877  
FXUS02 KWBC 151420  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1019 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
 
 
THE MODELS AND ENSEMBLES CONTINUE TO SHOW A PATTERN TRANSITION TO  
ONE WITH AN AMPLIFIED TROUGH MOVING ONSHORE ON THE WEST COAST AND  
PROGRESSING INLAND TO THE CENTRAL US NEXT WEEKEND. DOWNSTREAM FROM  
THE TROUGH IS A HIGH AMPLITUDE ERN US RIDGE. 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 TO THE  
SOUTHERN PLAINS...ESPECIALLY EVIDENT WITH THE ECMWF ECMWF/GEFS  
ENSEMBLE MEMBERS. THAT WOULD DELAY PROGRESSION OF THE DEEPEST  
HEIGHT FALLS OUT OVER THE CENTRAL US. HOWEVER...SINCE BOTH THE  
ECMWF AND GEFS ENSEMBLE MEANS SHOW A TROUGH THAT CONTINUES TO  
PROGRESS SLOWLY EAST...THAT WAS DEPICTED UNTIL BETTER AGREEMENT  
DEVELOPS.  
 
THE 00Z GFS WAS AN OUTLIER IN THE TIMING AND PHASING OF THE  
OVERALL PATTERN...WIT THE DAY 7/SUN 22 OCT PATTERN SHOWING A FLAT  
NORTHWESTERN US TROUGH AND MUCH LOWER AMPLITUDE UPPER MS VALLEY  
TROUGH THAN THE ECMWF AND BOTH THE ECMWF/GEFS MEANS. THE 06Z GFS  
RUN FLIPPED AND CAME INTO MUCH BETTER ALIGNMENT ON THE WAVE NUMBER  
AND AMPLITUDE OF THE UPPER TROUGH AS IT PROGRESSES FROM THE  
WESTERN TO CENTRAL US. THE CANADIAN WAS AGAIN NOT USED IN THE WPC  
FORECAST BLEND AS A FAST OUTLIER..COUNTERED BY SLOWER CANADIAN  
ENSEMBLES.  
 
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 06Z GFS/00Z ECMWF...AND 06Z  
GEFS/00Z ECMWF ENSEMBLE MEANS FOR DAYS 3-6 BEFORE PLACING LESS  
WEIGHTING ON THE 06Z GFS DAY 7 (SUN 22 OCT) AS THE FRONTAL  
PROGRESSION BECOMES FASTER THAN THE ECMWF MEAN/GEFS  
MEAN/OPERATIONAL ECMWF IN THE UPPER MS VALLEY/GREAT LAKES.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT A RATHER STORMY/WET PERIOD FOR THE NWRN US/CA WITH LESSER  
AMOUNTS OF PRECIP SPREADING INLAND ACROSS THE WEST...SUPPORTED BY  
SYNOPTIC SCALE ASCENT IN THE VICINITY OF THE UPPER JET AUGMENTED  
BY TOPOGRAPHIC LIFT. A SERIES OF EMBEDDED SHORTWAVES TAP AVAILABLE  
PACIFIC MOISTURE TO PRODUCE WIDESPREAD PRECIPITATION INCLUDING  
SOME HEAVY TERRAIN ENHANCED PRECIPITATION IN THE PACIFIC  
NORTHWEST...ESPECIALLY THE RANGES OF WA STATE. SEVERAL INCHES OF  
RAIN ARE POSSIBLE IN THE WA OLYMPICS AND CASCADES. POST-FRONTAL  
COLD SURGES DOWN THROUGH THE WRN STATES WILL LEAD TO LIKELY  
MOUNTAIN SNOWS NEXT WEEKEND WITH LOWERING SNOW LEVELS.  
 
THE UPPER TROUGH MOVING INTO THE PLAINS NEXT WEEKEND DRIVES A COLD  
FRONT ACROSS THE REGION. EAST OF THE FRONT...MOISTURE RETURN  
GRADUALLY INCREASES. THE INCREASE IN MOISTURE AND STRENGTH OF THE  
LOW LEVEL JET MAY FUEL RAINFALL/CONVECTIVE POTENTIAL EXPANSION  
LATER SAT INTO NEXT SUN OVER THE CENTRAL TO SOUTHERN PLAINS AND  
MID MS VALLEY.  
 
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/LA COASTS.  
 
THE UPPER RIDGE LEADS TO WARMER THAN NORMAL CONDITIONS ACROSS MOST  
OF THE EASTERN US...INCLUDING HIGH TEMPERATURES 10-15 DEGREES  
ABOVE NORMAL IN THE UPPER MS VALLEY...GREAT LAKES...AND OH VALLEY  
LATE THIS WEEK INTO THE WEEKEND...WHEN THE ANOMALOUS WARMTH  
SPREADS INTO THE APPALACHIANS AND NORTHEAST.  
 
PETERSEN  
 

 
 
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