492  
FXUS02 KWBC 150701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST FRI DEC 15 2017  
 
VALID 12Z MON DEC 18 2017 - 12Z FRI DEC 22 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
SHORTWAVE ENERGY SPLITS OVER THE WRN US TO CARVE OUT A WET SWRN US  
MID-UPPER LEVEL TROUGH INTO MON THAT SHEARS PROGRRESSIVELY OUT  
ACROSS THE US SRN TIER STATES EARLY-MID NEXT WEEK. MEANWHILE...A  
SERIES OF NRN STREAM SYSTEMS PROGRESS OVERTOP THROUGH THE CENTRAL  
AND ERN US NRN TIER STATES. THE 00 UTC GFS AND 12/00 UTC ECMWF  
HAVE CONVERGED ON THEIR SOLUTIONS FOR THIS TIME  
PERIOD...BOLSTERING FORECAST CONFIDENCE BACK TO AVERAGE  
LEVELS...ALBEIT WITH SOME PESKY CONTINUED EMBEDDED SYSTEM TIMING  
VARIANCES. A BLEND OF THE 00 UTC GFS/ECMWF AND PERHAPS A BIT OF  
SMOOTHING ENSEMBLES SEEMS REASONABLE.  
 
FORECAST SPREAD AND UNCERTAINTY INCREASE MID-LATER NEXT WEEK AS  
GUIDANCE STRUGGLES WITH THE EXTENT/DETAILS OF ADDITIONAL POTENT  
SHORTWAVE TROUGH ENERGY DIGGING FROM THE ERN PACIFIC DOWN THROUGH  
THE WRN STATES IN RESPONSE TO SHARP UPSTREAM MID-UPPER RIDGE  
AMPLIFICATION. THE 00 UTC GFS HAS ABANDONED THE DEVELOPMENT OF A  
SWRN US CLOSED LOW LATE NEXT WEEK CHAMPIONED BY EARLY GFS RUNS.  
HOWEVER...SUPPORTING TROUGH DIGGING THROUGH THE WEST COAST INTO  
THE SWRN US MAY STILL BE AMPLE SO USED THE MOST AMPLIFIED OF THE  
ENSEMBLE MEANS...THE ECMWF ENSEMBLES...INTO DAY 6/7 GIVEN UPSTREAM  
SUPPORT FROM SUCH A AMPLIFIED UPSTREAM RIDGE ALOFT OVER THE  
PACIFIC. THIS ENERGY IS THEN INJECTED INTO/UNDERNEATH A BROAD  
LONGWAVE THROUGH OVER THE CENTRAL TO ERN US.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
IT REMAINS THE CASE THAT THE GULF COAST/SRN TIER STATES WILL  
REMAIN AN AREA OF INTEREST FOR HEAVY RAINFALL POTENTIAL WITH  
EJECTING W-CENTRAL U.S. SHORTWAVE ENERGY/ASSOCIATED COLD FRONT AS  
WELL AS A POSSIBLE LEADING STATIONARY BOUNDARY PROVIDING FOCUS FOR  
THIS ACTIVITY. BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS APPEARS  
TO BE SUN-TUE...THEN OUT ACROSS THE ERN STATES. MEANWHILE...THE  
APPROACH/ARRIVAL OF THE NORTHEAST PACIFIC UPPER TROUGH WILL LIKELY  
BRING A PERIOD OF MEANINGFUL RAIN/HIGHER ELEVATION SHOW TO FAVORED  
TERRAIN OF THE PACIFIC NORTHWEST AND NRN ROCKIES. PRECIP SHOULD BE  
CONFINED TO FAR NRN AREAS EARLY NEXT WEEK AND THEN PUSH SOUTHWARD  
MID-LATER WEEK WITH SOME MOISTURE WORKING MORE INTO CA. THE  
POTENTIAL FOR THE COMBINATION OF SNOW AND WIND WILL HAVE TO BE  
MONITORED OVER PARTS OF THE ROCKIES/PLAINS AS LOW PRESSURE EMERGES  
INTO THE CENTRAL U.S. LATER WEEK. OVER THE EAST...MOST SNOW SHOULD  
BE CONFINED TO NRN TIER LOCATIONS WITH A COMBINATION OF  
SYNOPTIC/LOW PASSAGE AND LAKE EFFECT ACTIVITY.  
 
SCHICHTEL/RAUSCH  
 

 
 
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