422  
FXUS02 KWBC 100506  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1206 AM EST WED JAN 10 2018  
 
VALID 12Z SAT JAN 13 2018 - 12Z WED JAN 17 2018  
   
..ICE STORM TO EXIT THE NORTHEAST SATURDAY
 
 
   
..OVERVIEW
 
 
A DYNAMIC SYSTEM IN THE EAST ON SATURDAY WILL EXIT INTO THE  
CANADIAN MARITIMES BY SUNDAY AS THE WEATHER TURNS QUIET FOR A  
COUPLE DAYS OVER THE LOWER 48. A CLIPPER SYSTEM WILL WHISK DOWN  
THROUGH THE GREAT LAKES NEXT WEEK AND A PACIFIC SYSTEM WILL PUSH  
INTO WA/OR/NORCAL TUESDAY AS THE UPPER RIDGE YIELDS TO STRONG  
ONSHORE FLOW.  
 
   
..GUIDANCE ASSESSMENT/PREFERENCES
 
 
FOR THE SYSTEM IN THE EAST SAT/SUN, THE MESSY CONSENSUS HAS (FOR  
SEVERAL DAYS) POINTED TO A LOW NEAR THE WV/MD PANHANDLES WHICH  
LIES NEAR THE 12Z ECMWF/CANADIAN (AND 18Z GEFS MEAN) AND REMAINS  
THE PREFERENCE. THE 18Z/12Z GFS RUNS WERE STILL A BIT QUICKER  
WHILE THE UKMET WAS SLOWER. THOUGH THE 12Z ECMWF ENSEMBLE MEAN  
JUMPED A BIT QUICKER FROM AN OTHERWISE STABLE POSITION, IT MAY BE  
A RESULT OF NORTHERN LOWS (PERHAPS TRIPLE POINT LOWS) TUGGING THE  
MEAN ISOBAR TO THE NORTHEAST. PARENT LOW CLUSTERING STILL RESIDED  
NEAR THE ECMWF HRES. SYSTEM WILL PULL THROUGH MAINE EARLY SUNDAY  
AND THEN AWAY FROM NEW ENGLAND. IN ITS WAKE WILL BE HIGH PRESSURE  
AND THEN A CLIPPER SYSTEM. ENSEMBLES HAVE TRENDED MUCH STRONGER  
(BUT STILL ONLY ABOUT 1024MB) WITH THIS SYSTEM BUT GIVEN THE  
HIGHER PRESSURE ON EITHER SIDE THIS ONLY MEANS THAT THE SPREAD HAS  
DECREASED ENOUGH SO THAT THEY CAN RESOLVE THE FEATURE AMONG THE  
20/50 MEMBERS. USED A CONSENSUS APPROACH FOR THE FORECAST NEXT  
WEEK WHICH INCLUDED A MINORITY GFS WEIGHTING, BUT OTHERWISE LEANED  
ON THE ECMWF AND ECMWF ENSEMBLE MEAN (AND GEFS MEAN).  
 
IN THE WEST, QUESTION HAS BEEN HOW LONG THE RIDGE MAY HOLD BEFORE  
THE PACIFIC FLOW PUSHES IT INLAND. THE ECMWF ENSEMBLE MEAN WAS  
QUICKEST TO INDICATE SUCH AN EVOLUTION BUT HAS CONTINUED TO TREND  
A BIT SLOWER. THE GEFS MEAN HAS BEEN MORE CONSISTENT WITH A SLOWER  
EROSION OF THE RIDGE BUT THE 18Z/12Z GFS WERE MUCH MORE AGGRESSIVE  
(AND FLATTER) TO BREAK DOWN THE RIDGE. THOUGH THIS REMAINS A  
POSSIBILITY, PUT MORE WEIGHT IN THE ECMWF WHICH WAS A BIT SLOWER  
THAN ITS ENSEMBLE MEAN AND WAS ALSO CLOSE TO THE CANADIAN. BY NEXT  
WEDNESDAY, ALL ENSEMBLES SHOW THE BREAKDOWN OF THE RIDGE WITH THE  
LEAD SYSTEM LIKELY PUSHING THROUGH THE INTERIOR WEST AS ANOTHER  
LOOMS OFFSHORE. ECMWF-LED CONSENSUS SERVED WELL AS A STARTING  
POINT.  
 
   
..WEATHER HIGHLIGHTS
 
 
WELL ABOVE AVERAGE TEMPERATURES WILL BE SHUNTED OUT TO SEA AS THE  
SYSTEM MOVES THROUGH NEW ENGLAND ON SATURDAY, ENDING THE BRIEF  
WARM SPELL. HIGH PRESSURE WILL RETURN TEMPERATURES TO BELOW  
AVERAGE BY ABOUT 10-20 DEGREES. CLIPPER SYSTEM WILL BRING LIGHT  
SNOW TO THE GREAT LAKES (WITH LOCAL ENHANCEMENT) AND ANOTHER PUSH  
OF COLDER AIR (SUB-ZERO HIGHS FOR MUCH OF THE DAKOTAS AND  
MINNESOTA ON MONDAY). TO THE WEST, ABOVE AVERAGE TEMPERATURES WILL  
RULE INTO NEXT WEEK THANKS TO THE UPPER RIDGE. PRECIPITATION  
CHANCES WILL INCREASE ALONG THE COAST STARTING MONDAY AND THEN  
WORK THEIR WAY INLAND NEXT TUE-WED THROUGH THE CENTRAL/NORTHERN  
ROCKIES. SNOW LEVELS WILL BE ON THE HIGHER SIDE TO START BUT THEN  
DROP BEHIND THE FRONT.  
 
FRACASSO  
 

 
 
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