509  
FXUS02 KWBC 171601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT TUE OCT 17 2017  
 
VALID 12Z FRI OCT 20 2017 - 12Z TUE OCT 24 2017  
   
..PATTERN OVERVIEW/MODEL GUIDANCE/PREFERENCES
 
 
DURING THE UPCOMING MEDIUM RANGE PERIOD....THE PATTERN WILL BE  
HIGHLY AMPLIFIED AND PROGRESSIVE WITH A LONGWAVE TROUGH INITIALLY  
OVER THE WESTERN HALF OF THE COUNTRY. THE ATTENDANT HEIGHT FALLS  
ARE FORECAST TO SLIDE EASTWARD IN TIME WITH THE PRIMARY FORCING  
LIFTING INTO SOUTHERN SASKATCHEWAN/MANITOBA BY EARLY SATURDAY. AS  
THIS OCCURS...FOCUS SHIFTS TOWARD THE SOUTHERN EXTENT OF THE  
TROUGH WHICH MAY SEPARATE AND SLOW DOWN BASED ON SOME OF THE  
RECENT MODEL GUIDANCE. THIS PARTICULAR FEATURE IS PRIMARILY SHOWN  
DURING PAST RUNS OF THE ECMWF WITH PLENTY OF ENSEMBLE SUPPORT VIA  
SPAGHETTI PLOTS. TIMING/STRENGTH DIFFERENCES ARE  
NUMEROUS...PARTICULARLY BETWEEN INDIVIDUAL RUNS OF THE ECMWF. THE  
12Z CYCLES OF THE ECMWF HAVE SHOWN A TENDENCY TO PINCH OFF A  
CLOSED LOW ALONG THE SOUTHERN/CENTRAL GULF COAST WHILE THE 00Z  
RUNS FAVOR MORE PROGRESSION. MOST RECENTLY...THE 00Z GFS MIMICS  
THAT 00Z ECMWF RUN WHICH SHOW A MODEST SHORTWAVE IN THE VICINITY  
OF THE SOUTHEASTERN U.S. COAST BY 24/1200Z. MEANWHILE...THE  
NORTHERN STREAM LONGWAVE TROUGH ADVANCING THROUGH THE EASTERN U.S.  
WILL ATTAIN A NEGATIVE TILT INDUCING A PRONOUNCED AREA OF LOW  
PRESSURE OVER THE NORTHEASTERN STATES. MODELS HAVE SHOWN A  
TENDENCY TO SLOW RELATIVE TO PRIOR CYCLES BUT THE GENERAL SYNOPTIC  
PATTERN IS WELL AGREED UPON.  
 
ACROSS THE WESTERN U.S...IN THE WAKE OF THE INITIAL HEIGHT FALLS  
TRAVERSING THROUGH THE INTERMOUNTAIN WEST...THERE SHOULD BE A  
BRIEF PERIOD OF QUASI-ZONAL FLOW LATE IN THE WEEKEND BEFORE  
HEIGHTS RAPIDLY RISE. BY EARLY NEXT WEEK...A WELL ESTABLISHED  
UPPER RIDGE ENCOMPASSES THE ENTIRE WESTERN U.S. WITH 00Z CMC BEING  
VERY AGGRESSIVE SUGGESTING A 600-MB MID-LEVEL HEIGHT CONTOUR OVER  
WESTERN OR. RELATIVE TO THE MODEL CONSENSUS...ITS SOLUTION SEEMS  
QUITE EXTREME.  
 
GIVEN SUFFICIENT MODEL AGREEMENT THROUGH DAY 5/SUNDAY...FAVORED A  
MULTI-MODEL OPERATIONAL BLEND LED BY THE 00Z ECMWF/GFS WITH  
SMALLER CONTRIBUTIONS FROM THE 00Z UKMET. GRADUALLY INCORPORATED  
MORE ENSEMBLE MEANS INTO THE PICTURE INTO NEXT WEEK GIVEN  
INCREASING UNCERTAINTIES WITH SOME OF THE DETAILS OF EACH SYSTEM.  
KEPT 60 PERCENT OF THE 00Z ECMWF/ECMWF ENSEMBLE MEAN IN THE  
PICTURE GIVEN ITS CONSISTENCY IN FAVORING THE LOWER HEIGHTS ACROSS  
THE SOUTHEASTERN U.S. HOWEVER...ITS FLIP-FLOPPING BETWEEN RUNS IS  
SOMEWHAT CONCERNING WHICH KEEPS CONFIDENCE AROUND AVERAGE AT BEST.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
IT WILL BE QUITE WARM AHEAD OF THE ADVANCING FRONTAL SYSTEM WITH  
15 TO 20 DEGREE ABOVE AVERAGE READINGS LIKELY ACROSS THE NORTHERN  
PLAINS/UPPER MIDWEST ON FRIDAY. GIVEN THE MILD OVERNIGHT  
TEMPERATURES EXPECTED ON SATURDAY MORNING OVER THE CENTER OF THE  
COUNTRY...SOME DAILY RECORDS MAY BE BROKEN. AS THE LONGWAVE TROUGH  
PUSHES EASTWARD...TEMPERATURES CLOSER TO CLIMATOLOGY ARE IN THE  
FORECAST. TOWARD THE END OF THE PERIOD...WELL ABOVE NORMAL  
READINGS ARE LIKELY OVER THE WEST COAST GIVEN THE RETURN OF THE  
UPPER RIDGE. TEMPERATURES MAY BE 10 TO 15 DEGREES ABOVE AVERAGE  
ACROSS THE DESERT SOUTHWEST EARLY NEXT WEEK.  
 
IN TERMS OF PRECIPITATION...ZONAL FLOW ACROSS THE PACIFIC  
NORTHWEST WILL KEEP CONDITIONS WET DURING PART OF THE WEEKEND  
BEFORE HEIGHTS BEGIN TO RISE INTO NEXT WEEK.  
ELSEWHERE...WIDESPREAD RAINFALL IS LIKELY ALONG THE EASTWARD  
MIGRATING COLD FRONT. IT SHOULD BECOME MUCH MORE ACTIVE INTO NEXT  
WEEK AS THE SOUTHERN STREAM FEATURE COMES INTO PLAY ACROSS THE  
 
RUBIN-OSTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page