163  
FXUS02 KWBC 120650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
149 AM EST TUE DEC 12 2017  
 
VALID 12Z FRI DEC 15 2017 - 12Z TUE DEC 19 2017  
   
..OVERVIEW/GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AN EVOLVING PATTERN IS EXPECTED DURING THE PERIOD WITH A SHIFT  
FROM ARCTIC DOMINATION EAST OF THE GREAT PLAINS TO BEING MORE  
STRONGLY INFLUENCED BY PACIFIC AIR MASSES. ON 15/1200Z...AMPLIFIED  
FLOW ACROSS THE EASTERN THIRD OF THE COUNTRY WILL BE IN PLACE WITH  
A POTENT SHORTWAVE ACCELERATING FROM THE MID-SOUTH TOWARD THE  
SOUTHEASTERN U.S. ON FRIDAY. THIS SHOULD INDUCE AN AREA OF LOW  
PRESSURE OFF THE CAROLINA COAST WITH A TRACK EAST OF THE NC/VA  
BORDER BY 16/0000Z. QUICK PROGRESSION OF THIS WAVE IS EXPECTED  
WITH A POSITION EAST OF NOVA SCOTIA 12 HOURS LATER BEFORE PUSHING  
INTO THE CANADIAN MARITIMES. LOOKING BACK TO THE WEST COAST...A  
STAGNANT UPPER RIDGE WILL BE IN THE PROCESS OF ERODING AS A  
LOW-AMPLITUDE SHORTWAVE OVER THE EASTERN PACIFIC MOVES INTO THE  
PACIFIC NORTHWEST SOME TIME FRIDAY. PROGRESSIVE HEIGHT FALLS WILL  
MARCH EASTWARD TOWARD THE GREAT PLAINS WITH POTENTIAL NORTHERN  
STREAM INTERACTION WHICH LEADS TO SOME FORECAST UNCERTAINTY WITH  
TIMING AND DEPTH OF RELEVANT SURFACE CYCLONES. GUIDANCE IS  
SCATTERED ALL ABOUT BUT THIS SYSTEM SHOULD REACH THE EASTERN  
SEABOARD DURING THE DAY 6/7...DECEMBER 18/19. IN ITS WAKE...THE  
SYNOPTIC-SCALE PATTERN OVER THE WESTERN/CENTRAL U.S. SHOULD REMAIN  
DE-AMPLIFIED WITH THE NEXT SHORTWAVE AFFECTING THE NORTH-CENTRAL  
STATES EARLY NEXT WEEK. AT THE SAME TIME...LOW-AMPLITUDE RIDGING  
IS LIKELY OVER THE WEST COAST WHILE LOWER HEIGHTS LURK WELL  
OFFSHORE.  
 
MODEL DIFFERENCES ARE COMMONPLACE THROUGHOUT THE FORECAST WHICH  
LED TO SOME BELOW AVERAGE CONFIDENCE FOR THE DAY 6/7...DECEMBER  
18/19 PERIOD. AT THE START...THERE ARE DETAIL DIFFERENCES WITH THE  
EVOLUTION OF THE EASTERN U.S. LONGWAVE TROUGH. THE LARGEST OUTLIER  
REMAINS THE PAST SEVERAL RUNS OF THE UKMET WHICH HAS HAD A  
TENDENCY TO BE SLOWER/DEEPER WITH THE SOUTHERN EXTENT OF HEIGHT  
FALLS. RECENT CYCLES OF THE GFS/ECMWF HAVE BEEN IN CLOSER  
AGREEMENT AND HAVE MADE A WESTWARD SHIFT BASED ON MULTI-DAY  
ENSEMBLE LOW PLOTS. THE 00Z GFS HAS MOVED A TOUCH BIT WEST OF ITS  
PRECEDING RUN WHICH WOULD MARK A SNOWFALL THREAT FOR SECTIONS OF  
THE MID-ATLANTIC THROUGH FRIDAY NIGHT. EXCEPT FOR A FEW RANDOM  
ENSEMBLE MEMBERS...A VAST MAJORITY OF THE 90 CMC/GEFS/ECMWF  
ENSEMBLE MEMBERS WOULD BE WELL EAST OF NEW ENGLAND BY 16/1200Z.  
SHIFTING FOCUS TO THE WESTERN/CENTRAL U.S...MODEL UNCERTAINTY IS  
NOTED QUITE EARLY WITH THE 00Z/12Z CMC BEING DECIDEDLY SLOWER  
EJECTING SHORTWAVE ENERGY INTO THE PACIFIC NORTHWEST ON FRIDAY.  
EVEN WITH THE 00Z GUIDANCE ARRIVING...THEY ADD MORE QUESTIONS THAN  
ANSWERS TO THE PATTERN OVER THE WEEKEND. THE 00Z CMC/UKMET HAVE  
MOVED TOWARD A SOLUTION WHICH FAVORS A BROAD UPPER LOW PEELING OFF  
FROM THE PREVAILING WESTERLIES WITH AN EVENTUAL POSITION OVER THE  
DESERT SOUTHWEST BY SUNDAY. THIS IS A GENERAL SHIFT FROM THEIR  
PRECEDING RUNS AS WELL AS THE MORE PROGRESSIVE GFS/ECMWF SUITES.  
WHILE THE 564-DM ENSEMBLE SPAGHETTI PLOT SHOWS SOME 12Z GEFS/ECMWF  
ENSEMBLE MEMBERS WITH THIS SCENARIO...IT STILL APPEARS TO BE A  
LOWER PROBABILITY SOLUTION AT THIS TIME. CONSIDERING THE 00Z/18Z  
GFS VERSUS THE 12Z ECMWF...THE LATTER CONTINUES TO BE MUCH QUICKER  
WHILE EVEN BECOMING QUICKER THAN ITS PARENT ENSEMBLE MEAN BY AS  
EARLY AS SATURDAY. AT THE SURFACE...THERE IS THE POTENTIAL FOR TWO  
DOMINANT WAVES...ONE ACROSS THE GREAT LAKES WHILE THE OTHER WILL  
EMERGE OUT THE TX/OK PANHANDLES. ENSEMBLE LOW SPREAD IS LESS WITH  
THE SOUTHERN WAVE WHILE IMMENSE UNCERTAINTY DOMINATES THE FORMER  
TO THE NORTH. RUN-TO-RUN CONTINUITY IS POOR WITH INTENSITY AND  
POSITION WHICH SIGNIFICANTLY DECREASES THE FORECAST CONFIDENCE  
HERE. WHILE THIS SYSTEM SLIDES TOWARD THE EAST COAST...RENEWED  
HEIGHT FALLS WILL LIKELY REACH THE NORTH-CENTRAL U.S. BY EARLY  
NEXT WEEK.  
 
