254  
FXUS02 KWBC 141558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1158 AM EDT TUE AUG 14 2018  
 
VALID 12Z FRI AUG 17 2018 - 12Z TUE AUG 21 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
GUIDANCE CONTINUES TO AGREE ON A STEADY AMPLIFICATION OF THE LARGE  
SCALE PATTERN TOWARD A CENTRAL/EAST-CENTRAL NORTH AMERICA TROUGH.  
THIS EVOLUTION WILL BE IN RESPONSE TO A DEEP UPPER TROUGH/LOW THAT  
DEVELOPS TO THE SOUTH OF THE ALASKA PENINSULA AND THE DOWNSTREAM  
RIDGE WHICH THEN BUILDS OVER WESTERN CANADA AND VICINITY. WITHIN  
THIS AGREEABLE SCENARIO A FAIR AMOUNT OF SPREAD ARISES AMONG  
INDIVIDUAL MODEL RUNS AND ENSEMBLE MEMBERS FOR SPECIFICS BY THE  
LATTER HALF OF THE PERIOD.  
 
DURING THE FIRST HALF OF THE FORECAST (FRI INTO EARLY SUN) A  
GENERAL BLEND OF LATEST OPERATIONAL MODELS PROVIDED A GOOD  
STARTING POINT TO REPRESENT CONSENSUS IDEAS-- EASTWARD PROGRESSION  
OF AN INITIAL GREAT LAKES TO PLAINS SHORTWAVE PUSHING ALONG A WAVY  
FRONT, A WEST COAST SHORTWAVE ALOFT EMERGING INTO THE HIGH PLAINS  
BY SUN AND BEGINNING TO SUPPORT A DEFINED SURFACE WAVE, AND  
CANADIAN ENERGY STARTING TO PUSH A FRONTAL BOUNDARY INTO THE  
NORTHERN PLAINS.  
 
AFTER EARLY SUN THE PREFERRED GUIDANCE BLEND RAPIDLY TRENDED  
TOWARD THE 06Z GEFS/00Z ECMWF MEANS WITH A PRONOUNCED INCREASE IN  
GUIDANCE SPREAD AND RUN TO RUN VARIABILITY FOR DETAILS WITHIN THE  
AMPLIFYING NORTH AMERICAN TROUGH AS WELL AS ACROSS PORTIONS OF THE  
WEST. OVER THE CENTRAL/EASTERN STATES THE BIG QUESTION WILL BE  
HOW NORTHERN STREAM FLOW PUSHING SOUTH FROM CANADA INTERACTS WITH  
SHORTWAVE ENERGY/SURFACE LOW PRESSURE EXPECTED TO BE OVER THE  
PLAINS AROUND SUN-MON. FOR THE TIME BEING THE ENSEMBLE MEANS  
OFFER GOOD AGREEMENT/CONTINUITY WITH LOW PRESSURE REACHING THE  
GREAT LAKES BY EARLY DAY 7 TUE. HOWEVER THE FULL SPREAD OF  
ENSEMBLE MEMBERS/MODEL RUNS IS SUFFICIENTLY GREAT TO KEEP  
CONFIDENCE IN SPECIFICS IN THE MIDDLE TO LOWER PORTION OF THE  
SPECTRUM. OVER THE WEST COAST AND FARTHER INLAND, THE ENSEMBLE  
MEANS AGREE FAIRLY WELL WITH TELECONNECTION-FAVORED FLOW RELATIVE  
TO UPSTREAM FEATURES. IN PARTICULAR THEY SHOW THE SPLIT FLOW  
CONFIGURATION SUGGESTED BY NEGATIVE HEIGHT ANOMALIES THAT BECOME  
ESTABLISHED NEAR THE CENTRAL ALEUTIANS IN THE D+8 MULTI-DAY MEANS.  
AFTER PASSAGE OF THE WEEKEND SHORTWAVE THERE IS NOT A LOT OF  
SUPPORT FROM THE FAVORED PATTERN FOR ADDITIONAL SHORTWAVE ENERGY  
OVER THE INTERIOR WEST SUCH AS SEEN IN THE 00Z PARALLEL (FV3) GFS  
OR A MINORITY OF ENSEMBLE MEMBERS.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
GREAT LAKES/NEW ENGLAND LOW PRESSURE DURING THE FIRST HALF OF THE  
PERIOD AND TRAILING FRONTAL BOUNDARY THAT EVENTUALLY STALLS OVER  
THE PLAINS THEN RETURNS NORTHEAST AS A WARM FRONT WILL PROVIDE A  
MULTI-DAY FOCUS FOR SHOWERS AND THUNDERSTORMS. WEST COAST  
SHORTWAVE ENERGY THAT LIKELY REACHES THE PLAINS BY SUN SHOULD  
PROVIDE ADDITIONAL ENCOURAGEMENT FOR ORGANIZED CONVECTION. OVER  
THE FIVE-DAY PERIOD THERE WILL BE A FAIRLY BROAD AREA WITH SOME  
DEGREE OF HEAVY RAINFALL POTENTIAL FROM THE HIGH PLAINS INTO PARTS  
OF THE EAST. LESS AVAILABLE MOISTURE SHOULD EQUATE TO LIGHTER  
RAINFALL ALONG ONE OR MORE FRONTS DROPPING INTO THE NORTHERN  
STATES FROM CANADA. ELSEWHERE, THE AFOREMENTIONED WEST COAST  
SHORTWAVE MAY PRODUCE SOME RAINFALL OVER PARTS OF THE  
NORTHWEST/NORTHERN ROCKIES EARLY IN THE PERIOD. TERRAIN/DIURNAL  
HEATING WILL PROMOTE SHOWERS/STORMS OVER PARTS OF THE SOUTHWEST  
WITH EXTREME SOUTHERN AREAS LIKELY TO SEE THE HIGHEST RAINFALL  
TOTALS WITH GREATER AVAILABLE MOISTURE. FLORIDA AND THE GULF  
COAST REGION SHOULD SEE DIURNAL ACTIVITY AS WELL.  
 
THE LARGE SCALE PATTERN EVOLUTION WILL BRING AN EXPANDING AREA OF  
BELOW NORMAL TEMPERATURES TO THE CENTRAL U.S. BY SUN-TUE.  
CURRENTLY EXPECT THE CORE OF COOLEST HIGH TEMPERATURES (10-15F  
BELOW NORMAL) TO BE CENTERED OVER THE NORTH-CENTRAL HIGH PLAINS  
SUN-MON. BEFORE THIS TREND EXPECT WARM READINGS (UP TO 10-15F  
ABOVE NORMAL) ACROSS EXTREME NORTHERN AREAS LATE THIS WEEK.  
DAYTIME HIGHS OVER THE INTERIOR WEST WILL TREND GRADUALLY WARMER  
WITH TIME AS RIDGING ALOFT REBUILDS WHILE MIN TEMPS SHOULD BE  
ABOVE NORMAL THROUGH THE PERIOD. THE EAST WILL LIKELY BE WITHIN A  
FEW DEGREES OF NORMAL FOR HIGHS ON MOST DAYS BUT WITH PERIODS OF  
CLOUDS/RAINFALL HELPING TO KEEP MIN TEMPS WARMER RELATIVE TO  
NORMAL.  
 
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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