040  
FXUS02 KWBC 141432  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1032 AM EDT SAT OCT 14 2017  
 
VALID 12Z TUE OCT 17 2017 - 12Z SAT OCT 21 2017  
 
...A COOLING AND STORMY WEATHER PATTERN NEXT WEEK TO DEVELOP OVER  
THE WEST...  
   
..PATTERN AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MODELS AND ENSEMBLES SHOW THE PATTERN TRANSITIONING TO ONE OF  
INCREASED WRN U.S. TROUGHING AND BUILDING HEIGHTS OVER THE ERN  
STATES. THERE IS NOW GOOD AGREEMENT AND ABOVE NORMAL CONFIDENCE  
WITH THE OVERALL FLOW PATTERN FROM THE 00-06Z GFS/GEFS...12/00Z  
ECMWF AND 12/00Z ECMWF ENSEMBLE MEAN. THERE IS ALSO ABOVE NORMAL  
CONTINUITY THROUGH THE PERIOD.  
 
THE LATEST 06Z GEFS RUN TRENDED TOWARD WRN U.S. MID-UPPER LEVEL  
TROUGH TIMING AND AMPLITUDE NEXT WEEKEND MORE IN LINE WITH THE  
ECMWF/ECMWF ENSEMBLES. LINGERING MODEL DIFFERENCES ARE MAINLY  
RELATED TO THE LESS PREDICTABLE TIMING AND STRENGTH OF EMBEDDED  
SHORTWAVES. THE CANADIAN REMAINS A FAST OUTLIER MOVING THE UPPER  
TROUGH MORE QUICKLY ACROSS THE WESTERN AND CENTRAL US WITH THE  
CANADIAN ENSEMBLES SEVERAL HOURS SLOWER.  
CONSEQUENTLY...THE CANADIAN GLOBAL WAS NOT USED IN THE FORECAST  
BLEND.  
WPC SURFACE FRONTS/PRESSURES AND 500 MB PROGS...QPF...AND SENSIBLE  
WEATHER GRIDS WERE DERIVED FROM A BLEND OF THE WELL CLUSTERED  
00Z-06Z GFS/00Z ECMWF AND 06Z GEFS/00Z ECMWF ENSEMBLE MEANS AND  
WPC CONTINUITY.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT A STORMY/WET PERIOD FOR THE PACIFIC NORTHWEST/CA AND THEN  
THE NORTHERN ROCKIES WED-THU AS THE CYCLONE MOVES TOWARDS THE  
BRITISH COLUMBIA COAST WITH STRONG ONSHORE FLOW  
ADVECTS COPIOUS MOISTURE INLAND FROM THE EASTERN PACIFIC WITH  
STRONG TOPOGRAPHIC LIFT PRODUCING HEAVY RAIN IN THE WA OLYMPIC AND  
CASCADE RANGES.  
 
THU NIGHT-FRI THE UPPER LEVEL TROUGH PHASES WITH A CLOSED LOW OVER  
THE EASTERN PACIFIC AND PRODUCES A THREAT OF RAINS IN  
CA...INCLUDING IN AREAS BESET WITH RECENT DRY CONDITIONS AND  
WILDFIRES. POST-FRONTAL COLD SURGES AT THE END OF THE WEEK INTO  
NEXT WEEKEND LEADS TO A THREAT FOR HEAVY MOUNTAIN SNOWS WITH  
LOWERING SNOW LEVELS IN CA TO THE NORTHERN ROCKIES. WITH THE  
PROGRESSION OF THE TROUGH ACROSS THE SOUTHWEST THE THREAT FOR  
PRECIP NEXT WEEKEND EXTENDS INTO THE RANGES OF NV/UT/CO AND AS FAR  
SOUTH AS SOUTHERN CA TO NORTHERN AZ AND NM.  
 
MOST OF THE ERN THIRD OF THE US WILL BE DRIER THAN NORMAL UNDER  
THE INFLUENCE OF HIGH PRESSURE. HOWEVER...PERSISTING MOISTURE AND  
COASTAL CONVERGENCE PRODUCES THE LIKELIHOOD OF SHOWERS ALONG THE  
EAST COAST OF FLORIDA...WITH AN INVERTED TROUGH PRODUCING SHOWERS  
ALONG THE TEXAS/SOUTHWEST LA COASTS. ABOVE NORMAL TEMPERATURES  
ARE FORECAST ACROSS A WIDE SWATH CENTERED ON THE NORTHERN PLAINS  
TO UPPER MS VALLEY DAYS 3-5...WITH VALUES 10-15 DEGREES ABOVE  
NORMAL COMMON. ON DAY 7 NEXT SAT THE MAX SHIFTS EAST TO COVER THE  
UPPER MS VALLEY AND GREAT LAKES/OHIO VALLEY IN TANDEM WITH THE  
UPPER RIDGE DRIFTING EAST.  
 
PETERSEN  
 

 
 
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