590  
FXUS02 KWBC 180248  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1047 PM EDT FRI AUG 17 2018  
 
VALID 12Z TUE AUG 21 2018 - 12Z SAT AUG 25 2018  
   
..PATTERN OVERVIEW
 
 
AN AMPLIFIED PATTERN -- WITH NOTABLE POSITIVE 500 HPA ANOMALIES  
NEAR WESTERN CANADA AND ACROSS ATLANTIC CANADA -- IS EXPECTED INTO  
AUGUST 22. THEREAFTER, THE WESTERN CANADIAN ANOMALY MOVES  
WESTWARD ACROSS THE NORTHEAST PACIFIC WHICH SHOULD CAUSE SOME  
RETROGRESSION TO THE UPPER TROUGH FROM THE GREAT LAKES TOWARDS  
MID-CONTINENT BY NEXT SATURDAY. THE GUIDANCE AGREES ON THESE  
IDEAS. THE BIGGER ISSUES INVOLVE THE SURFACE LOW NEAR THE  
MID-CONTINENT TROUGH NEXT SATURDAY, WHERE THE 18Z GFS WAS MORE  
WESTWARD AND THE 12Z CANADIAN MORE EASTWARD. OVERALL THOUGH, THIS  
ISSUE IS RATHER MINIMAL FOR A DAY 7-8 FORECAST.  
 
   
..MODEL PREFERENCES
 
 
SINCE THE GUIDANCE WAS GENERAL AGREEABLE, USED A COMPROMISE OF THE  
12Z CANADIAN, 12Z ECMWF, 12Z UKMET, AND 18Z GFS THROUGH THE PERIOD  
FOR 500 HPA HEIGHTS/PRESSURES/FRONTS/WIND GRIDS, WITH SLOWLY  
INCREASING PERCENTAGES OF THE 12Z NAEFS MEAN AND 12Z ECMWF FROM  
THURSDAY ONWARD WHICH EVENTUALLY MAKE UP 45% OF THE BLEND. FOR  
THE OTHER GRIDS (WEATHER, CLOUDS, PRECIPITATION CHANCES, DEW  
POINTS), A MORE EVEN BLEND OF THE ABOVE DETERMINISTIC AND ENSEMBLE  
GUIDANCE WAS USED. THE DAYS 4-7 QPF WERE AN EVEN BLEND OF THE 12Z  
ECMWF, 18Z GFS, 12Z CANADIAN, 00Z NATIONAL BLEND OF MODELS, AND  
18Z IN-HOUSE BIAS CORRECTED ENSEMBLE QPF.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE DEVELOPING SYSTEM TRACKING FROM THE PLAINS THROUGH GREAT LAKES  
INTO EASTERN CANADA AS WELL AS THE TRAILING COLD FRONT WILL LIKELY  
PRODUCE A BROAD AREA OF SHOWERS/THUNDERSTORMS--SOME WITH HEAVY  
RAINFALL--FROM THE EASTERN PLAINS/MS VALLEY INTO THE EASTERN U.S.  
TUESDAY INTO WEDNESDAY. RAINFALL/CONVECTION MAY ALSO FOCUS AT  
TIMES ALONG A LEADING FRONT THAT INITIALLY EXTENDS FROM THE  
WESTERN ATLANTIC BACK TO THE SURFACE LOW. PASSAGE OF THE COLD  
FRONT WILL BRING A DRYING TREND TO MANY AREAS IN THE EASTERN HALF  
OF THE COUNTRY BUT AREAS FROM THE MID-ATLANTIC TO THE EXTREME  
SOUTHEAST AND ALONG THE GULF COAST MAY SEE RAINFALL PERSIST OR  
RETURN LATE IN THE WEEK AS THE FRONT STALLS. MEANWHILE THE  
TROUGH/UPPER LOW INITIALLY NEAR THE WEST COAST MAY PRODUCE SOME  
SCATTERED RAINFALL OVER NORTHERN PARTS OF THE WEST DURING THE  
FIRST HALF OF THE WEEK. THEN SOME SHOWERS/STORMS MAY EMERGE OVER  
THE NORTHERN PLAINS/MS VALLEY LATER IN THE WEEK AS THE SHORTWAVE  
ENERGY EJECTS EASTWARD. MOST PRECIPITATION WITH THE UPPER TROUGH  
ARRIVING FROM THE PACIFIC LATE IN THE WEEK SHOULD REMAIN NORTH OF  
THE CANADIAN BORDER. MEANWHILE THE FOUR CORNERS STATES SHOULD  
CONTINUE TO SEE PERIODS OF DIURNALLY ENHANCED CONVECTION.  
 
THE MOST EXTREME TEMPERATURE ANOMALIES DURING THE PERIOD WILL  
LIKELY BE OF THE NEGATIVE VARIETY, WITH COOL TEMPERATURES OVER THE  
NORTHERN-CENTRAL PLAINS DURING THE FIRST HALF OF THE WEEK, THOUGH  
NO RECORDS ON THE COOL SIDE ARE EXPECTED. SOME LOCATIONS MAY SEE  
ONE OR TWO DAYS WITH HIGHS AROUND 10-15F BELOW NORMAL. THIS COOL  
AIR WILL MODIFY AS IT PROGRESSES EASTWARD, THOUGH REFRESHINGLY LOW  
DEW POINTS -- THE LOWEST IN 3-4 WEEKS -- ARE EXPECTED TO  
INFILTRATE THE MID-ATLANTIC STATES. THE WEST WILL TEND TO SEE  
ABOVE NORMAL MIN TEMPERATURES (NEAR RECORD WARM VALUES) MOST OF  
NEXT WEEK BUT WITH ABOVE NORMAL HIGHS TENDING TO BE EARLY IN THE  
WEEK. THE OVERALL EVOLUTION TOWARD LESS AMPLIFIED FLOW ALOFT OVER  
MUCH OF THE LOWER 48 BY LATE IN THE WEEK SHOULD LEAD TO MANY AREAS  
SEEING TEMPERATURES WITHIN A FEW DEGREES OF NORMAL BY NEXT FRIDAY  
AND SATURDAY.  
 
ROTH/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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