763  
FXUS02 KWBC 110508  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
108 AM EDT MON JUN 11 2018  
 
VALID 12Z THU JUN 14 2018 - 12Z MON JUN 18 2018  
   
..PATTERN OVERVIEW
 
 
A TRANSITION STILL LOOKS IN ORDER FOR THE FOR THE FLOW PATTERN  
ACROSS NORTH AMERICA OVER THE NEXT WEEK. A BLOCKING UPPER HIGH  
INITIALLY IN PLACE NORTHWEST OF HUDSON BAY SHOULD BREAK DOWN, WITH  
SOMEWHAT MORE PROGRESSIVE FLOW TAKING HOLD BY EARLY NEXT WEEK FROM  
THE CANADIAN PRAIRIES/U.S. NORTHERN PLAINS TO NEW ENGLAND. FARTHER  
SOUTH, HOWEVER, BLOCKING IS EXPECTED TO INCREASE, WITH ANOMALOUS  
UPPER RIDGING LIKELY TO DEVELOP BY THE WEEKEND NEAR THE COAST OF  
BRITISH COLUMBIA, AND RIDGING SHIFTING EAST FROM THE  
SOUTHERN/CENTRAL PLAINS TOWARD THE OHIO VALLEY. THIS PATTERN WILL  
FAVOR DEVELOPMENT OF AN AMPLIFIED AND SLOW-MOVING UPPER TROUGH  
ACROSS THE INTERIOR WEST BY LATE IN THE WEEK. HEIGHT FALLS DIGGING  
INTO THE NORTHEASTERN U.S. BY LATE IN THE WEEK SHOULD SHOW SOME  
DEGREE OF AMPLIFICATION AS WELL, BUT THE SPEED OF EASTWARD  
PROGRESSION INTO THE ATLANTIC OCEAN IS SOMEWHAT UNCLEAR AT THIS  
TIME AS THE FEATURE WILL LIE AT THE INTERFACE BETWEEN MORE  
PROGRESSIVE NORTHERN STREAM FLOW AND THE BLOCKING TENDENCIES  
FARTHER SOUTH.  
 
   
..MODEL GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE WPC FORECAST DURING DAYS 3-5 (THU-SAT) WAS BASED ON A  
MULTI-MODEL DETERMINISTIC BLEND INCLUDING THE 12Z ECMWF/GFS/UKMET.  
THIS BLEND REPRESENTS A CONSENSUS SOLUTION, FAVORABLY RESOLVING  
RELATIVELY MINOR TIMING/AMPLITUDE DIFFERENCES WITH SHORTWAVE  
ENERGY DIGGING ACROSS THE NORTHEAST/MID-ATLANTIC, AND WITH THE  
UPPER TROUGH BEGINNING TO AMPLIFY ACROSS THE PACIFIC NORTHWEST  
DURING THAT TIME PERIOD. THE GFS HAS CONTINUED TO HANG ONTO THE  
POTENTIAL FOR LOW PRESSURE DEVELOPMENT IN THE GULF OF MEXICO BY  
SAT, BUT HAS TRENDED TOWARD THE ECMWF (LESS ORGANIZED AND FARTHER  
WEST) WITH EACH CONSECUTIVE RUN.  
 
