138  
FXUS02 KWBC 140450  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1249 AM EDT TUE AUG 14 2018  
 
VALID 12Z FRI AUG 17 2018 - 12Z TUE AUG 21 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
THE MODELS SHOW THE LARGE SCALE PATTERN BECOMING MORE AMPLIFIED  
WITH TIME OVER THE WEEKEND INTO EARLY NEXT WEEK AS A HIGH  
AMPLITUDE TROUGH IN THE NORTH PACIFIC SOUTH OF ALASKA AMPLIFIED  
THE EAST PACIFIC TO WEST COAST OF CANADA RIDGE, IN TURN FAVORING  
THE DEVELOPMENT OF THE TROUGH OVER THE CENTRAL US EARLY NEXT WEEK.  
 
AT THE START OF THE FORECAST PERIOD ON FRIDAY, AN AREA OF LOW  
PRESSURE MOVES FROM NEAR LAKE HURON TO NEAR OR OVER NORTHERN MAINE  
SAT MORNING AND ACROSS THE CANADIAN MARITIMES SUNDAY.  
ATTENTION THEN TURNS UPSTREAM TO THE PATTERN AMPLIFICATION OUT  
WEST LATE SAT INTO SUNDAY.  
THE 12Z ECMWF BECOMES MORE AMPLIFIED THAN OTHER SOLUTIONS WITH THE  
500 MB TROUGH AND ASSOCIATED SURFACE THAT DEVELOPS FURTHER SOUTH  
SUN NIGHT INTO MONDAY IN THE MID MS VALLEY AT A TIME WHEN THE  
MEANS HAVE LOW PRESSURE IN THE UPPER MS VALLEY. THE 18Z GFS WAS  
THE STRONGEST WITH THE LOWEST PRESSURE ON 12Z MON WITH ITS LOW IN  
MN. DUE TO THE BIG DIFFERENCE AMONG THE GFS AND ECMWF AND MUCH  
BETTER AGREEMENT ON BOTH THE LOCATION AND INTENSITY OF THE LOW  
AMONG THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS, GREATER WEIGHTING WAS  
GIVEN TO THE ENSEMBLE MEANS, WITH LESS TO THE ECMWF IN CASE IT IS  
CORRECT ON THE GREATER AMPLIFICATION OF THE TROUGH.  
 
THE HIGHER HEIGHTS OUT WEST PERSIST OVER AZ AND CA WITH LOWERING  
HEIGHTS PROVIDING RELIEF FROM SOME OF THE MORE EXTREME  
TEMPERATURES IN THE NORTHWEST. BY THE END OF THE FORECAST PERIOD,  
THERE IS GOOD AGREEMENT AMONG THE 12Z ECMWF , THE 12Z ECMWF  
ENSEMBLE MEAN, AND GEFS MEAN ON THE ANTICYCLONE CENTER OVER AZ 00Z  
WED 22 AUG.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
IT SHOULD BE QUITE WARM ACROSS THE NORTH-CENTRAL U.S. THROUGH  
FRIDAY AS HIGH TEMPERATURES ARE FORECAST TO BE 10-15 DEGREES ABOVE  
NORMAL IN MUCH OF NORTHERN ND AND MN. A BUILDING UPPER RIDGE ALONG  
THE WEST COAST WILL ALLOW TEMPERATURES TO PERSIST ABOVE NORMAL BY  
5-10 DEGREES ACROSS MOST AREAS INLAND FROM THE COAST ACROSS THE  
PACIFIC NORTHWEST AND NORTHERN ROCKIES. ABOVE NORMAL READINGS  
ALSO PERSIST ACROSS INTERIOR CA ACROSS THE GREAT BASIN.  
 
WITH THE DEVELOPING CENTRAL US TROUGH EARLY NEXT, TEMPERATURES  
WILL SWITCH FROM BEING ABOVE NORMAL OVER THE WEEKEND IN THE  
NORTHERN PLAINS TO UPPER MS VALLEY TO MODERATE TO NORMAL OR EVEN A  
FEW DEGREES BELOW NORMAL BY TUE 21 AUG.  
 
MULTIPLE ROUNDS OF SHOWERS AND THUNDERSTORMS ARE LIKELY FROM THE  
MIDDLE OF THE COUNTRY EASTWARD INTO THE GREAT LAKES AND NEW  
ENGLAND. THE BEST FOCUS WILL INITIALLY BE NEAR THE LIFTING SURFACE  
WAVE WITH WIDESPREAD SHOWERS ACROSS NORTHERN NEW YORK AND NEW  
ENGLAND FRI/FRI NIGHT.  
 
WITH THE FLOW EXPECTED TO AMPLIFY THIS WEEKEND ACROSS THE ROCKIES,  
ROBUST QPF AMOUNTS ARE NOTED IN MANY MODEL SOLUTIONS WITH A  
PARTICULAR FOCUS RAINFALL ACROSS THE CENTRAL PLAINS INTO THE  
MIDDLE OR LOWER MISSISSIPPI VALLEY. AS THE TROUGH DEVELOPS EARLY  
NET WEEK, SHOWERS INCREASE ACROSS THE NORTHERN PLAINS TO UPPER MS  
VALLEY AND THEN MOVE OUT OF THE MID MS VALLEY INTO THE OHIO AND TN  
VALLEYS.  
 
ACROSS THE SOUTHWEST, TERRAIN AIDS IN THE DEVELOPMENT OF DIURNAL  
SHOWERS/STORMS IN THE CENTRAL TO SOUTHERN ROCKIES AND MOGOLLON RIM  
OF AZ, WITH HEAVIER AMOUNTS FURTHER SOUTH OVER SOUTHERN ARIZONA  
WHERE HIGHER PRECIPITABLE WATER AIR IS AVAILABLE.  
 
PETERSEN/SANTORELLI  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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