659  
FXUS02 KWBC 141600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST THU DEC 14 2017  
 
VALID 12Z SUN DEC 17 2017 - 12Z THU DEC 21 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE FIRST HALF OF THE FORECAST PERIOD WILL FEATURE A GENERAL  
FLATTENING OF CONUS FLOW WITH THE EJECTION OF INITIAL TROUGHING  
BETWEEN STRONG EASTERN PACIFIC AND BAHAMAS/CARIBBEAN RIDGES. THEN  
A NORTHEAST PACIFIC TROUGH WILL AMPLIFY INTO THE WESTERN U.S. IN  
RESPONSE TO A STRENGTHENING RIDGE UPSTREAM WHILE SOUTHWESTERLY  
FLOW OR FLAT RIDGING SHOULD BECOME ESTABLISHED OVER THE EASTERN  
STATES. GUIDANCE HAS BEEN SOMEWHAT AGREEABLE WITH THE OVERALL  
EVOLUTION OF MEAN FLOW BUT WITH A LOT OF SPREAD FOR DETAILS OF THE  
EJECTING WEST-CENTRAL U.S. TROUGH AND MID-LATE PERIOD ARRIVAL OF  
NORTHEAST PACIFIC TROUGHING.  
 
THE MODELS AND ENSEMBLES ARE STILL IN THE PROCESS OF RESOLVING THE  
DETAILS WITHIN THE OVERALL NORTHERN PLAINS INTO NORTHWEST MEXICO  
TROUGH AS OF DAY 3 SUN, WITH MEANINGFUL RUN TO RUN DIFFERENCES IN  
SOME GUIDANCE. IN CONTRAST TO 24 HOURS AGO, THE 00Z UKMET/CMC NOW  
CLUSTER BETTER WITH THE SLOWER 00Z/06Z GFS VERSUS THE ECMWF WHICH  
TO VARYING DEGREES HAS TENDED TO BE ON THE FASTER SIDE OF THE  
ENVELOPE FOR ENERGY THAT EJECTS ACROSS THE CENTRAL LATITUDES OF  
THE LOWER 48. CURRENT ENSEMBLE MEANS ALSO SIGNAL GREATER  
POTENTIAL FOR THE SLOWER GFS TYPE SCENARIO. HOW MUCH LINGERING  
TROUGHING IS COMPOSED OF FOUR CORNERS/ROCKIES ENERGY VERSUS  
NORTHWESTERN MEXICO ENERGY IS STILL VERY UNCERTAIN THOUGH, AND THE  
EXTENT TO WHICH INCOMING NORTHEAST PACIFIC FLOW INFLUENCES THE  
EJECTING SHORTWAVE REMAINS AN UNRESOLVED UNCERTAINTY.  
 
MEANWHILE IN RECENT DAYS MODELS/ENSEMBLE MEMBERS HAVE BEEN QUITE  
VARIED FOR THE TIMING/AMPLITUDE OF TROUGHING THAT HAS BEEN  
FORECAST TO MOVE INTO THE NORTHWEST, WITH DIFFERENCES IN HOW MUCH  
ENERGY GETS PULLED OFF FROM THE SOUTHWEST PART OF THE TROUGH AND  
EVOLUTION OF THE UPSTREAM RIDGE PLAYING A ROLE IN THE BROAD  
GUIDANCE SPREAD. MOST 00Z/06Z GUIDANCE HAS SIGNIFICANTLY IMPROVED  
CLUSTERING RELATIVE TO PRIOR RUNS, WITH MORE TROUGHING THAN  
YESTERDAY'S SOLUTIONS ON AVERAGE BUT PERHAPS CLOSER TO WHAT HAD  
BEEN ADVERTISED 36-48 HOURS AGO. 00Z GEFS ENSEMBLES WERE HOLDING  
OUT FOR STRONGER WESTERN RIDGING/SLOWER TROUGH ARRIVAL BUT THE 06Z  
GEFS HAS TRENDED TOWARD THE REMAINING CONSENSUS. NOTE THAT 00Z  
ECMWF SHORTWAVE DETAILS WITHIN AND UPSTREAM FROM THE TROUGH BECOME  
OUT OF SYNC WITH THE MAJORITY CLUSTER BY DAY 7 THU.  
TELECONNECTIONS RELATIVE TO THE RIDGE OVER THE NORTHEAST PACIFIC  
AND TROUGH NEAR HAWAI'I SUPPORT THE IDEA OF A POSITIVELY TILTED  
MEAN TROUGH EXTENDING SOUTHWESTWARD FROM NORTH OF HUDSON BAY BUT  
SUGGEST THE WESTERN U.S. PART OF THE TROUGH COULD END UP BEING A  
LITTLE WEAKER AND/OR EASTWARD OF WHERE INDICATED BY SOME GUIDANCE.  
 
