519  
FXUS02 KWBC 090655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 AM EST SAT DEC 09 2017  
 
VALID 12Z TUE DEC 12 2017 - 12Z SAT DEC 16 2017  
 
...A MIDWEST/GREAT LAKES THROUGH NORTHEASTERN U.S. HEAVY SNOW  
THREAT...  
 
   
..PATTERN OVERVIEW/PREDICTABILITY ASSESSMENT/PREFERENCES
 
 
WHAT APPEARS TO BE A SEMI-PERMANENT FEATURE TO START THE MONTH OF  
DECEMBER...A BROAD LONGWAVE TROUGH CONTINUES TO ENCOMPASS REGIONS  
EAST OF THE GREAT PLAINS. A SERIES OF EMBEDDED PERTURBATIONS IN  
THE FLOW WILL DRIVE WAVES OF LOW PRESSURE WHICH SHOULD BE CONDUITS  
TO WINTRY PRECIPITATION ANYWHERE FROM THE UPPER MIDWEST THROUGH  
THE GREAT LAKES INTO THE NORTHEASTERN U.S. ONE SUCH CYCLONE IS  
FORECAST TO BE IN THE PROCESS OF LIFTING AWAY FROM THE UPPER  
MID-ATLANTIC TOWARD NEW ENGLAND ON TUESDAY. THE NEXT REINFORCING  
SHOT OF ARCTIC AIR WILL BRING ADDITIONAL LOW PRESSURE THROUGH THE  
GREAT LAKES/OH VALLEY ON THURSDAY MORNING. TOWARD THE END OF THE  
PERIOD...DECEMBER 15/16...WHILE CYCLONIC FLOW REMAINS OVER THE  
EASTERN HALF OF THE NATION...THERE ARE HINTS AT THE FLOW RELAXING  
A BIT BASED ON SOME OF THE ENSEMBLE SPAGHETTI PLOTS. LOOKING FROM  
THE ROCKIES WESTWARD...AN UPPER RIDGE WILL REMAIN A FIXTURE IN THE  
FORECAST FOR MUCH OF THE PERIOD. 00Z GFS 500-MB HEIGHT ANOMALIES  
ARE AROUND 2 TO 2.5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY ALTHOUGH  
A PATTERN CHANGE IS SUGGESTED BY SOME OF THE GUIDANCE BY NEXT  
WEEKEND. BEFORE THIS OCCURS...A SHORTWAVE TRACKING ATOP THIS RIDGE  
WILL DIVE SOUTHEASTWARD THROUGH THE CENTRAL HIGH PLAINS WHILE  
EVENTUALLY JOINING THE PARENT DOWNSTREAM TROUGH. MID/UPPER-LEVEL  
HEIGHTS REBOUND QUICKLY ACROSS THE WESTERN STATES WITH THE  
EVENTUAL POTENTIAL FOR FEATURES OVER THE EASTERN PACIFIC TO BRING  
MORE SUBSTANTIAL HEIGHT FALLS INTO THE PACIFIC NORTHWEST BY DAY  
6/7...DECEMBER 15/16. LOOKING DOWN INTO THE SUBTROPICAL  
PACIFIC...A PERSISTENT UPPER LOW IS FORECAST TO DRIFT FROM  
NORTHWESTERN MX BACK INTO THE EASTERN PACIFIC AS IT BECOMES  
FURTHER CUT OFF FROM THE MID-LATITUDE FLOW.  
 
REGARDING THE STRONG SURFACE CYCLONE MOVING FROM THE LOWER GREAT  
LAKES TOWARD NOVA SCOTIA ON TUESDAY/WEDNESDAY...ENSEMBLE LOW PLOTS  
HAVE SHOWN MUCH STRONGER CLUSTERING IN THE 12Z MODEL CYCLE. 00Z  
MODELS HAVE REMAINED FAIRLY CLOSE TO THIS CONSENSUS SOLUTION WITH  
SURFACE PRESSURES IN THE 990-995 MB RANGE ON 12/1200Z. SOME  
GRADUAL INTENSIFICATION IS EXPECTED DURING THE FOLLOWING 24 HOURS  
WITH ADDITIONAL WAVE DEVELOPMENT OFF THE NEW ENGLAND COAST. STRONG  
COLD ADVECTION IN THE WAKE WILL BRING A SIGNIFICANT SHOT OF COLD  
AIR TO MUCH OF THE NORTHEASTERN QUARTER OF THE COUNTRY. BY  
MID-WEEK...1000-500 MB THICKNESSES DROP TO AROUND 500 DM ACROSS  
INTERIOR NY. LOOKING FARTHER UPSTREAM...FURTHER HEIGHT FALLS WILL  
BRING A PAIR OF SURFACE WAVES THROUGH THE GREAT LAKES/OH  
VALLEY...RESPECTIVELY. THERE IS SOME VARIABILITY IN WHETHER THERE  
WILL BE A SINGLE OR PAIR OF WAVES. WHILE THE PAST FOUR RUNS OF THE  
GFS AGREE ON ONLY A NORTHERN WAVE...THE 12Z/00Z ECMWF PREFER ONLY  
A MORE SOUTHERN WAVE. RECENT RUNS OF THE CMC/UKMET SEEM TO FLICKER  
OVER THE NUMBER OF CYCLONES PRESENT. NOT A TRIVIAL FORECAST...BUT  
AT LEAST AGREE ON THE PATTERN AT HAND. THIS CONGLOMERATE OF WAVES  
WILL EVENTUALLY REACH THE NORTHEASTERN U.S. BY DAY 6/FRIDAY.  
TOWARD THE END OF THE PERIOD...THE 00Z GFS/12Z ECMWF ARE AT ODDS  
WITH ONE ANOTHER WITH LONGWAVE TROUGH PLACEMENT AS THE LATTER IS  
MUCH QUICKER AS REFLECTED BY ITS ENSEMBLE MEAN. BACK OVER THE  
WESTERN U.S...ALL SOLUTIONS AGREE ON MAINTAINING A STRONG POSITIVE  
HEIGHT ANOMALY FOR A MAJORITY OF THE FORECAST PERIOD. WHERE THEY  
DO DIFFER IS WITH THE HANDLING OF THE MENTIONED SHORTWAVE MOVING  
TOWARD THE CENTRAL HIGH PLAINS ON WEDNESDAY MORNING. AS HAS BEEN  
THE CASE IN RECENT MODEL CYCLES...THE ECMWF CONTINUES TO FAVOR A  
SOUTHWARD TRAJECTORY INTO THE SOUTHWESTERN U.S. WHICH IS SHOWN IN  
NO OTHER MODEL SUITE. WILL SUGGEST THIS IS OUTLYING UNTIL OTHERS  
JOIN THE ECMWF SOLUTION. BY THE DECEMBER 15-16 TIMEFRAME...THE  
GUIDANCE SHOW SOME POTENTIAL FOR AMPLIFIED FLOW TO ERODE THE  
NORTHERN EXTENT OF THE RIDGE. HOWEVER...SUCH A SCENARIO IS NOT  
AGREED UPON YET BUT DOES REMAIN PLAUSIBLE GIVEN THE NUMBER OF  
ENSEMBLE MEMBERS WHICH SHOW IT.  
 
