215  
FXUS02 KWBC 130506  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
105 AM EDT WED JUN 13 2018  
 
VALID 12Z SAT JUN 16 2018 - 12Z WED JUN 20 2018  
 
...TROPICAL MOISTURE TO BRING POTENTIALLY HEAVY RAINS TO PORTIONS  
OF THE SOUTHWEST AND WESTERN GULF COAST...  
 
   
..PATTERN OVERVIEW
 
 
THE BREAKDOWN OF A BLOCKING RIDGE ACROSS HUDSON BAY WILL ALLOW FOR  
MORE PROGRESSIVE NORTHERN STREAM FLOW TO TAKE HOLD FROM THE  
CANADIAN PRAIRIES INTO THE GREAT LAKES AND NORTHEASTERN U.S.  
MEANWHILE FARTHER SOUTH, THE FLOW WILL BECOME MORE AMPLIFIED AND  
LESS PROGRESSIVE AS AN ANOMALOUS UPPER RIDGE AXIS BUILDS OFF THE  
NORTH AMERICA WEST COAST, AND SUBTROPICAL RIDGING MIGRATES  
EASTWARD FROM THE SOUTHERN/CENTRAL PLAINS TO THE OHIO/TENNESSEE  
VALLEYS AND THE SOUTHEAST. THIS WILL FAVOR A RELATIVELY AMPLIFIED  
AND SLOWLY-EVOLVING FLOW PATTERN ACROSS THE CONUS, WITH A  
PERSISTENT AND PERHAPS RETROGRADING TROUGH/UPPER LOW ACROSS THE  
INTERIOR WEST. THE WESTERN TROUGH WILL TRANSPORT MOISTURE  
ASSOCIATED WITH THE REMNANTS OF HURRICANE BUD INTO THE  
SOUTHWESTERN U.S. AND EVENTUALLY INTO THE PLAINS, WHERE A  
LINGERING STATIONARY FRONTAL BOUNDARY WILL MEANDER THROUGH MOST OF  
THE MEDIUM RANGE PERIOD. ADDITIONALLY, A SLOW-MOVING TROUGH/AREA  
OF DISTURBED WEATHER MOVING WEST ACROSS THE GULF OF MEXICO IS  
EXPECTED TO APPROACH THE WESTERN GULF COAST STATES BY SAT-SUN,  
POTENTIALLY LINGERING FOR A FEW DAYS DUE TO THE BLOCKY PATTERN  
SURROUNDING THE FEATURE.  
 
   
..MODEL GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE WPC FORECAST WAS INITIALLY BASED ON A HEAVILY DETERMINISTIC  
MULTI-MODEL BLEND, INCLUDING THE 12Z ECMWF/UKMET AND 18Z GFS  
DURING DAYS 3-5 (SAT-MON). MODELS CONSENSUS HAS CONTINUED TO  
IMPROVE WITH RESPECT TO THE TIMING OF THE BREAKDOWN OF THE  
BLOCKING UPPER HIGH ACROSS HUDSON BAY, AND THE DEVELOPING  
PROGRESSIVE HIGHER LATITUDE FLOW IN ITS WAKE. FARTHER SOUTH, AS  
THE RIDGE AMPLIFICATION OCCURS OFF THE WEST COAST, MODELS HAVE  
CONTINUED A SLOW TREND TOWARD MORE TROUGH/UPPER LOW AMPLIFICATION  
ACROSS THE WEST. FARTHER EAST, MODELS HAVE TRENDED TOWARD A MORE  
PROGRESSIVE SOLUTION WITH RESPECT TO SHORTWAVE ENERGY EXPECTED TO  
AMPLIFY OFF THE EAST COAST ON SAT, WITH MOST SOLUTIONS NOW TAKING  
THIS FEATURE EAST AND OUT TO SEA MORE QUICKLY THAN IN RECENT DAYS.  
ADDITIONALLY, MODELS SHOW GOOD CONSENSUS THAT THE UPPER RIDGE  
INITIALLY CENTERED ACROSS THE LOWER/MID-MISSISSIPPI VALLEY SHOULD  
SHOW A SLOW MIGRATION EASTWARD TOWARD THE OHIO/TENNESSEE VALLEYS.  
IMPROVED MODEL CONSENSUS AND RESULTANT FORECAST CONFIDENCE DURING  
THE DAY 3-5 TIME FRAME WAS SUFFICIENT TO JUSTIFY MAJORITY USE OF  
DETERMINISTIC SOLUTIONS.  
 
