685  
FXUS02 KWBC 151546  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1146 AM EDT SUN JUL 15 2018  
 
VALID 12Z WED JUL 18 2018 - 12Z SUN JUL 22 2018  
 
16Z UPDATE:  
 
SIMILAR TO THE PREVIOUS FORECAST ISSUANCE, THERE IS ENOUGH  
SYNOPTIC SCALE AGREEMENT THROUGH THURSDAY TO MERIT A MULTI-MODEL  
DETERMINISTIC BLEND. BY THURSDAY NIGHT, THE 00Z ECMWF IS WEST OF  
THE MODEL CONSENSUS, INCLUDING THE EC MEAN, REGARDING THE AXIS OF  
THE SHORTWAVE TROUGH/CLOSED LOW CROSSING THE NORTHERN PLAINS AND  
UPPER MIDWEST. THIS CONTINUES THROUGH THE REMAINDER OF THE  
FORECAST PERIOD AS THE LONG WAVE TROUGH EVOLVES OVER THE EASTERN  
THIRD OF THE NATION. HOWEVER, ITS UPPER LOW POSITION FALLS WITHIN  
THE RANGE OF THE 576DM ENSEMBLE MEMBER SOLUTIONS AND IS NOT  
CONSIDERED AN OUTLIER AT THIS TIME. THE 00Z UKMET IS ON THE  
FASTER SIDE OF THE GUIDANCE AND IT ALSO FALLS WITHIN THE ENSEMBLE  
SPREAD. THERE HAS ALSO BEEN A STRONGER TREND IN THE DETERMINISTIC  
GUIDANCE OVER THE PAST 24 HOURS, AND THE LATEST WPC FORECAST TAKES  
THIS INTO ACCOUNT. THE FEATURE WITH THE GREATEST MODEL AGREEMENT  
IS THE BUILDING UPPER LEVEL RIDGE OVER THE HIGH PLAINS AND  
EXTENDING OVER THE INTER-MOUNTAIN WEST. THE PREVIOUS OVERNIGHT  
DISCUSSION IS APPENDED BELOW FOR REFERENCE.  
 
D. HAMRICK  
---------------------------   
..OVERVIEW
 
 
THE FLOW PATTERN ACROSS THE CONTIGUOUS U.S. WILL BECOME MORE  
AMPLIFIED DURING THE MEDIUM RANGE, WITH AN UPPER-LEVEL RIDGE  
EXPANDING FROM THE GREAT BASIN TO THE SOUTHERN PLAINS, AND A  
SERIES OF UPPER WAVES GRADUALLY CARVING OUT A TROUGH ACROSS THE  
GREAT LAKES/OHIO VALLEY. A SHORTWAVE ORIGINATING IN THE GULF OF  
ALASKA WILL SKIRT THE PACIFIC NORTHWEST BY LATE IN THE WEEK,  
BRIEFLY LOWERING HEIGHTS ACROSS THAT REGION. THE INTENSIFYING  
UPPER RIDGE FARTHER SOUTH WILL HELP ENSURE THAT ANY SUCH HEIGHT  
FALLS IN THE NORTHWEST ARE SHORT-LIVE, HOWEVER.  
 
   
..GUIDANCE EVALUATION AND PREFERENCES
 
 
MODEL SPREAD WAS SUFFICIENTLY LOW TO START THE FORECAST ON DAY 3  
(WED) WITH A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE  
GFS/ECMWF/CMC/UKMET). THE TREND IN THE GUIDANCE NOTED IN RECENT  
DAYS TOWARD MORE AMPLIFICATION OF FEATURES SEEMS TO HAVE  
CONTINUED. MODEL CONSENSUS IS NOW GOOD WITH RESPECT TO THE UPPER  
TROUGH INITIALLY ACROSS THE GREAT LAKES ON WED, WHICH LIFTS OUT  
ACROSS NEW ENGLAND ON THU. THE AFOREMENTIONED TREND ALSO APPLIES  
TO SHORTWAVE ENERGY CROSSING THE NORTHERN PLAINS/UPPER MIDWEST  
WED-THU AND THEN AMPLIFYING FURTHER ACROSS THE GREAT LAKES/OHIO  
VALLEY ON FRI (DAY 5). MODELS CONTINUE TO VARY WITH RESPECT TO THE  
AMPLITUDE OF THIS FEATURE, BUT GIVEN THE TRENDS, CANNOT RULE OUT  
THE MORE AMPLIFIED SOLUTIONS (SUCH AS THE CMC AND UKMET). THUS,  
THE MULTI-MODEL DETERMINISTIC BLEND CONTINUED TO SERVE AS A BASIS  
FOR THE FORECAST THROUGH DAY 5.  
 