REGARDING THE MODEL PREFERENCE...INITIALLY FAVORED A 3-WAY  
COMBINATION OF THE 18Z GFS AND 12Z ECMWF/UKMET WITH MORE EMPHASIS  
ON THE FORMER TWO MODELS. THIS HELPED MAINTAIN A DEEPER SOLUTION  
DURING THE DAY 3/4...DECEMBER 12/13 PERIOD. AS UNCERTAINTY GREW  
WITH DETAILS OF THE UPSTREAM SYSTEM MOVING THROUGH THE GREAT  
LAKES...GRADUALLY INCREASED ENSEMBLE MEAN INFLUENCES FROM THE 18Z  
GEFS MEAN AND 12Z NAEFS/ECMWF ENSEMBLE MEANS. DID KEEP 30 PERCENT  
OF THE OPERATIONAL MODELS...18Z GFS/12Z ECMWF...INTO DAY 7 GIVEN  
DECENT SYNOPTIC-SCALE AGREEMENT. THIS ALLOWS FOR GENERAL  
NORTHWESTERLY FLOW ALOFT OVER THE PACIFIC NORTHWEST AS THE RIDGE  
ERODES A BIT.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
GIVEN THE STAGNANT HIGH-AMPLITUDE PATTERN IN PLACE...PRONOUNCED  
TEMPERATURE ANOMALIES WILL RULE THE COUNTRY. HOWEVER...EXPECTED  
HIGHS/LOWS DO NOT SEEM TO BE EXTREME ENOUGH TO BREAK ANY  
WIDESPREAD DAILY TEMPERATURE RECORDS. GENERALLY  
SPEAKING...READINGS SHOULD BE AROUND 10 TO POSSIBLY 20 DEGREES  
ABOVE CLIMATOLOGY OVER THE WESTERN HALF OF THE U.S. THE EXCEPTION  
APPEARS TO BE AREAS OF THE PACIFIC NORTHWEST/CENTRAL GREAT BASIN  
WHICH WILL LIKELY HAVE A PERSISTENT LOW-LEVEL TEMPERATURE  
INVERSION THUS KEEPING TEMPERATURES CLOSER TO AVERAGE. FROM THE MS  
RIVER VALLEY EASTWARD...THE INFLUENCE OF ARCTIC AIR IN THE  
FORECAST WILL KEEP HIGHS 10 TO 20 DEGREES BELOW AVERAGE. THE  
COLDEST AIR WILL BE MOVE THROUGH WEDNESDAY AS HIGHS STRUGGLE TO  
GET ABOVE THE FREEZING MARK OVER THE LOWER OH VALLEY/CENTRAL  
APPALACHIANS/MID-ATLANTIC.  
 
SNOWFALL WILL BE COMMONPLACE ANYWHERE FROM THE UPPER MIDWEST  
THROUGH THE GREAT LAKES INTO THE CENTRAL/NORTHERN APPALACHIANS AND  
NEW ENGLAND. PERSISTENT CYCLONIC FLOW WILL KEEP THE LAKE EFFECT  
TRAIN GOING ALTHOUGH SOME BREAKS ARE LIKELY GIVEN SHIFTS IN THE  
PREVAILING LOW-LEVEL WIND DIRECTION. THE INITIAL SYSTEM ON  
TUESDAY/WEDNESDAY COULD BRING MODERATE/HEAVY SNOW TO AREAS OF NEW  
ENGLAND...PARTICULARLY FARTHER INLAND. ELSEWHERE...A RETURN OF  
PRECIPITATION IS LIKELY LATE IN THE PERIOD OVER THE PACIFIC  
NORTHWEST AS THE UPPER RIDGE POSSIBLY BUCKLES.  
 
RUBIN-OSTER  
 

 
 
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