290  
FXUS02 KWBC 151600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST FRI DEC 15 2017  
 
VALID 12Z MON DEC 18 2017 - 12Z FRI DEC 22 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DURING THE FIRST HALF OF NEXT WEEK NORTHERN STREAM FLOW ACROSS  
CANADA AND NORTHERN TIER U.S. WILL PUSH ALONG FAIRLY STRONG  
CANADIAN LOW PRESSURE AND TRAILING FRONT CROSSING THE  
CENTRAL/EASTERN STATES, WHILE A SOUTHERN ROCKIES/EXTREME NORTHWEST  
MEXICO SHORTWAVE WILL ACCELERATE EASTWARD. A NORTHEAST PACIFIC  
TROUGH WILL APPROACH THE NORTHWESTERN U.S. TUE AND THEN  
PROGRESS/AMPLIFY TO THE SOUTH AND EAST THEREAFTER IN RESPONSE TO A  
BUILDING/SHARPENING UPSTREAM RIDGE. THIS EVOLUTION WILL YIELD A  
BROAD POSITIVELY TILTED TROUGH EXTENDING FROM NEAR HUDSON BAY INTO  
NORTHWEST MEXICO BY THE END OF NEXT WEEK. BAHAMAS/CARIBBEAN  
RIDGING WILL PERSIST THROUGH THE PERIOD WITH SOME FLUCTUATIONS IN  
STRENGTH.  
 
TYPICAL MODEL/ENSEMBLE BEHAVIOR SUGGESTS FURTHER ADJUSTMENTS MAY  
BE IN STORE BUT AT LEAST FOR THE TIME BEING THE CONSIDERABLE  
GUIDANCE SPREAD AND RUN TO RUN VARIABILITY FOR SIGNIFICANT  
FEATURES OVER RECENT DAYS HAS SETTLED DOWN SOMEWHAT SO CONFIDENCE  
IN THE OVERALL FORECAST IS GRADUALLY IMPROVING.  
 
THE RECENT SLOWER TREND IN THE ECMWF HAS NARROWED THE SOLUTION  
ENVELOPE A BIT FOR THE EJECTING SOUTHERN ROCKIES SHORTWAVE. THE  
00Z ECMWF AND SOME GEFS/ECMWF ENSEMBLE MEMBERS SHOW POTENTIAL FOR  
THE FEATURE TO EJECT IN TWO PIECES, ONE CLOSE TO THE MAJORITY  
GFS/UKMET/CMC CLUSTER AND A SECOND THAT TRAILS BY ABOUT 12-18  
HOURS. NO MODEL SYSTEM HAS BEEN IDEAL BUT THE GFS HAS GENERALLY  
HELD ONTO THE MOST STABLE EVOLUTION OF THIS FEATURE IN RECENT DAYS.  
 
INTO DAY 6 THU THE LATEST MODEL/ENSEMBLE MEAN RUNS AGREE WELL IN  
PRINCIPLE FOR THE UPPER TROUGH AMPLIFYING INTO THE WEST. OVER THE  
PAST COUPLE DAYS SOME GEFS MEMBERS HAVE BEEN MORE HESITANT TO  
BRING IN LEADING HEIGHT FALLS COMPARED TO MOST OTHER GUIDANCE SO  
PREFERENCE WOULD BE TO UNDERWEIGHT THE GEFS MEAN TO SOME EXTENT.  
ENSEMBLE MEANS END UP CONVERGING SOMEWHAT TOWARD THE END OF THE  
PERIOD THOUGH. BY DAY 7 FRI ALL THREE OF THE MAJOR ENSEMBLE  
SYSTEMS SHOW THE GREATEST SPREAD ALOFT OVER THE WEST  
COAST/SOUTHWEST/GREAT BASIN AND OVER/NEAR THE GREAT LAKES. NOT  
SURPRISINGLY THE OPERATIONAL GUIDANCE SHOWS MORE DIVERGENCE WITH  
DETAILS ALOFT BY NEXT FRI AS WELL. THE WESTERN AREA OF SPREAD  
MAKES SENSE AS TELECONNECTION-FAVORED FLOW DIFFERS DEPENDING ON  
THE RELATIVE DOMINANCE OF THE CORE OF POSITIVE HEIGHT ANOMALIES  
OFF THE ALASKA PANHANDLE AND NEGATIVE HEIGHT ANOMALIES JUST NORTH  
OF HAWAI'I, WITH THE FORMER SUPPORTING MORE SOUTHWEST U.S.  
TROUGHING THAN THE LATTER. AT THE SAME TIME THE  
STRENGTH/EVOLUTION OF WHAT APPEARS TO BE A FAIRLY LIKELY UPPER MS  
VALLEY/GREAT LAKES WAVE BY THU-FRI IS FAIRLY UNCERTAIN GIVEN THE  
SOLUTION SPREAD FOR DETAILS ALOFT, WITH OPERATIONAL RUNS DIFFERING  
BY 10-20MB. RECENT ECMWF RUNS HAVE TENDED TO BE ON THE STRONGER  
SIDE OF THE ENVELOPE WHILE THE GFS HAS VARIED BETWEEN THE STRONGER  
(00Z RUN) AND WEAKER (06Z RUN) AND CMC RUNS HAVE BEEN INCONSISTENT  
AS WELL. FOR BOTH OF THESE AREAS OF UNCERTAINTY THE DETERMINISTIC  
MANUAL FORECAST REPRESENTS A FAIRLY CONSERVATIVE MODEL/ENSEMBLE  
BLENDED APPROACH WHILE AWAITING IMPROVED CLUSTERING/STABILITY.  
 
