762  
FXUS02 KWBC 311602  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1101 AM EST SUN DEC 31 2017  
 
VALID 12Z WED JAN 03 2018 - 12Z SUN JAN 07 2018  
 
...HEAVY SNOW LIKELY OVER NEW ENGLAND ON THURSDAY WITH CONTINUED  
FRIGID CONDITIONS EAST OF THE ROCKIES...  
 
   
..PATTERN OVERVIEW
 
 
PERSISTENT UPPER TROUGHING WILL REMAIN A FIXTURE IN THE FORECAST  
ACROSS THE EASTERN U.S. WITHIN THIS NEGATIVE HEIGHT ANOMALY...A  
STRENGTHENING SOUTHERN STREAM FEATURE WILL IGNITE A ROBUST AREA OF  
LOW PRESSURE OVER THE WESTERN ATLANTIC WITH A TRACK REACHING NOVA  
SCOTIA BY LATE THURSDAY. IN ITS WAKE...STRONG COLD ADVECTION WILL  
BRING REINFORCING SHOTS OF ARCTIC TO AREAS EAST OF THE ROCKIES.  
THE MID/UPPER-LEVEL PATTERN MAY RELAX INTO THE END OF NEXT WEEKEND  
AS LOWER AMPLITUDE FLOW TRAVERSES TOWARD THE GREAT PLAINS. ACROSS  
THE WEST...AN UPPER RIDGE INITIALLY PARKED OVER THE REGION WILL  
GRADUALLY MOVE EASTWARD REACHING THE ROCKIES BY DAY 6/JANUARY 6.  
THIS IS IN RESPONSE TO LOWERING HEIGHTS ACROSS THE EASTERN PACIFIC  
AS MULTIPLE SYSTEMS TAKE AIM AT THE WEST COAST. THE INITIAL  
SHORTWAVE WILL SHEAR WHILE PUSHING INTO NORTHERN CA ON THURSDAY  
MORNING WHILE A MORE PRONOUNCED TROUGH LIKELY MOVES THROUGH AROUND  
FRIDAY/SATURDAY. THIS PACIFIC ENERGY IS FORECAST TO TRANSLATE  
EASTWARD WHICH ULTIMATELY HELPS DECREASE THE AMPLITUDE OF THE  
MID-LEVEL FLOW ACROSS THE U.S.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
AMPLIFIED FLOW ACROSS THE EASTERN U.S. WILL CONTINUE THROUGHOUT  
MUCH OF THE MEDIUM RANGE PERIOD. A POTENT SHORTWAVE WITHIN THE  
SOUTHERN STREAM IS EXPECTED TO MOVE THROUGH THE FL PANHANDLE  
MIDDAY WEDNESDAY WITH EVENTUAL PHASING IN THE OFFSHORE WATERS.  
MULTI-DAY MODEL COMPARISONS AS WELL AS ANALYSIS OF THE ENSEMBLE  
LOW PLOTS SUGGEST A FAIRLY CONSISTENT FORECAST IN PLACE. THE  
HIGHEST PROBABILITY WOULD BE A CYCLONE TRACK INTO THE VICINITY OF  
NOVA SCOTIA BY THURSDAY EVENING/NIGHT. THERE IS ENOUGH SCATTER  
AMONG THE 90 ENSEMBLE MEMBERS THAT A PUSH TOWARD THE WEST OR EAST  
IS STILL POSSIBLE WHICH WOULD SIGNIFICANTLY CHANGE IMPACTS ACROSS  
THE NORTHEASTERN U.S. AS WAS THE ISSUE THE PAST FEW DAYS...THE 00Z  
UKMET CAME IN MUCH TOO SLOW WHILE THE 00Z CMC EVENTUALLY DEPICTS A  
PERSISTENT DUAL-LOW STRUCTURE BEGINNING THURSDAY EVENING. IN ITS  
WAKE...STRONG COLD ADVECTION WILL ENSUE AS THE NORTHERN STREAM  
TROUGH CROSSES THE GREAT LAKES/NEW ENGLAND. THIS SHOULD ULTIMATELY  
ENSURE A COLD END TO THE WEEK FOR LOCATIONS EAST OF THE ROCKIES.  
TOWARD THE END OF NEXT WEEKEND...A MIXED SIGNAL EXISTS IN THE  
GUIDANCE BUT MODELS DO SEEM TO AGREE ON A RELAXATION OF THE FLOW  
AS PACIFIC AIR MASSES MOVE INTO THE PICTURE.  
 
