352  
FXUS02 KWBC 300553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1252 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 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.  
ADDITIONALLY, CUTOFF SOUTHERN STREAM ENERGY FROM THE PACIFIC WILL  
AFFECT THE WEST COAST AND SOUTHWEST DURING THE PERIOD.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS INITIALLY SHOW SIMILAR SOLUTIONS ON DAY 3 (TUE), BUT SPREAD  
INCREASES STEADILY THROUGH THE PERIOD AS MODELS STRUGGLE WITH A  
RAPID TRAIN OF NORTHERN STREAM SHORTWAVE ENERGY AND THEIR  
INTERACTION WITH LOWER AMPLITUDE SOUTHERN STREAM ENERGY. A  
MULTI-MODEL DETERMINISTIC BLEND (ECMWF/GFS/UKMET) WAS A SUFFICIENT  
BASIS FOR THE FORECAST DURING DAYS 3-5 (TUE-THU). TIMING/INTENSITY  
DIFFERENCES AMONG THE MODELS WERE RELATIVELY SMALL WITH SHORTWAVE  
ENERGY SKIRTING THE NORTH CENTRAL U.S. DURING THIS TIME, AS WELL  
AS A COLD FRONT PUSHING OFF THE EASTERN SEABOARD. WITH THE CUTOFF  
PACIFIC ENERGY NEARING THE WEST COAST DURING THIS TIME, MODELS  
HAVE COME TO SOMEWHAT BETTER CONSENSUS AFTER STRUGGLING, WITH A  
GENERAL TREND TOWARD A SLOWER AND MORE AMPLIFIED SOLUTION NOTED.  
 
AFTER THU, MODEL CONSENSUS BEGINS TO DEGRADE QUITE QUICKLY, WITH  
HANDLING OF NORTHERN STREAM ENERGY DIVING SOUTH ACROSS CENTRAL  
CANADA SEEMINGLY CHANGING WITH EACH MODEL CYCLE. THE ECMWF HAS  
SHOWN PARTICULARLY POOR RUN-TO-RUN CONSISTENCY IN THIS PORTION OF  
THE FORECAST. MODELS SHOW GENERAL AGREEMENT THAT ONE OF THE MORE  
SIGNIFICANT SHORTWAVES SHOULD BE CROSSING ALBERTA AND SASKATCHEWAN  
ON WED, BUT SOLUTIONS DIVERGE SHARPLY BY THU AS TO WHETHER THE  
WAVE MOVES EAST ACROSS THE GREAT LAKES (AS SHOWN BY THE GFS AND  
PRIOR ECMWF RUNS), OR BECOMES HIGHLY AMPLIFIED ACROSS THE  
NORTHERN/CENTRAL PLAINS, ALLOWING HEIGHTS TO RISE ALONG THE  
EASTERN SEABOARD. AS MENTIONED ABOVE, D+8 ANOMALIES SUGGEST THAT  
FLOW SHOULD GRADUALLY BECOME MORE PROGRESSIVE AS SYSTEMS LEAVE THE  
STRONG BLOCKING PATTERN PREVALENT ACROSS THE THE NORTH PACIFIC.  
GIVEN THIS, AND LITTLE SUPPORT FROM THE ENSEMBLE MEANS, THE  
DETERMINISTIC ECMWF WAS NOT FAVORED DURING DAYS 6-7. ONE ASPECT OF  
THE FORECAST SHOWING RELATIVELY HIGHER CONFIDENCE DURING THE LATE  
MEDIUM RANGE IS THE SOUTHERN STREAM SYSTEM. MODELS SHOW GENERAL  
AGREEMENT THAT THIS UPPER WAVE SHOULD CROSS THE SOUTHWEST AND  
APPROACH THE SOUTHERN PLAINS BY LATE NEXT WEEK. SOME TIMING  
DIFFERENCES EXIST, BUT ENSEMBLE MEANS APPEAR WELL REPRESENTATIVE  
OF A CONSENSUS SOLUTION HERE. GIVEN THE RELATIVELY POOR  
CONSISTENCY IN THE GUIDANCE FOR MOST AREAS, THE FORECAST WAS  
SHIFTED NEARLY EXCLUSIVELY TOWARD THE ECENS/NAEFS MEANS DURING  
DAYS 6-7. THIS SOLUTION KEEPS LOWER HEIGHTS IN PLACE ACROSS THE  
EASTERN U.S. THAN SHOWN BY THE ECMWF.  
 
   
..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, WITH  
AREAS FROM THE NORTHERN/CENTRAL PLAINS TO THE UPPER MIDWEST/GREAT  
LAKES LIKELY AT THE CORE OF THE COLD AIR. HIGH TEMPERATURES ACROSS  
THOSE AREAS WILL RANGE FROM 10 TO PERHAPS MORE THAN 20 DEG F BELOW  
AVERAGE THROUGH MUCH OF NEXT WEEK. 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. THE ROCKIES/GREAT BASIN WILL NOT BE IMMUNE TO THE COLD  
EITHER, WITH HIGH TEMPERATURES 5-15 DEG BELOW AVERAGE THROUGH MUCH  
OF NEXT WEEK, THE RESULT OF A STRONG POLAR SURFACE HIGH REMAINING  
LOCKED IN PLACE.  
 
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. A WAVE  
OF LOW PRESSURE 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 ENERGY 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/ROCKIES BY THU AND THE SOUTHERN PLAINS BY FRI. A  
NUMBER OF MODEL SOLUTIONS SUGGEST THE POTENTIAL FOR HEAVY RAIN  
ACROSS THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY BY NEXT  
FRI, BUT GIVEN THE TIME RANGE (DAY 7), CONFIDENCE IS MODEST AT  
BEST.  
 
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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