627  
FXUS02 KWBC 120703  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
202 AM EST FRI JAN 12 2018  
 
VALID 12Z MON JAN 15 2018 - 12Z FRI JAN 19 2018  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD BEGINS ON MONDAY WITH A WESTERN RIDGE AND  
A LARGE CENTRAL AND EASTERN U.S. TROUGH THAT WILL GENERALLY  
TRANSLATE EASTWARD OVER THE COURSE OF NEXT WEEK AS A LARGE SCALE  
TROUGH BECOMES ESTABLISHED OVER THE WEST COAST. THE RIDGE SHOULD  
GENERALLY BECOME LESS AMPLIFIED ONCE IT REACHES THE CENTRAL U.S.  
WITH A COUPLE OF SHORT WAVE IMPULSES EMBEDDED WITHIN IT. THE  
PREDOMINANT SYSTEMS THAT MERIT ATTENTION WILL BE A STRONG CLIPPER  
SYSTEM THAT WILL CROSS THE NORTHEAST U.S., AN ARCTIC INTRUSION  
ACROSS THE NORTH-CENTRAL U.S., AND MORE HEAVY PRECIPITATION FOR  
THE WEST COAST AS A COUPLE OF STORM SYSTEMS AFFECT THAT REGION.  
 
   
..GUIDANCE ASSESSMENT/PREFERENCES
 
 
THE LATEST MODEL GUIDANCE INDICATES THE STRONGEST AGREEMENT WITH  
RESPECT TO THE EASTERN U.S. TROUGH AND NORTHEAST U.S. SURFACE LOW  
FOR THE BEGINNING OF NEXT WEEK. A BROAD UPPER LOW WITH SUB-530DM  
HEIGHTS TRACKING FROM THE UPPER GREAT LAKES TO NEW ENGLAND IS  
HANDLED WELL BY BOTH THE DETERMINISTIC GFS AND ECMWF, AND THEIR  
RESPECTIVE MEANS AGREE WELL. THE CMC DIFFERED MORE BEYOND DAY 4  
WITH A STRONGER SYSTEM THAT IS DISPLACED TO THE NORTH OF THE MODEL  
CONSENSUS. THIS PATTERN FAVORS AN OCCLUDED SURFACE LOW CROSSING  
THE GREAT LAKES WITH A TRIPLE POINT LOW LIKELY BECOMING THE  
DOMINANT LOW UPON REACHING THE NEW ENGLAND COASTAL WATERS. THERE  
ARE STILL SOME DIFFERENCES REGARDING THE DETAILS OF THE TRIPLE  
POINT LOW EVOLUTION, BUT THE AMOUNT OF MODEL SPREAD HAS DECREASED  
SOME. THERE IS ALSO DECENT MODEL AGREEMENT CONCERNING THE ARCTIC  
SURFACE HIGH THAT FOLLOWS BEHIND THIS COLD FRONT THROUGH THE  
MIDDLE OF THE WEEK.  
 
THE MOST UNCERTAIN PORTION OF THE EXTENDED FORECAST INVOLVES  
SHORTWAVE ENERGY OVER THE NORTH PACIFIC THAT LEAD TO NOTEWORTHY  
DIFFERENCES BY DAY 4 WEST OF THE ROCKIES, WITH THE OPERATIONAL  
ECMWF MOST PRONOUNCED WITH A LEAD SHORTWAVE TROUGH CROSSING THE  
PACIFIC NORTHWEST ON TUESDAY. THE MEANS INDICATE A LESS AMPLIFIED  
SOLUTION THAT IS CLOSER TO THE GFS, SO IT WAS GIVEN SLIGHTLY MORE  
CONSIDERATION IN THE FORECAST PROCESS. DIFFERENCES BECOME MORE  
NOTICEABLE OVER THE PLAINS BY THURSDAY WITH MULTIPLE SHORTWAVES  
TRYING TO BREAK DOWN THE RIDGE AND REDUCING PREDICTABILITY BY THE  
END OF THE WEEK. HOWEVER, THERE IS DECENT AGREEMENT IN THE LARGE  
SCALE TROUGH THAT APPROACHES THE WEST COAST FOR THE END OF THE  
WEEK.  
 
   
..WEATHER HIGHLIGHTS
 
 
A WELL DEVELOPED CLIPPER SYSTEM IS FORECAST TO DELIVER ANOTHER  
ROUND OF SNOWFALL TO THE GREAT LAKES, FOLLOWED BY A FRESH SURGE OF  
ARCTIC AIR BEHIND THE TRAILING COLD FRONT. FRIGID CONDITIONS ARE  
EXPECTED FROM EASTERN MONTANA TO THE UPPER MIDWEST ON MONDAY AND  
TUESDAY WITH HIGHS FAILING TO REACH ZERO DEGREES IN MANY CASES.  
OVERNIGHT LOWS COULD EASILY REACH THE MINUS 20S ACROSS PARTS OF  
NORTH DAKOTA AND MINNESOTA, WITH SOME MINUS 30S POSSIBLE. IT WILL  
ALSO BE QUITE COLD ACROSS THE OHIO VALLEY AND NORTHEAST U.S. GIVEN  
THE MAGNITUDE OF THE SURFACE HIGH THAT FOLLOWS BEHIND THE CLIPPER  
SYSTEM. SOME RECORD LOW TEMPERATURES WILL BE POSSIBLE WHERE  
RADIATIONAL COOLING IS MAXIMIZED.  
 
ABOVE AVERAGE TEMPERATURES ARE EXPECTED NEXT WEEK FOR THE  
SOUTHWESTERN U.S. AND EXTENDING INTO THE GREAT BASIN.  
PRECIPITATION INCREASES AGAIN OVER THE PACIFIC NORTHWEST AND  
NORTHERN CALIFORNIA MONDAY WITH MULTIPLE ROUNDS OF UNSETTLED  
CONDITIONS FOR THE COAST THROUGH THURSDAY. EAST OF THE CASCADES,  
THERE SHOULD BE AT LEAST SOME RESPITE BETWEEN THE FIRST AND SECOND  
SYSTEMS. THE SYSTEM TOWARDS THE END OF THE WEEK ADVECTS MORE  
MOISTURE INTO THE REGION WITH SOME LOCATIONS POSSIBLY RECEIVING A  
FEW INCHES OF RAIN AND OVER A FOOT OF MOUNTAIN SNOW.  
 
D. HAMRICK  
 

 
 
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