FOR THE UPDATED FORECAST, A BLEND OF 00Z/06Z OPERATIONAL GUIDANCE  
(00Z-06Z GFS AND 00Z ECMWF/UKMET/CMC) REPRESENTED THE MOST COMMON  
ASPECTS OF LATEST GUIDANCE EARLY IN THE PERIOD. IN PARTICULAR  
THIS BLEND DOWNPLAYED FASTER ECMWF SHORTWAVE PROGRESSION OVER THE  
ROCKIES/PLAINS AROUND MON. TYPICALLY DECREASING  
CONFIDENCE/INCREASING SPREAD ADVOCATED FOR INCORPORATING 06Z  
GEFS/00Z ECMWF MEANS INTO THE FORECAST ALONG WITH SOME OPERATIONAL  
COMPONENT DAY 5 TUE ONWARD, LEADING TO MOSTLY ENSEMBLE MEAN  
WEIGHTING/20 PCT GFS INPUT BY DAY 7 THU.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE GULF COAST/SOUTHERN TIER STATES WILL BE AN AREA OF INTEREST  
FOR HEAVY RAINFALL POTENTIAL WITH EJECTING WEST-CENTRAL U.S.  
SHORTWAVE ENERGY/ASSOCIATED COLD FRONT AS WELL AS A POSSIBLE  
LEADING STATIONARY BOUNDARY PROVIDING FOCUS FOR THIS ACTIVITY.  
SPECIFICS OF TIMING/COVERAGE ARE STILL UNCERTAIN GIVEN THE  
DIFFICULTY THAT MODELS HAVE EXHIBITED SO FAR WITH THE DYNAMICS  
ALOFT. BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS APPEARS TO BE  
IN THE SUN-TUE PERIOD. MEANWHILE THE APPROACH/ARRIVAL OF THE  
NORTHEAST PACIFIC UPPER TROUGH WILL LIKELY BRING A PERIOD OF  
MEANINGFUL RAIN/HIGHER ELEVATION SHOW TO FAVORED TERRAIN OF THE  
PACIFIC NORTHWEST AND NORTHERN ROCKIES. PRECIP SHOULD BE CONFINED  
TO FAR NORTHERN AREAS DURING THE FIRST HALF OF THE PERIOD AND THEN  
PUSH SOUTHWARD GOING INTO MIDWEEK WITH SOME MOISTURE POSSIBLY  
EXTENDING INTO NORTHERN CA. TOWARD THE END OF THE PERIOD THE  
POTENTIAL FOR THE COMBINATION OF SNOW AND WIND WILL HAVE TO BE  
MONITORED OVER PARTS OF THE ROCKIES/PLAINS AS LOW PRESSURE EMERGES  
INTO THE CENTRAL U.S.. OVER THE EAST, SNOW SHOULD BE CONFINED TO  
FAIRLY NORTHWARD LOCATIONS WITH A COMBINATION OF SYNOPTIC AND LAKE  
EFFECT ACTIVITY.  
 
FOR MOST OF THE PERIOD WARMER THAN NORMAL TEMPS WILL PREVAIL OVER  
THE MAJORITY OF THE LOWER 48. THE NORTHERN PLAINS/ROCKIES AND  
SOUTHEAST SHOULD BE PARTICULARLY WARM FOR MINS DURING MON-WED AND  
MON-TUE RESPECTIVELY. THE CENTRAL PLAINS WILL LIKELY SEE THE  
HIGHEST MAX TEMP ANOMALIES AFTER SUN. EXCEPTIONS TO THE WARMTH  
WILL BE BELOW NORMAL READINGS OVER NEW ENGLAND AND INTERIOR  
WEST/ROCKIES ON DAY 3 SUN AND A SURGE OF BELOW NORMAL TEMPS INTO  
THE WEST AND NORTHERN ROCKIES/PLAINS DAY 7 THU.  
 
RAUSCH  
 

 
 
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