416  
FXUS02 KWBC 191600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 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  
DURING MUCH OF THE MEDIUM RANGE PERIOD. 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 THROUGH MIDWEEK. THIS  
PERSISTENT TROUGH WILL EJECT UPPER LEVEL SHORTWAVES EASTWARD  
ACROSS NORTH AMERICA, SOME OF WHICH WILL AMPLIFY AS THEY ENCOUNTER  
INCREASING BAROCLINICITY ACROSS THE CONUS. LARGER SCALE EAST  
PACIFIC TROUGHING WILL LIKELY MOVE INTO THE WEST LATER THIS WEEK.  
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
 
 
LATEST GUIDANCE IS SHOWING SOME SIMILAR TENDENCIES RELATIVE TO  
EACH OTHER THOUGH PERHAPS WITH SOME SUBTLE HINTS AT CONVERGENCE.  
MULTI-DAY TRENDS FOR THE STORM TRACKING NORTHEAST FROM THE  
MIDWEST/GREAT LAKES HAVE BEEN SLOWER, CONTINUING TO FAVOR HEDGING  
SOMEWHAT SLOWER THAN GFS/GEFS RUNS. HOWEVER COMPARED TO YESTERDAY  
THERE IS A BIT LESS OF A CONSENSUS OVER HOW MUCH SLOWER TIMING  
THAN THE GFS IS OPTIMAL AND UPSTREAM FLOW IS STILL FLAT ENOUGH TO  
KEEP THE DOOR OPEN TO FASTER PROGRESSION AT SOME POINT. THERE ARE  
ALSO SOME MODERATE BUT IMPORTANT TRACK DIFFERENCES WITH THE 00Z  
ECMWF/ECMWF MEAN SOMEWHAT ON THE SOUTHERN SIDE OF THE SPREAD  
VERSUS OTHER CURRENT SOLUTIONS AND CONTINUITY. MEANWHILE THE  
MAJORITY OF GUIDANCE HAS TRENDED A LITTLE MORE OFFSHORE FOR THE  
EASTERN PACIFIC WAVE EXPECTED TO BE OFF THE PACIFIC NORTHWEST  
COAST BY DAY 4 TUE. ENSEMBLE SPREAD IS STILL CONSIDERABLE FOR  
THIS FEATURE WITH ECMWF/CMC/UKMET RUNS STRONGER THAN THE 00Z/06Z  
GFS. A COMPROMISE APPROACH LOOKS BEST OVER THIS REGION. FOR THE  
FIRST PART OF THE FORECAST PERIOD, A BLEND OF THE 00Z ECMWF/06Z  
GFS/00Z CMC IN ORDER OF DECREASING WEIGHT PROVIDED THE BEST  
ACCOUNT FOR PREFERENCES WITH BOTH SYSTEMS OF INTEREST.  
 
CONFIDENCE IS STILL LOW WITH LEADING PACIFIC ENERGY THAT CROSSES  
THE LOWER 48 THROUGH DAY 6 THU. GFS RUNS CONTINUE TO BE  
SLOWER/SHARPER THAN ECMWF RUNS-- ALBEIT WITH THE 06Z GFS TRENDING  
A LITTLE FASTER/WEAKER-- BUT 00Z UKMET/CMC RUNS DO ARGUE FOR A  
MORE PRONOUNCED FEATURE THAN THE ECMWF.  
 
