050  
FXUS02 KWBC 290648  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
147 AM EST WED NOV 29 2017  
 
VALID 12Z SAT DEC 02 2017 - 12Z WED DEC 06 2017  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
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.  
 
IN CONTRAST TO THE RELATIVE LARGE SCALE AGREEMENT, INDIVIDUAL  
MODEL RUNS AND ENSEMBLE MEMBERS HAVE BEEN EXHIBITING VERY POOR  
AGREEMENT AND CONTINUITY WITH RESPECT TO THE DETAILS OF THIS  
PATTERN TRANSITION. THE PRIMARY FOCUS OF GUIDANCE SPREAD  
CONTINUES TO BE WITH THE TROUGH FORECAST TO MOVE INTO THE WEST BY  
DAY 4 SUN. DURING THE FIRST PART OF THE PERIOD RECENT GUIDANCE  
HAS SHOWN A GENERAL SLOWER TREND-- A COMBINATION OF SLIGHTLY  
DELAYED NORTHERN STREAM AMPLIFICATION AND MORE GUIDANCE SUGGESTING  
THAT SOME ENERGY IN THE SOUTHERN PART OF THE TROUGH MAY GET PULLED  
OFF WELL OFFSHORE (WHICH MANY GFS RUNS HAVE BEEN SHOWING IN RECENT  
DAYS). ONCE THE TROUGH MOVES INLAND THE BIG QUESTION IS WHAT  
PROPORTION OF ENERGY CONTINUES EASTWARD VERSUS COLLECTING  
OVER/NEAR THE SOUTHWEST U.S.. TELECONNECTIONS RELATIVE TO THE  
UPSTREAM RIDGE SEEM TO SUPPORT A WEAKNESS THAT IS MOST SIMILAR TO  
THE ENSEMBLE MEANS IN PRINCIPLE, AT LEAST LEANING AWAY FROM THE  
COMBINED DEEPEST/SLOWEST PART OF THE SOLUTION SPREAD. THE  
PREFERRED BLEND OF 12Z OPERATIONAL GUIDANCE (GFS/ECMWF/UKMET/CMC)  
REPRESENTED A MEASURED ADJUSTMENT SLOWER TO REFLECT RECENT TRENDS  
EARLY IN THE PERIOD. BY MID-LATE PERIOD AN AVERAGE OF THE 12Z  
GFS/ECMWF/CMC FIT WITHIN THE GENERAL FRAMEWORK OF THE MEANS SO A  
MINORITY WEIGHT OF EACH HELPED TO PROVIDE A LITTLE EXTRA  
DEFINITION TO THE 12Z GEFS/ECMWF MEANS. THE RESULTING BLEND  
MAINTAINED GOOD CONTINUITY ASIDE FROM SLIGHTLY SLOWER TIMING OF  
THE SOUTHWEST ENERGY AND LOW PRESSURE TRACKING FROM THE NORTHERN  
TIER INTO CANADA. THE 18Z GFS/GEFS SEEMED TO HAVE LOWER  
PROBABILITY. THE 18Z GFS HAD THE SOUTHWEST UPPER LOW TRACING A  
WIDER ARC MON ONWARD AND NORTHERN STREAM ENERGY BECOMING FASTER  
THAN CONSENSUS OVER THE EAST LATE IN THE PERIOD. THE 18Z GEFS  
MEAN SHOWED MORE ELONGATION TO SOUTHWEST FLOW THAN OTHER MEANS.  
 
IF ANYTHING, NEW 00Z GUIDANCE THUS FAR ADDS EVEN MORE UNCERTAINTY.  
THERE NOW SEEMS TO BE A BIT OF A FASTER ADJUSTMENT WITH THE  
TROUGH AS IT ARRIVES. THEN THE GFS/UKMET CLOSE OFF A COMPACT  
UPPER LOW ALONG THE CENTRAL CA COAST BY LATE MON WHILE THE CMC  
SHOWS A PROGRESSIVE UPPER LOW WHOSE ENERGY REACHES UP TO THE MS  
VALLEY BY DAY 7 WED.  
 
AHEAD OF THE WESTERN TROUGH, GUIDANCE IS STILL HAVING TROUBLE  
RESOLVING COMPACT UPPER LOW ENERGY FORECAST TO BE SOMEWHERE OVER  
OR OFFSHORE NORTHWEST MEXICO AS OF EARLY SAT. GFS RUNS HAVE  
TENDED TO BE ON THE FAST/WEAK SIDE OF THE SPREAD BUT THE 00Z  
UKMET/CMC HAVE MADE A NOTABLE EASTWARD ADJUSTMENT HALFWAY OR MORE  
TOWARD THE 00Z GFS, LEAVING THE 12Z ECMWF/ECMWF MEAN MORE ON THEIR  
OWN. FARTHER EAST THERE WILL BE A SHORTWAVE CROSSING THE EASTERN  
STATES DURING THE WEEKEND AND FEEDING INTO A WESTERN ATLANTIC MEAN  
TROUGH. 12Z/18Z GFS RUNS WERE STRONGEST WITH THIS SHORTWAVE WITH  
THE AFOREMENTIONED BLEND TONING DOWN THIS ASPECT OF THE GFS. THE  
00Z GFS BACKED AWAY SIGNIFICANTLY, WITH OTHER GUIDANCE FAIRLY FLAT  
AS WELL.  
 
   
..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/EAST-CENTRAL U.S. WILL BECOME A FOCUS FOR RAINFALL OF  
VARYING INTENSITY. TOWARD MIDWEEK THE FRONT MAY DECELERATE FOR A  
TIME, LEADING TO HIGHER RAINFALL AMOUNTS. ENSEMBLE GUIDANCE SHOWS  
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 LOWER 48  
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.  
 
RAUSCH  
 

 
 
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