934  
FXUS02 KWBC 121548  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1148 AM EDT TUE JUN 12 2018  
 
VALID 12Z FRI JUN 15 2018 - 12Z TUE JUN 19 2018  
 
...TROPICAL MOISTURE TO BRING POTENTIALLY HEAVY RAINS TO PORTIONS  
OF THE SOUTHWEST AND WESTERN GULF COAST...  
   
..16Z UPDATE
 
 
00Z MODELS/ENSEMBLES HAVE MOSTLY STAYED THE COURSE SINCE THE  
PREVIOUS ISSUANCE, AND A REFRESH OF THE LATEST GUIDANCE (INCLUDING  
THE 00Z GFS/ECMWF/ECMWF ENS MEAN AND 06Z GFS/GEFS) OFFERED A  
REASONABLE STARTING POINT WITH FEW CHANGES TO THE FRONTS/WEATHER  
GRIDS. THIS INCLUDED THE NORTHWARD PROGRESSION OF HURRICANE BUD IN  
THE PACIFIC INTO NORTHWESTERN MEXICO AND INTO ARIZONA (LIKELY JUST  
ITS MOISTURE BY THEN), THE LINGERING OF TROUGHING IN THE GREAT  
BASIN, AND THE RIDGING IN THE MIDWEST/SOUTHEAST/MID-ATLANTIC.  
 
FRACASSO  
 
   
..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 MISSISSIPPI  
VALLEY. THIS WILL FAVOR A RELATIVELY AMPLIFIED AND SLOWLY-EVOLVING  
FLOW PATTERN ACROSS THE CONUS, WITH A PERSISTENT TROUGH/UPPER LOW  
ACROSS THE INTERIOR WEST AND NEAR/OFF THE EASTERN SEABOARD. 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 (FRI-SUN). MODELS SEEM TO HAVE A SIGNIFICANTLY  
BETTER HANDLE NOW ON THE TIMING 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, AND A SLOWER  
EASTWARD PROGRESSION (AS WAS THE THINKING OVER RECENT DAYS).  
FARTHER EAST, MODELS CONTINUE TO SHOW VARIATION WITH THE DEPTH OF  
AN UPPER LOW FORECAST TO DEVELOP NEAR/OFF THE MID-ATLANTIC COAST  
AFTER A SHORTWAVES DIVES SOUTHEASTWARD ACROSS THE GREAT  
LAKES/NORTHEAST FRI-SAT. TIMING OF THE FEATURES IS SIMILAR AMONG  
THE VARIOUS MODELS, HOWEVER, THUS A BLEND SHOULD SHOULD REPRESENT  
A CONSENSUS SOLUTION IN TERMS OF DEPTH AT THIS TIME. 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 (MON-TUE) THE PATTERN REMAINS LARGELY STABLE WITH  
VERY LITTLE IN THE WAY OF SIGNIFICANT CHANGES. MODELS NOW SHOW A  
GENERAL CONSENSUS THAT THE WESTERN UPPER LOW MAY LINGER ACROSS THE  
FOUR CORNERS REGION OR GREAT BASIN INTO EARLY NEXT WEEK WHILE THE  
UPPER RIDGE OVER THE CENTRAL U.S. INTENSIFIES AND THEN MIGRATES  
EASTWARD TOWARD THE MISSISSIPPI VALLEY, AND MORE PROGRESSIVE FLOW  
PERSISTS WITH ANOTHER SHORTWAVE CROSSING THE GREAT  
LAKES/NORTHEAST. SOME DISCREPANCY EXISTS AS TO THE EVOLUTION OF  
THE UPPER LOW OFF THE EAST COAST, WITH THE ECMWF EVENTUALLY  
ABSORBING THE FEATURES BACK INTO THE NORTHERN STREAM FLOW BY TUE,  
WHILE THE GFS DRIFTS THE FEATURE FARTHER SOUTH TOWARD THE BAHAMAS.  
AT THIS TIME, GIVEN THE PATTERN, ACROSS MOST OF THE CONUS, WOULD  
EXPECT THE UPPER LOW TO REMAIN SEPARATED FROM THE NORTHERN STREAM  
FLOW FOR A LONGER PERIOD OF TIME (AS WAS EVEN INDICATED BY  
PREVIOUS ECMWF RUNS). OTHERWISE, MODELS SHOW TYPICAL  
TIMING/INTENSITY VARIATIONS WITH THESE FEATURES AS WOULD BE  
EXPECTED FOR ALMOST A WEEK OUT. 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 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  
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.  
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  
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.  
 
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. 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 STATIONARY 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 ACROSS THE MISSISSIPPI VALLEY. HIGH TEMPERATURE ANOMALIES OF  
+10 TO +18 DEG F ARE EXPECTED FRI-SUN ACROSS AREAS FROM THE  
CENTRAL PLAINS TO THE MID-MISSISSIPPI VALLEY, WITH HIGHS  
APPROACHING 100 DEGREES FOR SOME AREAS. FORECAST HIGHS AND LOWS  
ARE EXPECTED TO APPROACH BUT MOSTLY FALL SHORT OF RECORD WARM/HOT  
VALUES THROUGH THE PERIOD. BY MON, THE GREATEST TEMPERATURE  
ANOMALIES WILL SHIFT A BIT EAST FROM THE MID-MISSISSIPPI VALLEY  
INTO THE OHIO VALLEY. 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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