706  
FXUS02 KWBC 120625  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
124 AM EST SUN NOV 12 2017  
 
VALID 12Z WED NOV 15 2017 - 12Z SUN NOV 19 2017  
   
..PATTERN OVERVIEW
 
 
PROGRESSIVE FLOW PATTERN IS EXPECTED TO REMAIN IN PLACE ACROSS THE  
CONUS THROUGH THE MEDIUM RANGE DOWNSTREAM OF A HIGHLY AMPLIFIED  
FLOW PATTERN ACROSS THE NORTH/CENTRAL PACIFIC. REX BLOCK WHICH HAS  
BEEN PERSISTENT IN THE VICINITY OF THE DATE LINE SHOWS LITTLE SIGN  
OF BREAKING DOWN OVER THE NEXT WEEK, ALTHOUGH A SLIGHT WESTWARD  
PROPAGATION IS POSSIBLE. TELECONNECTIONS ASSOCIATED WITH THESE  
ANOMALY CENTERS SUPPORT WEAK RIDGING ACROSS THE SOUTHERN  
ROCKIES/PLAINS WITH A PROGRESSIVE FLOW PATTERN ACROSS THE MAJORITY  
OF THE CONUS, WET CONDITIONS FOR MUCH OF THE WEST COAST, AND THE  
POTENTIAL FOR PERIODIC INTRUSIONS OF COLD CANADIAN AIR SOUTHWARD  
INTO THE U.S. NORTHERN TIER.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
AS WITH THE PAST COUPLE DAYS, MODELS CONTINUE TO SHOW RELATIVELY  
LOW SPREAD DURING THE FIRST COUPLE DAYS OF THE MEDIUM RANGE.  
TIMING/AMPLITUDE DIFFERENCES ARE FAIRLY SMALL WITH A SHORTWAVE  
CROSSING THE GREAT LAKES/NEW ENGLAND DAYS 3-4 (WED-THU) AND THE  
ASSOCIATED SURFACE FRONTAL SYSTEM. THE GFS AND ECMWF SHOWED  
SUFFICIENT AGREEMENT THAT A BLEND OF THESE TWO DETERMINISTIC  
SOLUTIONS SERVED AS A GOOD FORECAST STARTING POINT WED-THU.  
PACIFIC TROUGH THAT MOVES INTO THE WEST DAYS 4-5 (THU-FRI)  
CONTINUES TO BE THE GREATEST SOURCE OF UNCERTAINTY IN THE  
FORECAST, ALTHOUGH MODEL SPREAD HAS REDUCED A BIT OVER THE LAST 24  
HOURS. THE ECMWF (ON THE SLOW SIDE) AND GFS (ON THE QUICK SIDE)  
SOLUTIONS HAVE TRENDED TOWARD EACH OTHER, REPRESENTING INCREASING  
CONFIDENCE IN A SOLUTION SOMEWHERE BETWEEN THE TWO. SFC CYCLONE IS  
EXPECTED TO DEVELOP ACROSS THE NORTHERN PLAINS/UPPER MIDWEST LATE  
THU INTO FRI, AND GFS/ECMWF SOLUTIONS ARE NOW MUCH MORE TIGHTLY  
CLUSTERED AROUND THE ECENS/GEFS/NAEFS MEANS WITH RESPECT TO THIS  
SYSTEM COMPARED TO LAST NIGHT. SOMEWHAT GREATER UNCERTAINTY  
REMAINS FARTHER SOUTH WITH A SECOND FRONTAL WAVE FORECAST BY MOST  
GUIDANCE TO DEVELOP ACROSS THE CENTRAL PLAINS ON FRI BEFORE  
RAPIDLY MOVING NE AND BECOMING THE DOMINANT SFC LOW BY DAY 6 (SAT)  
ACROSS THE GREAT LAKES, WITH A SFC COLD FRONT SWEEPING QUICKLY  
EAST ACROSS THE APPALACHIANS AND EAST COAST. TIMING DIFFERENCES  
REMAIN AS TO HOW QUICKLY THE FRONT MOVES EAST, WITH THE GFS  
FASTEST AND THE ECMWF ON THE SLOW SIDE; AT THIS TIME A COMPROMISE  
APPROACH WAS TAKEN WITH RESPECT TO FRONTAL TIMING. THE WPC  
FORECAST FOR DAYS 5-7 SHOWED A GRADUAL TREND TOWARD HEAVIER  
ENSEMBLE WEIGHTING (ESPECIALLY THE 12Z ECENS/NAEFS) TO ACCOUNT FOR  
INCREASING UNCERTAINTY LATER IN THE PERIOD.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
PERSISTENT ONSHORE FLOW AND THE ARRIVAL OF A COUPLE LOW PRESSURE  
SYSTEMS (ONE ON WED AND ANOTHER SAT-SUN) WILL KEEP PRECIPITATION  
IN THE PICTURE ALONG THE WEST COAST THROUGH MUCH OF THE PERIOD.  
HEAVY RAIN AND MOUNTAIN SNOW ARE LIKELY ALONG WITH GUSTY WINDS,  
ESPECIALLY WED-THU WITH THE ARRIVAL OF THE STRONGER OF THE TWO  
SYSTEMS. A BRIEF LULL IS POSSIBLE LATE IN THE WEEK BEFORE THE  
SECOND SYSTEM APPROACHES, BRINGING ANOTHER ROUND OF PRECIP OVER  
THE WEEKEND. FARTHER EAST, SURFACE FRONT WILL BRING AREAS OF  
SHOWERS TO THE MS/OH VALLEYS WED-THU. MORE SIGNIFICANT PRECIP IS  
POSSIBLE WITH DEEP GREAT LAKES CYCLONE FRI-SAT FROM THE OH  
VALLEY/GREAT LAKES TO THE NORTHEAST. SUFFICIENT COLD AIR MAY BE IN  
PLACE NORTH OF THE SYSTEM TO PRODUCE SOME SNOWFALL POTENTIAL LATE  
IN THE WEEK FROM THE UPPER MIDWEST TO THE GREAT LAKES AND PERHAPS  
EVEN NORTHERN NEW ENGLAND.  
 
EXPECT A BROAD AREA OF ABOVE AVERAGE TEMPS (+5 TO 15 DEG F)  
THROUGH MUCH OF THE MEDIUM RANGE FROM THE SOUTHERN/CENTRAL ROCKIES  
OT THE PLAINS. NEXT SIGNIFICANT ROUND OF COLD AIR MAY REACH THE  
UPPER MIDWEST BY NEXT WEEKEND IN THE WAKE OF THE AFOREMENTIONED  
DEEP CYCLONE, WITH HIGH TEMPS 5 TO 15 DEG F BELOW AVERAGE BY  
FRI-SAT.  
 
RYAN  
 

 
 
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