167  
FXUS02 KWBC 181600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SAT AUG 18 2018  
 
VALID 12Z TUE AUG 21 2018 - 12Z SAT AUG 25 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
RIDGING AND ASSOCIATED POSITIVE HEIGHT ANOMALIES ALOFT BUILDING  
OVER THE PACIFIC, ALIGNED NEAR 150W LONGITUDE, WILL BECOME AN  
INCREASINGLY PROMINENT FEATURE OVER THE COURSE OF THE PERIOD. AS  
SUPPORTED BY TELECONNECTIONS, MODEL/ENSEMBLE MEAN CONSENSUS SHOWS  
THE ESTABLISHMENT OF A MEAN TROUGH FROM WESTERN CANADA TO NEAR THE  
NORTHERN CALIFORNIA COAST BY LATE WEEK. MEANWHILE FAIRLY  
AMPLIFIED EASTERN U.S. TROUGHING TUE-THU SHOULD FILL RAPIDLY  
THEREAFTER DUE TO THE COMBINATION OF UPSTREAM EVOLUTION AND AN  
ATLANTIC RIDGE WHOSE WESTERN PERIPHERY BUILDS CLOSER TO THE EAST  
COAST. EMBEDDED WITHIN THIS OVERALL FLOW EVOLUTION WHICH HAS  
FAIRLY GOOD AGREEMENT/CONSISTENCY IN THE GUIDANCE, THERE ARE  
MULTIPLE UNCERTAINTIES FOR SPECIFICS OF THE FORECAST. THE UPPER  
RIDGE COVERING SOUTHERN PORTIONS OF THE WEST WILL PERSIST THOUGH  
THE PERIOD ALBEIT WITH A SLIGHTLY WEAKER TREND AFTER MIDWEEK.  
 
EVEN AS OF THE START OF THE PERIOD GUIDANCE SHOWS SOME SPREAD AND  
CONTINUITY ADJUSTMENTS FOR THE DETAILS OF THE ELONGATED  
TROUGH/UPPER LOW OVER THE NORTHWEST AS OF EARLY TUE. AS A WHOLE,  
LATEST RUNS APPEAR A LITTLE MORE DIFFUSE WITH THE FEATURE THAN IN  
FORECASTS FROM 24-36 HOURS AGO. LATEST GFS RUNS LEAN ON THE  
DIFFUSE SIDE OF THE GUIDANCE SPREAD, ULTIMATELY EJECTING ENERGY IN  
AT LEAST A COUPLE SEPARATE PIECES WHILE THE 00Z CMC IS THE MOST  
CONCENTRATED AND REFLECTIVE OF YESTERDAY'S CONSENSUS. SCALE OF  
THE ENERGY AND SOME DEGREE OF RIDGING EXPECTED TO THE NORTH EARLY  
IN THE PERIOD LOWER PREDICTABILITY SO CONFIDENCE IN SPECIFICS IS  
NOT VERY HIGH. ON THE POSITIVE SIDE THERE IS AT LEAST CONTINUITY  
IN THE IDEA THAT EJECTING ENERGY WILL SUPPORT MODEST SURFACE  
WAVINESS ACROSS THE EXTREME NORTHERN TIER MID-LATE PERIOD.  
 
BEHIND THIS EJECTING SHORTWAVE THERE IS REASONABLE  
AGREEMENT/CONSISTENCY WITH THE ARRIVAL OF A NORTHEASTERN PACIFIC  
TROUGH AROUND DAY 5 THU. AFTER THIS TIME HOWEVER MODEL/ENSEMBLE  
SPREAD STEADILY INCREASES DUE TO UNCERTAINTY OF SPECIFICS IN  
UPSTREAM SHORTWAVE ENERGY AND HOW IT WILL AFFECT THE DAY 5 THU  
TROUGH. GUIDANCE BEHAVIOR THUS FAR LENDS ITSELF TO LOW CONFIDENCE  
BUT 06Z GFS/00Z ECMWF SOLUTIONS THAT BRING ANOTHER SHORTWAVE INTO  
THE MEAN TROUGH POSITION BY DAY 7 SAT ARE CERTAINLY PLAUSIBLE AND  
MERIT INCLUSION WITH THE GEFS/ECMWF MEANS THROUGH THE END OF THE  
FORECAST.  
 
