508  
FXUS02 KWBC 080641  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
140 AM EST FRI DEC 08 2017  
 
VALID 12Z MON DEC 11 2017 - 12Z FRI DEC 15 2017  
 
...A MIDWEST/GREAT LAKES THROUGH NORTHEASTERN U.S. HEAVY SNOW  
THREAT...  
 
   
..PATTERN OVERVIEW/PREDICTABILITY ASSESSMENT/PREFERENCES
 
 
LITTLE CHANGE IN THE SYNOPTIC-SCALE PATTERN IS EXPECTED AS A MEAN  
UPPER TROUGH CONTINUES TO DOMINATE EAST OF THE GREAT PLAINS.  
MULTIPLE EMBEDDED SHORTWAVES WILL ROTATE AROUND WITHIN THE DEEP  
CYCLONIC FLOW WHICH SHOULD FOSTER THE DEVELOPMENT OF NUMEROUS  
SURFACE WAVES. 500-MB HEIGHT ANOMALIES DURING THE PERIOD WILL  
MAXIMIZE IN THE 2 TO 3 STANDARD DEVIATION BELOW AVERAGE RANGE  
WHILE THE OPPOSITE PHASE DOMINATES THE WESTERN U.S. WHILE NOT AS  
STRONG BASED ON ANOMALIES...ITS PERSISTENCE IS WORTH NOTING WITH  
ONLY PERTURBATION PUSHING THROUGH THE UPPER INTERMOUNTAIN WEST BY  
MID-WEEK BEFORE THE RIDGE REBOUNDS IN ITS WAKE. WHILE THE LARGE  
SCALE EVOLUTION IS QUITE PREDICTABLE HERE...DETAILS ARE UNCERTAIN  
WITH THE LONGWAVE TROUGH OUT EAST GIVEN THE NUMBER OF EMBEDDED  
SYSTEMS TO NOTE. EACH SHORTWAVE MAY BE A CONDUIT TO WINTER WEATHER  
THREATS WITH THE REGION ENTRENCHED IN ARCTIC AIR.  
 
WITHIN THE BROAD LONGWAVE TROUGH...A CONVEYOR OF BELT OF  
SHORTWAVES WILL BE CLOSELY MONITORED...PARTICULARLY ONE SUCH  
IMPULSE TRACKING THROUGH THE UPPER MIDWEST/MIDDLE MS VALLEY ON  
11/1200Z. AS HEIGHT FALLS PROJECT EASTWARD...A DISCERNIBLE SURFACE  
WAVE MOVES THROUGH THE LOWER GREAT LAKES/OH VALLEY/CENTRAL  
APPALACHIANS BY THE FOLLOWING DAY. THERE IS PLENTY OF NORTH/SOUTH  
SPREAD WHICH MAKES THIS A CHALLENGING FORECAST GIVEN IMPLICATIONS  
OF WINTRY PRECIPITATION. THE 00Z GFS HAS NOT BUDGED FROM ITS  
PREVIOUS FORECAST WITH A WAVE PLACEMENT OVER FAR WESTERN NY ON  
TUESDAY MORNING. THE 00Z CMC TRENDED SLOWER PULLING ITS SOLUTION  
BACK TO JUST NORTH OF THE 00Z/18Z GFS. THIS MORE NORTHERN SOLUTION  
IS WHERE MOST OF THE GEFS MEMBERS HAVE CONGREGATED WHILE THE 12Z  
ECMWF ENSEMBLE MEMBERS ARE SCATTERED ALL OVER THE PLACE.  
FURTHER...THE 12Z ECMWF AND ITS PRECEDING RUNS HAVE BEEN  
INCONSISTENT ALTHOUGH THERE HAS BEEN A DEFINED SLOWING TREND.  
EVENTUALLY THIS WAVE WILL MOVE OFF THE NEW ENGLAND COAST BY LATE  
TUESDAY/EARLY WEDNESDAY WITH MANY ENSEMBLE MEMBERS SUGGESTING A  
SIGNIFICANT SNOWFALL THREAT TO SOME SECTIONS OF NEW ENGLAND. IN  
ITS WAKE...ADDITIONAL ARCTIC SURGES ARE EXPECTED BUT IT APPEARS  
THESE FURTHER REINFORCING COLD SHOTS WILL BE LESS AMPLIFIED IN  
NATURE. ACROSS THE WESTERN U.S...THE STOUT UPPER RIDGE REMAINS THE  
DOMINANT STORY WITH ONLY A MODEST SHORTWAVE PUSHING INTO THE UPPER  
INTERMOUNTAIN ON WEDNESDAY MORNING BEING AN OUTLIER TO THIS RULE.  
THERE IS SOME UNCERTAINTY WITH THE TRACK IT TAKES LATE IN THE WEEK  
WITH THE 12Z ECMWF DROPPING IT DUE SOUTH WHICH DIVERGES FROM THE  
GENERAL DOWNSTREAM CLUSTER. OTHERWISE...THE UPPER RIDGE IS  
FORECAST TO RE-ESTABLISH ITS PRESENCE FOR THE CONCLUSION OF THE  
WORK WEEK.  
 
