207  
FXUS02 KWBC 100529  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1228 AM EST FRI NOV 10 2017  
 
VALID 12Z MON NOV 13 2017 - 12Z FRI NOV 17 2017  
   
..PATTERN OVERVIEW
 
 
LARGE SCALE PATTERN WILL REMAIN FAIRLY STABLE THROUGH THE MEDIUM  
RANGE. PERSISTENT AND STRONG REX BLOCK ACROSS THE CENTRAL/NORTHERN  
PACIFIC WILL CONTINUE TO FAVOR WEAK RIDGING ACROSS THE SOUTHERN  
TIER WITH PROGRESSIVE FLOW PATTERN FROM THE NORTHWESTERN U.S. TO  
NEW ENGLAND. THIS PATTERN WILL ALSO CONTINUE TO FAVOR THE  
POTENTIAL FOR COLD AIR MASSES TO SPILL SOUTHWARD EAST OF THE  
ROCKIES, ESPECIALLY FROM THE NORTHERN PLAINS TO THE GREAT LAKES,  
AND POTENTIALLY HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MODEL AGREEMENT WAS SUFFICIENT DURING DAYS 3-4 (MON-TUE) TO  
JUSTIFY A MAJORITY DETERMINISTIC STARTING POINT FOR THE FORECAST.  
PAST COUPLE GFS SOLUTIONS ALONG WITH 12Z ECMWF ALL FELL WELL  
WITHIN CONSENSUS WITH TROUGH EXITING THE EAST COAST ON DAY 3 AND  
SHORTWAVE ENERGY EJECTING QUICKLY ACROSS THE PAC NW TO THE NRN  
ROCKIES DAYS 3-4. SPREAD AMONG DETERMINISTIC SOLUTIONS BEGINS TO  
GRADUALLY INCREASE FROM DAY 5 (WED) ONWARD, AND THE WPC FORECAST  
WAS TRENDED TOWARD HEAVIER ENSEMBLE MEAN WEIGHTING AS A RESULT.  
THE ECMWF WAS TOWARD THE SLOWER SIDE OF THE SPREAD WITH SHORTWAVE  
PROGRESSION ACROSS THE CANADIAN PRAIRIES TOWARD JAMES BAY WED-THU,  
WITH SIMILAR IMPLICATIONS FOR THE SURFACE COLD FRONT CROSSING THE  
CENTRAL U.S. THE 18Z GFS WAS TOWARD THE FASTER SIDE OF THE SPREAD  
FOR THE SAME SYSTEM WHILE THE 00Z RUN WAS A BIT SLOWER. ENSEMBLE  
MEANS SHOWED BETTER CONSENSUS, WITH THE 12Z ECENS/NAEFS/18Z GEFS  
MEANS SHOWING TIMING BETWEEN THE ECMWF/GFS SOLUTIONS. SPREAD ALSO  
INCREASES WITH NEXT SYSTEM APPROACHING THE PAC NW WED-THU. 12Z CMC  
WAS AN OUTLIER BY THU, SEEMING TO HANG UP THE AMPLIFYING UPPER  
TROUGH WELL OFFSHORE. GFS AND ECMWF BOTH MOVE THE TROUGH INLAND BY  
LATE WED, WITH THE GFS CONSISTENTLY A BIT FASTER THAN THE ECMWF.  
ENSEMBLE TRENDS SUGGEST A SLIGHT TREND TOWARD A SLOWER AND MORE  
AMPLIFIED TROUGH (AND HIGHLY AMPLIFIED UPSTREAM FLOW WOULD SEEM TO  
SUPPORT THIS IDEA AS WELL) SO AN EVOLUTION A BIT MORE TOWARD THE  
12Z ECMWF IS FAVORED AT THIS TIME. SURFACE WAVE SHOULD DEVELOP  
THU-FRI ALONG A LINGERING FRONTAL BOUNDARY ACROSS THE SOUTHERN  
ROCKIES, WITH MOST GUIDANCE SUGGESTING A DEEPENING CYCLONE ACROSS  
THE NORTHERN PLAINS/MIDWEST BY DAY 7 (FRI). WHILE MOST SOLUTIONS  
AGREE WITH THIS GENERAL IDEA, SPREAD IS QUITE LARGE AS TO THE  
EXACT SURFACE LOW EVOLUTION, AND THE PREFERENCE IS TO STICK CLOSE  
TO THE ENSEMBLE MEANS AT THIS TIME (ECENS/GEFS/NAEFS), ALL OF  
WHICH SHOW A SFC LOW CROSSING THE UPPER GREAT LAKES ON DAY 7.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
HEAVY RAIN AND MOUNTAIN SNOW APPEAR LIKELY ACROSS THE PACIFIC  
NORTHWEST THROUGH MUCH OF THE MEDIUM RANGE AS ONSHORE FLOW REMAINS  
PREVALENT AND A SERIES OF UPPER SHORTWAVES AND SURFACE LOW  
PRESSURE SYSTEM AFFECT THE REGION. A SURFACE FRONT CROSSING THE  
CENTRAL U.S. AND OH VALLEY TUE WED WILL BRING A ROUND OF  
PRECIPITATION TO THOSE REGIONS. AS THE FRONT STALLS AND ANOTHER  
WAVE OF LOW PRESSURE DEVELOPS ACROSS THE NORTHERN PLAINS THU-FRI  
ANOTHER ROUND OF RAIN IS POSSIBLE WITH PERHAPS SOME THUNDERSTORMS  
FROM THE MID-MS VALLEY TO THE OH VALLEY. BELOW AVERAGE  
TEMPERATURES (5-10 DEG F BELOW AVERAGE FOR MAX TEMPS) WILL PERSIST  
ACROSS THE EASTERN THIRD OF THE CONUS FROM EARLY-MID NEXT WEEK,  
BUT TEMPS SHOULD GRADUALLY MODERATE AS WARM ADVECTION TAKES HOLD  
AHEAD OF THE STRENGTHENING LOW PRESSURE SYSTEM ACROSS THE CENTRAL  
U.S. BY LATE IN THE WEEK. MEANWHILE, ABOVE AVERAGE TEMPERATURES  
WILL PERSIST ACROSS THE CENTRAL/SOUTHERN ROCKIES AND INTO MUCH OF  
THE PLAINS UNTIL AN ARCTIC COLD FRONT BEGINS TO SWEEP SOUTH INTO  
THE NORTHERN PLAINS BY THU-FRI.  
 
RYAN  
 

 
 
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