280  
FXUS02 KWBC 270645  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
244 AM EDT FRI JUL 27 2018  
 
VALID 12Z MON JUL 30 2018 - 12Z FRI AUG 03 2018  
   
..HOT TEMPERATURES OVER THE WEST GRADUALLY MODERATING
 
 
...HEAVY RAINFALL THREAT EXTENDING FROM THE CENTRAL PLAINS/MS  
VALLEY TO SOUTHEAST/EASTERN U.S...  
   
..PATTERN OVERVIEW AND WEATHER HIGHLIGHTS/THREATS
 
 
ENERGY DROPPING THROUGH THE PLAINS AS OF EARLY NEXT WEEK WILL  
CARVE OUT ANOTHER EASTERN U.S. UPPER TROUGH THAT SHOULD ACHIEVE  
ITS GREATEST DEPTH AROUND MIDWEEK BEFORE TRENDING WEAKER. THIS  
FEATURE WILL BRING ANOTHER MULTI-DAY THREAT OF CONVECTION/HEAVY  
RAINFALL TO PARTS OF THE EASTERN HALF OF THE U.S.. THE MOST  
AGREEABLE GUIDANCE SIGNAL FOR HIGHEST 5-DAY RAINFALL TOTALS EXISTS  
OVER THE SOUTHEAST. LOCATIONS FROM THE CENTRAL/SOUTH-CENTRAL  
PLAINS INTO MS VALLEY ARE MOST LIKELY TO SEE ENHANCED RAINFALL  
DURING THE FIRST HALF OF THE PERIOD, AS THE UPPER TROUGH IS IN THE  
PROCESS OF AMPLIFYING AND PLAINS TO GREAT LAKES SURFACE LOW  
PRESSURE EVOLVES. SOME AREAS OF MODERATE-HEAVY RAINFALL MAY  
EXTEND INTO PARTS OF THE GREAT LAKES/APPALACHIANS/MID-ATLANTIC.  
ONE NOTABLE DIFFERENCE BETWEEN THIS EPISODE AND THE PRECEDING  
MULTI-DAY EVENT IS THAT THE UPCOMING TROUGH ALOFT SHOULD BE  
ALIGNED A FEW DEGREES LONGITUDE FARTHER WEST DUE TO THE PROXIMITY  
OF A STRONG ATLANTIC RIDGE. THIS SHOULD ALLOW FOR SOME OF THE  
PRECIP SHIELD TO EXTEND INTO AREAS THAT DID NOT EXPERIENCE HEAVY  
RAINFALL OVER THE PAST WEEK. HOWEVER OTHER LOCATIONS WILL BE VERY  
SENSITIVE TO ANY MEANINGFUL ADDITIONAL ACTIVITY. THE PATTERN WILL  
PROMOTE BELOW NORMAL TEMPERATURES FROM THE PLAINS INTO THE  
SOUTHEAST WITH GREATEST NEGATIVE ANOMALIES OF MINUS 5-15F MOST  
LIKELY OVER PARTS OF THE PLAINS EARLY IN THE WEEK.  
 
A STRONG WESTERN U.S. RIDGE ALOFT WILL FLATTEN SOMEWHAT AS A  
COUPLE BUNDLES OF ENERGY COMBINE TO YIELD AN UPPER LOW THAT  
REACHES NEAR NORTHERN VANCOUVER ISLAND BY EARLY WED FOLLOWED BY  
SOME COMBINATION OF SPLITTING/PROGRESSION THEREAFTER. BY LATE  
WEEK THIS ENERGY SHOULD SUPPORT NORTHERN PLAINS LOW PRESSURE WITH  
A CORRESPONDING INCREASE IN CONVECTION. EXPECT THE COVERAGE OF  
ABOVE NORMAL TEMPERATURES OVER THE WEST TO DECREASE WITH TIME.  
HIGHEST ANOMALIES OVERALL SHOULD BE OVER THE NORTHWEST ON MON-TUE.  
THE GREAT BASIN/SOUTHWEST WILL LIKELY REMAIN QUITE WARM FOR MINS  
THROUGH THE END OF THE WEEK WITH HIGHS TENDING TO BE MORE MODESTLY  
ABOVE NORMAL (THOUGH STILL HOT ON AN ABSOLUTE BASIS). THE  
NORTHWEST SHOULD TREND BELOW NORMAL FOR HIGHS BY THE LATTER HALF  
OF THE WEEK.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
LATEST GUIDANCE MAINTAINS GOOD AGREEMENT IN THE OVERALL PATTERN  
EVOLUTION DURING THE PERIOD BUT SHOWS SOME EMBEDDED UNCERTAINTIES.  
THERE ARE MODEST DIFFERENCES IN THE STRENGTH OF THE WESTERN  
ATLANTIC RIDGE, AFFECTING SPECIFICS OF THE EAST-CENTRAL U.S.  
TROUGH. ON MOST DAYS THE CMC/CMC MEAN ARE SLIGHTLY WEAKER THAN  
THE MAJORITY WITH THE RIDGE THUS ALLOWING EASTERN U.S. TROUGH  
ENERGY TO EXTEND A LITTLE FARTHER EASTWARD. THUS FAR GUIDANCE HAS  
BEEN SOMEWHAT INCONSISTENT REGARDING POTENTIAL INTERACTION OF  
AMPLIFYING CANADIAN FLOW AND THE CORE OF THE EASTERN U.S. TROUGH  
BY WED. FARTHER WESTWARD, SOLUTIONS ARE STILL IN THE PROCESS OF  
RESOLVING THE DETAILS OF AN UPPER LOW THAT REACHES NEAR/JUST NORTH  
OF VANCOUVER ISLAND BY WED, COMPOSED OF ENERGY FROM TWO SEPARATE  
STREAMS. WHAT HAPPENS TO THIS FEATURE BY DAYS 6-7 THU-FRI IS  
PROBABLY THE LOWEST-CONFIDENCE PART OF THE FORECAST.  
POSSIBILITIES RANGE FROM A FAIRLY COHESIVE LOW/SHORTWAVE WHOSE  
TIMING VARIES BY SOLUTION TO SOME ENERGY EJECTING AHEAD WHILE  
LINGERING ENERGY REMAINS NEAR THE PACIFIC NORTHWEST. AT LEAST THE  
MODELS/MEANS SHARE THE COMMON IDEA OF A GENERAL AREAS OF SURFACE  
LOW PRESSURE OVER/NEAR THE NORTHERN PLAINS LATE IN THE PERIOD.  
 
BASED ON REASONABLE PATTERN AGREEMENT AND LOWER PREDICTABILITY OF  
MOST DETAIL DIFFERENCES, THE UPDATED FORECAST REFLECTED AN  
OPERATIONAL MODEL BLEND FOR DAYS 3-5 MON-WED. THE 18Z GFS/12Z  
ECMWF HAD MAJORITY WEIGHTING WHILE THE BLEND UTILIZED LESSER INPUT  
FROM THE 12Z UKMET/CMC. INCREASING DIVERGENCE OF SOME DETAILS  
ACROSS THE NORTHERN TIER IN PARTICULAR LED TO SLIGHTLY MORE THAN  
HALF TOTAL WEIGHTING OF THE 18Z GEFS/12Z ECMWF MEANS BY DAY 7 FRI.  
 
RAUSCH  
 
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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