873  
FXUS02 KWBC 030655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 AM EST MON DEC 03 2018  
 
VALID 12Z THU DEC 06 2018 - 12Z MON DEC 10 2018  
 
...COLD PATTERN OVER THE CONTIGUOUS U.S. WITH A SIGNIFICANT STORM  
SYSTEM LIKELY TO AFFECT AT LEAST THE SOUTHERN HALF OF THE U.S.  
LATE WEEK INTO THE WEEKEND...  
 
   
..OVERVIEW
 
 
UPPER-LEVEL FLOW ACROSS THE CONUS DURING THE MEDIUM RANGE WILL LIE  
IN A TRANSITION ZONE FROM HIGHLY AMPLIFIED/BLOCKED FLOW EXTENDING  
FROM EAST ASIA ACROSS THE NORTH PACIFIC TO SOMEWHAT MORE  
PROGRESSIVE FLOW ACROSS THE ATLANTIC OCEAN. ANOMALOUS RIDGING  
ACROSS SIBERIA WILL DISPLACE POLAR/ARCTIC AIR INTO CANADA; AND  
PERSISTENT RIDGING ACROSS WESTERN NORTH AMERICA WILL FAVOR  
TRANSPORT OF COLD AIR SOUTHWARD INTO THE CENTRAL/EASTERN U.S. A  
NUMBER OF SHORTWAVES WILL TRAVERSE AN ACTIVE NORTHERN STREAM FROM  
THE MIDWEST TO THE NORTHEAST DURING THE MEDIUM RANGE, WITH A  
STRONG COLD FRONT ACCOMPANYING THE MOST SIGNIFICANT OF THESE WAVES  
ACROSS THE PLAINS/MIDWEST, OHIO VALLEY, AND NORTHEAST THU-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, A WAVE OF LOW PRESSURE  
IS EXPECTED TO DEVELOP ALONG THE POLAR FRONT NEAR THE WESTERN 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.  
BY LATE IN THE MEDIUM RANGE (EARLY NEXT WEEK) THERE ARE SOME SIGNS  
THAT THE PATTERN MAY MODULATE SOMEWHAT, WITH ENSEMBLE GUIDANCE  
SUGGESTING THE WESTERN NORTH AMERICA RIDGE MAY BEGIN TO SHIFT  
EASTWARD TOWARD THE ROCKIES/HIGH PLAINS, ALLOWING HEIGHTS TO FALL  
ALONG THE WEST COAST AS AT LEAST PORTIONS THAT REGION PERHAPS  
ENTER A PERIOD OF STORMIER CONDITIONS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS CONTINUE TO HANDLE FEATURES IN BOTH THE NORTHERN AND  
SOUTHERN STREAMS SIMILARLY DURING DAYS 3-5 (THU-SAT), AND A BLEND  
OF THE ECMWF/GFS/FV3 SERVED AS A GOOD FORECAST STARTING POINT  
DURING THIS TIME PERIOD. SOME DIFFERENCES BEGIN TO EMERGE AND  
AMPLIFY FROM DAY 5 ONWARD, PARTICULARLY WITH SMALLER SCALE  
SHORTWAVE ENERGY CROSSING THE UPPER MIDWEST/GREAT LAKES SAT-SUN.  
DIFFERENCES ALSO EMERGE WITH THE SOUTHERN STREAM ENERGY AS IT  
REACHES THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY SAT  
NIGHT/SUN, WITH MODELS CONTINUING TO SHOW SOME DEGREE OF  
VARIABILITY ON TIMING AND AMPLITUDE. GIVEN THE POTENTIAL FOR  
PHASING INTERACTIONS BETWEEN THE TWO STREAMS, UNCERTAINTY  
CONTINUES TO BE COMPOUNDED. A COMPARISON OF THE LAST FEW ENSEMBLE  
CYCLES DOES REVEAL A MODEST REDUCTION IN SPREAD BY SUN-MON WITH  
RESPECT TO THE EVOLUTION OF THE LOW PRESSURE SYSTEM AS IT CROSSES  
THE SOUTHEAST STATES. BASED ON A CONSENSUS OF DETERMINISTIC AND  
ENSEMBLE GUIDANCE, A POTENTIALLY MULTI-CENTERED LOW PRESSURE  
