755  
FXUS02 KWBC 021600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SUN DEC 02 2018  
 
VALID 12Z WED DEC 05 2018 - 12Z SUN DEC 09 2018  
 
...COLD PATTERN OVER THE LOWER 48 WITH A SIGNIFICANT STORM SYSTEM  
LIKELY TO AFFECT AT LEAST THE SOUTHERN HALF OF THE U.S. LATE WEEK  
INTO THE WEEKEND...  
 
   
..OVERVIEW
 
 
THE CONTIGUOUS U.S. WILL LIE IN A TRANSITION ZONE BETWEEN HIGHLY  
AMPLIFIED/BLOCKED FLOW EXTENDING FROM EASTERN ASIA ACROSS THE  
NORTH PACIFIC AND SOMEWHAT MORE PROGRESSIVE FLOW ACROSS THE  
ATLANTIC OCEAN. ANOMALOUS RIDGING ACROSS SIBERIA WILL DISPLACE  
COLD POLAR/ARCTIC AIR TOWARD CANADA AND PERSISTENT RIDGING FROM  
WESTERN CANADA NORTH TO ALASKA WILL FAVOR TRANSPORT OF COLD AIR  
SOUTHWARD FROM CANADA INTO THE CENTRAL/EASTERN U.S. A NUMBER OF  
SHORTWAVES WILL TRAVERSE AN ACTIVE NORTHERN STREAM FROM THE  
MIDWEST TO THE NORTHEAST DURING THE PERIOD, WITH A STRONG COLD  
FRONT ACCOMPANYING THE MOST SIGNIFICANT OF THESE WAVES ACROSS THE  
PLAINS, OHIO VALLEY, AND NORTHEAST WED-FRI. FARTHER SOUTH AN  
ACTIVE SOUTHERN STREAM WILL CARRY PACIFIC ENERGY FROM NEAR  
CALIFORNIA EASTWARD ACROSS THE SOUTHERN STATES. AS THIS ENERGY  
REACHES THE SOUTHERN ROCKIES/PLAINS EXPECT IT TO DEVELOP A WAVE OF  
LOW PRESSURE ALONG THE POLAR FRONT NEAR THE WESTERN-CENTRAL GULF  
COAST BY SAT. MODELS AND ENSEMBLES CONTINUE TO SHOW VARYING  
DEGREES OF PHASING/INTERACTION OF THIS SOUTHERN STREAM WAVE WITH  
ADDITIONAL NORTHERN STREAM ENERGY BY THE WEEKEND, ALONG WITH  
LIKELY DEVELOPMENT OF A COASTAL LOW NEAR THE SOUTHEAST U.S. COAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
AS HAS GENERALLY BEEN THE CASE IN RECENT DAYS THE MAJORITY OF  
OPERATIONAL GUIDANCE SHOWS SIMILAR IDEAS DURING THE FIRST HALF OF  
THE MEDIUM RANGE FORECAST, WHICH TODAY SPANS THE WED INTO EARLY  
FRI TIME FRAME. THUS THE LATEST RUNS OF THE GFS/FV3 GFS AND  
ECMWF/UKMET/CMC ALL APPEAR TO PROVIDE USEFUL INFORMATION FOR  
DEPICTING IMPORTANT FEATURES DURING THIS PART OF THE FORECAST.  
OVERALL THE MODEL AVERAGE (WHILE DOWNPLAYING THE OCCASIONAL MORE  
EXTREME SOLUTION) HAS BEEN REMARKABLY CONSISTENT IN RECENT DAYS  
FOR THE SYSTEM NEARING CALIFORNIA AROUND MIDWEEK AND LIKELY TO  
OPEN UP INTO AN UPPER TROUGH AS IT HEADS INTO THE SOUTHWEST.  
FORECASTS FOR THE SYSTEM TRACKING ACROSS THE UPPER GREAT LAKES/NEW  
ENGLAND REGIONS THU-FRI HAVE BEEN A LITTLE MORE VARIABLE ONCE THEY  
PROGRESS BEYOND THE GREAT LAKES EARLY THU. THE LATEST CYCLE OF  
GUIDANCE HAS TRENDED TOWARD FASTER PROGRESSION SUCH AS HINTED AT  
BY SOME ECMWF/GFS RUNS AT DIFFERENT TIMES IN RECENT DAYS.  
 
