652  
FXUS02 KWBC 011507  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1004 AM EST FRI DEC 01 2017  
 
VALID 12Z MON DEC 04 2017 - 12Z FRI DEC 08 2017  
   
..BIG PATTERN CHANGE FOR THE CENTRAL/EASTERN STATES
 
 
15Z UPDATE...  
 
CONFIDENCE REMAINS HIGH WITH RESPECT TO A MAJOR LARGE SCALE  
PATTERN CHANGE EXPECTED TO UNFOLD ACROSS NORTH AMERICA OVER THE  
NEXT WEEK. CONFIDENCE IS ABOVE AVERAGE EARLY IN THE PERIOD THAT A  
DEEPENING CYCLONE WILL MOVE FROM THE CENTRAL PLAINS ON DAY 3 (MON)  
INTO THE GREAT LAKES BY DAY 4 (TUE). THE COLD FRONT ASSOCIATED  
WITH THIS CYCLONE WILL BE THE LEADING EDGE OF A MUCH COLDER AIR  
MASS SPREADING INTO THE CENTRAL/EASTERN STATES. MODELS ARE STILL  
STRUGGLING WITH SOME OF THE DETAILS SURROUNDING WHETHER A LOBE OF  
ENERGY ORIGINATING IN THE GULF OF AK OVER THE WEEKEND BECOMES  
QUICKLY ABSORBED IN THE NORTHERN STREAM FLOW/AMPLIFYING TROUGH, OR  
SEPARATES INTO A CUTOFF UPPER LOW ACROSS THE SOUTHWESTERN U.S. OR  
EVEN ACROSS THE EASTERN PACIFIC. THE ECMWF MADE A SUBSTANTIAL  
CHANGE WITH THE 12Z THU RUN FROM A MORE CONSOLIDATED TROUGH TO A  
DEEP CUTOFF LOW PERSISTING ACROSS THE E PAC/SOUTHWEST/NORTHERN  
MEXICO THROUGH DAY 7, AND THE MODEL PERSISTED WITH THIS SOLUTION  
IN THE 00Z RUN. THE GFS HAS BEEN A BIT MORE CONSISTENT FOR SEVERAL  
RUNS, SHOWING AN ELONGATED AREA OF VORTICITY EXTENDING WESTWARD  
INTO THE SOUTHWESTERN STATES FROM THE LARGER TROUGH. ENSEMBLE  
TRENDS HAVE MOVED IN A DIRECTION OPPOSITE OF THE DETERMINISTIC  
ECMWF, WITH MORE ECENS/NAEFS MEMBERS SHIFTING TO A MORE GFS-LIKE  
SOLUTION. GIVEN THAT THE ECMWF HAS BEEN PERSISTENT IN THIS NEW  
SOLUTION FOR TWO CONSECUTIVE RUNS, HOWEVER, CANNOT JUSTIFY  
THROWING IT OUT ENTIRELY. THIS HAS IMPLICATIONS FOR THE EASTERN  
U.S. TROUGH, WITH LESS/NO CUTOFF LIKELY MEANING A MORE AMPLIFIED  
LARGE SCALE EASTERN TROUGH AND THUS COLDER TEMPERATURES BY THE  
MIDDLE OF NEXT WEEK IN A GFS SCENARIO RELATIVE TO THE ECMWF. GIVEN  
THE ABOVE CONSIDERATIONS, THE UPDATED WPC FORECAST WAS INITIALLY  
BASED LARGELY ON A 06Z GFS/00Z ECMWF BLEND, WITH SLIGHTLY HEAVIER  
GFS WEIGHTING RELATIVE TO THE ECMWF BY DAYS 4-5. AFTER DAY 5,  
GIVEN THE CONTINUED SPREAD SURROUNDING WHAT HAPPENS ALOFT ACROSS  
THE SOUTHERN TIER, OPTED FOR HEAVIER ENSEMBLE (00Z ECENS/NAEFS)  
WEIGHTING THROUGH DAY 7.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0534 UTC)...  
   
