHERN  
WAVE ACROSS THE OH VALLEY ON 14/1200Z. MEANWHILE...THE LAST  
SEVERAL RUNS OF THE GFS ARE MORE CONSISTENT WITH A NORTHERN LOW  
PUSHING THROUGH THE LOWER GREAT LAKES AT THE SAME TIME. THEN THERE  
ARE MODELS LIKE THE 00Z CMC WHICH FAVOR THE EXISTENCE OF BOTH OF  
THESE SOLUTIONS. MEANWHILE...SOME SPREAD EXISTS ON THE SOUTHERN  
EXTENT OF THE EJECTING LONGWAVE TROUGH AS THE 12Z ECMWF/UKMET  
ATTEMPT TO SEPARATE SOME OF THIS ENERGY. THIS PARTICULAR SCENARIO  
WOULD ALLOW FOR A GULF COAST WAVE TO FORM SOUTH OF THE MS/AL COAST  
ON FRIDAY MORNING ALTHOUGH ITS SOLUTION IS A DISTINCT MINORITY  
AMONG THE 90 AVAILABLE ENSEMBLE MEMBERS. FURTHER...THE 00Z UKMET  
BACKED OFF ON THIS AS WELL WHICH WAS EXPECTED GIVEN THE DIVERGENCE  
FROM THE ENSEMBLE SPAGHETTI CLUSTERING. SHIFTING THE FOCUS  
WEST...GENERAL AGREEMENT EXISTS IN THE EVENTUAL EROSION OF THE  
MEAN UPPER RIDGE ACROSS THE WESTERN U.S. BY SOMETIME FRIDAY.  
MULTI-DAY ENSEMBLE SPAGHETTI PLOT COMPARISONS SHOW A SHIFT AWAY  
FROM PINCHING OFF A CLOSED LOW OVER THE SOUTHWESTERN STATES BY  
EARLY SATURDAY. THE 12Z CMC STILL SUGGESTED ITS EXISTENCE BUT HAS  
SINCE BACKED OFF VIA THE 00Z RUN. THE BIGGEST ISSUE NOW IS TIMING  
WITH THE 12Z ECMWF AS WELL AS PREVIOUS RUNS BEING ON THE QUICKER  
SIDE OF THE ENVELOPE. THERE IS THE POTENTIAL FOR A FAIRLY  
IMPRESSIVE SURFACE CYCLONE ACROSS THE GREAT LAKES BY SUNDAY  
MORNING BUT MODELS ARE INCONSISTENT WITH INTENSITY AND PLACEMENT.  
FOR INSTANCE...THE 12Z GFS TOOK THE SYSTEM DOWN TO A 983-MB  
PRESSURE ON 17/1200Z WHILE THE RUNS AFTERWARD WERE 1003/1002-MB.  
AS THIS SYSTEM ACCELERATES EASTWARD...ANOTHER DEEP CYCLONE MAY  
EMERGE OVER THE NORTH-CENTRAL BY DAY 7/DECEMBER 18 BUT DETAILS ARE  
AGAIN FUZZY AT BEST.  
 
INITIALLY WENT WITH AROUND 60 TO 80 PERCENT OF 12Z GFS/ECMWF/CMC  
FOR DAYS 3/4...THURSDAY/FRIDAY TO ACCOUNT FOR THE SYSTEM MOVING  
TOWARD THE EAST COAST. GENERALLY STAYED AWAY FROM ENSEMBLE MEANS  
AS THEY AVERAGED THE TWO LOWS MOVING THROUGH THE OH VALLEY/GREAT  
LAKES ON THURSDAY. DID QUICKLY REDUCE CONTRIBUTIONS OF THE 12Z  
ECMWF ON DAYS 4/5...FRIDAY/SATURDAY TO AROUND 10 TO 15 PERCENT  
GIVEN THE OUTLYING SOLUTION OVER THE SOUTHEASTERN U.S. BEYOND THIS  
PERIOD...PUSHED TOWARD A FULL ENSEMBLE MEAN APPROACH GIVEN  
UNCERTAINTIES WITH THE TIMING OF THE PROGRESSIVE TROUGH AS WELL AS  
THE POTENTIAL FOR DEEP SURFACE LOWS ALONG THE NORTHERN TIER. GAVE  
A SLIGHT EDGE TO THE 12Z GEFS MEAN RELATIVE TO THE 12Z ECMWF  
ENSEMBLE MEAN DURING THE END OF THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THERE WILL ONLY BE A BRIEF PERIOD OF BELOW AVERAGE TEMPERATURES  
AND THIS APPEARS TO BE CONFINED TO LOCATIONS EAST OF THE MS RIVER  
ON THURSDAY/FRIDAY. THE FINAL ARCTIC SURGE SHOULD BRING A  
REINFORCEMENT OF COLD AIR TO THE REGION WITH THE MOST ANOMALOUS  
TEMPERATURES BEING OVER NEW ENGLAND WITH ANOMALIES IN THE 10 TO 20  
DEGREE BELOW CLIMATOLOGY RANGE. THIS SUGGESTS HIGHS ON THURSDAY  
SHOULD STRUGGLE TO GET OUT OF THE UPPER SINGLE DIGITS AND TEENS  
OVER INTERIOR NEW ENGLAND. OTHERWISE...MUCH OF THE COUNTRY SHOULD  
SEE ABOVE AVERAGE TEMPERATURES AS THE FLOW FLATTENS AND BECOMES  
MORE PACIFIC DOMINANT. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES  
WILL RAISE TEMPERATURES INTO THE 50S AS FAR NORTH AS EASTERN MT ON  
FRIDAY. THIS IS EASILY 15 TO 20 DEGREES ABOVE AVERAGE.  
 
LOOKING AT PRECIPITATION DURING THE PERIOD...SOME LAKE EFFECT  
ACTIVITY IS POSSIBLE ALTHOUGH IT SHOULD BE MORE TAME GIVEN A  
CHANGING SYNOPTIC-SCALE PATTERN. WHILE PRECIPITATION CHANCES WILL  
LOOM THROUGHOUT THE PERIOD...IT SHOULD BE WARM ADVECTION DRIVEN  
VERSUS MESOSCALE BANDING RELATED. OUT WEST...CONDITIONS WILL BE  
MUCH MORE ACTIVE THAN THEY HAVE BEEN THIS MONTH GIVEN THE  
FLATTENING OF THE RIDGE. WIDESPREAD RAIN/SNOW SHOULD BLANKET THE  
PACIFIC NORTHWEST AND LOCATIONS DOWNSTREAM INTO THE CENTRAL GREAT  
BASIN/UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. GIVEN A LACK OF  
COLD AIR...SNOWFALL SHOULD BE RELEGATED TO THE HIGHER ELEVATIONS.  
THIS THREAT COULD CONTINUE THROUGH DAY 7/DECEMBER 18 GIVEN THE  
SEMI-ZONAL FLOW IN PLACE. ELSEWHERE...WARM ADVECTION SHOWERS ARE  
POSSIBLE FROM THE WESTERN GULF COAST TOWARD THE TN  
VALLEY/SOUTHEASTERN U.S. THIS WEEKEND ALTHOUGH MODEL SPREAD IS  
HIGH WITH THE THE 00Z GFS BEING WET WHILE THE 12Z ECMWF IS  
COMPLETELY DRY.  
 
RUBIN-OSTER  
 

 
 
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