FOR DAYS 3/4...WEDNESDAY/THURSDAY...MODELS AGREE ON PUSHING THE  
WESTERN U.S. RIDGE FARTHER DOWNSTREAM AS HEIGHTS BEGIN TO LOWER  
ACROSS THE EASTERN PACIFIC. THE INITIAL SHORTWAVE WILL LIKELY LIFT  
NORTHEASTWARD INTO NORTHERN CA ON THURSDAY MORNING BEFORE QUICKLY  
SHEARING. THE GUIDANCE BEGINS TO STRUGGLE BEYOND THIS AS SPLIT  
FLOW SETS UP OVER THE NORTHEASTERN/EASTERN PACIFIC. BY  
05/1200Z...THE 00Z UKMET SHOWS A POWERFUL TROUGH ACROSS NORTHERN  
ALBERTA WHICH ERODES MUCH OF THE INTERMOUNTAIN WEST RIDGE.  
ADDITIONALLY...THE 00Z/12Z ECMWF HAVE BEEN SEVERAL MILLIBARS  
STRONGER WITH THE WAVE MOVING TOWARD BRITISH COLUMBIA WHICH DID  
NOT HAVE A LOT OF ENSEMBLE SUPPORT AT THIS TIME. REGARDING THE  
LONGWAVE TROUGH PROGRESSION...THE ECMWF ENSEMBLE MEAN HAS BEEN THE  
MOST UNSTABLE AS IT CONTINUES TO SLOW RELATIVE TO PRIOR RUNS.  
HOWEVER...IT STILL SITS WAY AHEAD OF THE GLOBAL MEAN CONSENSUS.  
THIS DIFFERENT PROJECTS FORWARD INTO THE WEEKEND AS THE 00Z ECMWF  
ENSEMBLE MEAN IS OVER THE FOUR CORNERS REGION ON SUNDAY WHILE THE  
06Z GEFS/00Z NAEFS ENSEMBLE MEANS FAVOR ZONAL FLOW. ENSEMBLE  
SPAGHETTI PLOTS CONSIST OF A LOT OF SPREAD WHICH DIMINISHES  
CONFIDENCE IN THIS PATTERN CHANGE.  
 
