906  
FXUS02 KWBC 071530  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1029 AM EST SUN JAN 07 2018  
 
VALID 12Z WED JAN 10 2018 - 12Z SUN JAN 14 2018  
 
15Z UPDATE...  
 
A BLEND OF THE 06Z GFS/00Z ECMWF ALONG WITH THEIR RESPECTIVE  
ENSEMBLE MEANS SERVED AS A BASIS FOR THE EXTENDED FORECAST UPDATE  
THIS MORNING. CONFIDENCE REMAINS SUFFICIENTLY HIGH TO JUSTIFY  
MAJORITY USE OF DETERMINISTIC GUIDANCE THROUGH DAY 5 (FRI). MODELS  
CONTINUE TO SHOW SOME TIMING AND STRENGTH DIFFERENCES SURROUNDING  
THE SHORTWAVE EMERGING FROM THE SOUTHERN ROCKIES INTO THE PLAINS  
WED-THU. THE FASTER GFS AND SLOWER ECMWF SOLUTIONS SEEM TO BE  
TRENDING GRADUALLY TOWARD A CENTER POINT, THUS THE DESCRIBED  
BLENDED APPROACH SHOULD WORK WELL HERE. THE SAME HOLDS TRUE FOR  
RELATIVELY LOW AMPLITUDE SHORTWAVE ENERGY MOVING INTO THE PAC NW  
ON WED, WHICH AMPLIFIES AS IT ENTERS THE BASE OF THE CENTRAL U.S.  
TROUGH BY THU, WITH THE GFS BACKING OFF A BIT ON ITS MORE  
AMPLIFIED AND SOMEWHAT SLOWER SOLUTION SHOWN YESTERDAY. ADDITIONAL  
MORE INTENSE SHORTWAVE AND ASSOCIATED SURFACE FRONTAL SYSTEM ARE  
EXPECTED TO REACH THE PAC NW ON THU. WHILE MODELS HAVE SHOWN SOME  
RUN-TO-RUN VARIABILITY SURROUNDING THIS SYSTEM, THE DESCRIBED  
ECMWF/GFS BLEND IS WELL REPRESENTATIVE OF THE GUIDANCE CONSENSUS.  
THIS SHORTWAVE WILL SERVE TO FURTHER REINFORCE THE EASTERN TROUGH  
LATER IN THE PERIOD, AS RIDGING RETURNS ALONG THE NORTH AMERICA  
WEST COAST.  
 
BEYOND DAY 5, WHILE CONFIDENCE IS ABOVE AVERAGE WITH RESPECT TO  
THE LARGE SCALE PATTERN (WESTERN RIDGE/EASTERN TROUGH), SPREAD  
INCREASES A BIT SURROUNDING EXPECTED SURFACE LOW EVOLUTION FROM  
THE TN VALLEY TO THE NORTHEAST. THE 00Z ECMWF SEEMED ON THE  
NORTHERN SIDE OF THE ENSEMBLE SPREAD WITH ITS FORECAST LOW  
POSITION AT 12Z SAT. GIVEN THE SOMEWHAT INCREASED UNCERTAINTY,  
HEAVIER ECENS/WEIGHTING WAS USED DURING DAYS 6-7 (SAT-SUN). THIS  
RESULTS IN A FORECAST LOW TRACK ALONG/JUST EAST OF THE  
APPALACHIANS FRI-SAT, REACHING THE NEW ENGLAND COAST BY SAT NIGHT,  
NOT STRAYING TOO FAR FROM CONTINUITY.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0633Z)...  
   
