409  
FXUS02 KWBC 031600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST THU JAN 03 2019  
 
VALID 12Z SUN JAN 06 2019 - 12Z THU JAN 10 2019  
   
..OVERVIEW
 
 
LATEST GUIDANCE STILL GENERALLY SHOWS A LARGE SCALE PATTERN  
FEATURING AN EASTERN PACIFIC MEAN TROUGH, CENTRAL U.S. RIDGE, AND  
EAST COAST/WESTERN ATLANTIC MEAN TROUGH WITH A MODERATE DEGREE OF  
PROGRESSION FOR EMBEDDED FEATURES. INDIVIDUAL MODELS AND ENSEMBLE  
MEANS STILL DIVERGE SIGNIFICANTLY ON SOME IMPORTANT DETAILS  
THOUGH. ALSO TELECONNECTIONS RELATIVE TO BEST-DEFINED HEIGHT  
ANOMALY CENTERS SUGGEST SOME POTENTIAL CONFLICTS, OR AT LEAST SOME  
TRANSITIONS, IN FAVORED FLOW OVER SOME AREAS. THESE ISSUES TEMPER  
CONFIDENCE IN SOME FORECAST SPECIFICS. THE MOST COMMON SIGNALS  
CONTINUE TO BE FOR A PERIOD OF ACTIVE WEATHER ALONG THE WEST COAST  
DURING THE PERIOD AND SOME EFFECTS FROM A GREAT LAKES/NEW ENGLAND  
SYSTEM DURING THE FIRST HALF OF NEXT WEEK.  
 
   
..GUIDANCE EVALUATION AND PREFERENCES
 
 
THIS CYCLE'S FORECAST EMPHASIZED THE 00Z ECMWF/FV3 GFS AND TO SOME  
EXTENT 00Z UKMET DURING THE FIRST HALF OF THE PERIOD, WHILE SLOWLY  
ADDING 06Z GEFS/00Z ECMWF MEAN INPUT WITH TIME. THEN THE REST OF  
THE FORECAST PHASED OUT THE FV3 GFS WHILE FURTHER INCREASING  
ENSEMBLE MEAN WEIGHT AND HOLDING ONTO MINORITY INCLUSION OF THE  
00Z ECMWF.  
 
THE MOST PROMINENT ISSUES THAT ARISE IN THE FIRST THREE DAYS OF  
THE PERIOD INVOLVE THE CHARACTER OF SHORTWAVE ENERGY PROGRESSING  
EASTWARD FROM THE WEST COAST FOLLOWED BY FLOW EVOLUTION OVER THE  
NORTHWESTERN U.S./SOUTHWESTERN CANADA. THERE IS STILL SIGNIFICANT  
MODEL/ENSEMBLE SPREAD REGARDING THE DISTRIBUTION OF ENERGY WITHIN  
THE OVERALL WEST COAST SHORTWAVE. GFS/ECMWF RUNS HAVE TENDED TO  
BE FAIRLY PHASED AND PROGRESSIVE BUT THE 00Z CMC AND A NUMBER OF  
00Z GEFS/ECMWF/CMC ENSEMBLE MEMBERS STILL SHOW POTENTIAL FOR MORE  
SEPARATION BETWEEN NORTHERN AND SOUTHERN STREAMS. AT THE SAME  
TIME GFS RUNS AND TO A SLIGHTLY LESSER EXTENT CMC DEPICT SHORTWAVE  
ENERGY REACHING THE NORTHERN WEST COAST AND BRITISH COLUMBIA BY  
MON--LEADING TO A SURFACE LOW TRACK FARTHER SOUTHEAST THAN THE  
MAJORITY OF OTHER GUIDANCE. THIS GFS/CMC CLUSTER SEEMS LESS  
PROBABLE SINCE THIS LOCATION APPEARS TO LIE WITHIN AN AREA MORE  
FAVORABLE FOR MEAN RIDGING, BETWEEN AN UPSTREAM PACIFIC TROUGH AND  
THE LEADING TROUGH HEADING INTO THE PLAINS. THE COMBINATION OF  
ABOVE ISSUES LEAD TO EITHER A FAST PROGRESSION OF THE NORTHEASTERN  
SYSTEM BY DAY 5 TUE (GFS) OR HAVING IT WIPED OUT (CMC). LATEST  
GEFS MEANS FAVOR A SOLUTION MUCH CLOSER TO OTHER ENSEMBLE MEANS  
AND ECMWF RUNS. THE 00Z FV3 GFS IS MUCH CLOSER TO THIS PREFERENCE  
IN PRINCIPLE AS WELL.  
 
