453  
FXUS02 KWBC 190638  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
137 AM EST FRI JAN 19 2018  
 
VALID 12Z MON JAN 22 2018 - 12Z FRI JAN 26 2018  
   
..OVERVIEW
 
 
A FAIRLY PROGRESSIVE FLOW REGIME WILL BE IN PLACE ACROSS THE CONUS  
THROUGH THE MEDIUM RANGE. A BUILDING UPPER RIDGE EXTENDING FROM  
THE NORTH CENTRAL PACIFIC OCEAN TO THE BERING SEA THROUGH THE  
PERIOD WILL PROMOTE CONTINUED TROUGHING IN THE GULF OF ALASKA AND  
OFF THE NORTH AMERICAN WEST COAST. THIS PERSISTENT TROUGH WILL  
SERVE TO CONTINUALLY EJECT UPPER LEVEL SHORTWAVES EASTWARD ACROSS  
NORTH AMERICA WHICH WILL AMPLIFY AS THEY ENCOUNTER INCREASING  
BAROCLINICITY ACROSS THE CONUS. THE END RESULT WILL BE A PATTERN  
MORE REMINISCENT OF EARLY SPRING THAN ONE OF MID-WINTER, WITH  
FREQUENT AND PERHAPS RELATIVELY INTENSE SURFACE CYCLONES ACROSS  
THE CENTRAL U.S., CHARACTERIZED BY STRONG THERMAL GRADIENTS ACROSS  
THEM.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
WHILE MODELS/ENSEMBLES SHOW GOOD AGREEMENT WITH RESPECT TO THE  
LARGE SCALE PATTERN DESCRIBED ABOVE, SOME SIGNIFICANT DIFFERENCES  
EXIST WITH RESPECT TO THE DETAILS OF THE FORECAST. DEEP UPPER LOW  
AND ASSOCIATED SURFACE CYCLONE EXPECTED TO BE LOCATED ACROSS THE  
CENTRAL PLAINS/UPPER MS VALLEY ON MON HAS SHOWN SOME IMPROVEMENT  
AMONG THE GUIDANCE, WITH THE 00Z GFS MOVING TOWARD THE CONSENSUS  
(A SOMEWHAT SLOWER SOLUTION THAT WAS SHOWN BY THE 12Z/18Z RUNS).  
THE GUIDANCE CONSENSUS SUGGESTS A LOW TRACK ACROSS THE GREAT LAKES  
EARLY TUE BEFORE LIFTING NORTH INTO EASTERN QUEBEC AND ONTARIO.  
THE OTHER AREA OF CONTENTION EARLY IN THE PERIOD IS A  
SHORTWAVE/LOW PRESSURE SYSTEM APPROACHING THE WEST COAST. THE  
ECMWF REMAINS FASTER/STRONGER WITH THIS SHORTWAVE ENERGY RELATIVE  
TO THE GFS/CMC, AND NEITHER MODEL SEEMS TO BE TRENDING  
SIGNIFICANTLY TOWARD THE OTHER. THE ECMWF WAS GENERALLY WELL  
CENTERED WITH RESPECT TO ITS ENSEMBLE MEMBERS, AND HAS BEEN  
CONSISTENT, THUS WAS NOT COMFORTABLE THROWING THE SOLUTION OUT AT  
THIS TIME. GIVEN THESE CONSIDERATIONS, A BLEND OF THE  
ECMWF/GFS/CMC ALONG WITH THE ECMWF ENSEMBLE MEAN WAS USED A  
STARTING POINT FOR THE FORECAST DURING DAYS 3-4 (SUN-MON).  
 
