329  
FXUS02 KWBC 171600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST SUN DEC 17 2017  
 
VALID 12Z WED DEC 20 2017 - 12Z SUN DEC 24 2017  
 
...ABOVE AVERAGE CONFIDENCE IN THE POTENTIAL FOR AN OUTBREAK OF  
ARCTIC AIR ACROSS INTO THE CENTRAL U.S. BY NEXT WEEKEND...  
 
   
..OVERVIEW
 
 
GUIDANCE CONTINUES TO SHOW A DRAMATIC AMPLIFICATION/SHARPENING OF  
EASTERN PACIFIC RIDGING THROUGH ALASKA/NORTHWEST CANADA INTO THE  
ARCTIC, SUPPORTING THE ESTABLISHMENT OF A DEEP MEAN TROUGH FROM  
NEAR HUDSON BAY SOUTHWEST INTO THE LOWER 48 AND A FLOW OF  
INCREASINGLY COLD AIR SOUTHWARD THROUGH CANADA AND MOST LIKELY  
DIRECTED INTO THE ROCKIES/PLAINS. TELECONNECTIONS RELATIVE TO THE  
POSITIVE HEIGHT ANOMALY CENTER ASSOCIATED WITH THE RIDGE, EVEN  
WITH SOME DISAGREEMENT IN EXACT LOCATION OF THE CENTER IN LATEST  
D+8 CHARTS, AGREE WITH MOST GUIDANCE THAT HIGHLIGHTS THE NORTHERN  
INTO CENTRAL PLAINS AS BEING THE PRIME FOCUS FOR COLDEST  
TEMPERATURE ANOMALIES BY NEXT WEEKEND. BETWEEN THE MEAN TROUGH  
AND A PERSISTENT RIDGE OFF TO THE SOUTHEAST, A FRONTAL ZONE SHOULD  
SET UP FROM THE EXTREME SOUTHERN PLAINS/NORTHERN MEXICO  
EAST-NORTHEAST TO NEAR THE SOUTHEAST COAST. THE COMBINATION OF  
ADVANCING COLD AIR AND MOISTURE FLOW BETWEEN THE UPPER TROUGH AND  
RIDGE SHOULD ULTIMATELY PROMOTE INCREASED POTENTIAL FOR PRECIP OF  
VARIOUS TYPES WHETHER LATE IN THE FORECAST PERIOD OR JUST BEYOND.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONTINUES TO SHOW SOME SPREAD/VARIABILITY WITH THE  
WEAKENING SHORTWAVE/SURFACE REFLECTION CROSSING THE SOUTHERN TIER  
AROUND MIDWEEK. AFTER BEING EARLY TO SIGNAL A TREND TOWARD SLOWER  
PROGRESSION, THE PAST COUPLE ECMWF RUNS HAVE BEGUN TO ACCELERATE A  
BIT AGAIN. AS OF THE 00Z/06Z CYCLES GUIDANCE IS FAIRLY EVENLY  
DISTRIBUTED WITHIN THE TIMING SPECTRUM WHILE THE 00Z ECMWF IS ON  
THE NORTHERN SIDE OF THE SPREAD FOR THE SURFACE WAVE BY LATE WED.  
AN AVERAGE OF LATEST OPERATIONAL GUIDANCE APPEARS BEST GIVEN  
LINGERING UNCERTAINTY FOR THIS RELATIVELY SMALL SCALE FEATURE.  
 
LOOKING AT THE DETAILS OF THE AMPLIFYING TROUGH OVER THE LOWER 48  
AND ASSOCIATED SYSTEMS, IT HAS BECOME EVIDENT OVER THE PAST DAY  
THAT ECMWF-BASED GUIDANCE SEEMS TO BE LEADING THE TRENDS. THE  
CONSENSUS SOLUTION IS FAIRLY STABLE WITH THE INITIAL  
SHORTWAVE/EMBEDDED UPPER LOW MOVING INTO THE WEST WED-THU WITH THE  
TRENDS BECOMING INCREASINGLY NOTICEABLE AFTER THAT TIME.  
SHORTWAVE ENERGY ROUNDING THE UPSTREAM RIDGE AROUND MIDWEEK AND  
DROPPING SOUTHWARD THEREAFTER HAS DEVELOPED MORE SOUTHWESTWARD  
AMPLITUDE SINCE YESTERDAY, ULTIMATELY LEADING TO FASTER EJECTION  
OF THE INITIAL WED-THU SHORTWAVE OVER THE WEST. AT THE SAME TIME  
THIS EVOLUTION SEEMS TO BE HOLDING BACK NORTHERN TIER HEIGHT FALLS  
THUS SLOWING THE SURFACE SYSTEM THAT IS STILL ADVERTISED TO TRACK  
FROM THE CENTRAL PLAINS THROUGH THE GREAT LAKES INTO EASTERN  
CANADA. RECENT GFS/GEFS-BASED GUIDANCE HAS BEEN LAGGING  
ECMWF/ECMWF MEAN TRENDS FOR THESE ASPECTS OF THE FORECAST BY AT  
LEAST ONE 12-HOURLY CYCLE. THE 00Z CMC/UKMET ARE ON THE EXTREME  
SIDE OF THE ENSEMBLE SPREAD FOR HOW AMPLIFIED THE UPSTREAM  
SHORTWAVE ENERGY BECOMES AND APPEAR LOW IN PROBABILITY GIVEN  
TELECONNECTION-FAVORED FLOW BUT WILL HAVE TO BE MONITORED GIVEN  
RECENT TRENDS.  
 
