380  
FXUS02 KWBC 291533  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1033 AM EST WED NOV 29 2017  
 
VALID 12Z SAT DEC 02 2017 - 12Z WED DEC 06 2017  
   
..OVERVIEW
 
 
THE MOST AGREEABLE ASPECT OF THE MEDIUM RANGE FORECAST IS THE  
TRANSITION OF THE MEAN PATTERN FROM ONE OF LOW AMPLITUDE AND  
PROGRESSION TOWARD A HIGH AMPLITUDE/PERSISTENT REGIME. THE END  
RESULT BY DAY 7 WED SHOULD BE A STRONG RIDGE ORIENTED NORTH-SOUTH  
NEAR THE WEST COAST WITH A DOWNSTREAM TROUGH EXTENDING SOUTHWEST  
FROM THE UPPER MS VALLEY/GREAT LAKES. BY THE D+8 TIME FRAME  
RECENT MULTI-DAY MEANS AGREE THAT THE POSITIVE HEIGHT ANOMALY  
CENTER ASSOCIATED WITH THE WEST COAST RIDGE SHOULD BE NEAR  
VANCOUVER ISLAND. TELECONNECTIONS RELATIVE TO THAT CENTER SUPPORT  
THE ESTABLISHMENT OF A FAIRLY DEEP EASTERN TROUGH BUT ANOTHER CORE  
OF POSITIVE HEIGHT ANOMALIES NEAR THE SOUTHERN TIP OF GREENLAND  
OFFERS POTENTIAL FOR THE TROUGH TO BE SOMEWHAT BROADER/LESS DEEP.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL AGREEMENT AND FORECAST CONFIDENCE IS AVERAGE TO SLIGHTLY  
ABOVE AVERAGE DURING THE EARLY PORTION OF THE MEDIUM RANGE, AND  
THEN DEGRADES VERY QUICKLY THROUGH TIME. DURING DAYS 3-4  
(SAT-SUN), A MULTI-MODEL BLEND APPROACH WAS TAKEN. UPPER ENERGY  
DEAMPLIFYING ACROSS THE SOUTHWESTERN STATES/NORTHERN MEXICO SHOWS  
A RANGE OF SOLUTIONS AMONG THE GUIDANCE, AND ENSEMBLE MEMBERS HAVE  
SHOWN A TREND TOWARD WEAKENING THIS FEATURE MORE QUICKLY. THE 06Z  
GFS AND 00Z UKMET WERE IN THE MIDDLE OF THE SPREAD HERE WITH THE  
00Z ECMWF STILL SLOWEST/STRONGEST AND THE 00Z CMC ON THE  
FASTER/WEAKER SIDE. FARTHER NORTH, SHORTWAVE ENERGY AMPLIFYING AS  
IT MOVES INTO THE WESTERN U.S. SHOWS A REASONABLY WELL-CLUSTERED  
RANGE OF SOLUTIONS THROUGH DAY 4, WITH THE UKMET/CMC ON THE DEEPER  
SIDE AND THE GFS/ECMWF SLOWER TO DEEPEN. THUS, A BLEND OF THE 00Z  
ECMWF/UKMET/CMC AND 06Z GFS COMPRISED A MAJORITY OF THE FORECAST  
DURING THIS TIME FRAME, WITH CONFIDENCE AVERAGE TO SLIGHTLY ABOVE  
AVERAGE.  
 
