923  
FXUS02 KWBC 130654  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
153 AM EST WED DEC 13 2017  
 
VALID 12Z SAT DEC 16 2017 - 12Z WED DEC 20 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE SYNOPTIC-SCALE PATTERN WILL FEATURE A SPLIT FLOW REGIME WITH A  
NUMBER OF MODEL DIFFERENCES WHICH MAKE THIS A CHALLENGING AND LOW  
CONFIDENCE FORECAST AHEAD. TO START WITH...A PRONOUNCED UPPER  
TROUGH WILL BE IN THE PROCESS OF EXITING NEW ENGLAND ON 16/1200Z  
WHILE RENEWED HEIGHT FALLS LURK UPSTREAM IN THE VICINITY OF THE  
NORTHERN/CENTRAL ROCKIES. DETACHED FROM THE PREVAILING WESTERLIES  
IS AN ELONGATED UPPER LOW INITIALLY POSITIONED OVER CENTRAL MX ON  
SATURDAY MORNING. THIS LATTER FEATURE IS EXPECTED TO QUICKLY  
BECOME ABSORBED BY THE MID-LATITUDE TROUGH PUSHING THROUGH THE  
CENTRAL U.S. BY EARLY SUNDAY. AS WILL BE MENTIONED IN THE NEXT  
SECTION...MODEL UNCERTAINTY PLAGUES THE EASTWARD PROGRESSION OF  
THIS LONGWAVE TROUGH WITH THE POTENTIAL FOR SOME SHORTWAVE ENERGY  
TO PEEL OFF AND SEPARATE OVER THE FOUR CORNERS REGION. THE SYSTEM  
SHOULD EVENTUALLY REACH THE EAST COAST BY ANYWHERE BETWEEN DAY  
5-7...DECEMBER 18-20...DEPENDING ON THE SELECTED MODEL. A MYRIAD  
OF SURFACE WAVES ARE NOTED ON OR IN THE VICINITY OF THE  
PROPAGATING BOUNDARY WITH PERHAPS CYCLONES OVER THE GREAT  
LAKES/LOWER MS VALLEY/RED RIVER ON SUNDAY MORNING.  
WEAKENING/SHEARING OF THE MID-LEVEL THROUGH WILL WEAKEN THE  
SURFACE REFLECTION LATER INTO THE PERIOD. ALL THE  
WHILE...AMPLIFIED FLOW WITHIN THE NORTHERN STREAM WILL TAKE SHAPE  
OVER THE NORTHERN PLAINS/UPPER MIDWEST BY MONDAY ACCOMPANIED BY A  
DEEP SURFACE LOW OVER CENTRAL CANADA. BACK WEST...IN THE WAKE OF  
THE SYSTEM TRACKING TOWARD THE SOUTHERN HIGH PLAINS...A RETURN OF  
UPPER RIDGING IS EXPECTED THROUGH DAY 6/TUESDAY ALTHOUGH THERE ARE  
SOME HINTS THAT A CUT-OFF LOW MAY AFFECT SOUTHERN CA. EVENTUALLY A  
PATTERN CHANGE WILL BE UNDERWAY AS A POWERFUL POSITIVELY-TILTED  
TROUGH SWINGS SOUTHWARD FROM BRITISH COLUMBIA TOWARD THE PACIFIC  
NORTHWEST ON DAY 7/DECEMBER 20.  
 