..OVERVIEW
 
 
BUILDING RIDGING IN THE NW ATLANTIC AND DEEPENING TROUGHING SOUTH  
OF ALASKA WILL HELP RE-ESTABLISH THE RIDGE/TROUGH PATTERN ACROSS  
THE LOWER 48 LATER THIS WEEK INTO THE WEEKEND. A LEAD PACIFIC  
SYSTEM WILL BE CLOSELY FOLLOWED BY AN ARCTIC BOUNDARY THAT WILL  
BRING IN ANOTHER SURGE OF COLD AIR TO THE NORTHERN PLAINS/UPPER  
MIDWEST WED-THU. BY FRI-SAT, THAT SYSTEM WILL BECOME A BIT  
EXTENDED AS A NORTHERN SFC LOW MOVES INTO SOUTHERN CANADA BUT THE  
SOUTHERN PART OF THE FRONT SPINS UP ANOTHER SFC WAVE AND MOVES  
THAT NORTHEASTWARD. WHERE THAT TRACKS IS UNCERTAIN BUT SEEMS TO BE  
FAVORING A TRACK NEAR/JUST EAST OF THE SOUTHERN/CENTRAL  
APPALACHIANS TOWARD SOUTHERN NEW ENGLAND.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
DETERMINISTIC BLEND FOR WED-FRI OFFERED A GOOD STARTING POINT  
GIVEN THE BETTER CLUSTERING OF SOLUTIONS OVERALL. DIFFERENCES  
REMAINED IN HOW TO HANDLE THE WEAKENING CUTOFF LOW IN THE GULF  
(WITH MUCH BIGGER QPF REPERCUSSIONS) AND HOW THE TROUGH EVOLVES  
THROUGH THE PLAINS AFTER THE LEAD UPPER LOW WEAKENS AND LIFTS INTO  
THE WESTERN GREAT LAKES. MIDDLE GROUND SOLUTION WAS FAVORED GIVEN  
THE RELATIVE STABILITY IN RECENT RUNS OF THE ECMWF/GFS  
(SYNOPTICALLY) AS WELL AS THE ENSEMBLE MEANS (WITH DECREASED  
SPREAD). INCOMING SYSTEM INTO SW BRITISH COLUMBIA REMAINS A POINT  
OF CONTENTION AMONG THE GUIDANCE, BUT TENDED TO FAVOR THE  
GFS/ECMWF THERE.  
 
FOR THE WEEKEND, AS THE TROUGH AXIS CONTINUES EASTWARD, AT LEAST A  
COUPLE SFC WAVES WILL RIDE UP ALONG THE FRONT IN THE EAST AS THE  
COLD FRONT MARCHES THROUGH THE GULF. AS THE QUICKER NORTHERN  
PORTION OF THE FRONT MOVED THROUGH QUEBEC, COOLER AIR WILL SINK  
INTO THE NORTHEAST WHICH MAY SET THE STAGE FOR AN OVERRUNNING  
PRECIP EVENT (I.E. SNOW/SLEET/FREEZING RAIN) NORTH OF THE  
BOUNDARY. TRACK OF THE SFC LOW(S) FROM THE SOUTHEAST TOWARD NEW  
ENGLAND WILL DETERMINE WHO SEES RAIN VS A WINTRY MIX. CONSENSUS  
SEEMS TO FAVOR A TRACK JUST EAST OF THE APPALACHIANS RIGHT NOW BUT  
THE SFC HIGH TO THE NORTH COULD EASILY SQUASH THAT TRACK FARTHER  
TO THE SOUTHEAST. TRANSITIONED TO A 50/50 DETERMINISTIC/ENSEMBLE  
BLEND TO ROUND OUT THE FORECAST THAT INCLUDED THE 18Z GFS AND 12Z  
ECMWF/CANADIAN WITH THE GEFS/ECENS MEANS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
LOW PRESSURE IN THE PLAINS ON WEDNESDAY WILL MOVE NORTHEASTWARD  
INTO THE WESTERN GREAT LAKES ON THURSDAY AND SPREAD MODEST TO  
PERHAPS LOCALLY HEAVY SNOW TO THE NORTH OF ITS TRACK FROM NEBRASKA  
TO MINNESOTA/WISCONSIN. RAIN WILL INCREASE IN COVERAGE IN THE  
LOWER MISSISSIPPI VALLEY INTO THE SOUTHEAST AS THE SOUTHERN SFC  
LOW LIFTS NORTHEASTWARD LATE FRIDAY INTO SATURDAY. POTENTIAL  
EXISTS FOR ICING NORTH OF THIS LOW AND COLD FRONT TO ITS  
NORTHEAST. ANOTHER ROUND OF RAIN AND MOUNTAIN SNOWS WILL RETURN TO  
THE PACIFIC NORTHWEST AT THE END OF THE WEEK.  
 
TEMPERATURES WILL BE A BIT VARIABLE OVER THE CONUS BUT GENERALLY  
10-20 DEGREES BELOW AVERAGE BEHIND THE ARCTIC FRONT (STARTING IN  
MONTANA/NORTH DAKOTA ON WEDNESDAY) BUT 10-20 DEGREES ABOVE AVERAGE  
AHEAD OF THE FRONT. SOME AREAS IN THE NORTHEAST/MID-ATLANTIC WILL  
SEE HIGH TEMPERATURES 40-50 DEGREES WARMER THAN WHAT WAS OBSERVED  
THIS WEEKEND.  
 
FRACASSO  
 

 
 
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