745  
FXUS02 KWBC 171555  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1054 AM EST THU JAN 17 2019  
 
VALID 12Z SUN JAN 20 2019 - 12Z THU JAN 24 2019  
 
...MAJOR WINTER STORM SHIFTS FROM GREAT LAKES/OHIO  
VALLEY/APPALACHIANS TO MID-ATLANTIC/NORTHEAST SUNDAY INTO MONDAY...  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE WPC FORECAST WAS INITIALLY BASED ON A MULTI-MODEL  
DETERMINISTIC BLEND (INCLUDING THE ECMWF/GFS/CMC) DURING DAYS 3-5  
(SUN-TUE). THESE SOLUTIONS WERE RELATIVELY WELL CLUSTERED  
THROUGHOUT AND THE BLEND MAINTAINS REASONABLY GOOD FORECAST  
CONTINUITY. UNCERTAINTY INCREASES SIGNIFICANTLY BY DAYS 6-7  
(WED-THU) AS PACIFIC ENERGY UNDERGOES SIGNIFICANT AMPLIFICATION  
ACROSS THE CENTRAL U.S., WITH MODEL SOLUTIONS WAFFLING AS TO THE  
DEGREE OF AMPLITUDE OF THE TROUGH AS WELL AS POTENTIAL SEPARATION  
OF NORTHERN/SOUTHERN STREAM FLOW, WITH MAJOR IMPLICATIONS ON THE  
DEVELOPMENT/TRACK/INTENSITY OF ANY POTENTIAL SURFACE LOW PRESSURE  
SYSTEM ACROSS THE CENTRAL/EASTERN U.S. GIVEN THE DEGREE OF MODEL  
VARIABILITY, THE FORECAST WAS TRENDED QUICKLY TO MAJORITY ENSEMBLE  
MEAN (ECENS/NAEFS) WEIGHTING DURING DAYS 6-7, WITH EXCLUSIVE USE  
OF ENSEMBLE MEANS BY DAY 7.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
DEEP GULF OF MEXICO AND ATLANTIC MOISTURE WILL LIFT OVER THE  
EASTERN U.S. SUNDAY. HEAVY PRECIPITATION FOCUSES AROUND A MAIN  
LOW/FRONTAL SYSTEM, OVERRUNNING ARCTIC AIR TO THE NORTH. HEAVY  
SNOWS SHIFT FROM THE GREAT LAKES/OHIO VALLEY AND APPALACHIANS  
THROUGH THE NORTHERN MID-ATLANTIC/NORTHEAST, EXITING MONDAY. SOME  
LOCATIONS COULD SEE BLIZZARD CONDITIONS IN THE TIGHT PRESSURE  
GRADIENT BETWEEN THE SURFACE LOW AND THE STRONG ARCTIC HIGH.  
MEANWHILE, LINGERING HEAVY WARM-SECTOR RAINS/STRONG CONVECTION  
MOVES THROUGH THE SOUTHEAST/MID-ATLANTIC SUNDAY, WITH A TRANSITION  
ZONE BETWEEN SNOW AND RAIN TO INCLUDE SIGNIFICANT SLEET AND  
FREEZING RAIN.  
 
MEANWHILE, PACIFIC STORM SYSTEMS WILL RELOAD WITH NEW FOCUS AS A  
HEAVY PRECIPITATION THREAT INTO THE PACIFIC NORTHWEST THIS  
WEEKEND. WINTRY WEATHER SPREADS/MODERATES INLAND ACROSS THE  
NORTH-CENTRAL GREAT BASIN/ROCKIES LATER IN THE HOLIDAY WEEKEND AND  
ACROSS THE FULL U.S. NORTHERN TIER NEXT WEEK. AS MENTIONED ABOVE,  
MODELS HAVE WAFFLED ON THE DEGREE OF MID-UPPER LEVEL TROUGH  
AMPLITUDE/SEPARATION OVER THE SOUTH-CENTRAL U.S., AND THERE IS A  
LARGE DEGREE OF UNCERTAINTY WITH THE SPECIFICS OF THIS FLOW AND  
ASSOCIATED SURFACE SYSTEM RESPONSE. HOWEVER, MOST GUIDANCE  
SUPPORTS INCREASING GULF THEN ATLANTIC MOISTURE INFLOW AND  
PRECIPITATION PATTERN NEXT TUE-THU THAT WILL INCLUDE THE THREAT  
FOR A WIDESPREAD SWATH OF SNOW/ICE ON THE NORTHERN PERIPHERY OF  
THE LARGE PRECIPITATION SHIELD.  
 
UPSTREAM, SHARP SHORTWAVE DIGGING NEXT WED/THU OVER THE WEST TO  
THE LEE OF A BUILDING/AMPLIFIED EASTERN PACIFIC UPPER RIDGE MAY  
PROVIDE COOLING AND RISK OF A WINTRY WEATHER SWATH FROM THE  
NORTHWEST TO THE NORTH-CENTRAL INTERMOUNTAIN WEST/ROCKIES/PLAINS.  
 
RYAN/SCHICHTEL  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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