366  
FXUS02 KWBC 181411  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1010 AM EDT TUE SEP 18 2018  
 
VALID 12Z FRI SEP 21 2018 - 12Z TUE SEP 25 2018  
 
***PATTERN OVERVIEW AND MODEL GUIDANCE***  
 
A 500 MB TROUGH IS FORECAST FRIDAY JUST OFF THE PACIFIC NORTHWEST  
COAST. AS THE SYSTEM MOVES ONSHORE AND INLAND, TIMING DIFFERENCES  
BECOME GREATER, WITH A LARGE SPREAD AMONG THE 06Z GFS/00Z  
ECMWF/00Z CANADIAN MODELS (ECMWF FASTEST, CANADIAN SLOWEST), WITH  
A WIDE SPREAD IN THE ENSEMBLE MEMBERS. THE DIFFERENCES STEM FROM  
THE HANDLING OF A TROUGH OVER THE BERING SEA SAT, WITH THE 00Z  
ECMWF DEAMPLIFYING IT AND MOVING IT FASTER DOWNSTREAM, IN TANDEM  
WITH THE EJECTION OF THE PACIFIC NORTHWEST TROUGH.  
 
AS THE TROUGH MOVES EAST, THE ECMWF ALSO BECOMES THE MOST  
AMPLIFIED WITH THE SOUTHERN CANADA  
TROUGH, AS THE 50 ENSEMBLE MEMBERS HAVE LOWER AMPLITUDE THAN THE  
OPERATIONAL RUN TUE 25 SEPT. WITH LARGE RUN TO RUN CHANGES AND  
LITTLE SUPPORT FROM THE ECMWF ENSEMBLES, LITTLE WEIGHTING WAS  
GIVEN TO THE 00Z OPERATIONAL ECMWF.  
 
MANUAL PROGS BLENDED THE 00Z ECMWF ENSEMBLE MEAN WITH THE 06Z GFS,  
WITH LESS WEIGHTING TO THE 06Z GEFS MEAN THROUGH THE FORECAST.  
CONFIDENCE IS A LITTLE BELOW NORMAL GIVEN THE LARGE SPREAD OF  
SOLUTIONS EARLY TO MIDDLE PORTIONS OF NEXT WEEK.  
 
ON FRI 21 SEPT, THE MODELS TRACK A 500 MB TROUGH THAT MOVES OUT OF  
THE UPPER MS VALLEY AND GREAT LAKES, DEAMPLIFYING AS IT MOVES  
QUICKLY ACROSS THE NORTHEAST SAT. SURFACE HIGH PRESSURE DRIVES THE  
COLD FRONT SOUTH THROUGH THE LAKES AND INTO THE MID-ATLANTIC  
REGION AND OHIO VALLEY, WHERE IT STALLS. THE MODELS SHOW A 700 MB  
TROUGH DEVELOPING IN CO/NM AS A SOUTHERN EXTENSION OF THE NORTHERN  
STREAM TROUGH AND MOVE THIS TROUGH INTO THE SOUTHERN PLAINS, WITH  
A SURFACE WAVE RESULTING.  
 
***SENSIBLE WEATHER AND HAZARDS***  
 
THE NEXT HEAVY RAINFALL EVENT DURING THE MEDIUM RANGE PERIOD IS  
EXPECTED ACROSS THE SOUTH-CENTRAL PLAINS FOR THE UPCOMING WEEKEND  
AS TROPICAL MOISTURE SURGES NORTHWARD FROM MEXICO AND THE WESTERN  
GULF, WITH LIFT FROM AN APPROACHING 700 MB TROUGH. THE POTENTIAL  
EXISTS FOR SEVERAL INCHES OF RAIN ACROSS PORTIONS OF EASTERN  
OKLAHOMA INTO ADJACENT PORTIONS OF SOUTHEAST KS, SOUTHWEST MO, AND  
NORTH TEXAS. LESSER AMOUNTS ARE FORECAST EASTWARD ACROSS THE  
MIDWEST AND THE OHIO VALLEY REGION IN THE VICINITY OF A FRONTAL  
BOUNDARY. ANOTHER AREA OF MODERATE TO LOCALLY HEAVY RAINFALL WILL  
BE POSSIBLE ACROSS THE UPPER MIDWEST/UPPER MS VALLEY TOWARDS THE  
MIDDLE OF NEXT WEEK AS A SURFACE LOW DEVELOPS OVER THE NORTHERN  
PLAINS. LIKEWISE, AS THE WESTERN TROUGH PROGRESSES ACROSS THE  
NORTHERN ROCKIES AND ADJACENT HIGH PLAINS, SHOWERS ARE EXPECTED IN  
ADVANCE OF THE UPPER TROUGH.  
 
EXPECT TEMPERATURES TO BE WELL ABOVE AVERAGE ON FRIDAY ACROSS MUCH  
OF THE EASTERN U.S. AHEAD OF THE COLD FRONT, WITH AREAS OF 10 TO  
15 DEGREES ABOVE NORMAL FORECAST. TEMPERATURES MODERATE TO NEAR  
NORMAL AFTER THE COLD FRONT PASSES.  
 
ABOVE NORMAL TEMPERATURES ARE FORECAST ACROSS THE SOUTHWEST TUE 18  
SEPT. AND WED SEPT 19, AND ALSO OVER THE SOUTHEAST US UNDERNEATH  
THE UPPER HIGH. ONCE THE UPPER TROUGH PASSES, COOLER THAN NORMAL  
TEMPERATURES DEVELOP NEXT WEEK FROM THE NORTHERN ROCKIES TO THE  
NORTHERN PLAINS.  
 
PETERSEN  
 
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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