THE EARLY-MID PERIOD BLEND INCORPORATED OPERATIONAL  
GFS/ECMWF/UKMET/CMC GUIDANCE TO REPRESENT MAJORITY CLUSTER  
CONCEPTS FOR DETAIL. THEN THE FORECAST TRANSITIONED TO 35-60 PCT  
ENSEMBLE MEAN GUIDANCE FOR DAYS 6-7 THU-FRI RESPECTIVELY AS  
GUIDANCE SPREAD INCREASED.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
ASIDE FROM TYPICAL DETAIL ADJUSTMENTS GUIDANCE HAS BEEN FAIRLY  
STABLE IN SHOWING THE POTENTIAL FOR A PERIOD OF LOCALLY HEAVY  
RAINFALL OVER AND INLAND FROM THE GULF COAST REGION, SUPPORTED BY  
THE EJECTING SOUTHERN ROCKIES SHORTWAVE/ASSOCIATED FRONT AS WELL  
AS A LEADING STATIONARY FRONT. CURRENTLY THE MOST LIKELY AXIS OF  
HIGHEST RAINFALL EXTENDS FROM EAST/SOUTHEAST TX INTO GA. THE  
MAJORITY OF THIS EVENT SHOULD TAKE PLACE DURING THE FIRST HALF OF  
THE WEEK. OVER THE WEST, EXTREME NORTHERN AREAS WILL SEE SOME  
EARLY WEEK RAIN/HIGHER ELEVATION SNOW ACCOMPANYING A STATIONARY  
FRONT. THEN THE APPROACH/ARRIVAL OF THE NORTHEAST PACIFIC  
TROUGH/SURFACE SYSTEM WILL BRING A PERIOD OF ENHANCED MOISTURE  
FOCUS INTO THE PACIFIC NORTHWEST/NORTHERN ROCKIES, FOLLOWED BY A  
DRYING TREND BY THU-FRI. AS ASSOCIATED LOW PRESSURE TRACKS INTO  
THE PLAINS AND GREAT LAKES/NORTHEAST DURING THE LATTER HALF OF THE  
WEEK, EXPECT SNOW AND BREEZY TO WINDY CONDITIONS TO BE POTENTIAL  
THREATS IN THE COLD SECTOR. SNOWFALL OVER THE UPPER MS VALLEY AND  
UPPER GREAT LAKES WILL BE PARTICULARLY SENSITIVE TO THE EXACT  
STRENGTH OF THE SURFACE LOW. EARLIER IN THE WEEK A FAST MOVING  
FRONT CROSSING THE NORTHERN TIER WILL BRING A PERIOD OF BREEZY  
WEATHER AND SOME LAKE EFFECT SNOW IN ITS WAKE.  
 
LOCATIONS FROM THE NORTHERN PLAINS TO SOUTHEAST SHOULD SEE THE  
HIGHEST TEMPERATURE ANOMALIES IN THE EARLY-MID WEEK PERIOD. THOSE  
TWO AREAS HAVE THE BEST POTENTIAL TO SEE SOME MIN TEMPS AT LEAST  
15-20F ABOVE NORMAL WHILE SUCH ANOMALIES FOR MAX TEMPS MAY FOCUS  
CLOSER TO THE CENTRAL PLAINS/MS VALLEY ESPECIALLY AROUND MIDWEEK.  
SYSTEM PUSHING OUT OF THE WEST WILL BRING A MUCH COLDER TREND TO  
THE INTERIOR WEST THEN EXTENDING INTO THE PLAINS/MS VALLEY  
MID-LATE WEEK, WITH SOME HIGHS 10-20F BELOW NORMAL.  
 
RAUSCH  
 

 
 
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