584  
FXUS02 KWBC 070634  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
133 AM EST SUN JAN 07 2018  
 
VALID 12Z WED JAN 10 2018 - 12Z SUN JAN 14 2018  
   
..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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