950  
FXUS02 KWBC 180641  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
141 AM EST MON DEC 18 2017  
 
VALID 12Z THU DEC 21 2017 - 12Z MON DEC 25 2017  
 
...ABOVE AVERAGE CONFIDENCE IN THE POTENTIAL FOR AN OUTBREAK OF  
ARCTIC AIR ACROSS THE CENTRAL U.S. BY THE WEEKEND...  
 
   
..OVERVIEW
 
 
CONFIDENCE REMAINS ABOVE AVERAGE FOR A MAJOR PATTERN CHANGE ACROSS  
NORTH AMERICA DURING THE MEDIUM RANGE. A SPLIT FLOW PATTERN EARLY  
IN THE PERIOD (THU) WILL QUICKLY UNDERGO A SIGNIFICANT  
AMPLIFICATION, WITH AN EASTERN PACIFIC RIDGE AXIS EXTENDING FROM  
JUST OFF THE U.S. WEST COAST NORTH TO THE ARCTIC, AND A DOWNSTREAM  
MEAN TROUGH FROM NEAR HUDSON BAY SOUTHWEST INTO THE CONUS. THIS  
SETUP WILL RESULT IN A FLOW OF INCREASINGLY COLD AIR SOUTHWARD  
THROUGH CANADA, MOST LIKELY DIRECTED INTO THE ROCKIES/PLAINS/UPPER  
MIDWEST. TELECONNECTIONS ASSOCIATED WITH THE AREA OF MAXIMUM  
POSITIVE HEIGHT ANOMALIES ACROSS NORTHWEST CANADA (+2 TO +3  
STANDARD DEVIATIONS ON THE GEFS MEAN) SUPPORT THE IDEA SHOWN BY  
MOST GUIDANCE, WITH THE NORTHERN/CENTRAL PLAINS AND UPPER MIDWEST  
THE FAVORED AREAS FOR THE COLDEST TEMPERATURES BY THE WEEKEND.  
FARTHER EAST, THE SUBTROPICAL RIDGE IS FORECAST TO STRENGTHEN OVER  
THE NEXT WEEK, CENTERED OVER THE BAHAMAS OR SOUTH FLORIDA (ALSO  
SUPPORTED BY TELECONNECTIONS FROM THE NORTH PACIFIC RIDGE). THE  
CONTRAST BETWEEN THE FLOW OF ARCTIC AIR SOUTHWARD INTO THE CENTRAL  
U.S. AND WARM, MOIST SUBTROPICAL AIR TRANSPORTED NORTHWARD INTO  
THE SOUTHERN/SOUTHEASTERN U.S. WILL SUPPORT A PERSISTENT FRONTAL  
ZONE FROM THE EXTREME SOUTHERN PLAINS/NORTHERN MEXICO  
EAST-NORTHEAST TO NEAR THE SOUTHEAST COAST. EXPECT THIS FRONTAL  
ZONE TO SERVE AS A FOCUS FOR POTENTIAL ROUNDS OF PRECIPITATION AS  
LONG AS THIS PATTERN PERSISTS, WITH SOME POTENTIAL FOR OCCASIONAL  
WINTER WEATHER ON THE NORTHERN FRINGES OF THIS AREA GIVEN A RATHER  
SHARP THERMAL GRADIENT.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS SHOWED SUFFICIENT CONSENSUS EARLY IN THE MEDIUM RANGE TO  
SUPPORT THE USE OF A MAJORITY DETERMINISTIC BLEND INCLUDING THE  
12Z ECMWF/18Z GFS. THESE SOLUTIONS SHOWED MODEST TIMING AND  
STRENGTH DIFFERENCES DURING DAYS 3-5 (THU-SAT) IN ASSOCIATION WITH  
THE SHORTWAVE/UPPER LOW EXITING FROM THE FOUR CORNERS REGION INTO  
THE PLAINS, AND WERE GENERALLY WELL-CLUSTERED WITH THE ENSEMBLE  
MEANS. THE 12Z UKMET WAS INCLUDED IN THE BLEND ON DAY 3, BUT WAS  
EXCLUDED AFTER THAT TIME DUE TO DIFFERENCES AT THE SURFACE ACROSS  
THE PLAINS. AS THE SURFACE CYCLONE DEEPENS ACROSS THE GREAT LAKES  
LATE FRI INTO SAT, THE GFS AND ECMWF REMAINED VERY WELL CENTERED  
WITHIN THE SPREAD, BUT SHOWED SOME FAIRLY SIGNIFICANT DIFFERENCES  
IN TERMS OF INTENSITY (WITH THE ECMWF 10-12 HPA WEAKER THAN THE  
GFS).  
 