..OVERVIEW
 
 
THE WELL-ADVERTISED HEMISPHERIC REORGANIZATION HAS BEGUN TO TAKE  
SHAPE AS UPPER RIDGING MOVES THROUGH THE GULF OF ALASKA THIS  
WEEKEND TO THEN TAKE UP RESIDENCE OVER THE PANHANDLE AND WESTERN  
BRITISH COLUMBIA NEXT WEEK. THIS WILL DRIVE DEEP TROUGHING THROUGH  
THE CENTRAL/EASTERN LOWER 48 IN WHAT WILL BE A SHOCK TO THE SYSTEM  
GIVEN THE RECORD OR NEAR RECORD HIGHS SEEN OVER THE EASTERN HALF  
OF THE CONUS THE PAST WEEK. AFTER A ROBUST SYSTEM LIFTS INTO  
CANADA LATE MONDAY AND PUSHES ITS COLD FRONT OFF THE EAST COAST  
LATE WEDNESDAY, MUCH COLDER AND DRIER CANADIAN AIR WILL PERSIST  
EAST OF THE ROCKIES WITH THE EXCEPTION BEING AROUND THE GREAT  
LAKES AND POSSIBLY THE NORTHEAST COAST DEPENDING ON THE EASTWARD  
PROGRESSION OF THE FRONT.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THOUGH THE PAST SEVERAL DAYS HAS SEEN A LARGE AMOUNT OF WAVERING  
WITHIN THE OTHERWISE WELL-PREDICTED SYNOPTIC FLOW, THE RECENT  
GUIDANCE HAVE MOSTLY CONVERGED ENOUGH SO THAT A BLEND OF THE  
RECENT 18Z GFS AND 12Z ECMWF/UKMET/CANADIAN WAS SUFFICIENT AS A  
STARTING POINT THROUGH WEDNESDAY. THE 12Z ECMWF WAS PERHAPS LEAST  
LIKE THE OTHERS IN THE WEST WHERE THE CONTENTION AMONG THE  
GUIDANCE HAS BEEN MOST PREVALENT. THAT RUN WAS MOST AGGRESSIVE  
WITH A SMALL TRAILING VORT MAX DIVING SOUTH OUT OF THE GULF OF  
ALASKA THAT GETS SEPARATED FROM THE TROUGH EXITING THE ROCKIES ON  
MONDAY, THEN LINGERS AND DEEPENS JUST OFF CALIFORNIA LIKE VARIOUS  
PREVIOUS RUNS (FROM VARIOUS MODELS) HAVE SHOWN BUT FROM WHICH THEY  
HAVE TRENDED AWAY. NEVERTHELESS, SPLIT THE DIFFERENCE BETWEEN THE  
LATEST 12Z/30 ECMWF RUN WITH THE OLDER 00Z/30 ECMWF WHICH FIT THE  
GENERAL 12Z/18Z GUIDANCE CLUSTER BETTER AS THE REST OF THE 12Z/30  
ECMWF'S FORECAST WAS MORE IN LINE WITH THE CONSENSUS. INCORPORATED  
MORE ENSEMBLE MEAN WEIGHTING BY NEXT THU/FRI AS DIFFERENCES IN  
TIMING/AMPLITUDE INCREASED, BUT OVERALL ENSEMBLE SPREAD WAS  
RELATIVELY LOW.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PERSISTENT TROUGHING WILL ALLOW ARCTIC AIR TO FUNNEL DOWN THROUGH  
THE HIGH PLAINS AND MIDWEST SOUTHWARD AND EASTWARD NEXT WEEK  
BEHIND THE LEAD COLD FRONT MON/TUE. TEMPERATURES AHEAD OF THE  
FRONT WILL BE ABOUT 10-20 DEGREES ABOVE AVERAGE BUT WILL THEN DROP  
AS THE ARCTIC BOUNDARY SINKS SOUTHWARD TUE/WED. LATER IN THE WEEK,  
TEMPERATURES WILL BE ABOUT 5-15 DEGREES BELOW AVERAGE OVER MUCH OF  
THE COUNTRY EAST OF THE ROCKIES. THE WEST WILL SEE MODERATING  
TEMPERATURES AFTER MONDAY AS HEIGHTS RISE ALONG THE WEST COAST.  
 
PRECIPITATION WILL FOCUS ALONG/AHEAD OF THE FRONT MON-TUE IN THE  
EAST BUT THEN DECREASE IN COVERAGE AS COLDER/DRIER AIR TAKES HOLD.  
THE EXCEPTION WILL BE AROUND THE GREAT LAKES WHERE LAKE-EFFECT  
SNOW WILL BE A DAILY OCCURRENCE. TOTAL ACCUMULATIONS MAY BE  
SUBSTANTIAL FOR SOME FAVORED AREAS. ADDITIONAL LIGHT SNOW WILL BE  
POSSIBLE WITH CLIPPER SYSTEMS DIVING OUT OF CANADA. LATE NEXT  
WEEK, THE FRONT THAT WILL HAVE PUSHED OFF THE COAST MAY LINGER  
CLOSE ENOUGH TO THROW SOME PRECIPITATION BACK TO THE NORTHWEST  
SHOULD A SURFACE WAVE DEVELOP ALONG IT. THIS WOULD MEAN A THREAT  
OF AT LEAST SOME RAIN/SNOW TO THE MID-ATLANTIC AND NORTHEAST LATE  
NEXT WEEK BUT FOR NOW KEPT THE SYSTEM OFFSHORE.  
 
FRACASSO  
 

 
 
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