ALONG THE WEST COAST BY DAY 6 SAT THE ECMWF CONTINUES TO BE ON THE  
STRONGER SIDE OF GUIDANCE WITH SHORTWAVE ENERGY THAT ARRIVES FROM  
THE PACIFIC. THE 00Z ECMWF IS ON THE EXTREME SIDE OF THE FULL 00Z  
ECMWF/GEFS/CMC ENSEMBLE SPREAD, PLUS THE ENERGY IS HEADING INTO A  
REGION OF MEAN RIDGING BETWEEN A RAPIDLY AMPLIFYING UPSTREAM  
TROUGH AND THE WEST-CENTRAL U.S./NORTHERN MEXICO TROUGH, SO  
CONTINUE TO HAVE LOWER CONFIDENCE IN THE OPERATIONAL ECMWF  
SCENARIO. HOWEVER THERE ARE SOME ENSEMBLE MEMBERS THAT DO STILL  
HAVE A DECENT SHORTWAVE AND THE 12Z/01 CMC WAS FAIRLY STRONG  
BEFORE ADJUSTING TO MORE DEFLECTION IN THE LATEST 00Z RUN--SO THIS  
IS A LOWER PROBABILITY SCENARIO THAT CANNOT BE FULLY DISCOUNTED  
YET. SOLUTIONS FOR THE UPSTREAM AMPLIFYING TROUGH OFF THE WEST  
COAST ARE COMPARABLE THROUGH DAY 7 SUN.  
 
