071  
FXUS02 KWBC 061559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1158 AM EDT WED JUN 06 2018  
 
VALID 12Z SAT JUN 09 2018 - 12Z WED JUN 13 2018  
   
..PATTERN OVERVIEW
 
 
OVER THE COURSE OF THE PERIOD MOST GUIDANCE EXPECTS THE AMPLIFIED  
TROUGH-RIDGE-TROUGH PATTERN IN PLACE AT THE START OF THE WEEKEND  
TO EVOLVE TOWARD A MORE TYPICAL SUMMERTIME PATTERN WITH  
PROGRESSIVE FLOW ACROSS THE NORTHERN TIER STATES. HOWEVER THE  
DETAILS OF HOW THIS OCCURS ARE UNCERTAIN GIVEN SIGNIFICANT  
MODEL/ENSEMBLE CHANGES OVER THE PAST 12-24 HOURS. MEANWHILE UPPER  
RIDGING ANCHORED OVER THE SOUTHERN PLAINS AS OF SAT SHOULD WEAKEN  
THROUGH THE WEEKEND, AND THEN AFTER PASSAGE OF THE VIGOROUS UPPER  
TROUGH THROUGH THE WEST, REBUILD WITH A CENTER MOST LIKELY  
POSITIONED OVER THE SOUTHERN ROCKIES. FARTHER EASTWARD AN UPPER  
WEAKNESS OVER THE NORTHEASTERN GULF OF MEXICO/SOUTHEAST U.S. WILL  
GRADUALLY BECOME MORE ILL-DEFINED WITH TIME.  
 
   
..MODEL GUIDANCE/PREDICTABILITY EVALUATION/PREFERENCES
 
 
FROM THE LARGE SCALE EVOLUTION PERSPECTIVE THE GREATEST  
UNCERTAINTY ARISES BY THE LATTER HALF OF THE PERIOD, INVOLVING THE  
STRONG TROUGH ENERGY EMERGING FROM THE WESTERN U.S. AND SHAPE OF  
THE DOWNSTREAM RIDGE. OVER THE PAST 12-24 HOURS GFS/ECMWF RUNS  
AND THEIR ENSEMBLE MEANS HAVE CHANGED CONSIDERABLY TOWARD THE IDEA  
OF ALLOWING SOME OF THE EJECTING WESTERN U.S. FLOW TO CUT THROUGH  
THE DOWNSTREAM RIDGE AS AN UPPER HIGH CLOSES OFF OVER OR  
NORTH/NORTHWEST OF HUDSON BAY. THIS TREND IS QUITE PRONOUNCED  
JUST GOING FROM THE 00Z TO 06Z CYCLE OF THE GEFS MEAN. THE  
MULTI-DAY TREND TOWARD FASTER PROGRESSION OF THE WESTERN TROUGH  
FITS THIS EVOLUTION. THE 00Z CMC ENSEMBLE MEAN IN PARTICULAR AND  
PERHAPS TO SOME EXTENT OPERATIONAL CMC APPEAR TOO SLOW WITH THE  
OVERALL PATTERN GIVEN REMAINING GUIDANCE AND TRENDS, BUT THEY DO  
PROVIDE SOME RESERVATIONS ABOUT GOING COMPLETELY TOWARD THE LATEST  
MAJORITY SCENARIO WITHOUT ADDITIONAL MODEL RUNS TO BUILD SOME  
DEGREE OF CONFIDENCE.  
 
AS FOR SOME FINER DETAILS... GFS RUNS APPEAR SOMEWHAT  
OVER-DEVELOPED WITH A WAVE EXPECTED OVER THE LOWER GREAT LAKES AS  
OF EARLY SAT SO THE FORECAST BLEND DOWNPLAYS THAT ASPECT OF THE  
GFS. ALSO DOWNPLAYED FROM GFS GUIDANCE IS A WEST OF CONSENSUS  
TRACK FOR A COMPACT UPPER LOW THAT DROPS SOUTHWARD OVER EASTERN  
CANADA. WHILE IT FITS INTO MULTI-DAY TRENDS, THE 00Z ECMWF  
BECOMES FASTER THAN OTHER SOLUTIONS WITH NORTHERN PLAINS/UPPER MS  
VALLEY SHORTWAVE ENERGY BY EARLY NEXT WEEK. FARTHER UPSTREAM  
MODEL/ENSEMBLE SOLUTIONS ARE STILL CHAOTIC WITHIN A GENERALLY  
AGREED UPON BAND OF LOW AMPLITUDE PROGRESSIVE FLOW. THE  
COMBINATION OF NORTHERN PACIFIC AND NORTHERN CANADA HEIGHT ANOMALY  
CENTERS IN LATEST D+8 CHARTS SEEM TO TELECONNECT WITH A TENDENCY  
TOWARD MODEST TROUGHING NEAR THE WEST COAST AROUND OR JUST AFTER  
THE END OF THE FORECAST PERIOD. LATEST GEFS/ECMWF MEANS BEST  
REFLECT THIS IDEA. ELSEWHERE, BY THE END OF THE PERIOD GFS RUNS  
CENTER THE SOUTHERN ROCKIES RIDGE FARTHER EAST THAN MOST OTHER  
SOLUTIONS. BY DAY 7 WED GFS/GEFS MEMBERS ARE SHOWING SIGNALS OF  
LOW PRESSURE/MOISTURE EMERGING FROM THE WESTERN CARIBBEAN. THIS  
APPEARS SIMILAR TO THEIR BEHAVIOR LEADING UP TO ALBERTO, WITH  
PREMATURE DEVELOPMENT/MOISTURE TRANSPORT. GEFS MEMBERS HAVE  
ALREADY TRENDED SLOWER VERSUS YESTERDAY AND THE CMC--USUALLY A  
GOOD "EXTREME BOUND" GIVEN ITS HISTORICAL PROCLIVITY TOWARD  
TROPICAL DEVELOPMENT--IS STILL VERY SUPPRESSED THROUGH NEXT WED.  
OTHER SOLUTIONS SHOW MINIMAL DEVELOPMENT AS WELL. THUS AT THIS  
TIME THE PROBABILITY OF THE GFS/GEFS SCENARIO APPEARS QUITE LOW.  
 