CONTINUITY LOOKS BETTER THAN AVERAGE FOR THE WEAK CLIPPER SYSTEM  
EXPECTED TO BECOME ILL-DEFINED AS IT TRACKS TOWARD THE UPPER GREAT  
LAKES LATE IN THE WEEK. TRENDS FOR FLOW ALOFT OVER THE WEST LEAD  
TO A SLIGHTLY SLOWER SOUTHWARD PUSH OF THE TRAILING COLD FRONT  
OVER THAT PART OF THE COUNTRY. THEN THERE IS DECENT AGREEMENT  
ASIDE FROM TYPICAL DETAIL SPREAD THAT NORTHERN CANADA UPPER LOW  
ENERGY SHOULD STRETCH SOUTHWARD TOWARD THE NORTHERN TIER STATES,  
BRINGING A FRONT WITH EVEN COLDER AIR TO THE NORTHERN PLAINS BY  
EARLY DAY 7 SUN.  
 
BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FORECAST BLEND  
INCORPORATED MOSTLY OPERATIONAL GUIDANCE EARLY-MID PERIOD FOLLOWED  
BY A MORE EVEN MODEL/ENSEMBLE MEAN WEIGHTING FOR NEXT WEEKEND,  
WITH HIGHER PERCENTAGES GIVEN TO THE ECMWF/ECMWF MEAN GUIDANCE  
THAN GFS/GEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SYSTEM INITIALLY CROSSING THE WEST WILL BRING A PERIOD OF  
SNOW/LOW ELEVATION RAIN AND WINDY CONDITIONS WED-THU. THEN EXPECT  
A BAND OF SNOW TO EXTEND TO THE NORTHWEST OF LOW PRESSURE TRACKING  
FROM THE CENTRAL PLAINS NORTHEASTWARD THU ONWARD. CURRENTLY THE  
BEST POTENTIAL FOR MEANINGFUL SNOW SHOULD BE ACROSS THE UPPER MS  
VALLEY/UPPER GREAT LAKES. SOME SNOW MAY EXTEND BACK INTO THE  
PLAINS, AS WELL AS OVER NEW ENGLAND WHERE WARM ADVECTION WILL BE  
THE MORE LIKELY MECHANISM. THERE IS STILL A FAIR AMOUNT OF  
UNCERTAINTY OVER EXACTLY HOW STRONG THIS SYSTEM WILL BE, AND THUS  
FOR PRECISE INTENSITY OF SNOW/WINDS.  
 
FOR THE 5-DAY PERIOD THE AREA EXTENDING FROM FROM EASTERN  
TEXAS/LOWER MS VALLEY INTO THE SOUTHERN APPALACHIANS/CAROLINAS HAS  
THE HIGHEST PROBABILITY OF SEEING THE HEAVIEST RAINFALL. THIS  
WILL BE FROM A COMBINATION OF THE SYSTEM CROSSING THE SOUTHERN  
TIER AROUND WED, THE COLD FRONT CROSSING THE REGION LATER IN THE  
WEEK, AND THEN WITH OVERRUNNING PRECIP AS FLOW ALOFT BECOMES  
NEARLY PARALLEL TO THE FRONT THAT SHOULD SETTLE NEAR THE  
GULF/CAROLINA COASTS.  
 
THE ROCKIES/PLAINS WILL SEE A DRAMATIC REVERSAL FOR TEMPS FROM WED  
INTO THE WEEKEND, AS READINGS AS WARM AS 10-20F ABOVE NORMAL EARLY  
IN THE PERIOD ARE DISPLACED BY HIGHS AS MUCH AS 20-30F BELOW  
NORMAL BY NEXT WEEKEND. ANOMALIES FOR MIN TEMPS SHOULD BE  
SOMEWHAT LESS EXTREME. RELATIVE WARMTH OVER THE EAST WITH SOME  
ANOMALIES EXCEEDING PLUS 10F SHOULD EXTEND THROUGH FRI AND  
POSSIBLY SAT BEFORE FADING BY SUN WITH FRONTAL PASSAGE.  
 
RAUSCH  
 

 
 
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