AS THE SOUTHERN STREAM TROUGH ALOFT PROGRESSES EASTWARD AND LIKELY  
SUPPORTS SURFACE LOW PRESSURE DEVELOPMENT FROM THE WESTERN-CENTRAL  
GULF COAST LATE FRI-SAT WITH EASTWARD/NORTHEASTWARD PROGRESSION  
THEREAFTER, SO FAR THERE HAS BEEN DECENT CLUSTERING/STABILITY UP  
TO ABOUT EARLY SAT. THEN GREATER DIVERGENCE ARISES OWING TO  
DIFFERENCES IN STREAM INTERACTION AND THE INFLUENCE OF WEST  
COAST/WESTERN U.S. FLOW DETAILS. AT THIS TIME CONSIDERABLY MORE  
ENSEMBLE MEMBERS SIDE WITH THE IDEA OF GREATER EMPHASIS ON COASTAL  
LOW DEVELOPMENT BY EARLY DAY 7 SUN, VERSUS A MORE INLAND  
TRACK--INDICATING LOWER PROBABILITY FOR SIGNIFICANT PHASING ALOFT  
AND GREATER DOMINANCE OF THE LOW LEVEL COLD AIR. AMONG LATEST  
OPERATIONAL RUNS THE 00Z/06Z GFS RUNS ARE GENERALLY ON THE  
NORTHERN/SOUTHERN SIDES OF THE SPREAD RESPECTIVELY. LOOKING AT THE  
ENSEMBLE MEANS, THE ECMWF MEAN HAS BEEN SOMEWHAT MORE STABLE THAN  
GEFS MEANS OVER THE PAST COUPLE DAYS. THROUGH THE 00Z CYCLE THE  
GEFS HAD EXHIBITED SOMEWHAT OF A SLOWER TREND BUT THE 06Z RUN  
REVERSED THAT TREND. GIVEN THAT MOST TIMING ADJUSTMENTS OVER  
RECENT DAYS HAVE TENDED TO BE SLOWER RATHER THAN FASTER--PLUS THE  
CMC MEAN IS EVEN SLOWER--PREFERRED TO USE THE 00Z RUN FOR THE GEFS  
MEAN COMPONENT OF THE FORECAST LATE IN THE PERIOD. A  
MODEL/ENSEMBLE MEAN BLEND YIELDED A FORECAST VERY CLOSE TO  
CONTINUITY.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE SYSTEM NEARING THE SOUTHERN WEST COAST WILL INITIALLY BRING  
LOCALLY MODERATE/HEAVY PRECIPITATION (RAIN AND HIGH ELEVATION  
SNOW) TO PARTS OF CALIFORNIA AND THE SOUTHWEST AND SOMEWHAT LESS  
MOISTURE TO AREAS FARTHER NORTHWARD. AREAS NEAR COASTAL SOUTHERN  
CALIFORNIA AND FAVORED TERRAIN OVER ARIZONA SHOULD SEE THE HIGHEST  
TOTALS WITH THIS EVENT. FARTHER EASTWARD EXPECT AREAS OF HEAVY  
RAINFALL TO DEVELOP OVER THE SOUTHERN PLAINS AROUND THU NIGHT-FRI  
AND THEN CONTINUE EASTWARD INTO THE WEEKEND. THE FULL ARRAY OF  
MODEL/ENSEMBLE GUIDANCE HIGHLIGHTS AN AREA FROM ABOUT THE EASTERN  
HALF OF TEXAS INTO THE LOWER MISSISSIPPI VALLEY AS HAVING  
POTENTIAL TO SEE THE HIGHEST RAINFALL TOTALS FOR THE FULL EVENT  
BUT LOCATIONS FARTHER EASTWARD SHOULD SEE SOME HEAVY RAINFALL AS  
WELL. MEANWHILE THE COLD AIR IN PLACE WILL LIKELY PRODUCE SNOW AND  
POSSIBLY OTHER WINTRY PRECIPITATION TYPES IN THE NORTHERN PART OF  
THE MOISTURE SHIELD, REACHING THE APPALACHIANS AND EAST COAST  
DURING THE WEEKEND. CURRENTLY THE BEST POTENTIAL FOR SIGNIFICANT  
SNOWFALL EXISTS FROM THE SOUTHERN HIGH PLAINS/SOUTH-CENTRAL PLAINS  
INTO THE SOUTHERN/CENTRAL APPALACHIANS AND PARTS OF THE  
MID-ATLANTIC. NORTHERN STREAM SYSTEMS AFFECTING THE NORTHEAST  
QUADRANT OF THE NATION WILL BE RELATIVELY STARVED FOR MOISTURE BUT  
LAKE EFFECT ENHANCED SNOWFALL WILL BE POSSIBLE. SEE THE LATEST DAY  
4-7 WINTER WEATHER OUTLOOKS FOR DETAILS ON AREAS EXPECTED TO SEE  
THE GREATEST WINTER WEATHER THREAT IN THE MEDIUM RANGE.  
 
EXPECT BELOW NORMAL TEMPERATURES OVER A MAJORITY OF THE LOWER 48  
THROUGH THE PERIOD. THE MOST EXTREME ANOMALIES SHOULD BE OVER THE  
CENTRAL U.S. BEHIND THE FRONT THAT DROPS SOUTHWARD THROUGH THE  
PLAINS MID-LATE WEEK. SOME LOCATIONS MAY SEE HIGHS AT LEAST 15-20F  
BELOW AVERAGE. THE WEST SHOULD SEE NEAR TO BELOW NORMAL  
TEMPERATURES BUT WITH A GRADUAL MODERATING TREND FROM READINGS  
5-15F BELOW NORMAL ON WED. THE EASTERN U.S. WILL GENERALLY SEE  
HIGHS 5-15F BELOW NORMAL WITH SOME DAY-TO-DAY VARIABILITY. CLOUDS  
AND PRECIPITATION WILL BRING ONE OR MORE DAYS OF ABOVE AVERAGE MIN  
TEMPS TO SOME LOCATIONS ACROSS EXTREME SOUTHERN TIER LOCATIONS.  
 
RAUSCH/RYAN  
 
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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