KEPT THE FORECAST HEAVILY INFLUENCED BY THE OPERATIONAL MODELS  
THROUGH DAY 5/WEDNESDAY TO ACCOUNT FOR SUFFICIENT CERTAINTY IN THE  
PATTERN. ALSO...THIS WOULD ALLOW FOR BETTER DEPTH OF SURFACE  
FEATURES MOVING THROUGH THE GREAT LAKES AND INTO THE WESTERN  
ATLANTIC. GENERALLY UTILIZED A COMBINATION OF THE 18Z/12Z GFS WITH  
THE 12Z ECMWF/CMC WITH MINOR CONTRIBUTIONS FROM THE ENSEMBLE  
MEANS. BEYOND MID-WEEK...STARTED TO FOCUS MORE OF THE FORECAST ON  
THE 12Z ECMWF/NAEFS ENSEMBLE MEANS AS MODELS DIVERGE WITH DETAILS  
WHILE STILL KEEPING 30 PERCENT OF THE 18Z/12Z GFS AND 12Z ECMWF IN  
THE PICTURE.  
 
   
..SENSIBLE WEATHER/THREATS/HIGHLIGHTS
 
 
GIVEN THE PERSISTENT HIGH AMPLITUDE PATTERN OVER THE  
NATION...ABOVE NORMAL TEMPERATURES WILL REMAIN A FIXTURE ACROSS  
THE WESTERN U.S. WHILE COLD READINGS WILL DOMINATE EAST OF THE  
GREAT PLAINS. WHILE NO WIDESPREAD DAILY TEMPERATURES RECORDS WILL  
BE BROKEN...EXPECT ANOMALIES IN THE 10 TO 15 DEGREES ABOVE/BELOW  
NORMAL TO BE COMMONPLACE OVER THESE RESPECTIVE REGIONS. OVER  
SECTIONS OF THE NORTHWESTERN U.S. INTO THE CENTRAL GREAT  
BASIN...UPPER/SURFACE RIDGING WILL HELP MAINTAIN A SEMI-PERMANENT  
LOW-LEVEL TEMPERATURE INVERSION. THUS...THESE LOCATIONS MAY  
STRUGGLE TO REACH SOME OF THE WARMER TEMPERATURES ACROSS  
SURROUNDING AREAS. OVER THE SOUTHWESTERN U.S...OFFSHORE SURFACE  
GRADIENTS IN RESPONSE TO INTERIOR HIGH PRESSURE MAY BRING FURTHER  
FIRE WEATHER CONCERNS TO SOUTHERN CA. HOWEVER...DETAILS OF THE  
PATTERN ARE STILL SOMEWHAT UNKNOWN. REGARDING PRECIPITATION...THE  
BIG STORY WILL BE THE SNOWFALL THREATS FROM THE UPPER MIDWEST  
THROUGH THE GREAT LAKES/OH VALLEY AND INTO THE NORTHEASTERN U.S.  
AS USUAL...COLD CYCLONIC FLOW OVER THE MILD GREAT LAKES SHOULD  
INVIGORATE WIDESPREAD SNOW SHOWERS DOWNWIND OF ALL THE MAJOR  
LAKES. AND DEPENDING ON THE DEPTH/PLACEMENT OF THE SURFACE LOW...A  
CONSIDERABLE SNOWFALL THREAT WILL BE POSSIBLE ANYWHERE FROM THE  
UPPER MIDWEST TO THE NORTHEASTERN STATES. HOPEFULLY FUTURE MODEL  
RUNS WILL PAINT A MORE CONFIDENT PICTURE IN THE UPCOMING DAYS.  
 
RUBIN-OSTER  
 

 
 
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