110  
FXUS02 KWBC 010659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT SAT SEP 01 2018  
 
VALID 12Z TUE SEP 04 2018 - 12Z SAT SEP 08 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST GUIDANCE CONTINUES TO ADVERTISE THE ESTABLISHMENT OF A MEAN  
TROUGH EXTENDING FROM NORTHERN/WESTERN CANADA INTO THE  
NORTHEASTERN PACIFIC, IN RESPONSE TO STRONG UPSTREAM RIDGING  
LIKELY TO EXTEND THROUGH THE EASTERN BERING SEA FOR MUCH OF THE  
PERIOD. THE EXPECTED MEAN PATTERN COMPARES FAIRLY WELL TO WHAT IS  
SUGGESTED BY TELECONNECTIONS RELATIVE TO THE RIDGE'S CORRESPONDING  
POSITIVE HEIGHT ANOMALY CENTER BY THE D+8 TIME FRAME. PERHAPS  
MORE OF A CONCERN SHORTLY AFTER THE CURRENT FORECAST PERIOD, THE  
NORTHERN PART OF THE ENVELOPE FOR WHERE THE POSITIVE HEIGHT  
ANOMALY MAY EXIST COULD ALLOW FOR SOME UNDERCUTTING FLOW THAT  
WOULD ULTIMATELY FLATTEN THE PATTERN A BIT OVER THE NORTHWESTERN  
U.S.. ANOTHER WILD CARD WILL BE THE EXTRATROPICAL TRANSITION OF  
CURRENT SUPER TYPHOON JEBI AND ITS DOWNSTREAM EFFECTS. CURRENT  
GUIDANCE INDICATES THAT THE RIDGE TO THE EAST OF THIS EVOLUTION IS  
WHAT REINFORCES THE EASTERN BERING SEA MEAN RIDGE LATE IN THE  
WEEK. MEANWHILE ANOMALOUSLY STRONG EASTERN U.S. RIDGING ALOFT  
THROUGH MIDWEEK WILL GRADUALLY WEAKEN LATER IN THE PERIOD.  
 
THERE IS STILL A FAIR DEGREE OF SHORTWAVE DETAIL UNCERTAINTY  
WITHIN THE BAND OF PROGRESSIVE FLOW BETWEEN THE EASTERN RIDGE  
ALOFT AND AXIS OF THE DEVELOPING MEAN TROUGH UPSTREAM. THUS FAR  
THERE HAVE BEEN SOME DAILY OSCILLATIONS FOR TIMING OF THE LEADING  
SHORTWAVE FORECAST TO CROSS CENTRAL CANADA AND EXTREME NORTHERN  
TIER OF THE LOWER 48 TUE-WED, WITH THE PAST DAY OF GUIDANCE  
TRENDING FASTER IN CONTRAST TO OPPOSITE TRENDS THAT WERE SEEN FROM  
THE PRIOR 24 HOURS. GUIDANCE CLUSTERING HAS IMPROVED OUT TO ABOUT  
DAY 4 WED BEFORE DETAILS DIVERGE OVER EASTERN CANADA. LATEST  
TRENDS LIKEWISE LEAD TO A FASTER PROGRESSION OF THE LEADING COLD  
FRONT ACROSS THE NORTHEASTERN QUADRANT OF THE LOWER 48. BEHIND  
THIS FEATURE, MOST SOLUTIONS AGREE IN PRINCIPLE ON AMPLIFYING THE  
NORTHEASTERN PACIFIC TROUGH ALOFT (WITH EMBEDDED UPPER LOW). THIS  
FEATURE WOULD THEN EJECT INLAND LATE IN THE PERIOD. EXACT TIMING  
BECOMES VERY UNCERTAIN DUE TO QUESTION MARKS WITH IMPORTANT  
UPSTREAM DETAILS, BUT FOR NOW THERE IS ACTUALLY DECENT AGREEMENT  
REGARDING SURFACE LOW PRESSURE REACHING THE VICINITY OF THE  
NORTHERN PLAINS/SOUTHERN CANADA BY DAY 7 SAT. TO THE SOUTH OF THE  
EASTERN RIDGE ALOFT, GFS/ECMWF RUNS HAVE CONVERGED DECENTLY--AT  
LEAST OUT TO WED/THU--WITH RESPECT TO THE MID LEVEL ENERGY AND  
WEAK SURFACE REFLECTION EXPECTED TO TRACK WESTWARD NEAR THE GULF  
COAST. OTHER MODELS AND SOME ENSEMBLE MEMBERS STILL OFFER  
ALTERNATE DETAILS, SUGGESTING ONGOING POTENTIAL FOR EVENTUAL  
CHANGES IN THE FORECAST RELATIVE TO CURRENT CONSENSUS/CONTINUITY.  
 
