439  
FXUS02 KWBC 121600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST TUE DEC 12 2017  
 
VALID 12Z FRI DEC 15 2017 - 12Z TUE DEC 19 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST GUIDANCE CONTINUES TO SHOW A TRANSITION FROM AN AMPLIFIED  
WESTERN RIDGE/EASTERN TROUGH PATTERN DURING THE SHORT RANGE TIME  
FRAME TO A MORE PROGRESSIVE REGIME FEATURING MORE PACIFIC  
INFLUENCE. MULTI-DAY MEANS OF MOST OPERATIONAL AND ENSEMBLE  
SOLUTIONS AGREE FAIRLY WELL WITH THE OVERALL EVOLUTION ALOFT BUT  
THERE ARE SOME SIGNIFICANT EMBEDDED DETAIL UNCERTAINTIES THAT  
SIGNIFICANTLY LOWER CONFIDENCE IN SENSIBLE WEATHER EFFECTS OVER  
SOME AREAS.  
 
THE MOST PROMINENT UNCERTAINTY INVOLVES THE SHORTWAVE FORECAST TO  
REACH WESTERN NORTH AMERICA AROUND FRI. FROM THIS TIME ONWARD  
MODEL/ENSEMBLE SOLUTIONS INCREASINGLY DIVERGE REGARDING HOW MUCH  
ENERGY MAY SEPARATE OVER THE SOUTHWEST OR FOUR CORNERS REGION  
VERSUS CONTINUING ONWARD IN PROGRESSIVE FLOW. THE 00Z CMC WAS  
SOMEWHAT ON THE EXTREME SIDE OF THE ENVELOPE BUT THE FULL ARRAY OF  
00Z/06Z GUIDANCE DOES SUGGEST AN INCREASING POTENTIAL FOR MORE  
ENERGY TO CROSS THE SOUTHWEST THAN IN PREVIOUS CYCLES. THIS TREND  
IS ALSO EVIDENT WHEN COMPARING 06Z GFS/GEFS MEAN RUNS TO THEIR 00Z  
COUNTERPARTS. THROUGH DAY 4 SAT THE 06Z GFS/00Z ECMWF COMPARE  
MOST CLOSELY TO THE ENSEMBLE MEANS. AFTER THAT TIME THE  
FASTER/DEEPER 00Z ECMWF BECOMES AN OUTLIER TO THE FULL ENSEMBLE  
ENVELOPE AT SOME HEIGHT VALUES. GIVEN THE CONTINUED GUIDANCE  
SPREAD AND RELATIVELY NEW NATURE OF THE TREND TOWARD GREATER  
SEPARATION, PREFER A FAIRLY CONSERVATIVE ADJUSTMENT BY WAY OF  
GREATER ENSEMBLE MEAN EMPHASIS BY THE LATTER HALF OF THE PERIOD.  
AHEAD OF THIS FEATURE IS ALSO ANOTHER SIGNIFICANT UNCERTAINTY,  
LEADING SHORTWAVE ENERGY THAT MAY CLOSE OFF A COMPACT LOW ANYWHERE  
BETWEEN NORTHERN MEXICO AND SOUTHWEST OF SOUTHERN BAJA CALIF.  
THIS DIFFERENCE EARLY IN THE PERIOD LEADS TO CONSIDERABLE SPREAD  
IN WHETHER AND HOW QUICKLY NORTHEASTWARD EJECTION OCCURS. THE 00Z  
UKMET WAS THE NORTHEAST EXTREME WHILE THE NEXT-FASTEST 00Z ECMWF  
WAS MUCH FASTER THAN PREVIOUS RUNS. SOME DEGREE OF COMPROMISE  
APPEARS BEST AT THIS TIME.  
 
UPSTREAM EXPECT NORTHEAST PACIFIC TROUGHING TO BEGIN APPROACHING  
THE NORTHWEST TOWARD THE END OF THE PERIOD. THE 00Z ECMWF IS  
SOMEWHAT MORE AGGRESSIVE WITH THE HEIGHT FALLS THAN THE MAJORITY  
OF OTHER SOLUTIONS, BUT WITHIN THE ENSEMBLE SPREAD AND CLOSER TO  
RUNS FROM 2-3 CYCLES AGO THAN THE 12Z/11 RUN. PREFER A STARTING  
BLEND BETWEEN THE 00Z ECMWF MEAN AND SLIGHTLY SLOWER 06Z GEFS/00Z  
NAEFS MEANS WHILE AWAITING ANY FURTHER SUPPORT FOR THE ECMWF  
SCENARIO.  
 
ALONG THE EAST COAST EARLY IN THE PERIOD, NORTHEAST GULF/SOUTHEAST  
COAST WAVINESS SHOULD STRENGTHEN/CONSOLIDATE OFF THE NORTHEAST  
COAST AND CANADIAN MARITIMES IN RESPONSE TO THE UPPER TROUGH  
CROSSING THE EAST. THERE ARE STILL SOME NOTICEABLE DETAIL  
DIFFERENCES ALOFT OVER THE NORTHEAST BY DAY 4 SAT BUT  
MODEL/ENSEMBLE CLUSTERING IS MUCH BETTER THAN IN PREVIOUS DAYS.  
SOLUTIONS ON THE NORTHWEST SIDE OF THE ENVELOPE BY LATE FRI TENDED  
TO BE FROM SOME OF THE CMC ENSEMBLES.  
 
