449  
FXUS02 KWBC 271602  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1202 PM 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
 
 
MUCH OF THE CENTRAL AND EASTERN U.S. WILL HAVE A MULTI-DAY THREAT  
OF CONVECTION/HEAVY RAINFALL; LEADING TO AN ELEVATED RISK FOR  
FLOODING, ESPECIALLY FOR THE SOUTH APPALACHIAN REGION. 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. A STEADY INFLUX OF  
GULF MOISTURE FEEDING INTO THIS SYSTEM WILL INCREASE PRECIPITATION  
EFFICIENCY. AREAL AVERAGES OVER THE COURSE OF 7 DAYS ARE FORECAST  
TO BE 2 TO 4 INCHES, WITH ISOLATED AMOUNTS EXCEEDING 5 INCHES FOR  
THE SOUTHEAST AND MID-ATLANTIC REGIONS. SOME AREAS OF  
MODERATE-HEAVY RAINFALL MAY EXTEND INTO PARTS OF THE GREAT  
LAKES/APPALACHIANS/MID-ATLANTIC. 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.  
 
MUCH OF THE SOUTHWEST WILL CONTINUE TO BE HOT ALTHOUGH  
TEMPERATURES WILL BE MODERATING THROUGH THE EXTENDED PERIODS. THE  
RIDGE OVER THE WEST WILL BECOME SLIGHTLY FLATTEN AS A TROUGH/LOW  
PRESSURE PERSISTS NEAR THE THE PACIFIC NORTHWEST COAST. BY LATE  
WEEK THIS ENERGY SHOULD SUPPORT NORTHERN PLAINS LOW PRESSURE WITH  
A CORRESPONDING INCREASE IN CONVECTION. 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
 
 
THE LATEST GUIDANCE CONTINUES WITH THE SYNOPTIC PATTERN OF A RIDGE  
OVER THE WEST, A DIGGING TROUGH OVER THE CENTRAL U.S. AND A  
RETROGRADING RIDGE OVER THE ATLANTIC/EAST COAST.  
 
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.  
 
CAMPBELL/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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