597  
FXUS02 KWBC 211411  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1011 AM EDT FRI SEP 21 2018  
 
VALID 12Z MON SEP 24 2018 - 12Z FRI SEP 28 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
UPPER-LEVEL RIDGING IS EXPECTED TO STRENGTHEN OVER THE NORTHEAST  
PACIFIC AND FORM A CLOSED 500 MB HIGH. DOWNSTREAM OF THE  
AMPLIFYING RIDGE, AN UPPER-LEVEL TROUGH WILL ALSO BECOME AMPLIFIED  
FROM THE NORTHERN PLAINS AND MIDWEST TO THE GREAT LAKES AS THE  
PASSAGE OF MULTIPLE SHORTWAVES PROGRESSIVELY LOWERS HEIGHTS. A LOW  
PRESSURE SYSTEM ACCOMPANYING THE FIRST SHORTWAVE WILL CROSS THE  
EASTERN NORTHERN PLAINS/UPPER MS VALLEY MON-TUE, WITH ANOTHER  
FOLLOWING IN ITS WAKE BY WED 26 SEP-THU 27 SEP.  
 
MODEL/ENSEMBLE CONSENSUS WAS ABOVE AVERAGE WITH RESPECT TO THE  
LARGE SCALE PATTERN EVOLUTION. THEREFORE, A MULTI-MODEL  
DETERMINISTIC BLEND OF THE 00Z GFS/ECMWF/CANADIAN GLOBAL WAS USED  
AS A BASIS FOR THE WPC FORECAST DURING DAYS 3-5 (MON-WED). THE  
OVERNIGHT TREND WAS TO MOVE THE LOW PRESSURE A BIT SLOWER THAN  
PRIOR FORECASTS THAT CURRENT RUNS SHOW TO BE IN MANITOBA WED 26  
SEP AND NEAR THE MN BORDER WITH CANADA 12Z THU 27 SEP. THE 06Z  
GFS/GEFS MEAN AMPLIFIED THE 500 MB WAVE AND FORECAST SFC LOW  
PRESSURE FURTHER SOUTH FRI 28 SEP VS. CONTINUITY AND THE  
ECMWF/CANADIAN MODELS/ENSEMBLE MEANS, SO THESE SOLUTIONS WERE  
WEIGHTED LESS.  
 
THE 00Z ECMWF ENSEMBLE MEAN WAS BLENDED INTO THE FORECAST DAYS 6-7  
AS SPREAD STARTS TO DEVELOP OVER THE EASTERN PACIFIC. THERE IS  
GOOD MULTI-MODEL AGREEMENT ON A REX BLOCK DEVELOPING WITH LOW  
PRESSURE SOUTH OF THE AFOREMENTIONED CLOSED HIGH. TYPICAL TIMING  
DIFFERENCES ON THE LOW'S TRACK LEAD TO A BLEND OF THE MODELS AND  
00Z ECMWF ENSEMBLE MEAN.  
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
A SURGE OF MOISTURE FROM THE GULF OF MEXICO AHEAD OF THE COLD  
FRONT WILL SUPPORT AREAS OF LOCALLY HEAVY RAIN ACROSS MUCH OF THE  
LOWER MISSISSIPPI AND OHIO AND TN VALLEYS MON-WED, WITH SHOWERS  
AND STORMS EXTENDING NORTHEAST ACROSS THE GREAT LAKES INTO THE  
NORTHEAST AND MID-ATLANTIC AS WELL. THE FRONT WILL MOVE SLOWLY  
INTO THE EAST WITH SHOWERS AND STORMS THU AND FRI FOCUSED ON THE  
APPALACHIANS ACROSS THE SOUTHERN MID ATLANTIC, CENTRAL GULF COAST,  
AND FLORIDA. AN AREA OF SHOWERS DEVELOPS OVER THE EASTERN  
NORTHERN PLAINS TO THE UPPER GREAT LAKES THU 27 SEP -FRI 28 SEP.  
 
IN TERMS OF TEMPERATURES, THE A SUBSTANTIALLY COOLER AIR MASS  
MOVES ACROSS THE FRONT RANGE OF THE NORTHERN ROCKIES ACROSS THE  
NORTHERN PLAINS AS THE UPPER TROUGH AMPLIFIES. HIGH TEMPERATURES  
IN THE WAKE OF THE FIRST COLD FRONT MON-TUE WILL BE 5 TO 10 DEG F  
BELOW AVERAGE, WITH TEMPERATURES PERHAPS DROPPING TO 10-15 DEG  
BELOW AVERAGE BEHIND THE SECOND COLD FRONT LATER IN THE WEEK. AS  
HIGH PRESSURE BUILDS DOWN THE PLAINS, COLD AIR PENETRATES FURTHER  
SOUTH,REACHING THE SOUTHERN PLAINS BY NEXT FRI.  
 
MEANWHILE, AREAS ACROSS THE SOUTHWEST, SOUTHERN GREAT BASIN, AND  
SOUTHEAST ARE FORECAST TO REMAIN 5 TO 10 DEG ABOVE AVERAGE.  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page