682  
FXUS02 KWBC 171600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT FRI AUG 17 2018  
 
VALID 12Z MON AUG 20 2018 - 12Z FRI AUG 24 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
AN AMPLIFIED PATTERN OVER THE NORTHEASTERN PACIFIC AND  
NORTHWESTERN NORTH AMERICA FROM THE SHORT RANGE PERIOD INTO EARLY  
NEXT WEEK WILL ENCOURAGE DEEPENING OF A TROUGH OVER THE EASTERN  
PART OF THE CONTINENT WITH GREATEST DEPTH LIKELY TO BE ACHIEVED  
AROUND MIDWEEK. AN UPPER TROUGH WITH EMBEDDED LOW WILL LINGER  
OVER THE NORTHERN-CENTRAL WEST COAST DURING THE FIRST PART OF NEXT  
WEEK, WITH ENERGY FORECAST TO EJECT EASTWARD MID-LATE WEEK AS  
UPSTREAM TROUGHING ARRIVES INTO THE NORTHWEST U.S. AND WESTERN  
CANADA. TELECONNECTIONS RELATIVE TO THE STRENGTHENING  
EAST-CENTRAL PACIFIC RIDGE DURING THE LATTER HALF OF THE PERIOD  
PROVIDE SUPPORT FOR THE CONSENSUS TROUGH POSITION BY NEXT FRI AS  
WELL AS THE EVENTUAL FILLING OF THE EASTERN TROUGH. THE MEAN  
RIDGE OVER THE ROCKIES, ANCHORED BY A HIGH OVER OR NEAR NEW MEXICO  
FOR ONE OR MORE DAYS, SHOULD BECOME A LITTLE WEAKER/MORE  
SUPPRESSED WITH TIME.  
 
DURING THE FIRST HALF OF THE PERIOD GUIDANCE CONTINUES TO SIGNAL A  
MIDWEST THROUGH GREAT LAKES INTO EASTERN CANADA SYSTEM THAT MAY BE  
QUITE VIGOROUS COMPARED TO TYPICAL AUGUST SURFACE LOWS. HOWEVER  
SOLUTIONS THAT HAD BEEN ON THE STRONG SIDE OF THE ENVELOPE  
(ECMWF/UKMET AND AT TIMES THE CMC) HAVE EITHER BACKED OFF A BIT OR  
DELAYED DEVELOPMENT BY A FEW HOURS. THIS REDUCES THE PERCENTAGE  
OF GUIDANCE SHOWING THE POTENTIAL FOR SURFACE LOW PRESSURE TO BE  
DEEPER THAN CURRENT RECORDS FOR THE MONTH OVER THE CENTRAL GREAT  
LAKES AROUND TUE. EVEN THE ECMWF MEAN HAS RAISED ITS CENTRAL  
PRESSURE BY A FEW MB VERSUS ITS PRIOR RUN AS OF 12Z TUE. ON THE  
OTHER HAND THE 06Z GFS AND PARALLEL FV3 00Z GFS ARE CLOSER TO  
CONSENSUS THAN THE WEAK/SUPPRESSED 00Z OPERATIONAL GFS. THIS HAS  
BEEN THE CASE OVER AT LEAST THE PAST 2-3 DAYS. ONGOING  
UNCERTAINTY OVER EXACTLY HOW CANADIAN FLOW WILL INTERACT WITH THE  
CENTRAL U.S. SHORTWAVE RECOMMENDS MAINTAINING AN INTERMEDIATE  
SOLUTION--WHICH FOR THIS RUN IS SLIGHTLY WEAKER THAN IN THE  
PREVIOUS CYCLE. CONTINUITY LOOKS GOOD FOR THE TRAILING COLD FRONT  
THAT SHOULD REACH THE EAST COAST/GULF COAST BY WED-THU. THE FRONT  
WILL LIKELY STALL BY FRI AS EASTERN U.S. HEIGHTS ALOFT RISE, BUT  
WITH LOW CONFIDENCE FOR EXACT POSITION AND DETAILS OF ANY EMBEDDED  
WAVES.  
 
REGARDING THE TROUGH/UPPER LOW INITIALLY OVER THE WEST COAST,  
SCATTER AMONG SOLUTIONS APPEARS TYPICAL FOR THE TIME FRAME  
INVOLVED. MEAN RIDGING OVER THE INTERIOR WEST/ROCKIES MAY ARGUE  
AGAINST THE MOST AMPLIFIED SIDE OF THE ENVELOPE (00Z UKMET AND  
SOME 00Z ECMWF ENSEMBLES) AS THE SHORTWAVE EJECTS EASTWARD BUT  
THERE IS AN OVERALL SIGNAL THAT THE FEATURE WILL MAINTAIN BETTER  
DEFINITION THAN IN PREVIOUS CYCLES AS IT CONTINUES EASTWARD ACROSS  
THE NORTHERN TIER THROUGH THU-FRI. THIS TREND LEADS TO A MORE  
DEFINED SURFACE REFLECTION ACROSS THE NORTHERN PLAINS/UPPER MS  
VALLEY LATE IN THE WEEK. UPSTREAM, AS MENTIONED EARLIER THE  
INCOMING PACIFIC TROUGH SETTLES INTO A DAY 7 FRI POSITION (WESTERN  
CANADA INTO NORTHWEST U.S.) THAT COMPARES WELL TO TELECONNECTIONS  
RELATIVE TO THE UPSTREAM RIDGE. AN AVERAGE AMONG MODEL/ENSEMBLE  
MEAN SOLUTIONS SHOULD PROVIDE A GOOD STARTING POINT FOR THE  
FORECAST HERE.  
 
THE 06Z GFS AND 00Z ECMWF/ECMWF MEAN/CMC PROVIDED THE INPUT FOR  
TODAY'S MASS FIELD FORECAST DURING THE FIRST HALF OF THE PERIOD,  
YIELDING THE DESIRED CONSENSUS OR INTERMEDIATE SOLUTION AS  
APPROPRIATE. THIS BLEND LEANED TOWARD A MORE EVEN BLEND OF  
MODEL/ENSEMBLE MEAN SOLUTIONS LATER IN THE PERIOD AS BOTH CLUSTERS  
OF GUIDANCE SEEMED TO PROVIDE REASONABLE IDEAS BUT WITH  
OPERATIONAL MODEL DETAIL SPECIFICS BECOMING LESS UNCERTAIN.  
 
   
..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.  
DURING THE FIRST HALF OF THE WEEK. 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  
THROUGH LATE 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. SOME  
LOCATIONS MAY SEE ONE OR TWO DAYS WITH HIGHS AROUND 10-15F BELOW  
NORMAL. THIS COOL AIR WILL MODIFY AS IT PROGRESSES EASTWARD. THE  
WEST WILL TEND TO SEE ABOVE NORMAL MIN TEMPERATURES 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 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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