DIFFERENCES SURROUNDING THE WAVE REACHING THE WEST COAST ON DAY 4  
TRANSLATE DOWNSTREAM INTO THE ROCKIES/CENTRAL U.S. DAYS 5-7  
(TUE-THU). DUE TO ITS FASTER TIMING INITIALLY, THE ECMWF EJECTS  
LEADING SHORTWAVE ENERGY INTO THE SOUTHERN/CENTRAL PLAINS QUICKER  
THAN THE GFS AS WELL. GUIDANCE ARE GENERALLY IN AGREEMENT ON THIS  
BEING A RATHER ENERGETIC WAVE, FURTHER AMPLIFYING AS IT MOVES FROM  
THE FOUR CORNERS REGION INTO THE CENTRAL U.S. INTO A REGION OF  
STRONGER LOW-LEVEL THERMAL GRADIENT. INTERESTINGLY, THE 00Z GFS IS  
QUITE A BIT WEAKER WITH THIS WAVE ACROSS THE ROCKIES ON DAY 7  
RELATIVE TO ANY OF ITS RECENT RUNS OR TO THE ECMWF. ENSEMBLE MEANS  
SUGGEST A CONTINUED COMPROMISE MAY BE IN ORDER, AND THIS WAS  
GENERALLY THE APPROACH TAKEN. THE WPC FORECAST FOR DAYS 5-7 SHOWED  
GRADUALLY INCREASING ENSEMBLE (ECENS/GEFS) MEAN WEIGHTING THROUGH  
TIME, WITH MINORITY COMPONENTS OF THE ECMWF/GFS STILL INCLUDED.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
POTENTIALLY SIGNIFICANT SNOWFALL ALONG WITH GUSTY WINDS WILL BE  
POSSIBLE ACROSS PORTIONS OF THE UPPER MIDWEST/GREAT LAKES MON/MON  
NIGHT ON THE NORTH SIDE OF THE SURFACE LOW PRESSURE SYSTEM.  
FARTHER SOUTH, RAIN AND PERHAPS SOME THUNDERSTORMS WILL BE  
POSSIBLE ALONG/HEAD OF THE SURFACE COLD FRONT FROM THE GULF COAST  
TO THE OH VALLEY. AS THE SYSTEM REACHES THE NORTHEAST LATE MON  
INTO TUE, AREAS OF HEAVY RAIN WILL BE POSSIBLE ACROSS SOUTHERN NEW  
ENGLAND WHILE A WINTRY MIX OF PRECIPITATION IS POSSIBLE FOR  
NORTHERN NEW ENGLAND. MEANWHILE, THE WESTERN U.S. WILL SEE A  
PROLONGED PERIOD OF ONSHORE FLOW WITH SEVERAL UPPER-LEVEL IMPULSES  
AND SURFACE FRONTAL SYSTEMS ENHANCING PRECIPITATION AT TIMES.  
LOW-ELEVATION RAIN AND MOUNTAIN SNOWS ARE EXPECTED THROUGH MUCH OF  
THE PERIOD. SNOW LEVELS SHOULD DECREASE BY MID-WEEK AS HEIGHTS  
FALL IN RESPONSE TO THE AMPLIFYING UPPER TROUGH. THE SOUTH CENTRAL  
U.S. WILL SEE INCREASING POTENTIAL FOR SHOWERS AND THUNDERSTORMS  
BY LATE NEXT WEEK AS THE NEXT UPPER TROUGH/FRONTAL SYSTEM REACH  
THE PLAINS. THE DESCRIBED DIFFERENCES IN TIMING BETWEEN VARIOUS  
MODEL SOLUTIONS WILL HAVE AN IMPACT ON HOW QUICKLY LOW-LEVEL  
MOISTURE RETURN COMMENCES. THUS, CONFIDENCE IN THE SPECIFICS OF  
WHERE/WHEN ANY POTENTIAL PRECIPITATION WOULD OCCUR IS RELATIVELY  
LOW AT THIS TIME.  
 
MUCH OF THE EASTERN THIRD OF THE CONUS WILL SEE ABOVE AVERAGE  
TEMPS MON-TUE IN THE WARM SECTOR AHEAD OF THE INTENSIFYING MS  
VALLEY/GREAT LAKES CYCLONE. HIGH TEMPERATURES MAY REACH 10-20 DEG  
F ABOVE AVERAGE. GIVEN THE PROGRESSIVE PATTERN, ANY COOL DOWN  
BEHIND THE FRONT WILL BE RATHER BRIEF, WITH WARM AIR ADVECTION  
AHEAD OF THE NEXT DEVELOPING CYCLONE EXPECTED TO BOOST  
TEMPERATURES ABOVE AVERAGE ONCE AGAIN ACROSS MUCH OF THE CENTRAL  
U.S. BY THE MIDDLE OF NEXT WEEK.  
 
RYAN  
 

 
 
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