290  
FXUS02 KWBC 100556  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
155 AM EDT FRI AUG 10 2018  
 
VALID 12Z MON AUG 13 2018 - 12Z FRI AUG 17 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A PAIR OF STAGNANT UPPER LOWS WILL INITIALLY INHABIT THE MAP TO  
COMMENCE THE WEEK. THESE SHOULD BE FIXED OVER THE CENTRAL  
APPALACHIANS AND SOUTHERN HIGH PLAINS, RESPECTIVELY. BOTH FEATURES  
ARE EXPECTED TO GRADUALLY EVOLVE INTO OPEN WAVES WITH GRADUAL  
EASTWARD MIGRATION THE FOLLOWING DAYS. THE MOST STRONGLY FORCED  
SYSTEMS SHOULD REMAIN ALONG OR NORTH OF THE INTERNATIONAL BORDER  
WITH CANADA. STRONG HEIGHT FALLS MOVING ACROSS THE CANADIAN  
PRAIRIES ON MONDAY WILL EVENTUALLY LEAD TO AMPLIFIED FLOW SITTING  
OVER QUEBEC BY MID-WEEK WITH POSSIBLE SOUTHWARD EXTENSION INTO  
UPPER NEW ENGLAND. LURKING FARTHER UPSTREAM, A RETURN TO AMPLIFIED  
TROUGHING IS POSSIBLE BY DAY 7/AUGUST 17 ACROSS THE PACIFIC  
NORTHWEST AND BRITISH COLUMBIA. HOWEVER, THIS FEATURE IS MUCH MORE  
UNCERTAIN AND VERY MODEL DEPENDENT. ACROSS THE DESERT  
SOUTHWEST/FOUR CORNERS REGION, MODELS ADVERTISE A RETURN OF A  
594-DM MID-LEVEL RIDGE TOWARD THE MIDDLE/LATER PART OF NEXT WEEK.  
 
REGARDING THE UPPER LOW SITTING WEST OF THE MID-ATLANTIC, THERE IS  
AN EASTWARD TREND IN THE 12Z ENSEMBLE SOLUTIONS BY TUESDAY. THE  
12Z ECMWF AND MOST ITS ENSEMBLE MEMBERS AS WELL AS THE 12Z UKMET  
FAVOR A SLOWER PROGRESSION. THIS WOULD APPEAR A BIT MORE  
REASONABLE GIVEN THE TENDENCY FOR CUT-OFF LOWS TO BE SLOWER THAN  
THE GUIDANCE EXPECTS. AS THE MENTIONED UPPER LOW INITIALLY OVER  
THE SOUTHERN HIGH PLAINS BEGINS ITS EASTWARD MIGRATION, THIS WILL  
ULTIMATELY FORCE THE MID-ATLANTIC SYSTEM OUT TO SEA. THE LARGER  
MODEL UNCERTAINTIES LIE WITHIN THE ACTIVE FLOW BORDERING CANADA.  
REGARDING HEIGHT FALLS PUSHING FROM SASKATCHEWAN TO QUEBEC, THE  
PAST COUPLE OF ECMWF RUNS HAVE BEEN MORE AGGRESSIVE WHICH WOULD  
AID IN THE PRODUCTION OF A SUB-1000 MB CYCLONE JUST NORTH OF MAINE  
BY THURSDAY. HOWEVER, THE GFS SOLUTIONS REMAIN MORE SUBDUED WHICH  
WOULD CAST SOME DOUBT ON THE VALIDITY OF THE ECMWF. FARTHER WEST,  
ENSEMBLE SPAGHETTI PLOTS SHOW TREMENDOUS SCATTER WITH PHASE  
DIFFERENCES EVIDENT AS WELL. THE 12Z ECMWF ENSEMBLE MEAN DEPICTS  
MEAN RIDGING EXTENDING ACROSS WESTERN NORTH AMERICA WHILE THE 18Z  
GEFS/12Z NAEFS MEANS SHOW MORE ZONAL FLOW. CONSIDERING THE PAST  
FOUR GFS RUNS, THE PREVIOUS TWO FAVORED TROUGHING WHILE PRECEDING  
CYCLES SHOWED RIDGING. SUFFICE TO SAY, A VERY CHALLENGING  
FORECAST. FARTHER SOUTH, WHILE THE 12Z CMC DEPICTS A SHORTWAVE  
MOVING INTO CALIFORNIA, THE 18Z GFS/12Z ECMWF BEG TO DIFFER WITH A  
RETURN OF THE 594-DM MID-LEVEL RIDGE DURING THE DAY 6/7, AUGUST  
16-17 PERIOD. WITH THE ARRIVAL OF THE 00Z CMC, IT HAS SINCE BACKED  
OFF ON THIS LOW-AMPLITUDE TROUGH SCENARIO.  
 
