251  
FXUS02 KWBC 171559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST WED JAN 17 2018  
 
VALID 12Z SAT JAN 20 2018 - 12Z WED JAN 24 2018  
   
..OVERVIEW
 
 
MULTI-DAY MEANS CONTINUE TO SHOW THE ESTABLISHMENT OF BROAD  
CYCLONIC FLOW AS THE PREVAILING LARGE SCALE PATTERN OVER THE LOWER  
48, THOUGH WITH SOME MODEST DIFFERENCES AND RUN TO RUN VARIABILITY  
FOR THE PRECISE LOCATION OF THE MEAN TROUGH'S AXIS. WITHIN THIS  
PATTERN THERE SHOULD BE TWO PROMINENT PACIFIC SYSTEMS THAT BRING  
PERIODS OF HEAVY PRECIP TO THE CENTRAL/NORTHERN WEST COAST AND  
SOME MOISTURE FARTHER INLAND, WHILE A LEADING UPPER TROUGH  
REACHING THE WEST AS OF EARLY SAT WILL GENERATE A POTENTIALLY  
SIGNIFICANT PLAINS THROUGH GREAT LAKES STORM FROM SUN INTO EARLY  
NEXT WEEK.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
AS THE UPPER TROUGH INITIALLY OVER THE WEST PROGRESSES  
EAST/NORTHEAST, GUIDANCE IN PRINCIPLE IS SHOWING AN INCREASINGLY  
AGREEABLE SIGNAL THAT ASSOCIATED LOW PRESSURE WILL DEEPEN INTO A  
FAIRLY STRONG STORM SYSTEM AS IT TRACKS INTO THE MS VALLEY/GREAT  
LAKES. HOWEVER THERE IS STILL A MODERATE DEGREE OF UNCERTAINTY  
REGARDING THE EXACT SPEED AND TRACK OF THIS SYSTEM. OVER THE PAST  
DAY THE MOST PROMINENT TREND AMONG MOST MODELS AND ENSEMBLE  
MEMBERS HAS BEEN TOWARD SLOWER TIMING. ALSO THE 00Z/06Z GFS RUNS  
ARE MOST PRONOUNCED IN PHASING SOUTHERN CANADA ENERGY WITH THE  
UPPER TROUGH OF INTEREST AS IT REACHES THE GREAT LAKES. THESE  
CONSIDERATIONS SHOULD INITIALLY FAVOR TILTING THE FORECAST  
SOMEWHAT MORE TOWARD THE ECMWF/ECMWF MEAN VERSUS THE FASTER  
GFS/GEFS MEAN. THAT SAID, THE 00Z UKMET/CMC STILL SUGGEST THAT  
TIMING COULD BE SOMEWHAT FASTER THAN THE ECMWF-BASED SOLUTIONS  
PLUS THERE HAS BEEN A FASTER/LESS AMPLIFIED TREND WITH UPSTREAM  
ENERGY FLOWING INTO THE WEST EARLY NEXT WEEK. THEREFORE CURRENTLY  
PREFER ADJUSTING THE FORECAST ONLY ABOUT 2/3 TOWARD THE SLOWER  
ECMWF SCENARIO. AS FOR TRACK LATITUDE, THE 00Z CMC AND A NUMBER  
OF ITS ENSEMBLE MEMBERS LEAN TOWARD THE NORTHERN SIDE OF THE FULL  
SPREAD. THE LATITUDE OF GREATEST ENSEMBLE LOW DENSITY APPEARS  
FAIRLY STABLE OVER THE PAST DAY.  
 
UPSTREAM THE GUIDANCE IS FAIRLY STABLE/AGREEABLE WITH BEST DEFINED  
LOW PRESSURE TRACKING TOWARD THE BC COAST BY SUN BUT THERE ARE  
SMALLER SCALE/LOWER PREDICTABILITY QUESTIONS WITH RESPECT TO  
WAVINESS ALONG THE TRAILING FRONT. SUCH DETAILS WILL LIKELY TAKE  
INTO THE SHORT RANGE TIME FRAME TO GET RESOLVED SATISFACTORILY AND  
WILL INFLUENCE SPECIFICS OF TIMING/INTENSITY OF PRECIP. GUIDANCE  
HAS BEEN VARIABLE WITH THE DETAILS OF SUPPORTING FLOW ALOFT THAT  
STREAMS INTO THE WEST BY MON-TUE BUT THE GENERAL TREND HAS BEEN  
TOWARD A FASTER/FLATTER DEPICTION OF THIS ENERGY. AS A RESULT  
WHATEVER SYSTEM(S) REACH THE CENTRAL/EAST-CENTRAL U.S. BY TUE-WED  
ARE LESS DEFINED THAN IN SOME PREVIOUS FORECASTS.  
 