THROUGH DAY 4/THURSDAY...WAS ABLE TO UTILIZE A COMBINATION OF THE  
06Z/00Z GFS WITH THE 00Z ECMWF/CMC WITH MORE WEIGHT TOWARD THE  
GFS/ECMWF SOLUTIONS. THEREAFTER...REMOVED THE 00Z CMC DUE TO THE  
DUAL-LOW STRUCTURE OVER THE WESTERN ATLANTIC WITH DECREASING  
CONTRIBUTIONS FROM THE 00Z ECMWF AS WELL. DURING THE DAY  
5-7...JANUARY 5-7 PERIOD...PRIMARILY WENT WITH A COMBINATION OF  
THE 06Z/00Z GFS AND 06Z GEFS/00Z ECMWF/NAEFS ENSEMBLE MEANS. DID  
WEIGHT THE GEFS/NAEFS MORE HEAVILY TO ESTABLISH LOWER AMPLITUDE  
FLOW ACROSS THE SOUTHWESTERN U.S. LATE IN THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MULTIPLE ARCTIC INTRUSIONS WILL REINFORCE THE FRIGID TEMPERATURES  
EAST OF THE ROCKIES. THE GREAT PLAINS COULD SEE SOME RELIEF BY  
NEXT WEEKEND AS THE UPSTREAM RIDGES SLIDES OVERHEAD.  
UNFORTUNATELY...BRUTALLY COLD TEMPERATURES SHOULD BE THE NORM  
ACROSS THE EASTERN U.S. ADDITIONALLY...GUSTY WINDS SHOULD LOWER  
WIND CHILL VALUES INTO THE UNCOMFORTABLE AND LIKELY DANGEROUS  
RANGES FOR ANY LONG-TERM EXPOSURES. THROUGHOUT THE FORECAST  
PERIOD. GENERALLY SPEAKING...EXPECTS HIGHS/LOWS WILL BE AROUND 20  
TO 25 DEGREES BELOW CLIMATOLOGY WITH POSSIBLY SOME 30 DEGREE  
DEPARTURES MIXED IN. A NUMBER OF DAILY TEMPERATURE RECORDS WILL  
LIKELY BE BROKEN FOR BOTH OVERNIGHT LOWS AS WELL AS THE COLD  
MAXIMA. COMPARED TO FORECAST TEMPERATURES...THE DAY WHERE MANY  
RECORDS MAY BE BROKEN IS ON FRIDAY AS HIGHS STRUGGLE TO GET OUT OF  
THE SINGLE DIGITS AND TEENS ALONG THE I-95 CORRIDOR OVER NEW  
ENGLAND/MID-ATLANTIC. THIS ARCTIC BLAST SHOULD ALSO AFFECT THE  
SOUTHERN TIER OF THE U.S. AS OVERNIGHT READINGS DROP BELOW  
FREEZING DOWN ALONG THE ENTIRE GULF COAST INTO CENTRAL FL ON  
FRIDAY MORNING. ACROSS THE WESTERN HALF OF THE COUNTRY...MILD  
CONDITIONS ARE FORECAST TO PERSIST WITH ANOMALIES IN THE 10 TO 15  
DEGREE RANGE. ABOVE NORMAL READINGS SHOULD REACH THE GREAT PLAINS  
BY NEXT WEEKEND.  
 
ACROSS THE EASTERN U.S...A POTENTIAL WINTER STORM IS ON THE  
HORIZON FOR NEW ENGLAND GIVEN THE FAVORABLE WESTERN ATLANTIC LOW  
TRACK. BOUNDARY LAYER TEMPERATURES SHOULD BE PLENTY COLD TO KEEP  
THIS AN ALL SNOW EVENT OVER ALL OF NEW ENGLAND AND POSSIBLY DOWN  
INTO THE COASTAL UPPER MID-ATLANTIC. ANY WESTWARD MOVEMENT OF  
FUTURE TRACKS WOULD LEAD TO POTENTIAL IMPACTS FARTHER WEST INTO  
THE NORTHEASTERN U.S. OTHERWISE...EXPECT LINGERING RAINFALL ACROSS  
FL AS THE FRONTAL ZONE EXITS THE REGION. OVER THE GREAT  
LAKES...PERSISTENT CYCLONIC WILL FAVOR CONTINUED LAKE EFFECT SNOW  
GENERATION. VERY COLD TEMPERATURES ALOFT SHOULD SIGNIFICANTLY  
STEEPEN LAPSE RATES LEADING TO FAIRLY IMPRESSIVE LOCAL UPDRAFTS.  
OUT WEST...THE MULTITUDE OF SYSTEMS MOVING INLAND WILL FAVOR  
ACTIVE PRECIPITATION CHANCES ALONG THE WEST COAST AND INTO THE  
INTERIOR DURING MUCH OF THE PERIOD. THIS WILL BRING WELL NEEDED  
RAINFALL TO SOUTHERN/CENTRAL WHICH HAS BEEN DRY MUCH OF ITS RAINY  
SEASON. EVENTUALLY SOME WINTRY PRECIPITATION WILL MOVE INTO THE  
HIGHER ELEVATIONS OF THE SIERRA AS WELL AS THE INTERIOR MOUNTAIN  
RANGES.  
 
RUBIN-OSTER  
 

 
 
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