660  
FXUS02 KWBC 110550  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
149 AM EDT SAT AUG 11 2018  
 
VALID 12Z TUE AUG 14 2018 - 12Z SAT AUG 18 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A PAIR OF SLOW MOVING CUT-OFF UPPER LOWS WILL INITIALLY SITUATE  
OVER THE MID-ATLANTIC AND SOUTHERN/CENTRAL PLAINS, RESPECTIVELY.  
BOTH FEATURES SHOULD GRADUALLY SHIFT DOWNSTREAM AS THE ENTIRE  
SYNOPTIC-SCALE FLOW BUDGES EASTWARD. THE LATTER SYSTEM AFFECTING  
THE CENTRAL U.S. IS FORECAST TO EVENTUALLY BECOME ABSORBED WITHIN  
AMPLIFIED FLOW ACROSS THE GREAT LAKES ON DAY 5/6, AUGUST 16/17.  
RELATIVE TO YESTERDAY'S FORECAST, THIS BECOMES THE KEY FEATURE  
ALOFT WITH RECENT ECMWF RUNS BACKING OFF ON A SIGNIFICANT UPPER  
LOW ACROSS NEW BRUNSWICK DOWN INTO MAINE. FARTHER UPSTREAM, MODELS  
REMAIN UNCERTAIN ON THE POLEWARD EXTENT OF RIDGING ACROSS WESTERN  
NORTH AMERICA LATE IN THE PERIOD. IT DOES APPEAR NEARLY CERTAIN A  
594-DM MID-LEVEL RIDGE WILL RE-ESTABLISH ITS SEMI-PERMANENT SUMMER  
HOME ACROSS THE DESERT SOUTHWEST.  
 
AFTER EXHIBITING A BRIEF FASTER TREND WITH THE EXITING CLOSED LOW  
OVER THE MID-ATLANTIC, THE GUIDANCE HAVE TRENDED SLOWER AS  
ANTICIPATED. WEAK UPSTREAM HEIGHT FALLS SHOULD EVENTUALLY NUDGE  
THE SYSTEM OFFSHORE IN THE FOLLOWING DAYS ENTERING THE CANADIAN  
MARITIMES BY LATE THURSDAY. SHIFTING FOCUS TO THE LONGWAVE TROUGH  
SITTING OVER THE GREAT LAKES AND POINTS SOUTHWARD, THERE IS PLENTY  
OF ENSEMBLE SPREAD WITH REGARDS TO PLACEMENT AND MAGNITUDE.  
ADDITIONALLY, RUN-TO-RUN CONTINUITY IS LACKING SUGGESTING FUTURE  
SHIFTS IN THE NEWER GUIDANCE. PERHAPS NOT SURPRISINGLY, THE 00Z  
GFS/CMC DIVERGE FROM PREVIOUS SOLUTIONS WHICH CASTS FURTHER DOUBT  
ON ANYTHING BEYOND AN ENSEMBLE APPROACH FROM DAY 5/THURSDAY  
ONWARD. ELSEWHERE, THE UNKNOWN PHASE OF THE SYNOPTIC PATTERN  
ACROSS WESTERN NORTH AMERICA REMAINS AN ISSUE. HOWEVER, MULTI-DAY  
ECMWF ENSEMBLE MEAN COMPARISONS SHOW THE PROMINENT RIDGE A BIT  
FLATTER IN RECENT RUNS. ALL THE WHILE, THE SAME COMPARISON FOR THE  
GEFS/NAEFS SOLUTIONS SEEM TO MAINTAIN A QUASI-ZONAL PATTERN WITH  
PERHAPS A WEAK LONGWAVE RIDGING SIGNAL ACROSS THE PACIFIC  
NORTHWEST INTO WESTERN CANADA. ONE THING THAT APPEARS NEARLY  
CERTAIN IS A RETURN OF THE 594-DM MID-LEVEL RIDGE TO THE  
SOUTHWESTERN U.S. THIS HAS SHOWN UP IN THE PAST SEVERAL RUNS OF  
THE GFS/ECMWF.  
 
WITH REASONABLE OPERATIONAL MODEL AGREEMENT THROUGH WEDNESDAY,  
WENT WITH A MYRIAD OF SOLUTIONS ALTHOUGH LEANED MORE HEAVILY ON  
THE 12Z ECMWF AND 18Z/12Z GFS. WITH UNCERTAINTY BUILDING ACROSS  
MANY KEY FEATURES ON THE MAP, ADDED HEFTIER CONTRIBUTIONS FROM THE  
12Z ECMWF/NAEFS ENSEMBLE MEANS. BY EARLY NEXT WEEKEND, THIS  
BLENDED APPROACH WOULD SUPPORT A WEAK NEGATIVE HEIGHT ANOMALY  
ACROSS THE GREAT LAKES AND ITS VICINITY WHILE A MEAN RIDGE SETS UP  
ACROSS A LARGE CHUNK OF THE WESTERN U.S.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
A MILD PATTERN REMAINS IN PLACE FROM THE PACIFIC NORTHWEST  
EASTWARD THROUGH THE UPPER INTERMOUNTAIN/NORTHERN ROCKIES INTO THE  
NORTH-CENTRAL U.S. GENERAL DEPARTURES FROM CLIMATOLOGY SHOULD BE  
AROUND 5 TO 10 DEGREES ACCOMPANIED BY CONTINUED DRY CONDITIONS.  
HOWEVER, THE 00Z GFS DOES BRING A SWATH OF HEAVY RAINFALL INTO THE  
PICTURE LATE THURSDAY/EARLY FRIDAY WHILE THE 12Z ECMWF KEEPS THE  
AREA DRY. WITH WET WEATHER IN STORE FOR THE SOUTHERN/CENTRAL  
PLAINS, THIS SHOULD KEEP READINGS SLIGHTLY BELOW AVERAGE. MODELS  
SHOW SOME HEALTHY 24-HOUR AMOUNTS ANYWHERE FROM OKLAHOMA/KANSAS  
EAST-NORTHEASTWARD INTO MISSOURI/ILLINOIS WITH NUMBERS IN THE 1 TO  
2 INCH RANGE. OTHERWISE, DAILY CHANCE FOR THUNDERSTORMS ARE LIKELY  
ANYWHERE EAST OF THE MISSISSIPPI RIVER GIVEN THE SLOW MOVING  
LONGWAVE TROUGH COMBINED WITH A ROBUST WARM SECTOR. ACROSS THE  
DESERT SOUTHWEST, MONSOONAL CONVECTION SHOULD BREAK OUT EACH  
AFTERNOON/EVENING. THE 00Z GFS SHOWS VERY HIGH AMOUNTS ALTHOUGH  
THESE APPEAR CONTAMINATED BY CONVECTIVE FEEDBACK.  
 
RUBIN-OSTER  
 
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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