346  
FXUS02 KWBC 061558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EDT MON AUG 06 2018  
 
VALID 12Z THU AUG 09 2018 - 12Z MON AUG 13 2018  
   
..RECORD HEAT FOR THE INTERIOR NORTHWEST THURSDAY/FRIDAY
 
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
EJECTION OF THE EASTERN PACIFIC UPPER LOW AND ASSOCIATED TROUGH  
ENERGY ACROSS THE EXTREME NORTHWESTERN U.S. AND SOUTHERN CANADA  
WILL GRADUALLY SUPPRESS UPPER RIDGING OVER THE WEST AND PUSH THE  
NORTHERN PART OF THE INITIAL WESTERN RIDGE INTO THE PLAINS AND  
UPPER MS VALLEY. TOWARD THE START OF NEXT WEEK ADDITIONAL ENERGY  
MAY FEED INTO THE MEAN TROUGH BECOMING ESTABLISHED OVER WESTERN  
CANADA/NORTHWESTERN U.S., IN RESPONSE TO UPSTREAM AMPLIFICATION AS  
THE EXTRATROPICAL EVOLUTION OF TYPHOON SHANSHAN BRINGS A DEEP LOW  
INTO THE ALEUTIANS/BERING SEA AND RIDGING BUILDS OVER THE  
NORTHEASTERN PACIFIC. MEANWHILE EXPECT UPPER TROUGHING TO SHARPEN  
OVER THE EAST-CENTRAL U.S. WITH AN EMBEDDED UPPER LOW POSSIBLY  
CLOSING OFF DURING THE WEEKEND.  
 
REVIEWING LATEST GUIDANCE, THE CURRENT CYCLE HAS FURTHERED THE  
TREND TOWARD SOMEWHAT FASTER EJECTION OF THE EASTERN PACIFIC UPPER  
LOW/TROUGH ALBEIT WITH LESS OF AN ADJUSTMENT THAN SEEN 12-24 HOURS  
AGO. THIS IS A SENSITIVE PART OF THE FORECAST GIVEN EXPECTED  
INTERACTION WITH NORTHEASTERN PACIFIC ENERGY, AND THE FULL  
ENSEMBLE ENVELOPE IS SUFFICIENTLY WIDE TO SUGGEST POTENTIAL FOR  
MORE ADJUSTMENTS BY THE MODELS. LATEST MODEL/MEAN CONSENSUS THAT  
DIGS A BIT MORE ENERGY INTO THE PACIFIC NORTHWEST/WESTERN CANADA  
TROUGH BY THE END OF THE PERIOD SEEMS REASONABLE GIVEN THE  
AMPLIFICATION EXPECTED TO OCCUR UPSTREAM. OVER THE EASTERN U.S.,  
RECENT MODEL RUNS AND LATEST ENSEMBLE MEAN TRENDS SEEM TO SIGNAL A  
GREATER THAN EVEN CHANCE THAT AN UPPER LOW WILL CLOSE OFF WITHIN  
THE SHARPENING TROUGH. HOWEVER THERE HAS BEEN VARIABILITY ON  
WHERE THE UPPER LOW WILL CLOSE OFF AND THE 00Z ECMWF DOES NOT  
CLOSE OFF A LOW UNTIL EASTERN CANADA ENERGY DROPS INTO THE MEAN  
TROUGH AFTER THE END OF THE PERIOD.  
 
FOR DAYS 3-5 THU-SAT A BLEND OF LATEST OPERATIONAL GUIDANCE  
(00Z-06Z GFS AND 00Z ECMWF/UKMET/CMC) REPRESENTED CONSENSUS WHILE  
ACCOUNTING FOR EXISTING SPREAD OF TIMING/DETAILS. DAYS 6-7  
SUN-MON INTRODUCED 06Z GEFS/00Z ECMWF MEAN GUIDANCE WITH MODEL AND  
MEAN COMPONENTS GIVEN EQUAL WEIGHT BY LATE IN THE FORECAST.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
EXPECT NEAR OR ABOVE RECORD HIGH TEMPERATURES OVER MUCH OF THE  
INTERIOR NORTHWEST THURSDAY/FRIDAY AHEAD OF THE EASTERN PACIFIC  
UPPER LOW WITH 100S WIDESPREAD EAST OF THE CASCADES AND THROUGH  
THE SNAKE VALLEY IN IDAHO AS WELL AS THE MISSOURI RIVER VALLEY IN  
MONTANA. LIGHT PRECIPITATION WILL BE CONFINED TO  
COASTAL/NORTHERN/HIGHER ELEVATION AREAS OF WASHINGTON AS THE BEST  
UPPER SUPPORT TRAVELS THROUGH CANADA. HEAVIER CONVECTIVE  
PRECIPITATION IS LIKELY IN THE DEEPER MOISTURE ON THE EASTERN SIDE  
OF THE WESTERN U.S. UPPER HIGH, FROM NM/CO/AZ EASTWARD TO THE  
SOUTHERN PLAINS. IT IS LIKELY THAT SOME LOCATIONS WILL SEE SEVERAL  
INCHES OF RAIN OVER A FEW DAYS WHICH MAY LEAD TO LOCALIZED  
FLOODING. CLOUDS/RAINFALL SHOULD PROMOTE BELOW NORMAL HIGH  
TEMPERATURES OVER THIS REGION FOR MOST OF THE PERIOD.  
 
PRECIPITATION WILL ACCOMPANY AND PRECEDE A FRONTAL BOUNDARY AS IT  
PUSHES THROUGH THE GREAT LAKES AND INTO THE NORTHEAST VIA  
AFTERNOON STORMS. ANOTHER FRONT WILL LINGER ACROSS THE U.S.  
SOUTHERN TIER DURING THE PERIOD WHICH WILL HELP SPARK AFTERNOON  
SHOWERS/STORMS AND LOCALLY HEAVY RAINFALL IN THE WARM/HUMID  
AIRMASS. AS THE UPPER TROUGH DIGS/SHARPENS OVER THE EAST-CENTRAL  
U.S., MORE WIDESPREAD PRECIPITATION MAY BE FAVORED TO ITS EAST  
ALONG THE APPALACHIANS TO THE I-95 CORRIDOR.  
DISTRIBUTION/INTENSITY OF RAINFALL WILL BE SENSITIVE TO THE EXACT  
EVOLUTION OF THE UPPER TROUGH AND POSSIBLE LOW.  
 
RAUSCH/FRACASSO  
 
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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