BEHIND A WEAK GREAT LAKES SYSTEM ON FRI, ECMWF-BASED GUIDANCE HAS  
TRENDED WEAKER WITH A CLIPPER SYSTEM THAT HAS BEEN FORECAST TO  
TRACK INTO THE GREAT LAKES REGION BY EARLY DAY 5 SUN. THE  
GFS/GEFS MEAN HAVE TRENDED SOMEWHAT WEAKER IN THEIR 06Z RUNS BUT  
THERE IS STILL DECENT CONSENSUS FOR A BETTER DEFINED SYSTEM THAN  
IN THE 00Z ECMWF/ECMWF MEAN.  
 
ADDING TOGETHER THE ABOVE CONSIDERATIONS, THE STARTING BLEND  
CONSISTED OF THE 06Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF MEAN FOR  
DAYS 3-4 FRI-SAT. 00Z ECMWF INPUT WAS DECREASED BY DAY 5 SUN AND  
THE 06Z GFS DAY 6 MON, LEADING TO A HALF 00Z ECMWF MEAN AND 25 PCT  
EACH 06Z GEFS MEAN/00Z NAEFS BLEND BY DAY 7 TUE.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
OVER THE WEST EXPECT THE MAJORITY OF PRECIP COVERAGE TO EXTEND  
FROM THE PACIFIC NORTHWEST SOUTHEAST ACROSS THE ROCKIES, WITH  
HIGHEST TOTALS OF RAIN/HIGHER ELEVATION SNOW LIKELY TO BE OVER  
EXTREME NORTHERN AREAS. CONFIDENCE IN COVERAGE/INTENSITY OF  
ACTIVITY FARTHER SOUTH IS LOWER THAN DESIRED DUE TO GUIDANCE  
SPREAD WITH THE EVOLUTION OF THE UPPER TROUGH ARRIVING LATE THIS  
WEEK. FROM THE WEEKEND INTO EARLY NEXT WEEK, ISSUES WITH THIS  
TROUGH AFFECT PRECIP POTENTIAL FROM THE SOUTHEAST PLAINS/LOWER MS  
VALLEY NORTHEASTWARD. DEPENDING ON HOW THIS TROUGH EVOLVES AND  
HOW MUCH LEADING ENERGY EJECTS ACROSS NORTHERN MEXICO, SOME  
PRECIP-- THE MAJORITY LIKELY TO BE RAIN-- MAY BE ON THE HEAVIER  
SIDE. LATE THIS WEEK EXPECT A PERIOD OF MOSTLY LIGHT SNOW OVER  
THE GREAT LAKES WHILE CONSOLIDATING LOW PRESSURE OFF THE EAST  
COAST WILL BRING A BAND OF RAIN OVER/NEAR NORTHERN FLORIDA AND  
POSSIBLY A LITTLE SNOW TO AREAS ALONG THE NORTHERN HALF OF THE  
EAST COAST. CURRENTLY ANTICIPATE ANY EAST COAST PRECIP TO BE  
FAIRLY LIGHT BUT IT WOULD NOT TAKE MUCH OF A WESTWARD SHIFT IN  
SURFACE LOW TRACK TO INCREASE AMOUNTS. ANOTHER NORTHERN TIER  
SYSTEM SHOULD BRING A BAND OF LIGHT PRECIP, MOSTLY SNOW, INTO THE  
GREAT LAKES/NORTHEAST DURING THE WEEKEND. SOME POCKETS OF SNOW  
ASSOCIATED WITH UPSTREAM ENERGY MAY ARRIVE OVER THE NORTHERN TIER  
EARLY NEXT WEEK.  
 
THE MOST EXTREME AND PERSISTENT ANOMALIES FOR TEMPS WILL BE OVER  
THE NORTHERN PLAINS, WHICH SHOULD SEE MIN AND/OR MAX READINGS  
10-20F ABOVE NORMAL MOST DAYS WITH SOME LOCALLY HIGHER ANOMALIES  
POSSIBLE. SOME OF THIS WARMTH MAY EXTEND FARTHER SOUTH/SOUTHEAST  
AT TIMES. SOUTHWESTERN STATES/GREAT BASIN WILL ALSO BE ON THE  
WARM SIDE WITH HIGHS TENDING TO BE 5-15F ABOVE NORMAL. THE  
CURRENT COOL SPELL OVER THE EAST WILL LINGER INTO THE END OF THE  
WEEK WITH SOME TEMPS AS LOW AS 10-15F BELOW NORMAL OVER THE GREAT  
LAKES/NORTHEAST ON FRI. LESS EXTREME COOL READINGS WILL LINGER  
INTO SAT FOLLOWED BY NEAR TO ABOVE NORMAL TEMPS.  
 
RAUSCH  
 

 
 
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