076  
FXUS02 KWBC 110504  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1203 AM EST SAT NOV 11 2017  
 
VALID 12Z TUE NOV 14 2017 - 12Z SAT NOV 18 2017  
   
..PATTERN OVERVIEW
 
 
THE LARGE SCALE PATTERN WILL REMAIN FAIRLY STABLE THROUGH THE  
MEDIUM RANGE. A PERSISTENT AND STRONG REX BLOCK ACROSS THE  
CENTRAL/NORTHERN PACIFIC WILL CONTINUE TO FAVOR WEAK RIDGING  
ACROSS THE SOUTHERN TIER WITH PROGRESSIVE FLOW LIKELY TO EXTEND  
ACROSS THE REST OF THE CONUS. THIS PATTERN WILL PROMOTE  
POTENTIALLY HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST WITH AN  
OCCASIONAL PUSH OF COLD AIR INTO THE NORTHERN TIER STATES,  
ESPECIALLY FROM THE NORTHERN PLAINS TO THE GREAT LAKES.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MODEL AGREEMENT DURING DAYS 3-4 (TUE-WED) WAS SUFFICIENT TO  
JUSTIFY A MAJORITY DETERMINISTIC STARTING POINT FOR THE FORECAST,  
BASED PRIMARILY ON THE 12Z ECMWF/18Z GFS. SHORTWAVE AMPLIFYING  
ACROSS THE NORTHERN PLAINS/UPPER MIDWEST DAYS 3-4 SHOWS RELATIVELY  
MODEST SPREAD AMONG THE GUIDANCE. ANOTHER SHORTWAVE MOVING OFF THE  
NORTHEAST U.S. COAST ON DAY 4 SHOWS SOME FAIRLY MINOR  
TIMING/AMPLITUDE DIFFERENCES WHICH COULD PLAY A ROLE ON SURFACE  
CYCLOGENESIS ALONG A FRONTAL BOUNDARY WELL OFFSHORE. THE MOST  
SIGNIFICANT AREA OF UNCERTAINTY DURING THIS FORECAST CYCLE ARISES  
FROM AN UPPER TROUGH AMPLIFYING OFF/ALONG THE WEST COAST WED-THU.  
DETERMINISTIC SOLUTIONS SHOW A LARGE AMOUNT OF SPREAD SURROUNDING  
THIS SYSTEM AS TIME GOES ON, WITH THE 12Z ECMWF (SIMILAR TO THE  
PREVIOUS RUN) SHOWING A MUCH SLOWER AMPLIFICATION AND MOVEMENT OF  
THE TROUGH INLAND RELATIVE TO THE GFS. THE EXACT EVOLUTION OF THIS  
SYSTEM OFF THE WEST COAST SEEMS TO BE LARGELY DETERMINED BY THE  
ARRIVAL OF SHORTWAVE ENERGY ORIGINATING IN THE NORTHERN BERING SEA  
TODAY, WHICH IS FORECAST TO DIVE SOUTHWARD AROUND THE EASTERN EDGE  
OF THE NORTH PACIFIC RIDGE. GIVEN THE ORIGIN IN A HIGHLY DATA  
SPARSE REGION, THIS WOULD SOMETIMES FAVOR THE ECMWF SOLUTION -  
HOWEVER IN THIS CASE THAT DOES NOT SEEM SO CLEAR. THE 12Z ECMWF  
ENSEMBLE MEAN FAVORS A SOMEWHAT QUICKER TROUGH PROGRESSION INLAND  
AND SURFACE CYCLOGENESIS ACROSS THE NORTHERN PLAINS/MIDWEST BY THU  
NIGHT/FRI. IT SHOULD BE NOTED THAT SPREAD IN THE EC ENSEMBLE IS  
QUITE LARGE, AND WITH A NUMBER OF MEMBERS DEVELOPING A DEEP  
CYCLONE ACROSS THE UPPER MIDWEST/GREAT LAKES THE MEAN COULD BE  
BIASED TOWARD THESE DEEPER SOLUTIONS. GEFS/NAEFS MEANS ARE ALSO  
GENERALLY ON THE SAME PAGE AS THE EC MEAN. FURTHER, THE  
DETERMINISTIC ECMWF HAS SHOWN A RATHER LARGE DEGREE OF RUN-TO-RUN  
VARIABILITY BY LATE IN THE WEEK, WHERE THE GFS SOLUTIONS HAVE BEEN  
RELATIVELY STABLE BY COMPARISON. THUS, DETERMINISTIC ECMWF  
WEIGHTING WAS REDUCED SUBSTANTIALLY IN THE FORECAST BY DAYS 6-7  
(FRI-SAT), WITH A TREND TOWARD MAJORITY ENSEMBLE WEIGHTING (12Z  
ECENS/NAEFS) FROM DAY 5 ONWARD. THIS KEEPS A FORECAST GENERALLY  
WELL ALIGNED WITH CONTINUITY WHILE ADJUSTING TOWARD LATEST TRENDS  
IN THE GUIDANCE.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
HEAVY RAIN AND MOUNTAIN SHOW APPEAR LIKELY ACROSS THE PACIFIC  
NORTHWEST THROUGH MUCH OF THE PERIOD AS ONSHORE FLOW REMAINS  
PREVALENT AND A SERIES OF UPPER SHORTWAVES AND SURFACE LOW  
PRESSURE SYSTEMS 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. HIGH TEMPERATURES OF 5 TO  
10 DEG F BELOW AVERAGE WILL PERSIST ACROSS THE EASTERN U.S.  
TUE-WED BEFORE TEMPS GRADUALLY MODERATE BY LATE IN THE WEEK AS  
WARM ADVECTION TAKES HOLD AHEAD OF THE STRENGTHENING CENTRAL U.S.  
LOW PRESSURE SYSTEM. MEANWHILE, ABOVE AVERAGE TEMPS WILL PERSIST  
ACROSS THE CENTRAL/SOUTHERN ROCKIES AND INTO MUCH OF THE PLAINS  
UNTIL A COLD FRONT BEGINS TO SWEEP SOUTH INTO THE NORTHERN PLAINS  
BY THU-FRI, BRINGING ANOTHER ROUND OF CHILLY CANADIAN AIR.  
 
RYAN  
 

 
 
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