568  
FXUS02 KWBC 211600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST FRI DEC 21 2018  
 
VALID 12Z MON DEC 24 2018 - 12Z FRI DEC 28 2018  
   
..SIGNIFICANT CENTRAL U.S. STORM POTENTIAL AFTER CHRISTMAS
 
 
16 UTC UPDATE...  
 
NO SIGNIFICANT CHANGE IN FORECAST THINKING FROM OVERNIGHT. MODEL  
CONSENSUS DURING DAYS 3-5 (MON-WED) WAS SUFFICIENT TO USE A BLEND  
OF THE LATEST DETERMINISTIC GUIDANCE AS A FORECAST STARTING POINT  
(06Z GFS AND 00Z ECMWF/UKMET/FV3). MODELS/ENSEMBLES HAVE SHOWN A  
BIT OF A SLOWING TREND WITH THE SIGNIFICANT LOW PRESSURE SYSTEM  
AFFECTING THE CENTRAL U.S. BY THE MIDDLE OF NEXT WEEK. THE 00Z FV3  
WAS THE SIGNIFICANT OUTLIER AFTER DAY 5, CONTINUING TO RACE THE  
SYSTEM QUICKLY NORTHEASTWARD ACROSS THE GREAT LAKES MORE QUICKLY  
THAN THE CONSENSUS, WHILE THE 00Z ECMWF WAS ON THE SLOWER/WESTERN  
SIDE OF THE SPREAD. ENSEMBLE MEMBERS CONTINUE TO SHOW A RANGE OF  
SOLUTIONS WITH RESPECT TO TIMING, INTENSITY, AND TRACK FOR THIS  
LOW PRESSURE SYSTEM, AND WEIGHTING OF ENSEMBLE MEANS (ECENS AND  
NAEFS) WAS INCREASED DURING DAYS 6-7 TO ACCOUNT FOR THIS SPREAD.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 07 UTC)  
 
   
..OVERVIEW
 
 
THE UPPER FLOW NEXT WEEK WILL TURN INCREASINGLY AMPLIFIED AS A  
PACIFIC SYSTEM PUSHES THROUGH THE GREAT BASIN LATE MONDAY AND OUT  
OF THE FOUR CORNERS WEDNESDAY. IT WILL TRACK NORTHEASTWARD TOWARD  
THE GREAT LAKES NEXT THU/FRI AS DOWNSTREAM RIDGING BUILDS INTO THE  
EAST COAST. THE LARGE SCALE REMAINS QUITE PREDICTABLE BUT THE  
IMPORTANT DETAILS HAVE YET TO COME INTO FOCUS, INCLUDING THE EXACT  
LOCATION AND TYPE OF PRECIPITATION.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A MAJORITY DETERMINISTIC MODEL BLEND SERVED AS A GOOD STARTING  
POINT GIVEN THE GOOD ENSEMBLE AGREEMENT THROUGH THE END OF THE  
FORECAST PERIOD. THE 12Z/18Z GFS RUNS WERE JUST A BIT QUICKER THAN  
THE ENSEMBLE CONSENSUS (INCLUDING THE 12Z CANADIAN) WHILE THE 12Z  
ECMWF WAS A BIT SLOWER -- ALL NEARLY FITTING THEIR LONG-TERM  
BIASES. THE 18Z PARALLEL GFS (FV3) WAS CLOSER TO THE ENSEMBLE  
CONSENSUS AND WAS USED IN PLACE OF THE OPERATIONAL GFS BY NEXT  
THU/FRI. ECMWF ENSEMBLE MEAN HAS BEEN CONSISTENT IN THE TRACK OF  
THE SYSTEM OUT OF THE SOUTHWEST TOWARD ~LAKE MICHIGAN BUT THE  
ENSEMBLE SPREAD ENCOMPASSED ABOUT A 300 MILE RADIUS NEXT THU AND  
ABOUT A 375 MILE RADIUS NEXT FRIDAY, ROUGHLY THE EXTENT OF THE  
COLD SECTOR PRECIPITATION SHIELD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EARLY IN THE WEEK THE PRECIPITATION FOCUS WILL BE ON THE WEST --  
MOSTLY THE NORTHERN CALIFORNIA AND SOUTHERN OREGON LINE BUT ALSO  
INTO THE SIERRAS -- AS THE PACIFIC SYSTEM MOVES ASHORE. NW TO SE  
MOTION WOULD GENERALLY KEEP LOWER ELEVATION RAINFALL AMOUNTS DOWN  
BUT HIGHER ELEVATION SNOW COULD BE WELL OVER A FOOT ABOVE ~5000FT  
OR SO. LESSER RAIN/SNOW IS EXPECTED FROM NEVADA INTO THE ROCKIES  
WITH MANY AREAS SEEING AT LEAST MEASURABLE PRECIPITATION. BEST  
CHANCE OF SNOWFALL OVER SEVERAL INCHES WILL LIE AROUND THE UPPER  
LOW TRACK INCLUDING THE MOGOLLON RIM. AS THE SYSTEM STARTS TO LIFT  
NORTHEASTWARD, PRECIPITATION WILL EXPAND AROUND THE DEEPENING LOW  
AS WELL AS THROUGH THE WARM SECTOR NEARER TO THE GULF COAST.  
LOCATIONS OVER THE NORTHERN HALF OR THIRD OF THE PLAINS WOULD SEE  
SNOW WITH THIS SYSTEM BASED ON THE CURRENT MOST LIKELY SCENARIO.  
STRONG WINDS MAY ADD TO THE HAZARDOUS CONDITIONS. RAINFALL COULD  
BECOME LOCALLY HEAVY NEAR THE GULF COAST AS THE SYSTEM WRAPS UP  
THROUGH THE CENTRAL MISSISSIPPI VALLEY THURSDAY.  
 
TEMPERATURES WILL START NEAR OF A BIT ABOVE AVERAGE TO START THE  
WEEK BUT TREND COOLER THAN AVERAGE IN THE WEST BEHIND THE FRONT.  
THE CENTRAL/EASTERN STATES WILL TREND ABOVE AVERAGE AS THE LOW  
CENTER LIFTS INTO THE GREAT LAKES. ANOMALIES WILL BE IN THE +5 TO  
+15 DEG F RANGE BUT +10 TO +20 DEG F FOR OVERNIGHT MINS.  
 
FRACASSO  
 
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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