FROM DAY 5 (MON) ONWARD, MODEL/ENSEMBLE SPREAD INCREASES  
SUBSTANTIALLY, AND FORECAST CONFIDENCE PLUMMETS, ESPECIALLY BY DAY  
7 (WED). WHAT DOES SEEM CLEAR IS THAT THE AFOREMENTIONED UPPER  
TROUGH ENTERING THE WEST EARLY IN THE PERIOD WILL CONTINUE TO  
AMPLIFY AS IT CROSSES THE ROCKIES ON DAY 5. THIS IS WHERE  
SOLUTIONS DIVERGE INTO TWO DIFFERENT SCENARIOS, HOWEVER. ONE CAMP  
OF SOLUTIONS (REPRESENTED BY THE 00Z CMC) BREAKS OFF A PIECE OF  
THE AMPLIFYING TROUGH AND DEVELOPS A DEEP CUTOFF LOW ACROSS THE  
SOUTHWESTERN STATES/FOUR CORNERS REGION, WHICH THEN MOVES EAST  
INTO THE PLAINS BY DAYS 6-7. THE 00Z ECMWF/06Z GFS, ON THE OTHER  
HAND, KEEP THE ENERGY MORE CONSOLIDATED, WHICH RESULTS IN A FASTER  
PROGRESSION OF THE TROUGH EASTWARD, AND MUCH STRONGER HEIGHT FALLS  
ACROSS THE MIDWEST/GREAT LAKES BY LATE IN THE PERIOD. FURTHER  
COMPLICATING THE SCENARIO, BOTH THE ECMWF AND GFS HAVE FLIPPED  
BETWEEN BOTH OF THESE SCENARIOS OVER THE PAST SEVERAL RUNS (THE  
12Z/18Z TUE ECMWF/GFS DEPICTED A SOLUTION MUCH LIKE THE 00Z CMC).  
ECMWF ENSEMBLE MEMBERS HAVE BEEN GENERALLY CLUSTERED AROUND THE  
DETERMINISTIC ECMWF IN BOTH THE 00Z RUN AS WELL AS THE 12Z RUN  
FROM YESTERDAY, WHICH FURTHER REDUCES CONFIDENCE GIVEN THAT THE  
ECENS IS TYPICALLY MORE DISBURSED. THE NUMBER OF CMC ENSEMBLE  
MEMBERS KEEPING THE CUTOFF LOW HAS NOT DECREASED NOTICEABLY OVER  
THE PAST COUPLE RUNS, WHILE GEFS MEMBERS ARE SPLIT BETWEEN BOTH  
SCENARIOS, RESULTING IN A NAEFS MEAN THAT STILL SHOWS A  
SUBSTANTIAL POSSIBILITY OF EITHER SCENARIO. GIVEN THE UNCERTAINTY,  
THE WPC FORECAST WAS TRENDED HEAVILY TOWARD 00Z ECENS/NAEFS  
ENSEMBLE MEANS THROUGH THE LATTER PORTION OF THE MEDIUM RANGE  
PERIOD, REPRESENTING A COMPROMISE BETWEEN THE TWO SCENARIOS  
(ALTHOUGH AT THIS POINT THE PROBABILITIES SEEM NEARLY EQUAL FOR  
EITHER SCENARIO).  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
RAIN AND HIGHER ELEVATION SNOW WILL SPREAD FROM THE PACIFIC  
NORTHWEST SOUTHEASTWARD AS THE UPPER TROUGH MOVES INTO THE WEST.  
ONGOING GUIDANCE SPREAD FOR DETAILS ALOFT CONTINUES TO KEEP  
CONFIDENCE LOW IN DETERMINING ULTIMATE SOUTHEASTWARD EXTENT OF  
PRECIP AND WHICH AREAS OF TYPICALLY FAVORED WINDWARD FOCUSED  
TERRAIN WILL SEE HIGHEST TOTALS FOR THE PERIOD. EXPECT PRECIP  
COVERAGE TO DECREASE WITH TIME AS UPPER RIDGING BUILDS TOWARD THE  
WEST COAST. THE FOUR CORNERS STATES HAVE THE BEST CHANCE TO SEE  
LINGERING ACTIVITY LATE IN THE PERIOD BUT AGAIN WITH MODEST  
CONFIDENCE. DURING NEXT MON-WED THE LEADING COLD FRONT REACHING  
THE PLAINS/EASTERN U.S. WILL BECOME A FOCUS FOR RAINFALL OF  
VARYING INTENSITY. ENSEMBLE GUIDANCE SHOWS THE BEST PROBABILITY OF  
HEAVIER RAINFALL FROM NORTHEAST TX INTO THE TN VALLEY/LOWER OH  
VALLEY. SNOW MAY BE POSSIBLE AT SOME LOCATIONS IN THE NORTHERN  
TIER BEHIND THE FRONT.  
 
TEMPS WILL BE QUITE WARM AHEAD OF THE FRONT CROSSING THE CONUS  
DURING THE PERIOD, WITH PLUS 10-20F ANOMALIES POSSIBLE ON ONE OR  
MORE DAYS FROM THE INTERIOR WEST/ROCKIES TO JUST WEST OF THE  
APPALACHIANS. PACIFIC ORIGIN OF AIR BEHIND THE FRONT WILL LEAD TO  
FAIRLY MODEST NEGATIVE ANOMALIES ASIDE FROM PERHAPS HIGHS 10-15F  
BELOW NORMAL OVER THE SOUTHERN HIGH PLAINS LATE IN THE PERIOD.  
WEST COAST STATES SHOULD REBOUND TO ABOVE NORMAL TEMPS TOWARD  
MIDWEEK AS RIDGING ALOFT APPROACHES.  
 
RYAN/RAUSCH  
 

 
 
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