511  
FXUS02 KWBC 170612  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
211 AM EDT TUE JUL 17 2018  
 
VALID 12Z FRI JUL 20 2018 - 12Z TUE JUL 24 2018  
 
...DANGEROUS HEAT WAVE EXPECTED FOR THE SOUTHERN PLAINS BY LATER  
THIS WEEK...  
 
   
..OVERVIEW
 
 
MODEL CONSENSUS REMAINS HIGH THAT A PATTERN CHANGE WILL BE ONGOING  
AT THE START OF THE MEDIUM RANGE (FRI) ACROSS THE CONTIGUOUS U.S.,  
WITH AN UPPER-LEVEL RIDGE EXPANDING FORM THE GREAT BASIN TO THE  
SOUTHERN PLAINS, AND AMPLIFICATION OF A TROUGH ACROSS THE GREAT  
LAKES/OHIO VALLEY. EXPECT A SURFACE LOW PRESSURE SYSTEM TO  
ACCOMPANY THE AMPLIFYING UPPER TROUGH ACROSS THE MIDWEST/GREAT  
LAKES FRI THROUGH THE WEEKEND, WITH A SURFACE FRONTAL BOUNDARY  
TRAILING INTO THE SOUTHERN PLAINS. ADDITIONAL SHORTWAVE ENERGY  
CROSSING THE PACIFIC NORTHWEST AND WESTERN CANADA FRI-SAT WILL  
BRING A PACIFIC COLD FRONT INTO THE NORTHERN GREAT BASIN AND  
ROCKIES, REACHING THE PLAINS BY NEXT TUE.  
 
A LOOK AT HEMISPHERIC TELECONNECTIONS SHOWS SUPPORT FOR THIS  
PATTERN EVOLUTION ACROSS NORTH AMERICA, FURTHER INCREASING  
CONFIDENCE IN THE IMPENDING PATTERN CHANGE. AN ANOMALOUS AND  
PERSISTENT REX BLOCK ACROSS EASTERN ASIA FAVORS UPPER RIDGING  
ACROSS THE SOUTHWESTERN AND SOUTH CENTRAL U.S., WITH A RELATIVELY  
STRONG VORTEX ACROSS THE ARCTIC, AND AN EQUATORWARD EXTENSION IN  
THE FORM OF FAVORED TROUGHING ACROSS HUDSON BAY AND THE GREAT  
LAKES. GIVEN THE PERSISTENT NATURE OF THE AFOREMENTIONED REX  
BLOCK, WOULD EXPECT THAT THE UPCOMING PATTERN SHIFT ACROSS NORTH  
AMERICA WILL NOT BE A TRANSIENT ONE.  
 
   
..GUIDANCE EVALUATION AND PREFERENCES
 
 
MODELS SHOWED GOOD AGREEMENT WITH RESPECT TO THE LARGE SCALE FLOW  
PATTERN CHANGES DESCRIBED ABOVE, AND DIFFERENCES WERE LARGELY  
CONFINED TO TIMING/AMPLITUDE DIFFERENCES WITH INDIVIDUAL FEATURES.  
THUS, A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE  
ECMWF/GFS/UKMET/CMC) SERVED AS A GOOD BASIS FOR THE FORECAST  
THROUGH DAY 5 (SUN). ONE OF THE BIGGER AREAS OF DISAGREEMENT AMONG  
THE GUIDANCE IS IN THE PACIFIC NORTHWEST BY EARLY-MID NEXT WEEK,  
WITH SOME QUESTION AS TO HOW MUCH INFLUENCE THE UPPER RIDGE WILL  
CONTINUE TO EXERT, AND WHETHER ADDITIONAL PACIFIC SHORTWAVE ENERGY  
WILL BE ABLE TO LOWER HEIGHTS ONCE AGAIN (AS SHOWN BY THE GFS). BY  
THIS TIME PERIOD (DAYS 6-7) WEIGHTING OF ENSEMBLE MEANS  
(ECENS/NAEFS) WAS INCREASED TO COMPRISE A SLIGHT MAJORITY OF THE  
FORECAST, TO ACCOUNT FOR MODERATE SPREAD AMONG THE GUIDANCE BY  
THAT TIME.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE FRONTAL SYSTEM CROSSING THE GREAT LAKES/OHIO VALLEY FRI-SAT  
WILL BE ACCOMPANIED BY SCATTERED TO NUMEROUS SHOWERS AND  
THUNDERSTORMS, SOME OF WHICH COULD PRODUCE LOCALLY HEAVY RAINFALL.  
A LINGERING STATIONARY/WARM FRONT WILL ALSO PRODUCE CONVECTION  
ACROSS THE SOUTHEAST. THE STRONGEST SIGNAL AMONG THE GUIDANCE FOR  
HEAVY RAINFALL CONTINUES TO BE ALONG THE IMMEDIATE SOUTHEAST  
COASTLINE FRI-SAT, IN CLOSE PROXIMITY TO THE FRONTAL BOUNDARY. THE  
UPPER TROUGH IS EXPECTED TO LINGER ACROSS THE EASTERN U.S. INTO  
EARLY NEXT WEEK AS THE FRONTAL BOUNDARY BEGINS TO WASH OUT,  
KEEPING SHOWERS AND THUNDERSTORMS A POSSIBILITY FOR MUCH OF THE  
EAST. FARTHER WEST, THE FRONTAL SYSTEM CROSSING THE NORTHERN  
ROCKIES FRI-SAT WILL GAIN ACCESS TO DEEPER MOISTURE BY THE TIME IT  
REACHES THE NORTHERN PLAINS SUN-MON, BRINGING LOCALLY HEAVY  
CONVECTION TO THE CENTRAL/NORTHERN PLAINS.  
 
THE HEAT ACROSS THE SOUTHERN PLAINS WILL BE ONE OF THE BIGGER  
STORIES DURING THE MEDIUM RANGE. WITH THE UPPER RIDGE BUILDING  
OVERHEAD AND A STRONG SUBSIDENCE INVERSION LIKELY IN PLACE,  
TEMPERATURES WILL SOAR WELL PAST THE CENTURY MARK FOR MANY AREAS.  
HIGH TEMPERATURES ARE EXPECTED TO BE 5 TO 15 DEG F ABOVE AVERAGE  
ACROSS MUCH OF THE SOUTHERN PLAINS FROM FRI INTO NEXT WEEK, WITH A  
NUMBER OF RECORD HIGH TEMPERATURES AND RECORD HIGH MINIMUM  
TEMPERATURES POTENTIALLY IN JEOPARDY. HIGH DEW POINTS WILL COMBINE  
WITH THE HOT TEMPERATURES TO PRODUCE DANGEROUS HEAT INDEX VALUES  
OVER 110 DEG F FOR MANY LOCATIONS. LOW TEMPERATURES NEAR 80 DEG  
FOR MANY AREAS WILL ADD TO THE POTENTIALLY HAZARDOUS IMPACTS OF  
THE HEAT.  
 
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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