099  
FXUS02 KWBC 241553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1153 AM EDT FRI AUG 24 2018  
 
VALID 12Z MON AUG 27 2018 - 12Z FRI AUG 31 2018  
   
..PATTERN OVERVIEW AND WEATHER HIGHLIGHTS/HAZARDS
 
 
AMPLIFIED MID-UPPER LEVEL TROUGHING OVER THE NORTH-CENTRAL ROCKIES  
AND INTERMOUNTAIN WEST MONDAY WILL TREK EAST-NORTHEASTWARD OVER  
THE NORTH-CENTRAL U.S. AND THEN THROUGH EASTERN CANADA/NORTHEAST  
TUESDAY-THURSDAY. THIS WILL SUPPORT A WAVY FRONTAL SYSTEM ACROSS  
THE NORTHERN TIER WHICH WILL FOCUS PERIODS OF ORGANIZED  
CONVECTION, PARTICULARLY FROM THE MID-MISSISSIPPI VALLEY AND  
MIDWEST/GREAT LAKES EASTWARD. TO THE SOUTH, LESS PREDICTABLE  
MESOSCALE PROCESSES WITHIN THE HOT/HUMID SUMMERTIME AIRMASS  
(TEMPERATURES WELL ABOVE AVERAGE FOR LATE AUGUST) AND DEEP  
MOISTURE/INSTABILITY WILL ALSO GIVE RISE TO LOCALLY HEAVY RAINFALL  
WHICH MAY PRODUCE LOCAL RUNOFF ISSUES WITH REPEAT/TRAINING OF  
CELLS. UPSTREAM, A STRONG UPPER HIGH SOUTH OF THE GULF OF ALASKA  
WILL RETROGRADE WESTWARD AND ALLOW ANOTHER COOLING TROUGH INTO THE  
PACIFIC NORTHWEST MID-LATE WEEK WITH ENOUGH ONSHORE FLOW TO  
INITIATE SOME PRECIPITATION. TEMPERATURES WILL BE COOLER THAN  
AVERAGE IN THE NORTHWEST UNDER THE UPPER TROUGH.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS AND ENSEMBLES SHOW A SIMILAR LARGER SCALE PATTERN IN THE  
3-5 DAY PERIOD (MON-WED), BUT SMALLER SCALE DIFFERENCES THAT WILL  
DRIVE CONVECTIVE EVOLUTION WILL REMAIN UNRESOLVED UNTIL THE SHORT  
TERM. A BLEND OF THE 00Z/06Z GFS AND 00Z ECMWF/UKMET/CANADIAN WAS  
A GOOD STARTING POINT AS THEIR CONSENSUS LOW POSITIONS FIT THE  
TRENDS NICELY. INCORPORATED MORE ENSEMBLE WEIGHTING BY NEXT  
THU/FRI WITH SOME DETAILS PROVIDED BY THE 06Z GFS AND 00Z ECMWF AS  
THERE REMAINED UNCERTAINTY WITH THE UPSTREAM FLOW (RIPPLE EFFECTS  
FROM THE ENTRAINMENT OF EXTRATROPICAL LOWS ASSOCIATED WITH  
WEAKENING WEST PACIFIC TROPICAL SYSTEMS CIMARON AND SOULIK) AS  
WELL AS THE SOUTHWARD EXTENT OF THE COLD FRONT IN THE EAST AS IT  
BATTLES THE UPPER RIDGING IN PLACE MIDWEEK. TELECONNECTIONS TO  
UPSTREAM ANOMALIES OVER NORTHERN ALASKA (NEGATIVE) AND THE  
NORTHEASTERN PACIFIC (POSITIVE) WERE CONFLICTING FOR THE EASTERN  
US, THUS THE WAVERING OF THE MODELS/ENSEMBLES.  
 
FRACASSO/SCHICHTEL  
 
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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