FROM LATE DAY 5 ONWARD, THE LARGE SCALE TROUGH BEGINS TO AMPLIFY  
ACROSS THE CENTRAL U.S. A SHORTWAVE MOVING FROM THE PAC NW ON DAY  
5 INTO THE ROCKIES ON DAY 6 (SUN) WILL LOWER HEIGHTS SUFFICIENTLY  
TO BEGIN CARVING OUT THE TROUGH, ALLOWING A DEEP UPPER LOW NEAR  
HUDSON BAY TO DRIFT SOUTHWARD TOWARD THE NORTH CENTRAL U.S. MODELS  
STILL SHOW A MODERATE DEGREE OF SPREAD AS TO THE EXACT DETAILS OF  
THIS EVOLUTION, WITH RUN-TO-RUN VARIABILITY IN HOW QUICKLY THE  
UPPER LOW MOVES SOUTHWARD. REGARDLESS, THE FLOW PATTERN WILL  
SUPPORT TRANSPORT OF VERY COLD AIR FROM THE HIGH LATITUDES OF  
CANADA SOUTHWARD INTO THE NORTH CENTRAL U.S. BY DAYS 6-7,  
DETERMINISTIC SOLUTIONS BEGIN TO DIFFER EVEN FURTHER WITH THE  
EVOLUTION OF THE SHORTWAVE ENERGY AS IT MOVES EAST FROM THE  
ROCKIES. THE 12Z ECMWF/18Z GFS, ALONG WITH THEIR RESPECTIVE  
ENSEMBLE MEANS WERE MUCH SLOWER THAN THE 00Z GFS TO MOVE THIS  
ENERGY EAST. LOOKING BACK OVER THE PAST SEVERAL RUNS, MODELS HAVE  
VARIED WITH RESPECT TO THIS FEATURE, AND ANY TRENDS REMAIN  
DIFFICULT TO DISCERN. THUS, ENSEMBLE (12Z ECENS/18Z GEFS) WEIGHTED  
WAS BOOSTED TO A MAJORITY OF THE FORECAST BLEND BY SUN-MON.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT A BAND OF SNOW TO THE NORTHWEST OF LOW PRESSURE TRACKING  
FROM THE CENTRAL PLAINS NORTHEASTWARD THU ONWARD. CURRENTLY THE  
BEST POTENTIAL FOR MEANINGFUL SNOW APPEARS TO BE ACROSS THE UPPER  
MS VALLEY/UPPER GREAT LAKES. SNOW WILL ALSO BE POSSIBLE OVER NEW  
ENGLAND FRI/SAT WHERE WARM ADVECTION WILL BE THE LIKELY MECHANISM.  
THERE REMAINS SOME SPREAD OVER EXACTLY HOW STRONG THIS SYSTEM WILL  
BE, AND THUS FOR PRECISE INTENSITY OF SNOW/WINDS.  
 
THE AREA EXTENDING FROM FROM LOWER MS/TN VALLEYS INTO THE SOUTHERN  
APPALACHIANS HAS THE HIGHEST PROBABILITY OF SEEING THE HEAVIEST  
RAINFALL DURING THE MEDIUM RANGE. THIS WILL BE FROM A COMBINATION  
OF THE COLD FRONT CROSSING THE REGION LATER IN THE WEEK, AND THEN  
WITH OVERRUNNING PRECIP NORTH OF THE FRONT THAT SHOULD SETTLE NEAR  
THE GULF/CAROLINA COASTS BY OVER WEEKEND.  
 
THE CENTRAL U.S. WILL SEE A DRAMATIC REVERSAL FOR TEMPS FROM THU  
INTO THE WEEKEND, AS READINGS AS WARM AS 10-20 DEG F ABOVE NORMAL  
EARLY IN THE PERIOD ARE DISPLACED BY HIGHS AS MUCH AS 20-30 DEG F  
BELOW NORMAL BY NEXT WEEKEND. RELATIVE WARMTH OVER THE EAST WITH  
SOME ANOMALIES EXCEEDING +10 DEG F SHOULD EXTEND THROUGH SAT  
BEFORE BEGINNING TO FADE BY SUN-MON WITH FRONTAL PASSAGE.  
 
RYAN  
 

 
 
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