SYSTEM IS EXPECTED TO BE CROSSING THE COASTAL SOUTHEAST BY SUN  
MORNING. AFTER THAT TIME, THERE STILL REMAINS SIGNIFICANT  
UNCERTAINTY AS TO WHETHER THE LOW PRESSURE SYSTEM WILL TRACK  
NORTHEASTWARD ALONG/CLOSE TO THE COAST OR TAKE A TRACK MORE OUT TO  
SEA. GIVEN LARGE VARIABILITY AMONG THE GUIDANCE AS WELL AS  
RUN-TO-RUN VARIABILITY, AN ENSEMBLE MEAN (ECENS/GEFS) BASED  
CONSENSUS SOLUTION WAS USED DURING DAYS 6-7 (SUN-MON). THIS  
SOLUTION DEPICTS A DEEPENING LOW PRESSURE SYSTEM OFF THE COAST OF  
THE CAROLINAS MON MORNING, FAIRLY CLOSE TO THE POSITION SHOWN BY  
THE 18Z GFS. ALONG THE WEST COAST, MODEL AGREEMENT IS BETTER THAT  
THE UPPER RIDGE SHOULD SLOWLY PROPAGATE EASTWARD TO THE ROCKIES BY  
SUN-MON ALLOWING HEIGHT FALLS TO REACH THE WEST COAST. SOLUTIONS  
DIFFER AS TO EXACTLY HOW AMPLIFIED THE TROUGH WILL BE, WITH SOME  
MODEST RUN-TO-RUN VARIABILITY AS TO THE NATURE OF THE SYSTEM AS  
WELL (OPEN WAVE VERSUS A CLOSED UPPER LOW), SO THE ENSEMBLE BASED  
APPROACH IS IDEAL HERE BY DAYS 6-7 AS WELL.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE SOUTHERN STREAM SYSTEM WILL INITIALLY BRING LOCALLY MODERATE  
TO LOCALLY HEAVY PRECIPITATION (RAIN AND HIGH ELEVATION SNOW) TO  
PARTS OF CALIFORNIA AND THE SOUTHWEST. AREAS NEAR COASTAL SOUTHERN  
CALIFORNIA AND FAVORED TERRAIN OVER ARIZONA SHOULD SEE THE HIGHEST  
TOTALS WITH THIS EVENT. FARTHER EAST, EXPECT AREAS OF HEAVY  
RAINFALL TO DEVELOP OVER THE SOUTHERN PLAINS THU NIGHT-FRI AND  
THEN CONTINUE EASTWARD INTO THE WEEKEND. COLD AIR IN PLACE ON THE  
NORTHERN PERIPHERY OF THE SYSTEM WILL LIKELY PRODUCE WINTRY  
PRECIPITATION IN THE NORTHERN PART OF THE PRECIPITATION SHIELD,  
STARTING OVER THE SOUTHERN PLAINS THU-FRI AND REACHING THE  
APPALACHIANS AND EAST COAST DURING THE WEEKEND. CURRENTLY THE BEST  
POTENTIAL FOR SIGNIFICANT SNOWFALL EXISTS FROM THE  
SOUTHERN/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.  
 
BELOW AVERAGE TEMPERATURES WILL PREVAIL ACROSS MUCH OF THE CONUS  
DURING THE MEDIUM RANGE. THE MOST EXTREME ANOMALIES SHOULD BE OVER  
THE CENTRAL U.S. BEHIND THE FRONT THAT DROPS SOUTHWARD MID-LATE  
WEEK. SOME LOCATIONS MAY SEE HIGHS AT LEAST 15-20F BELOW AVERAGE.  
THE EASTERN U.S. WILL GENERALLY SEE HIGHS 5-15F BELOW NORMAL WITH  
SOME DAY-TO-DAY VARIABILITY. THE WEST WILL BEGIN THE PERIOD WED  
WITH HIGHS 5-10F BELOW AVERAGE, BUT TEMPERATURES WILL MODERATE  
THROUGH THE WEEKEND AS UPPER RIDGING SLOWLY SHIFTS EAST.  
TEMPERATURES MAY REACH SEVERAL DEGREES ABOVE AVERAGE ACROSS  
PORTIONS OF THE CENTRAL/NORTHERN ROCKIES AND THE NORTHERN HIGH  
PLAINS BY SUN-MON.  
 
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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