DURING DAYS 6-7 (SAT-SUN), THE PRIMARY QUESTIONS BECOME HOW MUCH  
AMPLIFICATION OCCURS WITH THE EASTERN U.S. TROUGH, AS WELL AS TO  
WHAT DEGREE AND FOR HOW LONG HEIGHTS FALL ACROSS THE PACIFIC  
NORTHWEST WITH THE SHORTWAVE PASSING ACROSS WESTERN CANADA. GIVEN  
THE INCREASINGLY AMPLIFIED FLOW ACROSS THE CONUS, OPTED TO LEAN A  
BIT TOWARD GREATER AMPLIFICATION OF THE TROUGH ACROSS THE GREAT  
LAKES/OHIO VALLEY. THE GFS HAS BEEN AMONG THE LEAST AMPLIFIED  
SOLUTIONS, BUT THE FV3 IS MUCH MORE AMPLIFIED, SIMILAR TO THE  
ECMWF AND CMC. THE GFS (PRIOR TO THE 00Z RUN) HAS ALSO BEEN AMONG  
THE LESS AMPLIFIED SOLUTIONS WITH THE WESTERN CANADA ENERGY. THUS,  
THE FORECAST DURING DAYS 6-7 WAS BASED ON A BLEND OF  
DETERMINISTIC/ENSEMBLE MEAN SOLUTIONS, INCLUDING THE ECMWF/CMC  
ALONG WITH THE ECENS/NAEFS MEANS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
A COLD FRONT EXPECTED TO BECOME STATIONARY FROM THE SOUTHEAST TO  
THE SOUTHERN/CENTRAL PLAINS WILL FOCUS SCATTERED TO NUMEROUS  
SHOWERS AND THUNDERSTORMS FROM MIDWEEK INTO NEXT WEEKEND. AREAS OF  
HEAVY RAIN WILL BE POSSIBLE, WITH GUIDANCE CONTINUING TO SHOW A  
RELATIVELY STRONG SIGNAL FOR HEAVY RAIN ACROSS PORTIONS OF THE  
SOUTHEAST, FROM THE COASTAL CAROLINAS TO NORTHERN FLORIDA. FARTHER  
NORTH, THE SHORTWAVE CROSSING THE NORTHERN PLAINS/MIDWEST WED-THU  
WILL GENERATE CONVECTION WITH LOCALLY HEAVY RAIN. THIS FEATURE  
SHOULD EVENTUALLY GENERATE A NEW WAVE ALONG THE AFOREMENTIONED  
SURFACE FRONT, WHICH IS FORECAST TO CROSS THE MIDWEST/GREAT LAKES  
FRI-SAT, PRODUCING ADDITIONAL CONVECTION AND LOCALLY HEAVY RAINS.  
THE AMPLIFYING UPPER TROUGH ACROSS THE GREAT LAKES/OHIO VALLEY BY  
NEXT WEEKEND COULD ALSO RESULT IN THE POTENTIAL FOR LATITUDINAL  
TRANSPORT OF DEEP TROPICAL MOISTURE ALONG THE EASTERN SEABOARD. A  
COUPLE DETERMINISTIC MODEL SOLUTIONS INDICATE THE POTENTIAL FOR  
WIDESPREAD CONVECTION AND AREAS OF HEAVY RAIN ACROSS PORTIONS OF  
THE MID-ATLANTIC AND NORTHEAST BY LATE IN THE WEEK INTO THE  
WEEKEND. ENSEMBLE PROBABILITIES SUGGEST THAT CONFIDENCE IN THIS  
SCENARIO IS RELATIVELY LOW AT THIS TIME, HOWEVER. FINALLY,  
MONSOONAL MOISTURE WILL RESULT IN SCATTERED AFTERNOON/EVENING  
CONVECTION FROM THE SOUTHERN GREAT BASIN TO THE SOUTHERN/CENTRAL  
ROCKIES THROUGH MUCH OF THE WEEK. CONVECTIVE ACTIVITY SHOULD  
GRADUALLY BECOME MORE SPARSE IN NATURE THROUGH THE WEEK AS THE  
UPPER RIDGE STRENGTHENS OVERHEAD.  
 
THE EXPANDING RIDGE FROM THE SOUTHWEST TO THE SOUTHERN PLAINS WILL  
RESULT IN RISING TEMPERATURES BY LATER IN THE WEEK. HIGH  
TEMPERATURES ACROSS THE SOUTHERN PLAINS ARE FORECAST TO BE 5 TO 15  
DEG F ABOVE AVERAGE FROM THU ONWARD, WITH TEMPERATURES EXPECTED TO  
SOAR WELL PAST THE 100 DEGREE MARK ACROSS MUCH OF TEXAS. A NUMBER  
OF RECORD HIGH TEMPERATURES ACROSS THE SOUTHERN PLAINS COULD BE IN  
JEOPARDY.  
 
RYAN  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE 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  
HTTP://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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