836  
FXUS02 KWBC 150900  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
400 AM EST MON JAN 15 2018  
 
VALID 12Z THU JAN 18 2018 - 12Z MON JAN 22 2018  
 
***PATTERN OVERVIEW***  
 
THE LARGE UPPER LEVEL TROUGH THAT WILL BE OVER THE EASTERN U.S.  
THROUGH THE MIDDLE OF THIS WEEK WILL BEGIN LIFTING OUT AT THE  
BEGINNING OF THE MEDIUM RANGE PERIOD ON THURSDAY. THE ARCTIC  
SURFACE HIGH WILL MOVE OFFSHORE BY THE END OF THE WEEK AND ALLOW  
WARMER RETURN FLOW FROM THE SOUTHWEST BY THE WEEKEND. THIS  
PATTERN CHANGE WILL BE INITIATED BY LOWERING HEIGHTS OVER THE  
WESTERN U.S. AS A STRONG PACIFIC STORM SYSTEM MOVES INLAND,  
BRINGING WIDESPREAD SHOWERS AND MOUNTAIN SNOW FOR MUCH OF THE WEST  
COAST AND INTERMOUNTAIN WEST REGION. THIS STORM REACHES THE  
PLAINS BY SATURDAY WITH INCREASING PROSPECTS FOR SHOWERS AND  
THUNDERSTORMS TO DEVELOP AS A STRONG SURFACE LOW TRACKS  
NORTHEASTWARD FROM THE CENTRAL PLAINS TO THE GREAT LAKES REGION.  
A SECOND PACIFIC SYSTEM WILL LIKELY AFFECT THE WEST COAST BY  
SUNDAY AND INTO MONDAY WITH A BRIEF PERIOD OF UPPER LEVEL RIDGING  
BETWEEN SYSTEMS FOR THE WESTERN U.S.  
 
***GUIDANCE ASSESSMENT/PREFERENCES***  
 
THERE ARE SOME MODEST MODEL DIFFERENCES AT THE BEGINNING OF THE  
PERIOD REGARDING THE SHORTWAVE IMPULSES CROSSING THE SOUTHERN  
PLAINS AND ALSO THE UPPER MIDWEST, WITH THE UKMET BEING THE MOST  
PROGRESSIVE. THE GFS AND ITS ENSEMBLE MEAN ARE FARTHER OFFSHORE  
WITH THE COASTAL LOW OFF THE EAST COAST, AND THE ECMWF HAS TRENDED  
SOME IN THAT DIRECTION.  
 
THE LARGER TROUGH REACHING THE WEST COAST BY FRIDAY, ALONG WITH  
THE SURFACE LOW OVER THE PLAINS, IS HANDLED WELL AMONG THE  
GUIDANCE GOING INTO THE WEEKEND, AND CONFIDENCE HAS IMPROVED TO  
ABOVE AVERAGE FOR A FORECAST AT THIS TIME RANGE. THE ENSEMBLE  
MEANS PORTRAY A GOOD IDEA OF WHERE THE SURFACE FRONT WILL BE, WITH  
SLIGHTLY MORE WEIGHTING TOWARDS THE EC MEAN AND OPERATIONAL ECMWF  
GIVEN A MORE CONSISTENT TREND WITH THE ECMWF. AFTER A BRIEF RIDGE  
SETS IN OVER THE WEST BEHIND THE DEPARTING STORM SYSTEM, BOTH THE  
GFS AND ECMWF ARE INDICATING HEIGHT FALLS OVER THE EASTERN PACIFIC  
BY SUNDAY, WITH THE ECMWF AMONG THE SOUTHERN SOLUTIONS AND THE GFS  
WELL TO THE NORTH. THE EC MEAN INDICATED A REASONABLE POSITION  
FOR THIS SURFACE LOW.  
 
***SENSIBLE WEATHER***  
 
THE EASTERN HALF OF THE NATION WILL BEGIN TO HAVE A STEADY  
MODERATION TREND IN TEMPERATURES TOWARDS THE END OF THIS WEEK AND  
ESPECIALLY INTO NEXT WEEKEND ACROSS THE CENTRAL AND EASTERN U.S.  
AHEAD OF THE NEXT STORM SYSTEM, WITH READINGS EXPECTED TO BE ABOVE  
AVERAGE BY POTENTIALLY 10 TO 20 DEGREES. IN THE PRECIPITATION  
DEPARTMENT, THE WEST COAST WILL LIKELY GET REPEATED ROUNDS OF  
MODERATE TO LOCALLY HEAVY RAINFALL, WITH SEVERAL INCHES OF RAIN  
POSSIBLE WEST OF THE CASCADES. THE STORM SYSTEM REACHING THE  
PACIFIC NORTHWEST AND NORTHERN CALIFORNIA LATE SUNDAY AND INTO  
MONDAY WILL LIKELY QUALIFY AS AN ATMOSPHERIC RIVER EVENT WITH A  
PLUME OF SUBTROPICAL MOISTURE ORIENTED TOWARDS THE COAST. HEAVY  
SNOW IS LIKELY FOR THE HIGHEST ELEVATIONS OF THE CASCADES, SIERRA  
NEVADA, AND THE CENTRAL AND NORTHERN ROCKIES FOR THE END OF THE  
WEEK, AND AGAIN BY LATE SUNDAY.  
 
AN ACTIVE WEATHER PATTERN IS ALSO EXPECTED ACROSS THE CENTRAL U.S.  
BY SUNDAY AND INTO EARLY MONDAY OF NEXT WEEK. THE INTENSIFYING  
SURFACE LOW OVER THE CENTRAL AND NORTHERN PLAINS IS EXPECTED TO  
RESULT IN A SURGE OF GULF MOISTURE EXTENDING NORTHWARD TO THE  
GREAT LAKES REGION, WITH SCATTERED TO NUMEROUS SHOWERS AND SOME  
THUNDERSTORMS EXPECTED. RAINFALL AMOUNTS COULD EXCEED AN INCH  
BEFORE THE COLD FRONT PASSES THROUGH. IN ADDITION, THE POTENTIAL  
FOR A MAJOR WINTER STORM IS INCREASING ACROSS THE DAKOTAS AND  
ESPECIALLY OVER MINNESOTA AND WISCONSIN AS A DEFORMATION ZONE WILL  
LIKELY DEVELOP ON THE NORTHWEST SIDE OF THE SURFACE LOW.  
 
D. HAMRICK  
 

 
 
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