AS DISCUSSED PREVIOUSLY...A LOW CONFIDENCE FORECAST AT BEST WITH  
ENSEMBLE SPAGHETTI PLOTS FROM DAY 4 LOOKING CLOSER TO WHAT ONE  
WOULD EXPECT ON DAY 6/7. WHILE THE UPPER TROUGH EXITING NEW  
ENGLAND APPEARS TO BE WELL AGREED UPON...THE OPPOSITE CAN BE SAID  
FOR THE EVOLVING SYSTEM PUSHING TOWARD THE HIGH PLAINS ON  
SATURDAY. INITIALLY...AN UPPER LOW OVER CENTRAL MX IS A NEW  
FEATURE ON THE MAP AS PREVIOUS DAYS HAD IT WELL WEST BACK TOWARD  
BAJA CA. WHILE EARLY PLACEMENT IS VARIABLE...ALL OF THE GUIDANCE  
AGREES ON LIFTING THIS CLOSED LOW NORTH-NORTHEASTWARD WITH GRADUAL  
ABSORPTION INTO THE APPROACHING LONGWAVE TROUGH. REGARDING THIS  
LATTER FEATURE...THE MOST STABLE SOLUTION ON A RUN-TO-RUN BASIS  
HAS BEEN THE GFS. IT HAS STUCK WITH ITS SLOWER FORECAST ALL ALONG  
AS IT ALLOWS FOR A LOBE OF VORTICITY TO PEEL OFF AND DRIFT  
EASTWARD FROM THE FOUR CORNERS REGION TO THE OZARKS FROM SUNDAY  
THROUGH TUESDAY. SUCH A SCENARIO HAS SOME NEIGHBORS...MOST NOTABLY  
THE 12Z CMC/UKMET. MEANWHILE...THE ECMWF SYSTEM HAS BEEN THE  
FASTEST GLOBAL MODEL ALL ALONG ALTHOUGH IT HAS MADE A DRAMATIC  
WESTWARD SHIFT DURING THE 12Z/00Z RUNS. UNFORTUNATELY...IT IS  
STILL THE QUICKEST PIECE OF GUIDANCE WHILE ALSO BEING THE MOST  
VIGOROUS IN TERMS OF POSITIVE VORTICITY ADVECTION OVER THE  
SOUTHEASTERN U.S. SOMEWHAT UNNERVING...THE 00Z CMC/UKMET HAVE  
TRENDED QUICKER BUT STILL REMAIN WEST OF THE MENTIONED ECMWF  
SOLUTIONS. FROM WHAT HAS BEEN SEEN OF THE 00Z ECMWF...IT HAS MOVED  
WESTWARD WHICH IS A GOOD SIGN AS THE EARLIER RUNS WERE FOR THE  
MOST PART REJECTED AS OUTLIERS. BUT IT STILL IS THE QUICKEST 00Z  
OPERATIONAL MODEL AT THIS TIME.  
 
CONSIDERING THE REMAINDER OF THE MAP...AMPLIFIED FLOW WITHIN THE  
NORTHERN STREAM WILL BEGIN TO AFFECT THE NORTHERN PLAINS/UPPER  
MIDWEST BY AS EARLY AS MONDAY. BASED ON THE ENSEMBLE SPAGHETTI  
PLOTS...TEMPORAL DIFFERENCES ARE ALSO EVIDENT HERE WITH THE 12Z  
ECMWF ENSEMBLE SYSTEM WELL AHEAD OF THE OTHER GUIDANCE. OVER THE  
WEST COAST...THE PAST THREE RUNS OF THE ECMWF ARE THE ONLY MODELS  
TO SHOW A CUT-OFF UPPER LOW MOVING TOWARD SOUTHERN CA ON DAY  
6/7...DECEMBER 19/20. BACKTRACKING SUGGESTS THIS SYSTEM ACTUALLY  
PEELS OFF FROM THE INITIAL HEIGHT FALLS MOVING THROUGH THE WESTERN  
U.S. ON SATURDAY. GIVEN NO OTHER MODEL SHOWS THIS EVOLUTION...WILL  
CONTINUE TO DISMISS IT AS AN OUTLIER. LOOKING NORTHWARD...A  
POSITIVELY-TILTED TROUGH IS FORECAST TO DIP DOWN FROM BRITISH  
COLUMBIA INTO THE PACIFIC NORTHWEST LATE IN THE PERIOD. RELATIVE  
TO THE REST OF THE PERIOD...IT CONSISTS OF THE LOWEST MODEL SPREAD  
DESPITE BEING A DAY 7 FORECAST. RUN-TO-RUN CONTINUITY IS A LITTLE  
SKETCHY THOUGH AS THE 00Z GFS SLOWED BY LEAST 12 HOURS WHICH WOULD  
KEEP HEIGHT FALLS OUT OF WESTERN WA ON DECEMBER 20.  
 