WITH REASONABLE MODEL CLUSTERING THROUGH DAY 4/TUESDAY, WENT WITH  
A COMBINATION OF OPERATIONAL SOLUTIONS. THIS CONSISTED OF A  
COMBINATION OF THE 18Z/12Z GFS AND 12Z ECMWF/CMC/UKMET WITH  
GREATER WEIGHTING ON THE SLOWER SOLUTIONS ACROSS THE EAST.  
THEREAFTER, GROWING SPREAD WITHIN THE ACTIVE SOUTHERN CANADIAN  
FLOW SUPPORTED INCLUSION OF ENSEMBLE MEANS. THERE WAS A TENDENCY  
TO INCLUDE A BIT MORE OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN TO HELP  
LOWER PRESSURES A BIT ACROSS EASTERN NORTH AMERICA BY THE MIDDLE  
OF NEXT WEEK. ALSO, THIS WOULD HELP BUILD HEIGHTS SLIGHTLY MORE  
OVER THE WESTERN U.S. WHICH HAS BEEN THE PREFERRED RIDGE LOCATION  
THIS SUMMER.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
DURING THE UPCOMING WEEK, BELOW AVERAGE TEMPERATURES SHOULD  
CONGREGATE ACROSS THE SOUTH-CENTRAL U.S. GIVEN WIDESPREAD CLOUD  
COVER AND RAINFALL IN RESPONSE TO THE SLOW MOVING UPPER LOW.  
ACROSS THE SOUTHERN PLAINS, SOME LOCATIONS MAY STRUGGLE TO GET OUT  
OF THE UPPER 70S TO LOWER 80S ON MONDAY WITH DEPARTURES AROUND 10  
TO 15 DEGREE BELOW CLIMATOLOGY. ALTHOUGH THESE ANOMALIES SHOULD  
BECOME LESS IMPRESSIVE DOWNSTREAM, COOLER CONDITIONS WILL PREVAIL  
GIVEN THE CONTINUED PRECIPITATION THREATS. FARTHER NORTH FROM THE  
PACIFIC NORTHWEST EASTWARD TO THE GREAT LAKES, INTERMITTENT  
PERIODS OF ABOVE AVERAGE TEMPERATURES ARE EXPECTED NEXT WEEK. IN  
PARTICULAR, MONDAY AS WELL AS LATE IN THE WEEK IT COULD BE AROUND  
10 TO 15 DEGREES ABOVE MID-AUGUST CLIMATOLOGY ACROSS THE  
NORTH-CENTRAL U.S. WITH HIGHS SOARING INTO THE LOW 90S.  
 
THE MOST FOCUSED AREAS OF RAINFALL SHOULD INITIALLY CONGREGATE  
WITH THE MENTIONED PAIR OF UPPER LOWS ACROSS THE MID-ATLANTIC AND  
SOUTH-CENTRAL U.S. EVENTUALLY THE BIGGER THREAT WILL EMERGE ALONG  
THE SLOW MOVING COLD FRONT WHICH TRACKS TOWARD THE EASTERN  
SEABOARD DURING THE WEEK. SOME OPERATIONAL MODELS ARE QUITE  
BULLISH WITH QPF ACROSS MISSOURI INTO ILLINOIS AND INDIANA WITH 2  
TO 3 INCH 24-HOUR AMOUNTS. OVER THE PACIFIC NORTHWEST, THE 12Z  
ECMWF RIDGE WOULD SUPPORT A DRY PERIOD ALTHOUGH A GREAT DEAL OF  
SPREAD REMAINS WHICH HOLDS DISCERNIBLE CONFIDENCE AT BAY. ACROSS  
THE DESERT SOUTHWEST, DAILY CHANCES FOR MONSOONAL THUNDERSTORMS  
ARE LIKELY ALTHOUGH MODEL AMOUNTS ARE ON THE LOWER SIDE OF THE  
SPECTRUM.  
 
RUBIN-OSTER  
 
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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