543  
FXUS02 KWBC 140653  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
153 AM EST SUN JAN 14 2018  
 
VALID 12Z WED JAN 17 2018 - 12Z SUN JAN 21 2018  
   
..PATTERN OVERVIEW
 
 
A LARGE EASTERN U.S. TROUGH IN PLACE AT THE BEGINNING OF THE  
FORECAST PERIOD ON WEDNESDAY IS FORECAST TO LIFT OUT BY THE END OF  
THE WEEK, FOLLOWED BY A BUILDING LOW AMPLITUDE RIDGE EAST OF THE  
ROCKIES. THE LARGE ARCTIC 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 AIR MASSES CLASH. A  
SECOND PACIFIC SYSTEM MAY BEGIN AFFECTING THE WEST COAST BY SUNDAY.  
 
   
..GUIDANCE ASSESSMENT/PREFERENCES
 
 
THE GUIDANCE IS IN RELATIVELY GOOD AGREEMENT ON THE SYNOPTIC SCALE  
TROUGHS NEAR BOTH COASTS FOR WEDNESDAY AND THURSDAY, BUT THE  
MODELS ARE STILL STRUGGLING ON A COUPLE OF LEAD SHORTWAVE IMPULSES  
AHEAD OF THE MAIN WESTERN U.S. TROUGH AS THEY TRAVERSE A LOW  
AMPLITUDE RIDGE OVER THE PLAINS, WITH THE SOUTHERN STREAM IMPULSE  
LIKELY BEING THE STRONGER OF THESE. THE UKMET APPEARED TOO STRONG  
WITH THE NORTHERN STREAM SHORTWAVE AND DID NOT HAVE MUCH ENSEMBLE  
SUPPORT. THE LARGER TROUGH REACHING THE WEST COAST BY FRIDAY,  
ALONG WITH THE SURFACE LOW, IS HANDLED WELL AMONG THE GUIDANCE  
THROUGH EARLY SATURDAY, AFTER WHICH THE GFS IS FASTER AND MUCH  
STRONGER WITH THE 500MB LOW OVER THE PLAINS. 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.  
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 CMC INDICATING A  
STRONGER STORM SYSTEM.  
 
   
..SENSIBLE WEATHER
 
 
THE EASTERN HALF OF THE NATION WILL START OFF QUITE COLD ON  
WEDNESDAY COURTESY OF THE ARCTIC HIGH SETTLING IN OVER THE REGION  
AND THE PRONOUNCED UPPER LEVEL TROUGH NEAR THE EAST COAST. HIGH  
TEMPERATURE ANOMALIES ON THE ORDER OF 15 TO 25 DEGREES BELOW  
AVERAGE ARE EXPECTED FROM TEXAS TO THE OHIO VALLEY, WITH READINGS  
STRUGGLING TO GET OUT OF THE TEENS FROM IOWA TO THE CENTRAL  
APPALACHIANS. THE MAJOR CITIES OF THE NORTHEAST U.S. SHOULD BE  
SPARED THE BRUNT OF THIS ARCTIC SURGE IN CONTRAST TO THE EXTREMELY  
COLD WEATHER ABOUT A WEEK AGO. A STEADY MODERATION TREND IN  
TEMPERATURES COMMENCES BY THE END OF THE 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 ON WEDNESDAY 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. PARTS OF NEW  
ENGLAND CAN ALSO EXPECT SIGNIFICANT SNOW FROM A COASTAL LOW THAT  
DEVELOPS OFFSHORE, WITH SOME SNOW LIKELY EXTENDING SOUTHWARD INTO  
PARTS OF THE MID-ATLANTIC REGION.  
 
D. HAMRICK  
 

 
 
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