DURING DAYS 3-4 TUE-WED A BLEND OF OPERATIONAL MODEL GUIDANCE  
(INCLUDING THE 12Z FV3 PARALLEL GFS) REPRESENTED CONSENSUS WELL  
WHILE RESOLVING DETAIL/TIMING DIFFERENCES THAT WERE WITHIN TYPICAL  
GUIDANCE ERROR. THE 12Z CMC TRENDED OUT OF PHASE VERSUS CONSENSUS  
FOR SOME ASPECTS OF FLOW ALOFT MID-LATE PERIOD WHILE INCREASING  
DETAIL DIFFERENCES AMONG MODEL RUNS FAVORED INCORPORATION OF SOME  
ENSEMBLE MEAN GUIDANCE. THUS DAYS 5-7 THU-SAT USED 30-60 PERCENT  
TOTAL OF THE 18Z GEFS/12Z ECMWF MEANS TO GO ALONG WITH THE  
GRADUALLY DECREASING WEIGHT OF 18Z GFS/12Z PARALLEL GFS/12Z ECMWF.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
MOIST FLOW AROUND THE EASTERN RIDGE ALOFT WILL BRING EPISODES OF  
MEANINGFUL RAINFALL ACROSS FLORIDA AND THE GULF COAST REGION AS  
WELL AS THE SOUTHERN HIGH PLAINS AND EXTENDING NORTHEASTWARD  
THROUGH THE UPPER MIDWEST/GREAT LAKES, AND EVENTUALLY THE  
NORTHEAST/MID-ATLANTIC AS THE RIDGE WEAKENS. THE WEAK SURFACE  
WAVE AND ENERGY ALOFT PROGRESSING WESTWARD ALONG THE GULF COAST  
SHOULD ENHANCE ACTIVITY OVER THAT REGION WHILE A SURFACE TROUGH  
MAY APPROACH FLORIDA LATE IN THE PERIOD. WEAK IMPULSES ALOFT WILL  
PLAY A ROLE IN LOCALLY HEAVY SOUTHERN HIGH PLAINS CONVECTION.  
CURRENTLY THE MOST COHERENT SIGNAL FOR HEAVY RAIN POTENTIAL  
APPEARS TO BE FROM THE CENTRAL PLAINS INTO UPPER GREAT LAKES, WITH  
A FRONTAL SYSTEM APPROACHING/CROSSING THE REGION EARLY IN THE  
PERIOD AND THEN STALLING OVER MID-LATITUDES OF THE CENTRAL-EASTERN  
U.S. BEFORE THE CENTRAL U.S. PART BEGINS LIFTING NORTHEAST AS A  
WARM FRONT. SOME LOCATIONS WITHIN THIS AREA HAVE ALREADY RECEIVED  
SIGNIFICANT RAINFALL RECENTLY, INCREASING THE POTENTIAL FOR  
HYDROLOGIC CONCERNS FROM THIS COMING WEEK'S RAINFALL. EXPECT SOME  
SHOWERS/STORMS TO REACH THE NORTHEAST/MID-ATLANTIC BY LATE  
WEEK/WEEKEND. NEW ENGLAND MAY SEE SOME LIGHTER RAINFALL EARLY IN  
THE PERIOD WITH A LEADING FRONT. THE PACIFIC NORTHWEST MAY SEE  
LIGHT PRECIPITATION LATE IN THE PERIOD WITH THE ARRIVAL OF  
SHORTWAVE ENERGY ALOFT.  
 
WARMEST TEMPERATURES RELATIVE TO NORMAL WILL EXTEND FROM THE  
MIDWEST INTO THE GREAT LAKES/MID-ATLANTIC/NORTHEAST WITH PLUS  
5-15F ANOMALIES. DEPARTURES FROM NORMAL SHOULD GENERALLY BE A  
LITTLE GREATER FOR MORNING LOWS, WITH TUE-WED BEING THE MOST  
LIKELY DAYS WHEN A NUMBER OF LOCATIONS MAY SEE NEW DAILY RECORDS  
FOR WARM LOWS. MEANWHILE THE COLD FRONT INITIALLY TRACKING  
EAST/SOUTHEAST FROM THE NORTHERN PLAINS WILL BRING BELOW NORMAL  
READINGS TO PARTS OF THE CENTRAL U.S. WITH SOME MINUS 10-15F  
ANOMALIES OVER THE NORTHERN PLAINS IN PARTICULAR TUE-WED.  
 
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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