WHILE THE EARLY PART OF THE FORECAST COULD BE SALVAGED VIA A 18Z  
GFS/12Z ECMWF BLEND...ENSEMBLE MEANS HAD TO BE INCORPORATED FROM  
DAY 4/SATURDAY ONWARD GIVEN EVERYTHING MENTIONED EARLIER. BY THE  
CONCLUSION OF THE WEEKEND...WAS ONLY ABLE TO KEEP 20 PERCENT OF  
THE OPERATIONAL MODELS IN THE PICTURE WHILE SPLITTING BETWEEN THE  
18Z GEFS AND 12Z ECMWF/NAEFS ENSEMBLE MEANS. BY THE REMAINING TWO  
DAYS OF THE PERIOD...WENT FULLY ENSEMBLE MEANS FAVORING 50 PERCENT  
OF THE 12Z ECMWF ENSEMBLE MEAN WHILE UTILIZING 25 PERCENT EACH OF  
THE 18Z GEFS/12Z NAEFS ENSEMBLE MEANS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE LAST SHOT OF ARCTIC AIR MOVING THROUGH THE EASTERN U.S. WILL  
MAINTAIN BELOW AVERAGE TEMPERATURES THROUGH SATURDAY. WHILE  
FORECAST TEMPERATURE ANOMALIES ARE REDUCED A BIT FROM WHAT WILL BE  
EXPERIENCED BEFORE THIS PERIOD...EXPECT HIGHS AROUND 5 TO 15  
DEGREES BELOW CLIMATOLOGY. THE MOST ANOMALOUS COLD AIR WILL BE  
OVER THE GREAT LAKES UP INTO NEW ENGLAND AS HIGHS STRUGGLE TO GET  
OUT OF THE TEENS AND 20S ON FRIDAY. HOWEVER...THE INCREASED  
DOMINANCE OF PACIFIC AIR MASSES SHOULD ALLOW FOR WELL ABOVE NORMAL  
TEMPERATURES TO PREVAIL...MAXIMIZING IN THE 1O TO 15 DEGREE RANGE  
ACROSS THE CENTRAL U.S. ON FRIDAY...FORECAST HIGHS WILL BE IN THE  
LOW/MID 50S AS FAR NORTH AS EASTERN MT.  
 
A FEW ACTIVE AREAS OF PRECIPITATION ARE LIKELY ON FRIDAY WITH THE  
EASTERN U.S. TROUGH FAVORING A MIXTURE OF RAIN/SNOW OVER THE GREAT  
LAKES WHILE SHOWERS FALL ALONG THE COLD FRONT ACROSS FL. DEPENDING  
ON THE TRACK OF THE COASTAL LOW...SOME RAIN/SNOW MAY PUSH BACK  
INTO THE MID-ATLANTIC. BACK WEST...THE EROSION OF THE MEAN UPPER  
RIDGE WILL ALLOW FOR A RETURN OF WIDESPREAD PRECIPITATION FROM THE  
PACIFIC NORTHWEST EASTWARD INTO THE UPPER INTERMOUNTAIN  
WEST/NORTHERN ROCKIES. QUASI-ZONAL FLOW BEING ESTABLISHED SHOULD  
KEEP CONDITIONS WET OVER THE PACIFIC NORTHWEST WITH UPSLOPE FLOW  
FAVORING HIGHER AMOUNTS OVER THE WA CASCADES. AS THE LEAD SYSTEM  
MOVES TOWARD THE CENTRAL U.S. THIS WEEKEND...SOME THREAT FOR  
LIGHT/MODERATE RAINFALL WILL BE POSSIBLE FROM THE ARKLATEX  
EASTWARD TO THE LOWER MID-ATLANTIC. HOWEVER...THIS IS HIGHLY MODEL  
DEPENDENT GIVEN THE VAST SPREAD AT HAND. THERE SHOULD ALSO BE  
ENOUGH COLD AIR IN PLACE TO MAINTAIN A SNOWFALL THREAT FROM THE  
LOWER GREAT LAKES INTO THE NORTHEASTERN U.S. ON SUNDAY/MONDAY.  
 
RUBIN-OSTER  
 

 
 
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