364  
FXUS02 KWBC 241600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT TUE JUL 24 2018  
 
VALID 12Z FRI JUL 27 2018 - 12Z TUE JUL 31 2018  
   
..HEAT WAVE LINGERS OVER THE WEST
 
 
...HEAVY RAINFALL THREAT TO SPREAD FROM THE CENTRAL PLAINS TO  
MID-SOUTH THEN AGAIN OVER THE EAST...  
 
   
..PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
A STRONG 594-DM MID-LEVEL RIDGE WILL REMAIN FIXED OVER THE  
SOUTHWESTERN U.S. FOR THE FORESEEABLE FUTURE THEREBY CONTINUING  
THE PERIOD OF ABOVE AVERAGE TEMPERATURES. ACROSS THE REMAINDER OF  
THE COUNTRY, AN UPPER LOW JUST NORTH OF THE GREAT LAKES SHOULD  
EVENTUALLY DRIFT NORTHWARD AND FILL WHILE CROSSING INTO NORTHERN  
QUEBEC OVER THE WEEKEND. ON ITS HEELS, ANOTHER SLOW MOVING BAND OF  
HEIGHT FALLS IS FORECAST TO SWEEP ACROSS CENTRAL U.S. EVENTUALLY  
ATTAINING A POSITION NEAR THE MISSISSIPPI RIVER BY DAY 7/JULY 31.  
AS A PERSISTENT WESTERN ATLANTIC RIDGE BEGINS RETROGRADING TO  
AREAS WEST OF BERMUDA, A SQUEEZE PLAY OF THE TWO FEATURES IS  
EXPECTED AIDING IN MORE HEAVY RAINFALL SOMEWHERE ACROSS THE  
EASTERN THIRD OF THE NATION.  
 
THROUGH THE WEEKEND, THE OPERATIONAL MODELS HAVE PROVEN TO CLUSTER  
RATHER WELL WITH MINIMAL NOISE IN THE ENSEMBLE SPAGHETTI PLOTS. IT  
APPEARS MORE OF THE UNCERTAINTY LOOMS MUCH FARTHER UPSTREAM INTO  
THE NORTHEASTERN PACIFIC. WHILE SOLID AGREEMENT EXISTS THROUGHOUT  
THE PERIOD WITH THE MEAN RIDGE OVER THE DESERT SOUTHWEST, THE SAME  
DOES NOT HOLD TRUE WITH THE NEXT LONGWAVE DIGGING INTO THE  
NORTH-CENTRAL U.S. BY SUNDAY. MULTI-DAY COMPARISONS HAVE SHOWN A  
SLOWING TREND AS THE UPPER RIDGE ACROSS THE FAR WESTERN ATLANTIC  
CONTINUES TO SHIFT WESTWARD. WITH REGARDS TO THE AREA OF LOWERING  
HEIGHTS, THE OPERATIONAL GFS RUNS HAVE HAD A TENDENCY TO BE EAST  
OF THE ECMWF SOLUTIONS. BUT THEIR RESPECTIVE ENSEMBLE MEANS SEEM  
TO BE MORE STABLE SUPPORTING ANOTHER UPPER LOW IN THE VICINITY OF  
THE MID-SOUTH AND THE CORRESPONDING RIDGE OFFSHORE OF THE EAST  
COAST. EACH COMPRISED OF 500-MB ANOMALIES IN THE 1.5 TO 2 SIGMA  
RANGE, TREMENDOUS POLEWARD MOISTURE FLUXES WOULD LIKELY SURGE UP  
THE SPINE OF THE APPALACHIANS POTENTIALLY TOWARD THE I-95 CORRIDOR  
BY NEXT TUESDAY. ELSEWHERE, ATOP THE MEAN UPPER RIDGE ACROSS THE  
DESERT SOUTHWEST, A WEAK SHORTWAVE COULD IMPACT THE PACIFIC  
NORTHWEST AS SHOWN BY THE ENSEMBLE MEANS BY EARLY NEXT WEEK.  
OTHERWISE, EXPECT THE CORRIDOR OF STRONG FORCING ACROSS THE HIGH  
LATITUDES TO REMAIN OVER NUNAVUT WITH THE SOUTHERN EXTENT OF  
HEIGHT FALLS UNLIKELY TO TRACK SOUTH OF THE INTERNATIONAL BORDER  
WITH CANADA.  
 
GIVEN RATHER SOLID MODEL AGREEMENT THROUGH DAY 5/SUNDAY, UTILIZED  
A MULTI-OPERATIONAL MODEL BLEND COMPRISED OF THE 06Z/00Z GFS AS  
WELL AS THE 00Z ECMWF/CMC/UKMET. HOWEVER, FOCUSED 80 PERCENT OF  
THIS BLEND TOWARD THE GFS/ECMWF SOLUTIONS GIVEN GREATER RUN-TO-RUN  
STABILITY. INTO NEXT WEEK, INCORPORATED MORE OF THE 06Z GEFS/00Z  
ECMWF ENSEMBLE MEANS TO THE MIX AS CERTAINTY DIMINISHES ACROSS THE  
CENTRAL/EASTERN U.S.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PERSISTENT COLD ADVECTION ACROSS THE MIDDLE OF THE COUNTRY SHOULD  
KEEP TEMPERATURES BELOW CLIMATOLOGY INTO THE WEEKEND. THIS WILL  
PARTICULARLY FOCUS FROM THE NORTHERN ROCKIES EASTWARD INTO THE  
NORTHERN/CENTRAL PLAINS AS WELL AS MUCH OF THE UPPER GREAT LAKES  
INTO THE MIDDLE MISSISSIPPI VALLEY. EXPECT NUMBERS TO BE AROUND 5  
TO 10 DEGREES BELOW AVERAGE FOR LATE JULY. MEANWHILE, THE WARM  
SPOT WILL RESIDE ALONG THE WEST COAST GIVEN THE PERSISTENCE OF  
UPPER RIDGING. ALTHOUGH HIGH TEMPERATURES SHOULD STAY CLOSE TO  
CLIMATOLOGY OVER THE DESERT SOUTHWEST AS READINGS STAY AROUND THE  
LOWER 110S, GREATER DEPARTURES ARE LIKELY UP INTO THE PACIFIC  
NORTHWEST, GENERALLY 5 TO 10 DEGREES ABOVE AVERAGE.  
 
A HEAVY RAINFALL SIGNAL CONTINUES TO SHOW UP IN THE GUIDANCE AS  
THE SLOW MOVING UPPER TROUGH SHIFTS FOCUS FROM THE CENTRAL U.S.  
TOWARD THE APPALACHIANS. THE INITIAL PROGRESSIVE MOVEMENT OF THIS  
NEGATIVE HEIGHT ANOMALY WOULD INITIALLY MITIGATE THE THREAT FOR  
MORE EXTENSIVE AND WIDESPREAD ISSUES. HOWEVER, BY EARLY NEXT WEEK,  
MODELS SEEM TO AGREE ON CUTTING OFF THE UPPER LOW NEAR THE  
MID-SOUTH WHILE MOISTURE FLUX ANOMALIES RAPIDLY RISE FARTHER  
DOWNSTREAM. THIS WOULD SUPPORT THE POTENTIAL FOR HEAVY RAINFALL  
OVER AREAS ALREADY BEING IMPACTED DURING THE LAST FEW DAYS. THE  
THREAT WILL CONTINUE TO BE MONITORED IN FUTURE SHIFTS AHEAD.  
 
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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