FARTHER EAST, MODEST GUIDANCE ADJUSTMENTS ARE ONGOING FOR THE  
EARLY PERIOD SURFACE LOW TRACKING NORTHEAST FROM THE CENTRAL GREAT  
LAKES, WITH AN EVOLUTION MORE COMMONLY SEEN IN THE COOL SEASON.  
THE PAST COUPLE DAYS OF RUNS HAVE BEEN TRENDING GRADUALLY SLOWER  
WHILE THE GUIDANCE SPREAD FOR DEPTH--AT LEAST WHILE OVER THE  
CENTRAL GREAT LAKES--HAS BEEN NARROWING IN ON THE MIDDLE OR  
SLIGHTLY WEAKER PORTION OF THE SPREAD FROM RECENT DAYS. THE MOST  
NOTICEABLE TREND IS IN THE ECMWF WHOSE OPERATIONAL RUNS HAVE  
TRENDED ABOUT 10MB WEAKER OVER THE PAST 36 HOURS TO YIELD A  
SOLUTION MUCH CLOSER TO LATEST GFS RUNS. THE 00Z UKMET/CMC STILL  
SHOW A SURFACE LOW THAT COULD BE STRONG ENOUGH TO CHALLENGE AUGUST  
LOW PRESSURE RECORDS THOUGH. AN INTERMEDIATE DEPTH STILL APPEARS  
TO BE THE BEST OPTION FOR THIS SYSTEM. AFTER SYSTEM PASSAGE,  
SOLUTIONS INCREASINGLY VARY WITH SHORTWAVE DETAILS OVER THE  
EASTERN U.S. AS WELL AS OVER CANADA (AFFECTING ANY FRONTS THAT  
COULD REACH NEW ENGLAND) ESPECIALLY AFTER DAY 4 WED. AS EASTERN  
U.S. HEIGHTS RISE EXPECT THE COLD FRONT TO STALL NEAR THE  
SOUTHEASTERN AND GULF COASTS WITH UNCERTAIN SPECIFICS ALOFT  
DETERMINING THE DETAILS FOR EMBEDDED WAVES.  
 
BASED ON THE LATEST ARRAY OF GUIDANCE THE FORECAST STARTED WITH  
THE 06Z GFS/00Z ECMWF AND LESSER INPUT FROM THE 00Z CMC AND 06Z  
GEFS/00Z ECMWF MEANS EARLY IN THE PERIOD, FOLLOWED BY PHASING OUT  
OF THE CMC AND INCREASING TOTAL ENSEMBLE MEAN WEIGHT TO 60 PERCENT  
BY DAYS 6-7 FRI-SAT.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
LOW PRESSURE TRACKING NORTHEAST FROM THE GREAT LAKES AND ITS  
TRAILING COLD FRONT WILL BRING SHOWERS AND THUNDERSTORMS INTO THE  
EASTERN STATES EARLY IN THE PERIOD, WITH SOME POCKETS OF STRONG  
AND/OR HEAVY CONVECTION POSSIBLE. DECELERATION OF THE FRONT  
TOWARD THE SOUTHERN MID-ATLANTIC/SOUTHEASTERN COASTS AND GULF  
COAST WILL LIKELY ALLOW FOR PERSISTENCE OF RAINFALL OVER THOSE  
AREAS. CURRENT GUIDANCE SUGGESTS BEST POTENTIAL FOR HIGHEST 5-DAY  
RAINFALL TOTALS ALONG THE NC/SC COAST. SOUTH OF THIS FRONT THERE  
WILL BE PERIODS OF CONVECTION OVER THE PENINSULA OF FLORIDA.  
TROUGH/UPPER LOW ENERGY INITIALLY OVER THE NORTHWEST MAY GENERATE  
SOME SCATTERED RAINFALL OVER NORTHERN AREAS OF THE WEST EARLY IN  
THE PERIOD, FOLLOWED BY POTENTIALLY HEAVIER LATE WEEK ACTIVITY  
FROM THE NORTHERN PLAINS INTO UPPER GREAT LAKES. THERE IS ALSO  
THE POSSIBILITY THAT SOME OF THE NORTHWEST ENERGY COULD ROUND THE  
LOWER LATITUDE WESTERN RIDGE AND GENERATE RAINFALL OVER PARTS OF  
THE SOUTHERN HALF OF THE PLAINS. MOST RAINFALL ASSOCIATED WITH  
THE UPPER TROUGH NEARING THE NORTHWEST BY THU SHOULD REMAIN NORTH  
OF THE CANADIAN BORDER. DEPENDING ON HOW THIS FEATURE PROGRESSES  
WITH TIME, IT MAY CONTRIBUTE TO NORTHERN TIER CONVECTION TOWARD  
FRI-SAT. EXPECT CONTINUED EPISODES OF DIURNALLY FAVORED  
CONVECTION OVER THE FOUR CORNERS STATES.  
 
PATTERN EVOLUTION WILL TEND TO HAVE A MODERATING EFFECT ON  
TEMPERATURES TOWARD LATE WEEK AND EARLY WEEKEND WITH MANY  
LOCATIONS SEEING MIN/MAX READINGS WITHIN A FEW DEGREES F OF NORMAL  
BY FRI-SAT. PARTS OF THE NORTHERN-CENTRAL PLAINS MAY SEE RATHER  
COOL TEMPERATURES TUE-WED WITH SOME LOCALIZED ANOMALIES IN THE  
MINUS 10-15F RANGE. SOME OF THIS COOL AIR WILL EXTEND EASTWARD  
BUT WITH LESS EXTREME ANOMALIES.  
 
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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