407  
FXUS02 KWBC 311605  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1104 AM EST MON DEC 31 2018  
 
VALID 12Z THU JAN 03 2019 - 12Z MON JAN 07 2019  
   
..OVERVIEW
 
 
MEDIUM RANGE PERIOD BEGINS WITH A DEEP CLOSED LOW OVER THE  
SOUTHERN PLAINS PROGRESSING EAST AND NORTHEASTWARD TO OFF THE  
MID-ATLANTIC COAST BY DAY 5 (SATURDAY), WHILE AT THE SAME TIME  
ADDITIONAL WAVES OF ENERGY SLIDE THROUGH NEAR THE U.S./CANADIAN  
BORDER IN THE NORTHERN STREAM. OUT WEST, THE FIRST SHORTWAVE  
CROSSES INTO THE PACIFIC NORTHWEST LATER THIS WEEK, WITH A DIGGING  
TROUGH APPROACHING AND POSSIBLY MOVING INLAND DAYS 5 AND BEYOND.  
THIS OVERALL PATTERN WOULD FAVOR HEAVY RAINFALL IN THE EASTERN  
U.S. THROUGH DAY 5, WITH THE FOCUS SHIFTING TO THE WEST COAST BY  
DAYS 5-7.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
ALBEIT MINOR TIMING DIFFERENCES, THERE FINALLY SEEMS TO BE A  
BETTER CONSENSUS ON THE EVOLUTION OF A DEEP CLOSED LOW MOVING  
ACROSS THE SOUTH, LIFTING TOWARDS THE MID-ATLANTIC AND OFF THE  
ATLANTIC COAST BY THIS WEEKEND. OVERNIGHT, THE LATEST TWO RUNS OF  
THE GFS BEGAN SHIFTING TOWARDS A SLOWER/DEEPER SOLUTION WHICH HAS  
CONTINUALLY RUN-AFTER-RUN BEEN DEPICTED BY BOTH THE ECMWF/UKMET  
THE PAST DAY OR TWO. THE LATEST RUNS OF THE CMC AND THE FV3-GFS  
CONTINUE TO BE ON THE FASTEST SIDE OF THE GUIDANCE ENVELOPE.  
COMPARING SPAGHETTI PLOTS OF THE ENSEMBLES FROM YESTERDAY AND  
TODAY CONTINUES TO SHOW A TREND TOWARDS A SLOWER PROGRESSION AND  
THE WPC BLEND FOR THIS SET OF PROGS FAVORS A MAJORITY  
ECMWF/UKMET/GFS BLEND FOR THIS SYSTEM. THIS RESULTED IN A SLIGHTLY  
SLOWER AND DEEPER UPPER LEVEL AND SURFACE LOW COMPARED TO THE  
PREVIOUS WPC SHIFT.  
 
AS THE TROUGH/CLOSED LOW EXITS THE EAST ON DAY 5, ANOTHER TROUGH  
IN THE EASTERN PACIFIC APPROACHES THE WEST COAST. THERE CONTINUES  
TO BE SOME DISAGREEMENT BOTH ON STRUCTURE AND TIMING WITH THIS  
SYSTEM AS A WHOLE. ON DAY 6, THE PAST TWO RUNS OF THE GFS HOLD  
ONTO A RATHER DEEP CLOSED LOW OFF JUST OFF SOUTHERN CALIFORNIA,  
WHILE THE ECMWF (AND TO SOME DEGREE THE CANADIAN) HAVE BEEN  
CONSISTENT WITH SHOWING JUST AN ELONGATED TROUGH. THE  
TROUGH/ENERGY MOVES INTO THE SOUTHWEST AROUND DAY 7, BUT THE  
TIMING ON THIS IS INCONSISTENT. THE ENSEMBLE MEANS GENERALLY  
FOLLOW SUIT WITH THEIR DETERMINISTIC COUNTERPARTS (WITH OBVIOUS  
SUPPRESSION), BUT A GENERAL BLEND BETWEEN THE THE ECENS/GEFS/ECMWF  
SEEMED TO BE A GOOD STARTING POINT, WITH SLIGHT WEIGHTING TOWARDS  
THE ECENS/ECMWF JUST SIMPLY DUE TO BETTER RUN TO RUN CONSISTENCY.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
CLOSED LOW LATTER HALF OF THIS WEEK SHOULD PROMOTE HEAVY RAINFALL  
POTENTIAL FROM THE LOWER MISSISSIPPI VALLEY TO THE CENTRAL  
ATLANTIC COAST. MODELS HAVE BEEN FAIRLY CONSISTENT IN SHOWING THE  
HEAVIEST AXIS OF RAINFALL NEAR A DEVELOPING SURFACE LOW TRACKING  
FROM THE CENTRAL GULF COAST TO THE CAROLINAS. MUCH OF THE PRECIP  
ASSOCIATED WITH THIS SYSTEM SHOULD BE IN THE FORM OF RAIN, BUT  
SOME CHANGEOVER TO SLOW IS POSSIBLE ON THE FAR NORTHERN EDGE OF  
THE PRECIPITATION SHIELD, THOUGH ANY MEANINGFUL ACCUMULATION  
POTENTIAL REMAINS LOW. OUT WEST, THE FIRST SHORTWAVE ON FRIDAY  
SHOULD BRING RAIN/MOUNTAIN SNOWS INTO THE PACIFIC NORTHWEST WHILE  
THE APPROACHING TROUGH THIS WEEKEND SPREADS PRECIPITATION INTO  
CALIFORNIA (WITH HEAVIER PRECIP LIKELY FOCUSED ALONG FAVORED  
TERRAIN IN SOUTHERN CALIFORNIA) AND EVENTUALLY MOVING INTO THE  
SOUTHWEST. HEAVY MOUNTAIN SNOWS ARE POSSIBLE ESPECIALLY FOR THE  
SIERRA NEVADAS AND HIGHER ELEVATIONS OF ARIZONA.  
 
A LOOK AT TEMPERATURE FORECASTS IN THE MEDIUM RANGE SHOW MUCH  
ABOVE NORMAL TEMPERATURES POSSIBLE ACROSS THE NORTHERN PLAINS ON  
DAY 3, WITH NEAR TO ABOVE NORMAL TEMPERATURES PERSISTING ACROSS  
ROUGHLY THE EASTERN HALF OF THE COUNTRY. THE ONLY CHANCE FOR BELOW  
TO MUCH BELOW NORMAL TEMPERATURES SHOULD BE DAYS 3-4 ACROSS THE  
FOUR CORNERS INTO THE SOUTHERN PLAINS WHERE AFTERNOON HIGHS COULD  
BE 10-20F BELOW NORMAL. THE WESTERN U.S. SHOULD STAY NEAR NORMAL  
THROUGHOUT THE MEDIUM RANGE PERIOD.  
 
SANTORELLI  
 
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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