460  
FXUS02 KWBC 011600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST SAT DEC 01 2018  
 
VALID 12Z TUE DEC 04 2018 - 12Z SAT DEC 08 2018  
 
...COLD PATTERN MORE TYPICAL OF MID-WINTER OVER MOST OF THE  
CONTIGUOUS U.S. NEXT WEEK...  
 
   
..OVERVIEW
 
 
NORTHEAST PACIFIC INTO WESTERN CANADA MEAN RIDGING ALOFT WILL  
PROMOTE A DOWNSTREAM MEAN TROUGH ACROSS THE REST OF CANADA AND  
INTO THE CENTRAL/EASTERN U.S. THIS PATTERN WILL SUPPORT AN  
EXTENDED PERIOD OF COLD WEATHER OVER A MAJORITY OF THE LOWER 48  
WITH PERIODIC REINFORCEMENT FROM FRONTS DROPPING SOUTHWARD FROM  
CANADA. THE MEAN TROUGH MAY FLATTEN/BROADEN TO SOME EXTENT DURING  
THE LATTER HALF OF THE PERIOD BUT FAIRLY DRAMATIC TROUGH  
AMPLIFICATION FORECAST OVER THE PACIFIC AROUND 140-150W LATE IN  
THE PERIOD WILL LIKELY ENCOURAGE PERSISTENCE/BUILDING OF THE  
WESTERN NORTH AMERICA MEAN RIDGE AND IN TURN PROMOTE TROUGHING  
FARTHER EAST. AT THE SAME TIME AN UPPER LOW OFFSHORE CALIFORNIA  
WILL STEADILY MOVE TOWARD THE SOUTHWEST U.S. AND LIKELY OPEN UP,  
WITH THE RESULTING TROUGH FORECAST TO REACH THE SOUTHERN  
ROCKIES/PLAINS AND NORTHERN MEXICO LATE IN THE PERIOD. THIS  
SYSTEM WILL BE A SIGNIFICANT PRECIPITATION PRODUCER FROM PORTIONS  
OF CALIFORNIA AND ARIZONA EASTWARD ACROSS THE SOUTHERN PLAINS AND  
EVENTUALLY INTO THE SOUTHEAST. BY THE LATTER HALF OF THE PERIOD  
COLD AIR ALREADY IN PLACE SHOULD LEAD TO SOME WINTRY WEATHER IN  
THE NORTHERN PART OF THE MOISTURE SHIELD. ISSUES REGARDING THE  
DEGREE OF NORTHERN STREAM INTERACTION WILL PROVIDE ONGOING  
UNCERTAINTY OVER THE ULTIMATE TRACK OF ASSOCIATED LOW PRESSURE AND  
NORTHERN EXTENT OF PRECIPITATION.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
DURING DAYS 3-5 TUE-THU AN OPERATIONAL MODEL CONSENSUS INCLUDING  
IDEAS FROM THE 06Z GFS AND 00Z ECMWF/UKMET/CMC REPRESENTS  
CONSENSUS WELL FOR SIGNIFICANT FEATURES AND ACCOUNTS FOR DETAIL  
DIFFERENCES SUFFICIENTLY SMALL IN SCALE TO HAVE LOW PREDICTABILITY  
AT THOSE TIME FRAMES. ADJUSTMENTS IN LATEST GUIDANCE ARE FAIRLY  
MODEST FOR THE SYSTEM APPROACHING CALIFORNIA AND THE CLIPPER  
FORECAST TO REACH NEAR LAKE SUPERIOR BY DAY 5 THU. BEHIND THE  
CLIPPER SYSTEM GUIDANCE TRENDS ARE STILL NUDGING THE FORECAST  
TOWARD A SLIGHTLY FASTER SOUTHWARD PROGRESSION OF COLD AIR OVER  
THE PLAINS/ROCKIES.  
 
