808  
FXUS02 KWBC 181536  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1136 AM EDT MON JUN 18 2018  
 
VALID 12Z THU JUN 21 2018 - 12Z MON JUN 25 2018  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS SHOW BROAD AGREEMENT ON THE LARGE SCALE FLOW PATTERN ACROSS  
THE CONUS DURING THE MEDIUM RANGE, WITH A MODESTLY PROGRESSIVE  
PATTERN, BUT WITH AT LEAST SOME CONTINUED TENDENCY FOR AT LEAST  
OCCASIONALLY BLOCKED FLOW AND AMPLIFICATION. AN UPPER RIDGE ACROSS  
CENTRAL CANADA PERSISTING THROUGH THE FORECAST PERIOD WILL SUPPORT  
TROUGH AMPLIFICATION ACROSS THE NORTHEASTERN U.S. (INITIALLY ON  
THU AND THEN AGAIN BY EARLY NEXT WEEK), AS WELL AS ACROSS THE  
PACIFIC NORTHWEST. MEANWHILE, ENERGY CROSSING FROM THE ROCKIES  
INTO THE CENTRAL U.S. WILL SEE POTENTIAL FOR DEEPENING AND/OR  
CUTTING OFF, MOVING THROUGH A REGION OF RELATIVELY WEAK UPPER FLOW  
BENEATH THE CANADIAN RIDGE.  
 
THE WPC FORECAST DURING DAYS 3-5 (THU-SAT) WAS BASED ON A MAJORITY  
DETERMINISTIC BLEND INCLUDING THE 00Z ECMWF/06Z GFS. THESE  
SOLUTIONS REPRESENTED CONSENSUS WILL WITH RESPECT TO THE TROUGH  
LIFTING OUT OF NEW ENGLAND AND THE UPPER LOW MOVING FROM THE  
CENTRAL PLAINS TOWARD THE MISSISSIPPI VALLEY BEFORE TRANSITIONING  
TO AN OPEN WAVE ACROSS THE GREAT LAKES. THIS BLEND ALSO  
SUFFICIENTLY REPRESENTS ADDITIONAL SHORTWAVE ENERGY CROSSING THE  
NORTHERN ROCKIES THU-FRI.  
 
MODEL DIFFERENCES BEGIN TO GROW AFTER 12Z SAT AS ENERGY CROSSES  
THE ROCKIES AND MOVES TOWARD THE PLAINS. THE GFS HAS BEEN  
RELATIVELY CONSISTENT IN TRYING TO CLOSE THE ENERGY OFF EITHER AS  
IT CROSSES THE ROCKIES OR AS IT NEARS THE HIGH PLAINS SUN/SUN  
NIGHT. THE ECMWF ON THE OTHER HAND HAS GENERALLY KEPT A MORE  
PROGRESSIVE SOLUTION, BUT WITH MORE SIGNIFICANT RUN-TO-RUN  
VARIABILITY. GIVEN THE AMPLIFIED NATURE OF THE FLOW ACROSS THE  
HIGHER LATITUDES FROM THE NORTH PACIFIC ACROSS CANADA, THE PATTERN  
SEEMS, AT LEAST CONCEPTUALLY, TO ALSO FAVOR GREATER POTENTIAL FOR  
SHORTWAVE ENERGY TO CLOSE OFF ACROSS THE CONUS. THE ENSEMBLE MEANS  
ALSO SEEM TO SUPPORT THIS IDEA, WITH A SIGNIFICANT NUMBER OF  
NAEFS/GEFS/ECENS MEMBERS SHOWING A SOLUTION AT LEAST SOMEWHAT  
CLOSER TO THE GFS (THE 00Z ECENS MEAN WAS SLOWER AND MORE  
AMPLIFIED THAN THE DETERMINISTIC ECMWF). FARTHER DOWNSTREAM,  
ADDITIONAL UNCERTAINTY ENTERS THE FORECAST WITH RESPECT TO TROUGH  
AMPLIFICATION ACROSS EASTERN NORTH AMERICA BY SUN-MON. THE 00Z/06Z  
GFS RUNS WERE HIGHLY AMPLIFIED BY MON, WITH SIGNIFICANT HEIGHT  
FALLS GETTING INTO NEW ENGLAND. THE ECMWF WAS SIMILAR BUT A BIT  
LESS AMPLIFIED THAN THE GFS. ENSEMBLE MEANS SHOWED GOOD CONSENSUS  
AT LEAST WITH RESPECT TO THE GENERAL TIMING OF ANY TROUGH  
AMPLIFICATION, HOWEVER, WITH THE MAIN DIFFERENCES PERTAINING TO  
THE AMPLITUDE. GIVEN THE INCREASED UNCERTAINTY, WEIGHTING OF  
ENSEMBLE MEANS (00Z ECENS/NAEFS) WAS BOOSTED TO A SLIGHT MAJORITY  
OF THE FORECAST WEIGHTING DURING DAYS 6-7. AMONG THE DETERMINISTIC  
COMPONENTS, THE 06Z GFS WAS WEIGHTED A BIT MORE RELATIVE TO THE  
00Z ECMWF, TO SUPPORT AT LEAST SOMEWHAT MORE AMPLIFICATION OF  
FEATURES.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
A FRONTAL BOUNDARY WITH MULTIPLE WAVES OF LOW PRESSURE WILL  
PERSIST FROM THE CENTRAL U.S. INTO THE OHIO VALLEY AND  
MID-ATLANTIC THROUGH MUCH OF THE FORECAST PERIOD, PROMOTING  
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS, WITH AREAS OF  
HEAVY RAINFALL POTENTIAL. MEANWHILE, ABOVE AVERAGE TEMPERATURES  
WILL CONTINUE FOR MUCH OF THE WESTERN U.S. AS WELL AS THE  
SOUTHEAST, WITH HIGH TEMPERATURES 5 TO 10 DEG F ABOVE AVERAGE  
POSSIBLE ACROSS THESE AREAS THROUGH AT LEAST A PORTION OF THE  
MEDIUM RANGE. THE LARGEST HIGH TEMPERATURE ANOMALIES ARE EXPECTED  
ACROSS PORTIONS OF CALIFORNIA AND OREGON FRI-SUN, WHERE HIGHS MAY  
REACH 10 TO 15 DEG F ABOVE AVERAGE.  
 
RYAN  
 
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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