DURING DAYS 6-7 (TUE-WED) THE PATTERN REMAINS LARGELY STABLE WITH  
VERY LITTLE IN THE WAY OF SIGNIFICANT CHANGES. MODELS (WITH THE  
EXCEPTION OF THE 12Z CMC) HAVE SHOWN A TREND OVER THE PAST DAY  
TOWARD RETROGRADING THE WESTERN U.S. UPPER LOW BY TUE-WED,  
BRINGING IT TO CALIFORNIA BY THE MIDDLE OF NEXT WEEK (A SOLUTION  
THAT THE GFS SHOWED LED THE OTHER MODELS ON). MEANWHILE, THE SLOW  
EASTWARD MIGRATION OF THE MISSISSIPPI VALLEY UPPER RIDGE SHOULD  
CONTINUE, AND MORE PROGRESSIVE FLOW SHOULD PERSIST FARTHER NORTH  
WITH ANOTHER SHORTWAVE CROSSING THE GREAT LAKES/NORTHEAST. MODELS  
SHOW A RANGE OF SOLUTIONS AS TO HOW FAR SOUTH INTO THE RIDGE THE  
NORTHERN STREAM FLOW/HEIGHT FALLS WILL BE ABLE TO PUSH ACROSS THE  
MID-ATLANTIC AND OHIO VALLEY BY THE MIDDLE OF NEXT WEEK, WITH THE  
GFS A BIT MORE AGGRESSIVE WITH THIS RELATIVE TO THE ECMWF.  
NONETHELESS, GIVEN THE EVOLUTION TOWARD A SEEMINGLY STABLE UPPER  
PATTERN, PREDICTABILITY SEEMS SLIGHTLY ABOVE AVERAGE FOR THE TIME  
FRAME. A MAJORITY OF ENSEMBLE MEANS (12Z ECENS/NAEFS) WAS USED AS  
A FORECAST BASIS FOR DAYS 6-7, BUT A SUBSTANTIAL COMPONENT OF THE  
DETERMINISTIC ECMWF/GFS WAS CONTINUED THROUGH THE END OF THE  
FORECAST PERIOD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A WEAKENING SURFACE FRONT AND MOIST AIR MASS WILL SUPPORT  
SCATTERED 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  
WEST-NORTHWEST TO NORTHWEST ACROSS THE GULF OF MEXICO IS ALSO  
EXPECTED TO BRING WIDESPREAD AND POTENTIALLY HEAVY RAINS TO  
PORTIONS OF TX AND LA FROM THE WEEKEND THROUGH EARLY NEXT WEEK,  
WITH MOISTURE POTENTIALLY SPREADING NORTH ACROSS THE SOUTHERN  
PLAINS AS WELL. FARTHER NORTH, A SHORTWAVE, SURFACE LOW PRESSURE  
SYSTEM CROSSING THE NORTHERN PLAINS AND UPPER MIDWEST (ALONG WITH  
A POTENTIAL INFLUX OF MOISTURE FROM THE EVENTUAL REMNANTS OF  
HURRICANE BUD) WILL PRODUCE AREAS OF CONVECTION WITH POTENTIALLY  
HEAVY RAINFALL. THE FRONTAL BOUNDARY APPEARS LIKELY TO STALL FOR A  
PERIOD OF TIME FROM THE CENTRAL PLAINS TO THE UPPER MIDWEST,  
RESULTING IN SEVERAL DAYS WITH SHOWER/THUNDERSTORM POTENTIAL, AND  
POTENTIAL FLOODING THREAT FOR SOME AREAS. MEANWHILE, MOISTURE  
ASSOCIATED WITH THE REMNANTS OF HURRICANE 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 ACROSS THE CENTRAL AND NORTHERN ROCKIES.  
 
TEMPERATURES WILL ALSO BECOME A BIGGER STORY DURING THE NEXT WEEK  
AS THE UPPER RIDGE DRIFTS EAST ACROSS THE MISSISSIPPI VALLEY  
TOWARD THE OHIO/TENNESSEE VALLEYS AND THE SOUTHEAST. HIGH  
TEMPERATURE ANOMALIES OF +10 TO +18 DEG F ARE EXPECTED SAT-SUN  
ACROSS AREAS FROM THE CENTRAL PLAINS TO THE OHIO VALLEY AND GREAT  
LAKES. BY MON-TUE AS THE RIDGE SHIFTS EAST, THESE TEMPERATURES  
ANOMALIES WILL REACH THE EASTERN SEABOARD. WIDESPREAD RECORD HIGH  
TEMPERATURES ARE NOT ANTICIPATED, MANY LOCATIONS WILL NEAR RECORD  
VALUES, AND A FEW RECORDS ARE POSSIBLE. HIGH DEW POINTS IN  
ADDITION TO THE HOT TEMPERATURES WILL MAKE FOR DANGEROUS HEAT  
INDEX VALUES FOR SOME AREAS, PARTICULARLY FOR URBAN AREAS WHERE  
WARM OVERNIGHT LOWS WILL REMAIN WARMER THAN SURROUNDING RURAL  
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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