477  
FXUS02 KWBC 161559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST TUE JAN 16 2018  
 
VALID 12Z FRI JAN 19 2018 - 12Z TUE JAN 23 2018  
   
..OVERVIEW
 
 
DURING THE COURSE OF THE PERIOD THE LONGWAVE PATTERN OVER THE  
LOWER 48 WILL SETTLE INTO BROAD MEAN TROUGH WHOSE AXIS WILL TEND  
TO BE ALIGNED OVER THE WEST-CENTRAL STATES. THIS PATTERN WILL  
FAVOR PERIODS OF ACTIVE WEATHER OVER THE WEST-- WITH HEAVY PRECIP  
POTENTIAL OVER THE CENTRAL/NORTHERN WEST COAST IN PARTICULAR--  
WHILE AN EMBEDDED WESTERN SYSTEM LATE THIS WEEK WILL EMERGE OVER  
THE PLAINS THIS WEEKEND AND TRACK NORTHEASTWARD. THIS SYSTEM WILL  
BRING THE POSSIBILITY OF A BAND OF SIGNIFICANT SNOWFALL AND WIND  
TO SOME LOCATIONS FROM THE CENTRAL PLAINS THROUGH UPPER  
MIDWEST/UPPER GREAT LAKES DURING THE WEEKEND INTO EARLY NEXT WEEK.  
 
   
..GUIDANCE ASSESSMENT/UNCERTAINTIES/PREFERENCES
 
 
BROADLY SPEAKING THE 00Z GFS/ECMWF, THEIR ENSEMBLE MEANS PLUS THE  
06Z GEFS MEAN, AS WELL AS THE 00Z CMC/UKMET, REPRESENT THE PATTERN  
FAIRLY WELL. ONE AREA OF RELATIVE CONTENTION IS WITH FLOW OVER  
THE NORTHEAST U.S./EASTERN CANADA EARLY-MID PERIOD, WITH LOWER  
HEIGHTS ALOFT IN THE ECMWF-BASED GUIDANCE BRINGING A SURFACE  
FARTHER SOUTHEAST INTO NEW ENGLAND THAN A MAJORITY OF OTHER  
SOLUTIONS. A COMPROMISE APPROACH LOOKS BEST WITH THIS ASPECT OF  
THE FORECAST. ELSEWHERE THE 06Z GFS APPEARS TO BE A LOWER  
PROBABILITY/FAST SCENARIO RELATIVE TO CONSENSUS. THIS INCLUDES  
THE FORECAST OF THE SHORTWAVE TRACKING OUT OF THE RIO GRANDE  
VALLEY, THE WESTERN U.S. THROUGH GREAT LAKES SYSTEM, AND EASTERN  
PACIFIC/WESTERN U.S. FLOW LATE IN THE PERIOD. AS A RESULT THE  
UPDATED FORECAST EMPHASIZED A 00Z OPERATIONAL MODEL BLEND DAYS 3-5  
FRI-SUN WHILE ADDING SOME 06Z GEFS/00Z ECMWF ENSEMBLE MEAN  
GUIDANCE BUT MAINTAINING SOME 00Z GFS/ECMWF/CMC INPUT BY DAYS 6-7  
MON-TUE.  
 
TAKING A CLOSER LOOK AT THE WEST COAST THROUGH GREAT LAKES/EASTERN  
CANADA SYSTEM, ASIDE FROM THE AFOREMENTIONED FAST 06Z GFS THERE  
HAS BEEN SOMEWHAT BETTER THAN AVERAGE AGREEMENT/STABILITY GIVEN  
THE TIME FRAME INVOLVED. DETAIL DIFFERENCES/ADJUSTMENTS ALOFT  
BECOME NOTICEABLE OVER THE PLAINS/MS VALLEY BY SUN-MON, LEADING TO  
UNCERTAINTY OVER EXACTLY HOW STRONG THE THE SYSTEM WILL BE. MOST  
RECENT OPERATIONAL RUNS HAVE BEEN SHOWING A FAIRLY STRONG SYSTEM,  
WHICH AT LEAST ARGUES FOR LEANING AWAY FROM GEFS MEANS WHICH IN  
SOME RUNS HAVE BEEN CONSIDERABLY WEAKER THAN THE ECMWF MEAN.  
ENSEMBLE LOW CLUSTERING IN THE 00Z CYCLE HAS IMPROVED COMPARED TO  
24 HOURS AGO WITH, WITH THE GREATEST DENSITY OF SURFACE LOWS OVER  
KS/NORTHWEST OK AS OF 12Z SUN AND IA INTO THE UPPER GREAT LAKES BY  
12Z MON. THE PREFERRED GUIDANCE BLEND YIELDS A SOLUTION WELL  
WITHIN THE SPREAD AND CLOSE TO OR A FRACTION SLOWER THAN  
CONTINUITY. REGARDLESS OF HOW WELL GUIDANCE APPEARS TO BE  
CLUSTERED, IT MAY STILL TAKE UNTIL MUCH CLOSER TO THE VALID TIME  
FOR IMPORTANT DETAILS OF THIS STORM TO BE RESOLVED.  
 