BASED ON THE ABOVE CONSIDERATIONS, THE FORECAST STARTED WITH  
VARIOUS COMPONENTS OF THE 00Z ECMWF/00Z-06Z GFS/00Z UKMET WITH A  
LITTLE 00Z GEFS/ECMWF ENSEMBLE INPUT EARLY-MID PERIOD. THE LATTER  
PART OF THE FORECAST INCREASED WEIGHT OF THE ENSEMBLE MEANS WHILE  
ALSO INCORPORATING SOME ELEMENTS OF CONTINUITY TO TEMPER RECENT  
CHANGES IN GUIDANCE FOR THE EVOLUTION OF THE NORTHERN  
U.S./EAST-CENTRAL CANADA PATTERN.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE STRONG UPPER TROUGH CROSSING THE NORTHWEST WILL BRING AN  
EPISODE OF ACTIVE WEATHER THIS WEEKEND WITH HIGHEST PRECIP TOTALS  
LIKELY TO BE OVER FAVORED TERRAIN IN WA/OR AND THE NORTHERN  
ROCKIES. SOME OF THE HIGHEST ELEVATIONS COULD SEE A LITTLE SNOW  
FROM THIS EVENT. THE REST OF THE PERIOD COULD FEATURE AN  
OCCASIONAL PERIOD OF RAIN OVER EXTREME NORTHERN AREAS BUT WITH LOW  
CONFIDENCE ON EXISTENCE/TIMING GIVEN THE FLAT NATURE OF MEAN FLOW  
ALOFT.  
 
THE COMBINATION OF SHORTWAVE ENERGY ALOFT AND WAVY FRONT FROM THE  
NORTH-CENTRAL PLAINS TO EAST COAST WILL PROMOTE AREAS OF SHOWERS  
AND STORMS/MESOSCALE CONVECTIVE COMPLEXES DURING THE WEEKEND, WITH  
SOME LOCALLY HEAVY RAINFALL. OVER THE EAST THE CONVECTIVE THREAT  
WILL PUSH SOUTHWARD ALONG WITH THE SURFACE FRONT, WHILE THE SYSTEM  
EMERGING FROM THE WEST SHOULD GENERATE ANOTHER PERIOD OF  
CONVECTION FROM THE NORTHERN PLAINS INTO MID-UPPER MS VALLEY, AND  
PERHAPS FARTHER EASTWARD, FROM ABOUT SUN NIGHT ONWARD. EXPECT  
DIURNALLY FAVORED CONVECTION OVER PARTS OF THE SOUTHEAST/FL WITH  
SOME ENHANCEMENT FROM THE WEAKNESS ALOFT.  
 
EXPECT THE MOST EXTREME TEMPERATURE ANOMALIES TO BE ASSOCIATED  
WITH THE STRONG UPPER TROUGH PASSING THROUGH THE WEST DURING THE  
WEEKEND. READINGS SHOULD REACH 10-20F ABOVE NORMAL IN THE LEADING  
WARM SECTOR FROM THE EASTERN SLOPES OF THE NORTHERN-CENTRAL  
ROCKIES INTO THE PLAINS. SOME LOCATIONS IN THE NORTHERN PLAINS  
MAY SEE RECORD WARM MINS AROUND SUN-MON. COLD FRONTAL PASSAGE  
WILL BRING A DAY OR TWO OF MINUS 10-15F ANOMALIES FOR HIGHS FROM  
THE NORTHERN HALF OF THE WEST COAST INTO THE NORTHERN ROCKIES.  
RISING HEIGHTS OVER THE WEST/SOUTHERN ROCKIES RIDGING ALOFT SHOULD  
LEAD TO ABOVE NORMAL TEMPERATURES OVER MUCH OF THE WEST BY NEXT  
TUE-WED. MEANWHILE THE MEAN TROUGH ALOFT OVER THE NORTHEAST AND  
VICINITY WILL LIKELY KEEP HIGHS NEAR TO BELOW NORMAL INTO EARLY  
NEXT WEEK ALONG THE EAST COAST.  
 
RAUSCH  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES CAN BE FOUND AT:  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
 

 
 
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