165  
FXUS02 KWBC 301600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST FRI NOV 30 2018  
 
VALID 12Z MON DEC 03 2018 - 12Z FRI DEC 07 2018  
 
...COLD PATTERN MORE TYPICAL OF MID-WINTER TO GRIP MUCH OF THE  
CONTIGUOUS U.S. NEXT WEEK...  
 
   
..OVERVIEW
 
 
A COLD PATTERN IS IN STORE FOR MUCH OF THE CONUS DURING THE MEDIUM  
RANGE PERIOD AS MEAN UPPER-LEVEL TROUGHING PERSISTS FROM THE  
NORTHERN PLAINS/UPPER MIDWEST TO NEW ENGLAND. UPSTREAM SUPPORT FOR  
THIS PATTERN SHOWS NO SIGNS OF WANING, WITH A HIGHLY AMPLIFIED AND  
MULTI-CENTER BLOCKING PATTERN EXTENDING FROM SIBERIA ACROSS ASIA  
TO THE NORTH PACIFIC, WHICH WILL MAINTAIN ANOMALOUS RIDGING FROM  
WESTERN BRITISH COLUMBIA INTO ALASKA. THE FLOW RESULTING FROM THIS  
PATTERN WILL FAVOR CONTINUED SOUTHWARD TRANSPORT OF POLAR/ARCTIC  
AIR THROUGH CANADA AND INTO THE CONUS. AN ACTIVE NORTHERN STREAM  
FLOW REGIME WILL RESULT IN A DOMINANT STORM TRACK FROM CENTRAL  
CANADA SOUTHWARD INTO THE CENTRAL/EASTERN U.S., WITH THE POTENTIAL  
FOR CYCLOGENESIS OFF THE EASTERN SEABOARD. WITH THE FLOW ACROSS  
THE NORTH ATLANTIC MUCH LESS BLOCKED THAN THAT FARTHER WEST,  
SYSTEMS SHOULD BECOME MORE PROGRESSIVE THROUGH TIME AS THEY MOVE  
EAST, WITH LIMITED THREAT FOR SIGNIFICANT COASTAL STORMS.  
MEANWHILE A FAIRLY DEEP UPPER LOW OVER THE EASTERN PACIFIC EARLY  
IN THE WEEK WILL STEADILY MOVE TOWARD CALIFORNIA THROUGH MIDWEEK,  
LIKELY OPENING UP AS THE SYSTEM PROGRESSES INTO THE SOUTHWEST  
U.S./NORTHWESTERN MEXICO LATE IN THE WEEK. THIS SYSTEM WILL BRING  
SIGNIFICANT PRECIPITATION TO PORTIONS OF CALIFORNIA/ARIZONA, AND  
POTENTIALLY THE SOUTHERN PLAINS TOWARD THE END OF THE WEEK.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
FOR ABOUT THE FIRST HALF OF THE PERIOD THE FORECAST EMPHASIZED THE  
06Z GFS/00Z FV3 GFS AND 00Z ECMWF ALONG WITH MODEST INCLUSION OF  
THE 00Z UKMET. THE CMC DIFFERED FROM CONSENSUS FOR BOTH THE UPPER  
LOW TRACKING TOWARD CALIFORNIA AND THE SHORTWAVE CROSSING THE  
NORTHEAST DURING THE FIRST HALF OF THE PERIOD. THE BLEND  
INTRODUCED 00Z GEFS/ECMWF MEAN INPUT BY MID-PERIOD WITH THE MEANS  
REACHING GREATER THAN HALF OF THE TOTAL WEIGHT LATE IN THE PERIOD.  
THE LATE PART OF THE FORECAST RETAINED SOME ASPECTS OF THE FV3  
GFS/ECMWF AS OPERATIONAL GFS RUNS WERE QUICKER THAN CONSENSUS FOR  
PROGRESSION OF PACIFIC FLOW UPSTREAM FROM THE SYSTEM REACHING THE  
SOUTHWEST AND HAD LOWER CONFIDENCE DETAILS WITHIN THE EASTERN  
NORTH AMERICA TROUGH.  
 
THERE IS STILL LOWER THAN DESIRED PREDICTABILITY IN EXACT DETAILS  
OF HOW RAPIDLY PROGRESSING NORTHERN STREAM SHORTWAVES WILL  
INTERACT WITH SOUTHERN STREAM FLOW. ONE ASPECT OF THE FORECAST  
THAT HAS BEEN IN FOCUS OVER RECENT DAYS HAS BEEN THE EVOLUTION OF  
SHORTWAVE ENERGY EMERGING FROM THE ROCKIES/PLAINS WITH VERY  
UNCERTAIN EFFECTS ON THE EASTERN U.S. SURFACE PATTERN AROUND DAY 4  
TUE. SOME SPREAD PERSISTS BUT THERE IS A NOTABLE TREND TOWARD  
KEEPING THE ENERGY ALOFT MORE SHEARED AND ANY SOUTHEAST U.S.  
FRONTAL WAVINESS MORE SUPPRESSED UNTIL IT REACHES THE WESTERN  
ATLANTIC. COMPARED TO YESTERDAY THE LATEST ENSEMBLE SURFACE LOW  
PLOTS VALID 12Z TUE ALSO DISPLAY A SIGNIFICANT REDUCTION OF  
MEMBERS SHOWING ANY WAVINESS TO THE NORTHWEST OF THE SOUTHEASTERN  
COAST.  
 