FROM DAY 6 (SUN) ONWARD, SPREAD BEGINS TO INCREASE WITH THE TWO  
AREAS OF UPPER TROUGH AMPLIFICATION - ACROSS THE INTERIOR WEST AND  
ALONG/OFF THE MID-ATLANTIC/NORTHEAST COAST. SIGNIFICANT QUESTION  
REMAINS AS TO HOW DEEP A POTENTIAL UPPER LOW ACROSS THE GREAT  
BASIN WILL BE, AND IF/HOW QUICKLY THE ENERGY WILL MOVE EAST ACROSS  
THE ROCKIES BY DAY 7 (MON). ENSEMBLE SPAGHETTI PLOTS FOR THE PAST  
COUPLE RUNS SHOW A TREND TOWARD MORE AMPLIFICATION AND A SLOWER  
PROGRESSION, AND THIS SEEMS TO BE SUPPORTED CONCEPTUALLY BY THE  
EVOLUTION TOWARD MORE FLOW BLOCKING AND THE ANOMALOUS RIDGE  
AMPLIFYING OFF THE WEST COAST. THE 12Z ECENS MEAN WAS LESS  
PROGRESSIVE THAN THE DETERMINISTIC ECMWF, AS WAS THE GFS. FARTHER  
EAST, THERE IS A GENERAL CONSENSUS THAT THE NORTHEAST/MID-ATLANTIC  
UPPER TROUGH SHOULD MOVE OFFSHORE, BUT CONFIDENCE IS LOW IN THE  
DEGREE OF AMPLIFICATION THAT WILL OCCUR. THE 12Z ECMWF WAS THE  
MOST AMPLIFIED DETERMINISTIC SOLUTION, CLOSING OFF AN ANOMALOUS  
UPPER LOW EAST OF NEW ENGLAND THAT PERSISTS INTO DAY 7. DUE TO  
LOWER CONFIDENCE IN THIS AREA, A MIX OF ENSEMBLE MEANS IS  
PREFERRED. MAJORITY WEIGHTING TOWARD THE 12Z ECENS/NAEFS ENSEMBLE  
MEANS WAS USED IN THE FORECAST FOR DAYS 6-7 TO ACCOUNT FOR  
INCREASED SPREAD, AND TO REFLECT A SOMEWHAT MORE AMPLIFIED AND  
SLOWER PROGRESSION OF THE WESTERN TROUGH THAN SHOWN BY THE  
DETERMINISTIC 12Z ECMWF.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A WEAKENING SURFACE FRONT AND MOIST AIR MASS WILL SUPPORT  
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS THROUGH THE  
WEEKEND, ESPECIALLY DURING THE AFTERNOONS AND EVENINGS, ACROSS THE  
SOUTHEASTERN STATES, WITH THE POTENTIAL FOR LOCALLY HEAVY  
RAINFALL. A BROAD TROUGH/AREA OF DISTURBED WEATHER MOVING WESTWARD  
ACROSS THE GULF OF MEXICO IS ALSO EXPECTED TO BRING THE POTENTIAL  
FOR MORE WIDESPREAD RAINS TO PORTIONS OF THE WESTERN GULF COAST  
STATES BY THE WEEKEND. FARTHER NORTH, A SHORTWAVE, SURFACE LOW  
PRESSURE SYSTEM CROSSING THE NORTHERN PLAINS AND UPPER MIDWEST  
WILL PRODUCE AREAS OF CONVECTION WITH POTENTIALLY HEAVY RAINFALL.  
THE FRONTAL BOUNDARY MAY SLOW OR STALL FOR A PERIOD OF TIME  
SAT-SUN FROM THE CENTRAL PLAINS TO THE UPPER MIDWEST, RESULTING IN  
SEVERAL DAYS WITH SHOWER/THUNDERSTORMS POTENTIAL, AND POTENTIAL  
FLOODING THREAT FOR SOME AREAS. CONFIDENCE IS LOW AT THIS TIME AS  
TO THE SPECIFICS OF WHAT AREAS WILL SEE THE HEAVIEST RAIN,  
HOWEVER. FINALLY, MOISTURE ASSOCIATED WITH TROPICAL STORM BUD  
SHOULD BEGIN TO SPREAD INTO THE SOUTHWESTERN U.S. AND THE ROCKIES  
BY THE WEEKEND, BRINGING THE POTENTIAL FOR AREAS OF HEAVY  
RAINFALL. INTERACTIONS WITH A STATIONARY FRONTAL BOUNDARY DRAPED  
FROM THE GREAT BASIN INTO THE CENTRAL ROCKIES COULD LEAD TO AN  
ENHANCEMENT OF HEAVY RAINFALL POTENTIAL. SOME MODEL SOLUTIONS  
EVEN SUGGEST THE POSSIBILITY THAT THE MOISTURE FROM BUD COULD  
SPREAD EAST INTO THE CENTRAL/NORTHERN PLAINS, FURTHER ENHANCING  
HEAVY RAIN POTENTIAL ALONG THE FRONTAL BOUNDARY.  
 
TEMPERATURES WILL ALSO BECOME A BIGGER STORY DURING THE NEXT WEEK  
AS THE UPPER RIDGE EXPANDS ACROSS THE CENTRAL U.S. AND THEN DRIFTS  
EAST INTO THE OHIO VALLEY. HIGH TEMPERATURE ANOMALIES OF +10 TO  
+18 DEG F ARE EXPECTED THU-SAT ACROSS AREAS FROM THE CENTRAL  
PLAINS TO THE MID-MISSISSIPPI VALLEY, WITH HIGHS APPROACHING 100  
DEGREES FOR SOME AREAS. BY SUN-MON, THE GREATEST TEMPERATURE  
ANOMALIES WILL SHIFT A BIT EAST FROM THE MID-MISSISSIPPI VALLEY  
INTO THE OHIO VALLEY. A FEW RECORD HIGH TEMPERATURES ARE POSSIBLE.  
HIGH DEW POINTS IN ADDITION TO THE HOT TEMPERATURES WILL MAKE FOR  
DANGEROUS HEAT INDEX VALUES FOR SOME AREAS.  
 
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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