THE GFS/GEFS CONTINUE TO LEAN ON THE SLOWER SIDE OF THE GUIDANCE  
SPREAD OVER THE LARGE SCALE TROUGH MOVING INTO THE WEST DURING THE  
LATTER HALF OF THE WEEK. HOWEVER THE SPREAD HAS NARROWED A BIT AS  
THE ECMWF HAS SLOWED DOWN VERSUS PREVIOUS RUNS. ALSO OF NOTE,  
RECENT ECMWF MEANS HAVE BEEN SIMILAR BUT BY WED-THU AT LEAST THE  
00Z RUN EXTENDS AN EAST-CENTRAL PACIFIC RIDGE FARTHER EAST INTO A  
REGION OF NORTHWESTERLY FLOW DEPICTED BY CONSENSUS INCLUDING THE  
OPERATIONAL ECMWF. THIS CONSIDERATION RECOMMENDS KEEPING THE  
ECMWF MEAN COMPONENT OF THE FORECAST SOMEWHAT LOWER THAN AVERAGE  
MID-LATE PERIOD. EVEN WITH THE DIFFERENCES THAT ARISE IN THE DAYS  
4-6 TIME FRAME, THERE IS ACTUALLY BETTER CONSENSUS AMONG MOST  
GUIDANCE (MINUS THE FLAT 00Z CMC MEAN) BY LATE DAY 7 FRI.  
BALANCING CONSIDERATIONS FOR THIS TROUGH AND LEADING SHORTWAVE  
ENERGY, THE MID-LATE PART OF THE FORECAST INCORPORATES A BLEND OF  
THE 00Z ECMWF/06Z GFS/00Z CMC AND THE 06Z GEFS/00Z ECMWF MEANS.  
 
   
..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 SYSTEM TRACKING OUT OF THE MIDWEST.  
FARTHER SOUTH, RAIN AND PERHAPS SOME THUNDERSTORMS WILL BE  
POSSIBLE ALONG/AHEAD OF THE SURFACE COLD FRONT FROM THE GULF COAST  
TO THE OH VALLEY. AS THE SYSTEM REACHES THE NORTHEAST LATE MON  
INTO TUE, ATLANTIC INFLOW MAY PROMOTE AREAS OF HEAVY RAIN ACROSS  
SOUTHERN NEW ENGLAND WHILE A WINTRY MIX OF PRECIPITATION IS  
POSSIBLE FOR NORTHERN NEW ENGLAND. THE PROPORTION OF PRECIP THAT  
FALLS AS SNOW OVER NEW ENGLAND WILL BE SENSITIVE TO THE EXACT  
SURFACE LOW TRACK, WITH CURRENT SPREAD WITHIN TYPICAL GUIDANCE  
ERROR FOR 3-5 DAYS OUT IN TIME. 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. EXPECT LOW-ELEVATION RAIN AND MOUNTAIN SNOWS THROUGH MUCH  
OF THE PERIOD, WITH HIGHEST TOTALS FROM NORTHERN CALIFORNIA  
THROUGH THE PACIFIC NORTHWEST. SNOW LEVELS WILL LIKELY DECREASE  
MID-LATE WEEK AS HEIGHTS ALOFT FALL IN RESPONSE TO THE AMPLIFYING  
UPPER TROUGH MOVING INTO THE REGION. 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. THE SAME IS TRUE IN ASSOCIATION WITH GULF MOISTURE THAT MAY  
STREAM ACROSS THE EXTREME SOUTHEAST/FLORIDA AROUND WED-THU, WITH  
PRECIP DEPENDENT ON LOW PREDICTABILITY SHORTWAVE SPECIFICS IN  
PROGRESSIVE FLOW ALOFT. THE OVERALL GUIDANCE AVERAGE HAS TRENDED  
SOMEWHAT LIGHTER OVER THE PAST DAY BUT NOT YET TO THE POINT OF THE  
DRIEST ECMWF-BASED GUIDANCE.  
 
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 WITH SOME PLUS 20F OR GREATER ANOMALIES POSSIBLE  
FOR MINS. 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 BY UP TO 10-20F ONCE AGAIN ACROSS MUCH OF THE  
CENTRAL/EAST-CENTRAL U.S. BY THE MIDDLE OF NEXT WEEK, WHILE MUCH  
OF THE WESTERN U.S. SHOULD SEE READINGS 5-15F BELOW NORMAL BY NEXT  
FRI.  
 
RAUSCH/RYAN  
 

 
 
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