501  
FXUS02 KWBC 230515  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
114 AM EDT SUN SEP 23 2018  
 
VALID 12Z WED SEP 26 2018 - 12Z SUN SEP 30 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
A HIGHLY AMPLIFIED FLOW PATTERN IS EXPECTED TO DEVELOP DURING THE  
SHORT RANGE AND PERSIST INTO THE MEDIUM RANGE PERIOD FROM THE  
NORTH PACIFIC ACROSS NORTH AMERICA. A RIDGE AXIS OFF THE NORTH  
AMERICA WEST COAST WILL GRADUALLY BE UNDERCUT BY AMPLIFYING WAVE  
ENERGY ALONG THE EASTERN PERIPHERY OF THE RIDGE, ALONG WITH A  
CUTOFF UPPER LOW OVER THE NORTH CENTRAL PACIFIC DRIFTING TOWARD  
THE U.S. WEST COAST. DOWNSTREAM OF THE RIDGE, A SERIES OF UPPER  
SHORTWAVES AND SUBSEQUENT PUSHES OF LOW-LEVEL COLD AIR WILL  
GRADUALLY LOWER HEIGHTS FROM THE NORTHERN PLAINS TO THE UPPER  
MIDWEST AND GREAT LAKES THROUGH THE PERIOD. MEANWHILE, UPPER  
RIDGING WILL REMAIN IN PLACE FROM THE SOUTHERN PLAINS TO THE  
SOUTHEAST, SETTING UP THE POTENTIAL FOR RATHER STRONG THERMAL  
GRADIENTS ACROSS A CENTRAL U.S. FRONTAL BOUNDARY BY NEXT WEEKEND.  
 
MODEL CONSENSUS REMAINED ABOVE AVERAGE FOR THE FIRST HALF OF THE  
MEDIUM RANGE, AND A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE  
ECMWF/GFS/CMC) WAS USED AS A BASIS FOR THE WPC FORECAST STARTING  
OUT THE PERIOD ON DAY 3 (WED) AND CONTINUING INTO DAY 5 (FRI).  
MODELS SHOWED RELATIVELY MINOR TIMING/INTENSITY DIFFERENCES WITH  
RESPECT TO A SHORTWAVE/SURFACE FRONT CROSSING THE EASTERN U.S.  
DURING THE TIME FRAME, AS WELL AS WITH A SECOND CLIPPER-LIKE LOW  
PRESSURE SYSTEM DIVING SOUTHEASTWARD INTO THE NORTHERN TIER. THE  
MORE SIGNIFICANT DIFFERENCES CONTINUE TO ARISE BY DAYS 6-7 WITH  
RESPECT TO THE AMPLITUDE AND EVOLUTION OF RELATIVELY VIGOROUS  
SHORTWAVE ENERGY ATTEMPTING TO DIVE SOUTH TOWARD THE PACIFIC  
NORTHWEST. ADDITIONAL COMPLICATIONS ARE INTRODUCED BY THE UPPER  
LOW OFF THE WEST COAST, WITH THE 12Z ECMWF MAKING A MAJOR CHANGE  
AND BRINGING THIS FEATURE TO THE NORTHWEST AND ABSORBING IT INTO  
THE NORTHERN STREAM MUCH MORE QUICKLY THAN PREVIOUS RUNS OR MOST  
OTHER DETERMINISTIC SOLUTIONS. ADDITIONAL VARIANCE IN THE  
INTENSITY OF THE PERSISTENT NORTHEAST PACIFIC/ALASKA RIDGE AXIS  
AND THE RESULTANT EFFECTS ON DOWNSTREAM FLOW ACROSS NORTH AMERICA  
RESULT IN A RATHER QUICK ESCALATION IN FORECAST UNCERTAINTY DURING  
THE DAY 6-7 (SAT-SUN) TIME FRAME. THUS, BOOSTED WEIGHTING OF  
ENSEMBLE MEANS (ECENS AND NAEFS) SUBSTANTIALLY DURING THE LATER  
MEDIUM RANGE TO COMPRISE A MAJORITY OF THE FORECAST BLEND.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
A COLD FRONT ASSOCIATED WITH THE FIRST IN A SERIES OF SHORTWAVES  
CROSSING THE NORTHERN TIER DURING THE PERIOD WILL EVENTUALLY  
BECOME STATIONARY, BRINGING LOCALLY HEAVY CONVECTION TO PORTIONS  
OF THE SOUTHERN/SOUTHEASTERN STATES WED-THU. THE SECOND  
CLIPPER-LIKE SYSTEM CROSSING THE NORTHERN PLAINS/MIDWEST WED-THU  
MAY WILL BRING AREAS OF RAIN ALONG WITH CHILLY TEMPERATURES. WITH  
A TREND IN THE GUIDANCE TOWARD KEEPING THE LOWEST HEIGHTS ACROSS  
CENTRAL CANADA LATE IN THE PERIOD, THIS WOULD LIKELY RESULT IN A  
QUASI-STATIONARY WEST-EAST FRONTAL BOUNDARY FROM THE CENTRAL  
ROCKIES INTO THE PLAINS/MIDWEST BY LATE IN THE WEEK, POTENTIALLY  
CHARACTERIZED BY A RATHER STRONG NORTH-SOUTH THERMAL GRADIENT  
ACROSS IT, AND POTENTIALLY PRODUCING AREAS OF PRECIPITATION  
ALONG/NORTH OF THE BOUNDARY.  
 
THE NORTH CENTRAL U.S. WILL BE THE FOCUS FOR BELOW AVERAGE  
TEMPERATURES DURING THE MEDIUM RANGE AS PASSAGE OF SUCCESSIVE COLD  
FRONTS GRADUALLY BRINGS COLDER TEMPERATURES. HIGH TEMPERATURES OF  
10 TO 20 DEG F BELOW AVERAGE ARE POSSIBLE THU-FRI, WITH  
TEMPERATURES POTENTIALLY 20-25 DEG BELOW AVERAGE BY SAT-SUN.  
MEANWHILE, THE SOUTHWEST AND SOUTHEAST WILL SEE CONTINUED  
TEMPERATURES 5 TO 10 DEG ABOVE AVERAGE.  
 
RYAN  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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