255  
FXUS02 KWBC 100626  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
125 AM EST SUN DEC 10 2017  
 
VALID 12Z WED DEC 13 2017 - 12Z SUN DEC 17 2017  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DEEP-LAYERED UPPER TROUGHING WILL REMAIN A FIXTURE IN THE FORECAST  
THROUGH FRIDAY ACROSS THE NORTHEASTERN QUARTER OF THE COUNTRY. THE  
LEAD SYSTEM WHICH IS EXPECTED TO BECOME NEGATIVELY-TILTED OVER NEW  
ENGLAND WILL BRING A REINFORCING SHOT OF ARCTIC AIR TO AREAS EAST  
OF THE MS RIVER. A PAIR OF DEEP SURFACE CYCLONES SHOULD BE IN THE  
PROCESS OF EXITING INTO FAR EASTERN CANADA WITH CONSIDERABLE  
STRENGTHENING EXPECTED ACROSS THE HIGHER LATITUDES. MEANWHILE...IN  
THE WAKE...SOME TRAILING MID-LEVEL PERTURBATIONS ARE FORECAST TO  
MOVE THROUGH THE GREAT LAKES/OH VALLEY WITH A PAIR OF WEAKER LOWS  
MOVING TOWARD THE NORTHEASTERN U.S. BY EARLY FRIDAY. THE  
SYNOPTIC-SCALE FLOW AT THAT POINT FLATTENS CONSIDERABLY WHICH WILL  
BRING AN END TO THE SEVERAL DAY PERIOD OF WELL BELOW NORMAL  
TEMPERATURES. ACROSS THE WESTERN U.S...THE INITIAL SET UP ON  
WEDNESDAY FEATURES THE EVER PERSISTENT UPPER RIDGE WHILE BEING  
BOUNDED BY A BROAD LOW OVER ALONG THE INTERNATIONAL BORDER OF  
AZ/NM WITH MX. THIS LATTER FEATURE SHOULD DRIFT SOUTH AND  
WEST...THUS BECOMING FURTHER DETACHED FROM THE PREVAILING  
WESTERLIES. TO THE NORTH...PROGRESSIVE HEIGHT FALLS REACHING THE  
PACIFIC NORTHWEST ON FRIDAY WILL BRING AN END TO THE STAGNANT  
UPPER RIDGE ANCHORED TO THE WEST COAST. THIS SYSTEM IS FORECAST TO  
ACCELERATE EASTWARD TOWARD THE MIDDLE OF THE COUNTRY ALTHOUGH  
THERE ARE HINTS IN SOME OF THE GUIDANCE THAT AN UPPER LOW MAY PEEL  
OFF AND LINGER OVER THE DESERT SOUTHWEST NEXT WEEKEND. BY DECEMBER  
17...GENERAL WEST-NORTHWESTERLY FLOW ALOFT WILL BE THE ESTABLISHED  
PATTERN ACROSS THE WEST WHILE AN UPPER RIDGE SITS OFFSHORE OVER  
THE EASTERN PACIFIC.  
 
EARLY IN THE PERIOD...THERE IS VERY LITTLE UNCERTAINTY IN THE  
FORECAST AS ALL SOLUTIONS DEPICT A PAIR OF CYCLONES LIFTING INTO  
EASTERN CANADA ON WEDNESDAY. WHERE THEY SEEM TO DIFFER IS  
REGARDING THE EVOLUTION OF THE NEXT LOW-AMPLITUDE WAVE PUSHING  
TOWARD THE GREAT LAKES/OH VALLEY. THE UNCERTAINTY RESIDES AROUND  
WHETHER THERE WILL BE A SINGLE OR PAIR OF LOWS WITH ADDITIONAL  
ISSUES WITH WHETHER THE NORTHERN/SOUTHERN ONE WILL DOMINATE. THE  
PAST SEVERAL RUNS OF THE GFS CONTINUE TO FAVOR A WAVE OVER THE  
GREAT LAKES WHICH IS WHERE OVER 80 PERCENT OF ITS ENSEMBLES  
RESIDE. THE CMC TO VARYING DEGREES SITS WITHIN THE GFS CAMP  
ALTHOUGH THE 00Z RUN MOVED SOUTHWARD. MEANWHILE...THE PREVIOUS  
THREE RUNS OF THE ECMWF ONLY FOCUS ON THE SOUTHERN MOST LOW MOVING  
THROUGH THE OH VALLEY WITH SUPPORT FROM THE UKMET SOLUTIONS.  
THUS...IT REMAINS A BATTLE OF THE GFS/CMC VERSUS THE ECMWF/UKMET.  
HOWEVER...THE PATTERN MOVING INTO NEXT WEEKEND IS AGREED UPON WITH  
MID-LEVEL HEIGHTS RELAXING AS THE FLOW DEAMPLIFIES. OUT WEST...THE  
PRIMARY ISSUES INVOLVE THE EROSION OF THE UPPER RIDGE AND HOW THAT  
OCCURS. AS MENTIONED EARLIER...THERE ARE SOME  
SOLUTIONS...PRIMARILY THE 00Z/18Z/12Z GFS WHICH PINCH OFF AN UPPER  
LOW ACROSS THE DESERT SOUTHWEST BY SATURDAY MORNING WITH THE  
REMAINDER OF THE HEIGHT FALLS ACCELERATING TOWARD THE CENTRAL U.S.  
ON THE OPPOSITE SIDE OF THE SPECTRUM ARE THE 12Z/00Z ECMWF WHICH  
ARE QUICKER WITH SUPPORT FROM THEIR ENSEMBLE MEMBERS.  
 
GIVEN SOLID OPERATIONAL MODEL AGREEMENT THROUGH DAY  
4/THURSDAY...KEPT AROUND 70 TO 80 PERCENT OF THE 18Z GFS AND 12Z  
ECMWF/CMC IN THE PICTURE TO MAINTAIN SUFFICIENT SURFACE DETAILS.  
GRADUALLY MOVED TOWARD A 50/50 SPLIT OF THE OPERATIONAL/ENSEMBLE  
GUIDANCE BY DAY 5/FRIDAY AS DETAILS IN THE PATTERN BECAME MORE  
UNCERTAIN ACROSS THE GREAT LAKES/OH VALLEY/NORTHEASTERN U.S. GIVEN  
LARGE-SCALE DIFFERENCES OVER THE WESTERN/CENTRAL U.S. ON DAYS  
6/7...DECEMBER 16/17...WENT WITH UP TO 70 PERCENT ENSEMBLE  
MEANS...18Z GEFS/12Z ECMWF ENSEMBLE TO CONCLUDE THE FORECAST  
PERIOD.  
 
   
..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. AND 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.  
 
RUBIN-OSTER  
 

 
 
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