143  
FXUS02 KWBC 150659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT SUN OCT 15 2017  
 
VALID 12Z WED OCT 18 2017 - 12Z SUN OCT 22 2017  
 
...A COOLING AND STORMY WEATHER PATTERN TO DEVELOP LATER WEEK OVER  
THE WEST...  
   
..PATTERN AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
RECENT MODELS AND ENSEMBLES CONTINUE TO SHOW A PATTERN TRANSITION  
OVER THE NATION TO ONE WITH AN AMPLIFIED AND COOLING WRN U.S.  
TROUGH AND WARMING ERN US RIDGE COUPLET. THERE IS GOOD AGREEMENT  
WITH ABOVE NORMAL CONFIDENCE AND CONTINUITY OF THE OVERALL FLOW  
PATTERN THROUGH THE WEEK. THERE IS SOME FORECAST SPREAD WITH THE  
EXTENT OF FLOW SEPARATION AND EVEN POTENTIAL CLOSED LOW  
DEVELOPMENT NEXT WEEKEND OVER THE SWRN US...ESPECIALLY EVIDENT  
WITH THE ECMWF AND SUBSET OF ECMWF/GEFS ENSEMBLE MEMBERS. THAT  
WOULD DELAY PROGRESSION OF THE DEEPEST HEIGHT FALLS OUT OVER THE  
CENTRAL US. HOWEVER...WPC PROGS INSTEAD DEPICT MORE MODERATE FLOW  
SEPARATION AND THE 00 UTC ECMWF HAS BACKED OFF FROM THE 12 UTC  
RUN.  
 
OTHERWISE...DESPITE TRANSITION THAT OFTEN PORTENDS INCREASING  
GUIDANCE UNCERTAINTY...THE GFS/GEFS AND ECMWF/ECMWF ENSEMBLES HAVE  
OVER PAST COUPLE OF DAYS GENERALLY CONVERGED UPON A REASONABLY  
SIMILAR WRN U.S. MID-UPPER LEVEL TROUGH POSITION AND AMPLITUDE  
INTO NEXT WEEKEND. LINGERING MODEL DIFFERENCES REMAIN MAINLY  
RELATED TO THE LESS PREDICTABLE TIMING AND STRENGTH OF EMBEDDED  
SHORTWAVES. ONE EXCEPTION CONTINUES TO BE THE CANADIAN MODEL. THE  
CANADIAN REMAINS A FAST OUTLIER MOVING THE UPPER TROUGH MORE  
QUICKLY OVER THE WRN AND CENTRAL U.S. CANADIAN ENSEMBLES REMAIN  
HOURS SLOWER AND TRENDS ARE TOWARD MORE AMPLIFIED FLOW IN THE BULK  
OF GUIDANCE. CONSEQUENTLY...THE CANADIAN WAS AGAIN NOT USED IN THE  
WPC FORECAST BLEND. WPC SURFACE FRONTS/PRESSURES AND 500 MB  
PROGS...QPF...AND SENSIBLE WEATHER GRIDS WERE PRIMARILY DERIVED  
FROM A COMPOSITE BLEND OF WELL CLUSTERED WPC CONTINUITY...THE  
GFS/ECMWF...AND GEFS/ECMWF ENSEMBLE MEANS FOR THE UPCOMING WEEK.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
AN ENERGETIC LEAD STORM TRACK PROGRESSES EASTWARD ACROSS SRN  
CANADA AND THE FAR NRN US TIER STATES THROUGH MID NEXT  
WEEK...ALONG WITH SOME SWATHS OF ORGANIZED PCPN HOLDING MAINLY  
NORTH OF THE BORDER. SUBSTANTIAL UPSTREAM FLOW THOUGH THEN DIGS  
DOWN ACROSS THE PACIFIC NORTHWEST INTO CA BEFORE TRAVERSING THE  
INTERMOUNTAIN WEST/SOUTHWEST TO THE ROCKIES THU-SAT. EXPECT A  
RATHER STORMY/WET PERIOD FOR THE NWRN US/CA THEN INLAND ACROSS THE  
WEST AS MAINLY POWERED BY HEIGHT FALLS/DYNAMIC AND TOPOGRAPHIC  
LIFT. A SERIES OF EMBEDDED SHORTWAVES TAP AVAILABLE PACIFIC  
MOISTURE TO PRODUCE WIDESPREAD PRECIPITATION INCLUDING SOME HEAVY  
TERRAIN ENHANCED PRECIPITATION. POST-FRONTAL COLD SURGES DOWN  
THROUGH THE WRN STATES WILL INCLUDE A THREAT FOR HEAVY MOUNTAIN  
SNOWS WITH LOWERING SNOW LEVELS...SO PLEASE SEE THE WPC MEDIUM  
RANGE WINTER WEATHER OUTLOOK PROBABILITIES FOR ADDITIONAL DETAILS.  
AMPLE ENERGY EJECTION INTO THE PLAINS HEADING THROUGH NEXT WEEKEND  
WOULD SPAWN CYCLOGENESIS AND FRONTOGENESIS WITH INITIALLY SLUGGISH  
RETURN GULF MOISTURE INFLOW KICKING MORE IN ERNEST LATE PERIOD.  
THIS MAY FUEL RAINFALL/CONVECTIVE POTENTIAL EXPANSION LATER SAT  
INTO NEXT SUN OVER THE CENTRAL US.  
 
MEANWHILE...MOST OF THE ERN THIRD OF THE COUNTRY 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.  
 
SCHICHTEL  
 

 
 
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