BY DAYS 6-7 FRI-SAT THE MODELS AND ENSEMBLES DIVERGE WITH VARIOUS  
ASPECTS OF THE FORECAST. RECENT ECMWF RUNS HAVE BEEN RATHER  
AGGRESSIVE TO BRING EASTERN PACIFIC SHORTWAVE ENERGY INTO THE WEST  
COAST RIDGE, EVENTUALLY LEADING TO FASTER TIMING FOR THE SYSTEM  
DEVELOPING NEAR THE GULF COAST. ENSEMBLE MEANS AND THE 06Z GFS  
TREND VERSUS THE 00Z RUN PROVIDE SUPPORT FOR LEANING AWAY FROM THE  
ECMWF SCENARIO FOR FRI-SAT. ON THE OTHER HAND THE 00Z FV3 GFS  
SHOWS MORE SOUTHWESTWARD ELONGATION OF EASTERN PACIFIC TROUGH  
ENERGY, LEADING TO SLOWER PROGRESSION DOWNSTREAM. LEADING HEIGHT  
FALLS OVER THE PLAINS DO NOT DIFFER AS MUCH FROM CONSENSUS SO THE  
00Z FV3 RUN IS ONLY SLIGHTLY SLOWER WITH THE SURFACE LOW VERSUS  
THE 06Z OPERATIONAL GFS BY DAY 7 SAT. INTERESTINGLY LATEST GFS  
RUNS REPRESENT AN ADJUSTMENT AWAY FROM A MORE ECMWF-LIKE SOLUTION  
THROUGH THE 12Z/30 CYCLE. THE OVERALL AMPLITUDE OF FLOW EVOLUTION  
APPEARS TO FAVOR WEAKENING AND/OR NORTHWARD DEFLECTION ANY ENERGY  
NEARING THE WEST COAST BY NEXT SAT. THE ECMWF ALSO STRAYS TO THE  
FAST SIDE FOR THE CLIPPER SYSTEM, WITH GFS RUNS HAVING TRENDED  
SLOWER TOWARD THE ENSEMBLE MEANS. BASED ON THESE CONSIDERATIONS  
THE LATTER PART OF THE FORECAST ADJUSTS TOWARD HALF OR MORE TOTAL  
WEIGHT OF THE 06Z GEFS/00Z ECMWF MEANS WITH SOME LINGERING  
MINORITY INPUT FROM THE 06Z GFS/00Z CMC.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE DOMINANT SYSTEM OF INTEREST FROM THE PRECIPITATION PERSPECTIVE  
WILL START TO BRING MOISTURE (RAIN AND HIGH ELEVATION SNOW) INTO  
CALIFORNIA TUE-WED AND ARIZONA THU, FOLLOWED BY AREAS OF HEAVY  
RAINFALL LIKELY DEVELOPING OVER THE SOUTHERN PLAINS LATE IN THE  
WEEK WITH EASTWARD PROGRESSION THEREAFTER. OVER SOUTHWESTERN  
AREAS THE HIGHEST TOTALS SHOULD BE OVER FAVORED COASTAL LOCATIONS  
AND ALONG THE SIERRA NEVADA RANGE AS WELL AS FAVORED TERRAIN IN  
ARIZONA. FARTHER EASTWARD THE BEST SIGNAL FOR HEAVIEST RAINFALL  
EXTENDS FROM ABOUT THE EASTERN HALF OF TEXAS AND LOWER MISSISSIPPI  
VALLEY. COLD AIR TO THE NORTH OF DEVELOPING LOW PRESSURE NEAR THE  
GULF COAST SHOULD SUPPORT AN AREA OF SNOW AND POSSIBLY OTHER  
WINTRY PRECIPITATION TYPES IN THE NORTHERN PART OF THE MOISTURE  
SHIELD--CURRENTLY MOST LIKELY OVER PARTS OF THE CENTRAL PLAINS/MID  
MISSISSIPPI VALLEY. THE GREAT LAKES AND VICINITY WILL SEE  
EPISODES OF LAKE EFFECT SNOW WITH THE BEST ACTIVITY LIKELY IN THE  
WAKE OF THE CLIPPER SYSTEM FORECAST TO TRACK NEAR LAKE SUPERIOR  
AROUND THU.  
 
MUCH OF THE CONTIGUOUS U.S. WILL SEE BELOW NORMAL HIGH  
TEMPERATURES THROUGH THE PERIOD. COLDEST TEMPERATURES VERSUS  
NORMAL SHOULD BE BEHIND THE FRONT DROPPING SOUTH THROUGH THE  
PLAINS MID-LATE WEEK WITH SOME HIGHS AT LEAST 15-20F BELOW NORMAL.  
EXTREME SOUTHERN LOCATIONS AHEAD OF THE SYSTEM PROGRESSING  
EASTWARD FROM OFFSHORE CALIFORNIA MAY SEE ONE OR MORE DAYS OF  
ABOVE AVERAGE MIN TEMPS DUE TO CLOUDS/PRECIPITATION.  
 
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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