BY DAYS 6-7 WED-THU THE 00Z FV3 GFS BECOMES FASTER THAN CONSENSUS  
WITH TROUGH ENERGY NEARING THE WEST COAST AS OF EARLY WED. AMONG  
OTHER GUIDANCE THERE IS A SPLIT BETWEEN FASTER ECMWF/ECMWF MEAN  
AND SLOWER GFS/GEFS MEAN SOLUTIONS WHILE THE CMC/CMC MEAN OFFER A  
COMPROMISE. GEFS/ECMWF MEANS HAVE BEEN FAIRLY CONSISTENT WITH  
THEIR RESPECTIVE IDEAS AND ARE WITHIN TYPICAL ERROR RANGE FOR DAYS  
6-7 FORECASTS SO CONFIDENCE IS NOT ESPECIALLY HIGH IN FAVORING ONE  
SOLUTION TOO MUCH OVER THE OTHER. HOWEVER THE FAIRLY RAPID  
APPROACH OF ANOTHER PACIFIC TROUGH (REACHING NEAR 145W BY EARLY  
NEXT THU IN MOST GUIDANCE) WOULD SEEM TO FAVOR AT LEAST HEDGING A  
LITTLE FASTER THAN THE GFS-BASED GUIDANCE ALONG THE WEST COAST  
LATE IN THE PERIOD.  
 
AS FOR HEIGHT ANOMALY CENTERS AND TELECONNECTIONS... D+8 MULTI-DAY  
MEANS REFLECT THE CONSENSUS PATTERN OF SOME EASTERN PACIFIC  
TROUGHING/NEGATIVE HEIGHT ANOMALIES WITH DOWNSTREAM EAST-CENTRAL  
U.S. POSITIVE ANOMALIES. HOWEVER STRONG AND PERSISTENT POSITIVE  
HEIGHT ANOMALIES OVER THE FAR NORTHEASTERN ATLANTIC AS WELL AS  
DEVELOPING NEGATIVE ANOMALIES OVER THE SOUTH-SOUTHWEST OF THE  
WESTERN ALEUTIANS BOTH POINT TO A WEST COAST/BC COAST RIDGE AND  
EASTERN TROUGH. THUS THERE IS THE RISK THAT SOME ASPECTS OF FLOW  
MAY DIFFER FROM CURRENT FORECAST OR AT LEAST EXPERIENCE SOME  
TRANSITIONS WITH TIME.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE WEST COAST WILL CONTINUE TO BE A FOCUS FOR SIGNIFICANT WEATHER  
DURING THE PERIOD, WITH NORTHERN CALIFORNIA AND THE PACIFIC  
NORTHWEST STILL SEEING THE GREATEST POTENTIAL FOR HIGHEST 5-DAY  
TOTALS (IN SOME CASES AT LEAST 5-10 INCHES LIQUID) OF RAINFALL AND  
MOUNTAIN SNOW. SOME OF THIS MOISTURE WILL EXTEND INTO THE  
NORTHERN ROCKIES. LEADING SYSTEM CROSSING THE WEST EARLY IN THE  
PERIOD AND THEN ANOTHER TOWARD NEXT WED-THU SHOULD SPREAD AREAS OF  
RAIN/SNOW ACROSS THE SOUTHERN HALF OF THE WEST AS WELL. EARLY IN  
THE PERIOD THERE IS STILL SOME UNCERTAINTY OVER THE DEGREE TO  
WHICH MOISTURE STREAMING NORTHEASTWARD FROM LOWER LATITUDES OF THE  
EASTERN PACIFIC WILL ENHANCE PRECIPITATION OVER ARIZONA AND THE  
SOUTHERN ROCKIES. CONFIDENCE IN SPECIFICS OF INDIVIDUAL PACIFIC  
SYSTEMS IS LOWER THAN DESIRED BUT EACH ONE MAY HAVE THE POTENTIAL  
OF PRODUCING A PERIOD OF STRONG WINDS IN ADDITION TO ENHANCING  
PRECIPITATION.  
 
THE ANTICIPATED PLAINS THROUGH GREAT LAKES/NEW ENGLAND SYSTEM FROM  
EARLY MON ONWARD WILL LIKELY PRODUCE MEANINGFUL PRECIPITATION OVER  
A DECENT PORTION OF THE NORTHEAST QUADRANT OF THE LOWER 48, WITH  
HIGHEST TOTALS LIKELY OVER THE GREAT LAKES/NORTHEAST. GREATEST  
SNOWFALL POTENTIAL CONTINUES TO EXTEND FROM THE UPPER GREAT LAKES  
INTO THE NORTHEAST, WITH ACCUMULATIONS OF POTENTIALLY HEAVY SNOW  
OVER THE LATTER REGION PARTICULARLY SENSITIVE TO SYSTEM TIMING  
WHICH IS FAR FROM BEING DECIDED IN THE GUIDANCE.  
 
EXPECT MIN TEMPERATURES TO BE ABOVE NORMAL OVER NEARLY THE ENTIRE  
LOWER 48 DURING THE PERIOD. HIGHEST DEPARTURES FROM NORMAL MAY  
REACH PLUS 20-30F OVER THE CENTRAL/EAST-CENTRAL PART OF THE  
COUNTRY DURING THE FIRST HALF OF THE PERIOD. FOR HIGH  
TEMPERATURES, THE ACTIVE PATTERN OVER THE WEST WILL TEND TO KEEP  
READINGS NEAR TO SOMEWHAT BELOW NORMAL OVER A MAJORITY OF THE  
REGION. ABOVE NORMAL HIGHS SHOULD PREVAIL OVER MOST AREAS FROM  
THE ROCKIES EASTWARD, THOUGH THE NORTHEAST MAY SEE SOME SLIGHTLY  
BELOW NORMAL HIGHS AFTER PASSAGE OF A FRONT AND THEN THE GREAT  
LAKES/NORTHEAST STORM.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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