AN UPSTREAM SYSTEM WILL TRACK TOWARD THE BC COAST/VANCOUVER ISLAND  
AROUND SUN-MON WITH SHORTWAVE ENERGY TO THE SOUTH AND WEST PUSHING  
INTO THE WEST EARLY NEXT WEEK. ENSEMBLES SHOW A FAIR AMOUNT OF  
SPREAD BUT THE 00Z GFS/ECMWF/CMC ALL FIT INTO THE ENSEMBLE MEAN  
PATTERN SO SOME OF THEIR SOLUTIONS COULD BE MAINTAINED THROUGH THE  
END OF THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT THE HEAVIEST PRECIP DURING THE PERIOD TO OCCUR FROM THE  
PACIFIC NORTHWEST INTO NORTHERN CA WITH FAVORED TERRAIN POSSIBLY  
SEEING SEVERAL INCHES LIQUID IN LOWER ELEVATION RAIN/MOUNTAIN  
SNOW. SOME OF THIS MOISTURE WILL EXTEND INTO THE WEST WITH THE  
NORTHERN-CENTRAL ROCKIES SEEING A RELATIVE MAXIMUM OF PRECIP  
ALBEIT WITH LOWER ABSOLUTE TOTALS THAN SOME LOCATIONS ALONG THE  
WEST COAST. LEADING SYSTEM EMERGING FROM THE WEST AND THEN  
TRACKING NORTHEAST AS IT STRENGTHENS WILL BRING POTENTIAL FOR A  
BAND OF MODERATE/HEAVY SNOW FROM THE CENTRAL PLAINS THROUGH UPPER  
GREAT LAKES. IF THE STORM AND ASSOCIATED WINDS BECOME  
SUFFICIENTLY STRONG THERE MAY BLOWING/DRIFTING OF SNOW POSSIBLY TO  
THE EXTENT OF BLIZZARD CONDITIONS. WARM SECTOR RAINFALL OVER THE  
EASTERN HALF OF THE COUNTRY SHOULD BE MOSTLY ON THE LIGHT-MODERATE  
SIDE BUT WITH ISOLATED POCKETS OF HEAVIER ACTIVITY POSSIBLE. THE  
UPPER SHORTWAVE TRACKING OUT OF THE RIO GRANDE VALLEY MAY SUPPORT  
SOME CONVECTION ALONG THE TX COAST LATE THIS WEEK.  
 
THE MOST PRONOUNCED ANOMALIES FOR TEMPS WILL BE IN THE WARM SECTOR  
OF THE WESTERN U.S. THROUGH GREAT LAKES SYSTEM. LOW TEMPS IN  
PARTICULAR MAY BE 15-25F ABOVE NORMAL OVER THE NORTH CENTRAL  
ROCKIES/NORTHERN PLAINS FRI WITH THIS AXIS OF WARMTH PROGRESSING  
EAST/SOUTHEAST WITH TIME, REACHING THE EAST COAST STATES BY NEXT  
MON. HIGH TEMPS WILL A BIT LESS EXTREME BUT STILL 10-20F ABOVE  
NORMAL OVER SOME AREAS. THE SOUTHEAST WILL START OUT QUITE CHILLY  
ON FRI, WHILE COOL AIR WILL SPREAD ACROSS MUCH OF THE WEST AND TO  
SOME EXTENT THE NORTHERN PLAINS.  
 
RAUSCH  
 

 
 
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