FARTHER WESTWARD, THE CONSENSUS/MAJORITY CLUSTER IS HOLDING UP  
FAIRLY WELL FOR THE UPPER LOW APPROACHING CALIFORNIA. CMC RUNS  
HAVE BEEN CONSISTENTLY STRAYING FROM THE MAJORITY CLUSTER IN SOME  
FASHION, MOST COMMONLY ON THE SLOW SIDE. THE 00Z RUN IS INITIALLY  
FAST AND THEN BECOMES SLOWER THAN CONSENSUS. BY MID-LATE PERIOD  
THE ECMWF/ECMWF HAVE TRENDED SOMEWHAT SLOWER OVER THE PAST 1-2  
DAYS TO YIELD FAIRLY GOOD TIMING AGREEMENT IN TODAY'S FORECAST.  
THE 00Z GEFS RUN WAS PREFERRED OVER THE 06Z VERSION AS THE FORMER  
MAINTAINED A BETTER DEFINED AND SLOWER SOUTHWESTERN  
SYSTEM/UPSTREAM RIDGE. TOWARD DAY 7 FRI THE CMC ENSEMBLE MEAN  
BECOMES QUESTIONABLY MORE PROGRESSIVE OVER THE EASTERN PACIFIC.  
 
00Z/06Z SOLUTIONS AGREE IN GENERAL PRINCIPLE WITH THE FORECAST OF  
A STRONG SHORTWAVE DROPPING THROUGH WESTERN CANADA TOWARD THE  
NORTHERN PLAINS TUE-WED. MOST GUIDANCE PICKED UP ON THIS FEATURE  
IN THE 12Z/29 CYCLE WITH PRIOR RUNS MUCH FLATTER/MORE SHEARED.  
THIS FEATURE IS PROVIDING A STRONGER PUSH OF COLD AIR INTO THE  
NORTHERN ROCKIES AND SOUTHWARD THROUGH THE PLAINS AFTER EARLY WED.  
BY DAY 7 FRI THERE ARE PRONOUNCED DIFFERENCES REGARDING HOW FAR  
SOUTH THE FRONT REACHES. THE BEST CLUSTERING CONSISTS OF THE 00Z  
FV3 GFS/GEFS MEAN/ECMWF/ECMWF MEAN WHILE OPERATIONAL GFS RUNS HOLD  
THE COLD AIR MUCH FARTHER NORTH. FRONTAL POSITION WILL LIKELY  
PROVE TO BE VERY IMPORTANT IN DETERMINING PRECIPITATION TYPE AS  
MOISTURE FROM THE CALIFORNIA/SOUTHWEST SYSTEM REACHES THE SOUTHERN  
PLAINS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
AS DESCRIBED ABOVE, THE FLOW PATTERN WILL FAVOR SOUTHWARD  
TRANSPORT OF POLAR/ARCTIC AIR INTO THE CONUS THROUGH MUCH OF THE  
MEDIUM RANGE. IN A RELATIVELY RARE OCCURRENCE, NEARLY THE ENTIRE  
CONUS WILL SEE BELOW AVERAGE TEMPERATURES BY LATE NEXT WEEK. FOR  
THE FIVE-DAY PERIOD EXPECT THE MOST CONSISTENTLY LOW ANOMALIES TO  
BE OVER/JUST NORTH OF THE GREAT BASIN AND OVER THE  
NORTHERN-CENTRAL HIGH PLAINS. HIGH TEMPERATURES ACROSS THOSE AREAS  
AND AT TIMES NEARBY LOCATIONS WILL TEND TO RANGE FROM 10 TO AT  
LEAST 20 DEG F BELOW AVERAGE ON MULTIPLE DAYS. AFTER A BRIEF WARM  
UP ALONG/AHEAD OF A FRONTAL BOUNDARY, BELOW AVERAGE TEMPS (5-10  
DEG F) WILL RETURN TO THE EASTERN SEABOARD BY TUE-WED, WITH TEMPS  
DROPPING FURTHER BY LATER WEEK AS ADDITIONAL ROUNDS OF COLD AIR  
ARRIVE.  
 
IN TERMS OF PRECIPITATION, THIS WILL BE A RELATIVELY DRY PATTERN  
FOR MUCH OF THE NORTHERN TIER AS LOW PRESSURE SYSTEMS TRAVERSING  
THE NORTHWESTERLY FLOW REMAIN RELATIVELY MOISTURE-STARVED.  
PERIODS OF LAKE EFFECT SNOW ARE POSSIBLE THOUGH. ONE OR MORE LOW  
PRESSURE WAVES ALONG THE FRONT ACROSS THE SOUTHEAST ON MON-TUE  
SHOULD FOCUS MORE WIDESPREAD RAINS FOR PORTIONS OF FLORIDA AND THE  
COASTAL SOUTHEAST BEFORE QUICKLY MOVING OUT INTO THE ATLANTIC  
OCEAN. THE SLOW-MOVING PACIFIC SYSTEM WILL INCREASE RAIN AND  
MOUNTAIN SNOWS ALONG THE WEST COAST BY TUE/WED BEFORE SPREADING  
PRECIPITATION INTO THE SOUTHWEST AND PORTIONS OF THE SOUTHERN  
GREAT BASIN BY THU AND THE SOUTHERN PLAINS BY FRI. SOME AREAS OF  
HEAVY RAINFALL MAY BE POSSIBLE AS THE MOISTURE SHIELD SPREADS  
ACROSS THE SOUTHERN PLAINS AND POSSIBLY INTO THE LOWER MISSISSIPPI  
VALLEY. WINTRY WEATHER MAY ALSO BE A THREAT AT SOME LOCATIONS  
DEPENDING ON PRECISE LOCATION OF THE FRONT EXPECTED TO DROP  
SOUTHWARD THROUGH THE PLAINS MID-LATE WEEK.  
 
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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