565  
FXUS02 KWBC 130700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST SAT JAN 13 2018  
 
VALID 12Z TUE JAN 16 2018 - 12Z SAT JAN 20 2018  
   
..OVERVIEW
 
 
THE BEGINNING OF THE MEDIUM RANGE PERIOD ON TUESDAY WILL FEATURE A  
PRONOUNCED EASTERN U.S. TROUGH ALOFT AND AN EXPANSIVE SURFACE HIGH  
SETTLING SOUTH FROM CANADA. A NARROW RIDGE AXIS OVER THE  
INTERMOUNTAIN REGION IS FORECAST TO BREAK DOWN GOING THROUGH THE  
MIDDLE OF THE WEEK AS A FEW WEAK SHORTWAVE DISTURBANCES TREK  
ACROSS THE CENTRAL U.S. THE PATTERN SHOULD BECOME SLIGHTLY LESS  
AMPLIFIED TO CLOSE OUT THE WEEK WITH THE EASTERN TROUGH LIFTING  
OUT ACROSS THE ATLANTIC AND WEAK RIDGING OVER THE PLAINS. A  
STRONGER TROUGH SETTLES IN OVER THE WEST COAST BY THURSDAY AND  
RESULTS IN HEIGHT FALLS ACROSS MUCH OF THE WESTERN U.S. BY THE END  
OF THE FORECAST PERIOD.  
 
   
..GUIDANCE ASSESSMENT/PREFERENCES
 
 
MODELS AND ENSEMBLES ARE IN REASONABLE AGREEMENT THROUGH EARLY  
WEDNESDAY ON THE SYNOPTIC SCALE ACROSS THE CONTINENTAL U.S. BEFORE  
NOTEWORTHY DIFFERENCES EMERGE REGARDING THE BREAK DOWN OF THE  
RIDGE AND THE RATE OF DEPARTURE FOR THE UPPER TROUGH OVER THE EAST  
COAST. A GENERAL COMPROMISE OF THE GFS/ECMWF/CMC ALONG WITH SOME  
OF THE EC MEAN APPEARED TO WORK WELL THROUGH DAY 4. THE ECMWF HAS  
MORE PRECIPITATION OVER EAST COAST FOR THE MIDDLE OF THE WEEK WITH  
A SURFACE LOW FORMING CLOSER TO THE COAST THAN THE GFS, AND THIS  
TREND WITH THE ECMWF HAS BEEN CONSISTENT OVER THE PAST 24 HOURS.  
A BLEND OF THE GEFS/ECMWF ENSEMBLE MEANS SEEM TO BEST REPRESENT  
THE MORE PREDICTABLE WEATHER FEATURES FOR THE SECOND HALF OF THE  
FORECAST PERIOD, ALONG WITH LESS OF THE DETERMINISTIC GFS AND  
ECMWF.  
 
THE PROSPECTS FOR AN UNSETTLED WEATHER PATTERN ACROSS THE WESTERN  
U.S. FOR THE MIDDLE TO END OF THE WEEK REMAIN WELL NOTED IN THE  
GUIDANCE, WITH HEAVY RAIN, MOUNTAIN SNOW, AND WINDY CONDITIONS  
BECOMING INCREASINGLY LIKELY. THERE IS ALSO A GOOD SIGNAL FOR AT  
LEAST ONE COASTAL LOW TO PRIMARILY EFFECT THE MID ATLANTIC AND  
NORTHEAST U.S., WITH SOME UNCERTAINTY REMAINING WITH THE PROXIMITY  
OF THE COASTAL LOW TRACK AND INLAND EXTENT OF PRECIPITATION AS  
SHORTWAVE ENERGY ROTATES THROUGH THE AMPLIFIED MID-UPPER LEVEL  
TROUGH POSITION.  
 
   
..WEATHER/THREATS 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 A REINFORCING COLD FRONT. FRIGID CONDITIONS ARE  
EXPECTED ACROSS THE UPPER MIDWEST AND EXTENDING SOUTHEAST TO THE  
OHIO VALLEY ON TUESDAY, WITH HIGHS 20 TO 30 DEGREES BELOW NORMAL  
FOR MID JANUARY. OVERNIGHT LOWS COULD EASILY REACH THE MINUS 20S  
ACROSS PARTS OF NORTH DAKOTA AND MINNESOTA, AND SOME RECORD LOW  
TEMPERATURES WILL BE POSSIBLE WHERE RADIATIONAL COOLING IS  
MAXIMIZED UNDER THE SURFACE HIGH.  
 
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 TUESDAY WITH MULTIPLE ROUNDS OF UNSETTLED  
CONDITIONS FOR THE COAST THROUGH FRIDAY. 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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