276  
FXUS02 KWBC 151601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SAT DEC 15 2018  
 
VALID 12Z TUE DEC 18 2018 - 12Z SAT DEC 22 2018  
   
..SIGNIFICANT PRECIPITATION CONTINUES OVER THE NORTHWEST
 
 
   
..EAST COAST RAINSTORM BECOMING MORE LIKELY LATE NEXT WEEK
 
 
   
..OVERVIEW
 
 
GUIDANCE AGREES IN PRINCIPLE THAT FLOW ALOFT WILL RAPIDLY EVOLVE  
TOWARD A SHARP AND AMPLIFIED WESTERN RIDGE/EAST-CENTRAL TROUGH  
PATTERN BY THU, AS INCOMING PACIFIC SHORTWAVE ENERGY (CONTRIBUTING  
TO THE HEAVY NORTHWEST PRECIPITATION) DIGS SOUTHEASTWARD AFTER  
ROUNDING THE STRENGTHENING WESTERN RIDGE. BY THE LATE  
WEEK/WEEKEND TIME FRAME, GLOBAL MODELS ARE GETTING INTO BETTER  
AGREEMENT THAT A SIGNIFICANT CYCLONE WILL LIKELY DEVELOP ALONG THE  
EAST COAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
LATEST MODELS AND ENSEMBLES HAVE COME INTO MUCH BETTER AGREEMENT  
ON THE OVERALL EVOLUTION OF THE SYNOPTIC PATTERN ACROSS THE U.S.  
THROUGH THE END OF NEXT WEEK. THE MOST NOTABLE FEATURE ON THE MAP  
IS REGARDING THE AMPLITUDE OF AN UPPER-LEVEL TROUGH AS IT  
PROGRESSES EASTWARD ACROSS THE PLAINS THROUGH THE MIDDLE OF NEXT  
WEEK AND HEADS TOWARD THE EAST COAST LATE NEXT WEEK. THE ECMWF  
HAS LED THE TREND TOWARD A MORE AMPLIFIED PATTERN SINCE YESTERDAY.  
ON THE OTHER HAND, THE MORE PROGRESSIVE GFS SOLUTIONS YESTERDAY  
HAVE GRADUALLY TRENDED MUCH CLOSER TO THOSE OF THE ECMWF. THE 00Z  
CANADIAN MODEL HAS ALSO SHOWN A FASTER SOLUTION TOWARD THE ECMWF  
SOLUTION AS WELL. THE CONVERGENCE OF GLOBAL MODELS TOWARD A  
HIGHER AMPLITUDE UPPER-LEVEL FLOW GIVES US HIGHER CONFIDENCE THAT  
A SIGNIFICANT CYCLONE WILL IMPACT MANY AREAS ALONG THE EAST COAST  
LATE NEXT WEEK. IT APPEARS THAT RAIN WILL BE THE PRIMARY  
PRECIPITATION TYPE UNDER STRONG SOUTHERLY FLOW AHEAD OF THE  
SYSTEM, WITH ONLY SOME WINTER PRECIPITATION NEAR THE CANADIAN  
BORDER OF NEW ENGLAND.  
 
A CONSENSUS APPROACH WAS ADAPTED FOR DAYS 3 AND 4 PRESSURE FIELDS  
USING THE 00Z ECMWF/EC MEAN AND 06Z GFS/GEFS. FOR DAY 5 ONWARD, A  
COMBINATION OF 00Z ECMWF/EC MEAN AND 06Z GEFS WERE USED.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A FRONTAL SYSTEM APPROACHING/REACHING THE NORTHWEST ON TUE WILL  
PROVIDE ONE EPISODE OF ENHANCED RAINFALL AND HIGHER ELEVATION SNOW  
TO THE REGION WITH ACTIVITY LIKELY EXTENDING INTO EXTREME NORTHERN  
CALIFORNIA. ANOTHER FRONT, ANCHORED BY A WAVE WHOSE STRENGTH AND  
TRACK ARE NOT YET WELL-RESOLVED IN THE GUIDANCE, SHOULD BRING  
ANOTHER EPISODE OF MEANINGFUL PRECIPITATION TO THE NORTHWEST LATE  
IN THE WEEK. SOME OF THE MOISTURE FROM THESE EVENTS WILL EXTEND  
INTO THE NORTHERN ROCKIES. ANOTHER AREA OF MOISTURE COULD REACH  
THE NORTHWEST NEXT WEEKEND BUT WITH LOWER CONFIDENCE. PARTS OF  
THE NORTHWEST MAY SEE PERIODS OF STRONG WINDS.  
 
THE UPPER TROUGH INITIALLY FROM THE PLAINS INTO MEXICO ALONG WITH  
LEADING WEAK SURFACE WAVE MAY PRODUCE AREAS OF RAINFALL FROM THE  
SOUTHERN PLAINS EASTWARD MID-LATE WEEK, WHILE AN APPROACHING  
NORTHERN STREAM FRONTAL SYSTEM MAY ALSO PROVIDE A FOCUS FOR AT  
LEAST LIGHT-MODERATE PRECIPITATION. INTERACTION OF THESE FEATURES  
MAY LEAD TO STRENGTHENING LOW PRESSURE NEAR THE EAST COAST BY  
FRI-SAT, SPREADING A SIZABLE MOISTURE SHIELD ACROSS THE EASTERN  
STATES. CURRENTLY THE BEST SIGNAL FOR HIGHEST RAINFALL TOTALS  
EXIST OVER PARTS OF THE FLORIDA PENINSULA AND ALONG THE NORTH  
CAROLINA COAST. DEPENDING ON DETAILS OF SYSTEM EVOLUTION THERE  
MAY RELATIVE MAXIMA OVER OTHER PARTS OF THE EAST. EXPECT MOST  
WINTRY WEATHER TO BE CONFINED FAIRLY FAR TO THE NORTH AND WEST.  
 
TEMPERATURES WILL BE ABOVE NORMAL OVER A MAJORITY OF THE LOWER 48  
DURING THE PERIOD. THE MOST EXTREME READINGS VERSUS NORMAL SHOULD  
BE OVER THE NORTHERN PLAINS WHERE ANOMALIES MAY EXCEED PLUS 20F  
FOR ONE OR MORE DAYS, ESPECIALLY FOR MIN TEMPS WHICH COULD  
APPROACH OR EXCEED DAILY RECORDS AT SOME LOCATIONS. ONE EXCEPTION  
TO THE WARMTH WILL BE NEW ENGLAND WHICH WILL BE ON THE CHILLY SIDE  
TUE BEHIND A DEPARTING UPPER LOW.  
 
KONG/RAUSCH  
 
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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