096  
FXUS02 KWBC 101559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST SUN DEC 10 2017  
 
VALID 12Z WED DEC 13 2017 - 12Z SUN DEC 17 2017  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS AND ENSEMBLES REASONABLY AGREE ALOFT WITH MAINTAINENCE OF A  
HIGHLY AMPLIFIED WRN NOAM RIDGE AND ERN NOAM TROUGH PATTERN INTO  
WED/THU. THIS PATTERN TRANSITIONS/DE-AMPLIFIES AND BECOMES  
INCREASINGLY PROGRESSIVE FRI-NEXT WEEKEND AS A SERIES OF ENERGETIC  
SYSTEMS WORK HEIGHT FALLS THROUGH THE WRN NOAM RIDGE POSITION. WPC  
CONTINUITY IS GOOD WITH THIS LARGER SCALE SCENARIO THAT OFFERS  
ABOVE NORMAL PREDICTABILITY. FORECAST PREDICTABILITY AND WPC  
CONTINUITY IS NOT AS GOOD WITH THE TIMING AND STRENGTH OF A SERIES  
OF SHORTWAVES EMBEDDED WITHIN THE OVERALL FLOW AND THE LOWER  
ATMOSPHERIC SYSTEM/WEATHER FOCUS RESPONSE. THIS IS HARD TO RESOLVE  
AT MEDIUM RANGE TIME SCALES AND VARIANCE HAS BEEN QUITE EVIDENT AT  
SHORTER TIME FRAMES.  
 
A MAIN SURFACE SYSTEM AND WINTER WEATHER THREAT DIFFERENCE WED/THU  
CONCERNS THE SEVERAL POTENTIAL LOWS THAT WOULD FOCUS SWATHS OF  
ENHANCED COLD AIR SNOWS FROM THE UPPER MIDWEST/GREAT LAKES THROUGH  
THE OH VALLEY/NRN MID-ATLANTIC AND NORTHEAST. SYSTEM PROGRESSIONS  
THROUGH THE FLOW AND MODEST MOISTURE SHOULD LIMIT QPF DESPITE  
FAVORABLE UPPER DYNAMICS. THE 00 UTC ECMWF/UKMET AND ECMWF  
ENSEMBLES OFFER A MORE DEFINED LEAD IMPULSE WED/THU THAN THE 00/06  
UTC GFS/GEFS AND 00 UTC CANADIAN THAT FOCUS ON AN UPSTREAM SYSTEM.  
WHILE BOTH SYSTEMS HAVE CREDANCE GIVING CURRENT SATELLITE IMAGERY  
SHOWING THEM OVER THE NERN PACIFIC...HAVE LEANED WPC GUIDANCE IN  
FAVOR OF THE LEAD SYSTEM THAT SEEMS TO BETTER FIT THE LARGER SCALE  
FLOW PATTERN. LATER...WPC PROGS OPT FOR A BLENDED COMPOSITE OF THE  
QUITE WELL CLUSTERED 06 UTC GEFS AND 00 UTC NAEFS/ECMWF ENSEMBLES  
IN LUE OF MUCH MORE RUN TO RUN INCONSISTENT DETAILS OF  
DETERMINISTIC MODEL GUIDANCE IN TRANSITIONAL FLOW WHOSE  
INCREASINGLY PROGRESSIVE/LESS AMPLIFIED FLOW PORTENDS AN INCREASE  
IN UNCERTAINTY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WELL BELOW AVERAGE TEMPERATURES WILL BE THE NORM EARLY ON EAST OF  
THE MS RIVER AS HIGHS STRUGGLE TO GET ABOVE THE FREEZING MARK FROM  
THE LOWER OH VALLEY/SOUTHERN APPALACHIANS/MID-ATLANTIC NORTHWARD.  
WITHIN THE HEART OF THE COLDER AIR...WEDNESDAY MORNING  
TEMPERATURES MAY BE BELOW 0 DEGREES ACROSS THE UPPER MIDWEST/GREAT  
LAKES. ON WEDNESDAY...GUSTY WINDS WILL ALSO BE POSSIBLE OVER THE  
NORTHEASTERN U.S. GIVEN THE TIGHT PRESSURE GRADIENT IN PLACE.  
GRADUALLY TEMPERATURES WILL REBOUND WHILE RETURNING TO NEAR  
CLIMATOLOGY BY NEXT WEEKEND. LOOKING FROM THE WEST COAST TO THE  
GREAT PLAINS...ABOVE NORMAL TEMPERATURES...GENERALLY IN THE 10 TO  
15 DEGREE RANGE ARE EXPECTED THROUGH FRIDAY. THE EXCEPTION WILL BE  
OVER SECTIONS OF THE PACIFIC NORTHWEST/CENTRAL GREAT BASIN/UPPER  
INTERMOUNTAIN WEST GIVEN A PERSISTENT LOW-LEVEL TEMPERATURE  
INVERSION WHICH SHOULD KEEP TEMPERATURES MUCH COOLER. THE EROSION  
OF THE UPPER RIDGE WILL REDUCE TEMPERATURE ANOMALIES INTO NEXT  
WEEKEND.  
 
INITIALLY...A WIDESPREAD THREAT OF LIGHT/MODERATE SNOW WILL BE  
POSSIBLE ANYWHERE FROM THE UPPER MIDWEST THROUGH THE GREAT  
LAKES/OH VALLEY AND INTO NEW ENGLAND. THIS SHOULD PERSIST THROUGH  
AT LEAST FRIDAY WITH PERHAPS A BETTER FOCUS NEAR THE PARENT  
SURFACE LOWS ARE WITHIN ANY PRONOUNCED MESOSCALE BANDING DOWNWIND  
OF THE GREAT LAKES. EVENTUALLY A RETURN TO A WET PATTERN IS IN  
STORE ACROSS THE PACIFIC NORTHWEST EASTWARD INTO THE UPPER  
INTERMOUNTAIN WEST/NORTHERN ROCKIES FROM DECEMBER 15-17. SOME  
HEAVIER ACTIVITY IS POSSIBLE OVER THE PACIFIC NORTHWEST LATER NEXT  
WEEKEND AS QUASI-ZONAL FLOW BECOMES ESTABLISHED. ACROSS THE  
SOUTHERN U.S...SOME MOISTURE RETURN AHEAD OF THE TROUGH SWEEPING  
THROUGH THE CENTRAL U.S. MAY BRING RAINFALL TO AREAS FROM THE  
ARKLATEX EASTWARD TO THE SOUTHERN APPALACHIANS.  
 
SCHICHTEL/RUBIN-OSTER  
 

 
 
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