GIVEN THE ROBUST AMOUNT OF MODEL SPREAD...IT WAS NECESSARY TO LEAN  
MORE HEAVILY ON THE ENSEMBLE MEANS THAN USUAL. THROUGH DAY  
4/SUNDAY...TOOK A MULTI-MODEL BLEND APPROACH WITH THE 12Z GFS/GEFS  
MEAN/UKMET BEING 80 PERCENT OF THE BLEND WITH ONLY 20 PERCENT OF  
THE 12Z ECMWF/ECMWF ENSEMBLE MEAN BEING UTILIZED.  
THEREAFTER...QUICKLY REMOVED THE 12Z ECMWF WHILE LEANING MUCH MORE  
HEAVILY ON THE 12Z GFS/GEFS MEAN/NAEFS ENSEMBLE MEAN. DID KEEP 20  
PERCENT OF THE 12Z ECMWF ENSEMBLE MEAN IN THE MIX DURING THE DAY  
5-7...DECEMBER 18-20 PERIOD TO ACCOUNT FOR GENERAL MODEL  
UNCERTAINTY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
GENERALLY SPEAKING...TEMPERATURES SHOULD BE AT OR ABOVE  
CLIMATOLOGY THROUGH THE FORECAST PERIOD. EXCEPTIONS INCLUDE THE  
EAST COAST ON SATURDAY IN THE WAKE OF A STRONG FRONTAL PASSAGE  
EXITING INTO THE WESTERN ATLANTIC. THIS EXTENDS DOWN ALONG THE  
GULF COAST AS THE BOUNDARY SWEEPS SOUTHWARD THROUGH CUBA ON  
16/1200Z. DEPARTURES FROM NORMAL WILL LIKELY ONLY BE IN THE 5 TO  
10 DEGREE RANGE. BY SUNDAY...COOLER WEATHER IS EXPECTED UNDERNEATH  
THE EVOLVING UPPER TROUGH ACROSS THE FOUR CORNERS/SOUTHERN HIGH  
PLAINS. OTHERWISE...THE ENTIRE COUNTRY CAN EXPECT MILD CONDITIONS  
FOR THE THIRD WEEK OF DECEMBER GIVEN THE DOMINANCE OF PACIFIC AIR  
MASSES. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES WILL ALLOW FOR  
HIGHS TO BE ANYWHERE FROM 10 TO 15 DEGREES ABOVE NORMAL OVER THE  
CENTRAL U.S. BY THE MIDDLE OF NEXT WEEK...HIGH TEMPERATURES MAY  
REACH THE MID 50S ALL THE WAY UP TO THE NEB PANHANDLE.  
 
WITH A LACK OF ARCTIC AIR TO WORK WITH...ANY WINTRY PRECIPITATION  
WILL LIKELY BE CONFINED TO HIGHER ELEVATIONS OVER THE PACIFIC  
NORTHWEST/UPPER INTERMOUNTAIN WEST/ROCKIES AS WELL AS ACROSS THE  
GREAT LAKES AND INTERIOR NEW ENGLAND. CONSIDERING PRECIPITATION IN  
GENERAL...A STREAK OF LIGHT/MODERATE RAIN/SNOW IS LIKELY OVER THE  
CENTRAL/NORTHERN ROCKIES AS THE UPPER TROUGH TRACKS SOUTHEASTWARD  
ON SATURDAY. INCREASING WARM ADVECTION AHEAD OF THIS SYSTEM WILL  
LEAD TO ENHANCED HEAVY RAINFALL CHANCES FROM THE ARKLATEX EASTWARD  
TO THE SOUTHEASTERN U.S. COAST. THERE ARE MANY MODELS SHOWING 2 TO  
3 INCH 24-HOUR AMOUNTS DURING PART OF THE PERIOD. TIMING IS OF  
COURSE AN ISSUE GIVEN THE PREPONDERANCE OF SOLUTIONS IN PLAY. BACK  
TO THE PACIFIC NORTHWEST...AN ATMOSPHERIC RIVER SIGNATURE SHOWS UP  
IN THE GUIDANCE BY THE EARLY/MIDDLE PART OF NEXT WEEK.  
HOWEVER...THERE ARE CHANCES THIS SETS UP NORTH OF THE  
INTERNATIONAL BORDER WITH CANADA AS SHOWN IN THE 00Z GFS.  
 
RUBIN-OSTER  
 

 
 
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