458  
FXUS02 KWBC 010622  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
122 AM EST SAT DEC 01 2018  
 
VALID 12Z TUE DEC 04 2018 - 12Z SAT DEC 08 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. 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 PACIFIC ENERGY MAY PLAY A ROLE IN  
FORMATION OF A LOW PRESSURE AREA ALONG A FRONT IN THE NORTHERN  
GULF OF MEXICO BY NEXT WEEKEND.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS SHOWED REASONABLY GOOD CONSENSUS DURING THE EARLY PORTION  
OF THE MEDIUM RANGE, AND A MULTI-MODEL DETERMINISTIC BLEND  
(INCLUDING THE ECMWF/GFS/UKMET) WAS USED AS A FORECAST STARTING  
POINT DURING DAYS 3-5 (TUE-THU). MULTIPLE SHORTWAVES WILL TRAVERSE  
AN ACTIVE NORTHERN STREAM DURING THIS PERIOD, THE STRONGEST OF  
WHICH SHOULD REACH THE NORTHERN PLAINS WED NIGHT/THU. MODELS  
CONTINUE TO SHOW TIMING/AMPLITUDE DIFFERENCES HERE, BUT THESE  
SHOULD BE RESOLVED REASONABLY WELL BY THE MULTI-MODEL BLEND.  
FARTHER SOUTH, CONSENSUS IS ALSO REASONABLY GOOD WITH THE CUTOFF  
PACIFIC ENERGY AS IT MOVES INTO CALIFORNIA AND BEGINS TO OPEN UP  
BY THU.  
 
SPREAD INCREASES BY DAYS 6-7 (FRI-SAT) AS THE SOUTHERN STREAM  
SYSTEM CROSSES THE SOUTHWEST AND REACHES THE SOUTHERN PLAINS/LOWER  
MISSISSIPPI VALLEY. MODELS SHOW THE POTENTIAL FOR SOME DEGREE OF  
INTERACTION/PHASING WITH NORTHERN STREAM ENERGY DURING THIS TIME,  
BUT AS WITH ANY SUCH SITUATION AT THIS TIME SCALE, MODELS ARE  
STRUGGLING TO RESOLVE THE MOST LIKELY SCENARIO GIVEN THE LARGE  
NUMBER OF POTENTIAL OUTCOMES. THERE APPEARS TO BE SOME DEGREE OF  
ENSEMBLE SUPPORT FOR FORMATION OF A WAVE OF LOW PRESSURE ALONG THE  
FRONTAL BOUNDARY IN THE NORTHERN GULF OF MEXICO BY SAT, WITH COLD  
HIGH PRESSURE NOSING INTO THE MIDWEST/NORTHEAST NORTH OF THE COLD  
FRONT. FARTHER WEST, ADDITIONAL SHORTWAVE ENERGY WILL LIKELY  
APPROACH THE WEST COAST BY NEXT SAT, BUT MODELS SHOW SPREAD AND  
RUN-TO-RUN INCONSISTENCIES ON TIMING/AMPLITUDE AND CHARACTER OF  
THIS ENERGY BASED ON INCREASING SPREAD, WEIGHTING OF ECENS AND  
GEFS ENSEMBLE MEANS WAS BOOSTED TO COMPRISE A MAJORITY OF THE  
FORECAST DURING DAYS 6-7.  
 
   
..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  
WILL RANGE FROM 10 TO AT LEAST 20 DEG F BELOW AVERAGE ON MULTIPLE  
DAYS. 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. 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 AND LOWER MISSISSIPPI VALLEY BY FRI, WITH AREAS OF HEAVY  
RAINFALL POSSIBLE. WINTRY WEATHER MAY ALSO BE A THREAT AT SOME  
LOCATIONS ON THE NORTHERN EDGE OF THE PRECIPITATION AREA,  
DEPENDING ON THE PRECISE LOCATION OF THE COLD FRONT EXPECTED TO  
DROP SOUTHWARD THROUGH THE PLAINS MID-LATE WEEK.  
 
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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