THE NEXT SYSTEM TO AFFECT THE WEST COAST SHOULD COME INTO THE  
PICTURE AROUND DAYS 6-7 TUE-WED. THERE IS MORE UNCERTAINTY WITH  
THIS SYSTEM AS IT INVOLVES A POTENTIAL INTERACTION OF MID-LATITUDE  
PACIFIC FLOW AND NORTHEAST PACIFIC UPPER LOW/TROUGHING. MINIMAL  
INTERACTION WOULD ALLOW THE MID-LATITUDE ENERGY TO BRING A WAVE  
INTO THE CENTRAL WEST COAST AS SUGGESTED BY THE 00Z CMC, SOME 00Z  
CMC ENSEMBLES, AND A COUPLE 00Z GEFS MEMBERS. ON THE OTHER HAND  
RECENT GFS RUNS, INCLUDING THE 06Z VERSION RELATIVE TO THE 00Z  
RUN, HAVE BEEN TRENDING TOWARD GREATER INTERACTION AND A FARTHER  
NORTHWEST SURFACE LOW TRACK WHICH THE ECMWF AND ENSEMBLE MEANS  
HAVE BEEN SHOWING. CURRENT PREFERENCE IS TO LEAN TOWARD THIS  
MAJORITY SCENARIO WITH A COMBINATION OF THE 06Z GFS/00Z ECMWF AND  
THEIR MEANS PROVIDING A GOOD BALANCE OF SYNOPTIC AGREEMENT VERSUS  
DETAIL.  
 
BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FORECAST STARTED  
WITH A 50/30/20 WEIGHT OF THE 00Z ECMWF/06Z GFS/00Z ECMWF MEAN  
RESPECTIVELY FOR DAYS 3-5 SAT-MON FOLLOWED BY GREATER INCLUSION OF  
ENSEMBLE MEAN GUIDANCE SUCH THAT THE TOTAL GEFS/ECMWF MEAN WEIGHT  
REACHED 60 PERCENT BY DAY 7 WED.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
SEPARATE STORM SYSTEMS WILL CONTRIBUTE TO AN ACTIVE WEATHER  
PATTERN OVER PORTIONS OF THE LOWER 48. LEADING STORM EMERGING  
FROM THE WEST SHOULD STRENGTHEN AS IT TRACKS FROM THE PLAINS  
THROUGH THE GREAT LAKES, BRINGING A BAND OF MODERATE/HEAVY SNOW  
THAT WILL MOST LIKELY BE ALIGNED FROM THE CENTRAL PLAINS THROUGH  
UPPER GREAT LAKES. WINDS MAY BE STRONG ENOUGH TO GENERATE  
BLIZZARD CONDITIONS AT TIMES. MEANWHILE IN THE WARM SECTOR EXPECT  
SCATTERED TO NUMEROUS SHOWERS, WITH THUNDERSTORMS POSSIBLE FROM  
EASTERN TEXAS TO THE OHIO VALLEY, AS DEEP GULF OF MEXICO MOISTURE  
SURGES NORTHWARD AHEAD OF THE COLD FRONT. SOME RAINFALL MAY BE  
LOCALLY MODERATE TO HEAVY. AS THE FRONT REACHES THE EAST COAST,  
ATLANTIC INFLOW COULD ENHANCE TOTALS OVER NEW ENGLAND AS WELL.  
THE TWO PRIMARY SYSTEMS AFFECTING THE WEST COAST WILL BRING  
EPISODES OF HEAVY RAIN/HIGHER ELEVATION SNOW TO NORTHERN  
CALIFORNIA AND PACIFIC NORTHWEST. THERE MAY BE BRIEF PERIODS OF  
STRONG WINDS AS WELL DEPENDING ON EXACT EVOLUTION OF INDIVIDUAL  
SYSTEMS. SOME OF THE MOISTURE FROM THESE SYSTEMS WILL EXTEND INTO  
THE ROCKIES.  
 
CONTINUE TO EXPECT THE MOST EXTREME ANOMALIES FOR TEMPS TO BE IN  
THE WARM SECTOR AHEAD OF THE INITIAL WESTERN/CENTRAL U.S. STORM  
AND COLD FRONT, WITH THE WARMTH PROGRESSING FROM THE PLAINS SAT TO  
THE EASTERN STATES IN LESS EXTREME FASHION BY MON-TUE. SOME MIN  
TEMPS MAY BE AS HIGH AS 20-30F ABOVE NORMAL SAT-MON WHILE HIGHS  
MAY REACH 10-20F ABOVE NORMAL. BEHIND THIS SYSTEM, ANTICIPATE  
HIGHS OF 5-15F BELOW NORMAL OVER MUCH OF THE SOUTHERN 2/3 OF THE  
WEST DURING THE WEEKEND.  
 
RAUSCH  
 

 
 
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