474  
FXUS02 KWBC 170529  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
129 AM EDT FRI AUG 17 2018  
 
VALID 12Z MON AUG 20 2018 - 12Z FRI AUG 24 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
GUIDANCE IS CONSISTENT IN SHOWING AN EVOLUTION TOWARD AN AMPLIFIED  
EASTERN US TROUGH WHILE MEAN RIDGING PERSISTS OVER THE SOUTHWEST  
AND SOUTHERN GREAT BASIN TO THE SOUTHERN HIGH PLAINS. GRADUAL  
DEAMPLIFICATION OF THE PATTERN OVER THE NORTHEASTERN PACIFIC  
SHOULD ULTIMATELY LEAD TO SHEARING AND EJECTION OF A TROUGH FROM  
THE WEEKEND INTO EARLY NEXT WEEK, WITH THE ARRIVAL OF A SUCCEEDING  
TROUGH IN THE PACIFIC NORTHWEST CENTERED ON EARLY FRI 24 AUG.  
 
LOW PRESSURE DEVELOPING IN THE PLAINS AND MOVING TOWARDS THE GREAT  
LAKES IS QUITE STRONG FOR AUGUST.  
OPERATIONAL ECMWF/UKMET RUNS HAVE TENDED TO BE ON THE DEEPER SIDE  
OF THE ENVELOPE WITH THE SURFACE LOW. THE DEPTH IN SOME OF THOSE  
RUNS WOULD BE SUFFICIENT TO EQUAL OR EXCEED LOWEST SEA LEVEL  
PRESSURE RECORDS FOR THE MONTH OF AUGUST. THE OPERATIONAL GFS  
RUNS HAVE GENERALLY BEEN ON THE WEAKER AND SLOWER SIDE OF THE  
SPREAD.  
 
OVERALL FAVOR AN OPERATIONAL 12Z ECMWF/12Z CANADIAN MODEL/12Z  
ECMWF ENSEMBLE MEAN/12Z NAEFS MEAN COMPROMISE, BUT NOT AS DEEP AS  
THE 12Z ECMWF/12-00Z UKMET GIVEN THE POSSIBILITY THAT FEEDBACK  
FROM CONVECTION IS DEEPENING THE LOWS TOO MUCH. THIS CONSENSUS  
ALSO ADDRESSED REMAINING NORTH-SOUTH SPREAD IN THE LOCATION/TIMING  
OF THE LOW TRACK. THERE HAS BEEN FAIRLY GOOD AGREEMENT/CONTINUITY  
REGARDING THE TRAILING FRONT REACHING THE EAST COAST/GULF COAST BY  
MIDWEEK.  
 
OVER THE WEST COAST/NORTHWEST THE MODELS AND ENSEMBLE MEANS ARE  
COMING INTO BETTER AGREEMENT OVER THE EVOLUTION OF THE UPPER  
TROUGH THAT ARRIVES AROUND THE START OF THE PERIOD, POSSIBLY  
DEVELOPS A CLOSED LOW BY MON, AND EVENTUALLY EJECTS/SHEARS OUT  
WITH THE APPROACH OF UPSTREAM TROUGH ENERGY.  
THUS EXPECT EJECTING WEST COAST ENERGY TO BE FAIRLY WEAK BY  
WED-THU. THE MODELS/ENSEMBLES ARE CONVERGING ON THE ARRIVAL O THE  
NEXT EASTERN PACIFIC TROUGH INTO THE PACIFIC NORTHWEST/NORTHWEST  
CA CENTERED ON EARLY FRI 24 AUG, WITH BETTER THAN AVERAGE  
AGREEMENT.  
 
A BLEND AMONG THE 12Z ECMWF/ECMWF ENSEMBLE MEAN/12Z CMC/18Z GEFS  
MEAN/12Z NAEFS MEAN WAS USED FOR THE HEIGHT AND ADJUSTED SEA LEVEL  
PRESSURE FORECASTS, AS WELL AS GRIDDED TEMPERATURE, DEWPOINT, ETC.  
FORECASTS. GREATER WEIGHTING TOWARDS THE MEANS WAS USED ON DAY 7  
FRI AS THE OPERATIONAL GFS AND CANADIAN WERE ON OPPOSITE ENDS OF  
THE TIMING SPECTRUM FROM THE NORTHERN ROCKIES ACROSS THE NORTHERN  
PLAINS FRI 24 AUG WITH BETTER AGREEMENT AMONG THE NAEFS/ECMWF/18Z  
GEFS ENSEMBLE MEANS.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE DEVELOPING SYSTEM TRACKING FROM THE PLAINS THROUGH GREAT LAKES  
INTO EASTERN CANADA WILL LIKELY PRODUCE A BROAD AREA OF  
SHOWERS/STORMS FROM SUN TO TUE FROM THE PLAINS INTO THE GREAT  
LAKES/OHIO VALLEY. EXPECT THE FRONT TRAILING FROM THE SURFACE LOW  
TO GENERATE SOME SHOWERS/THUNDERSTORMS INTO THE LOWER MS VALLEY  
AND TN VALLEY ACROSS THE SOUTHERN APPALACHIANS TO THE SOUTHEAST  
COAST AS WELL. THE POSSIBILITY OF A WAVY FRONT STALLING OFF THE  
MID ATLANTIC COAST COULD LEAD TO A PRONGED SHOWER THREAT NEXT THU  
23 AUG. THE SOUTHWEST WILL LIKELY SEE DIURNALLY FAVORED  
CONVECTION.  
 
PORTIONS OF THE NORTHERN/CENTRAL PLAINS SHOULD BE WITHIN THE CORE  
OF COOLEST DAYTIME TEMPERATURES VERSUS NORMAL (MINUS 10-15F  
ANOMALIES) FROM SUN-TUE. DEPARTURE OF GREAT LAKES LOW PRESSURE  
AFTER TUE AND SOUTHWARD/EASTWARD PROGRESSION OF THE TRAILING COLD  
FRONT WILL BRING A COOLING TREND INTO THE EASTERN STATES BUT WITH  
SMALLER ANOMALIES. EXPECT WARMEST HIGHS VERSUS NORMAL TO BE OVER  
PARTS OF THE PACIFIC NORTHWEST WITH SOME READINGS LOCALLY 10-15F  
ABOVE NORMAL MON-TUE. ONCE THE NEXT PACIFIC TROUGH COMES ONSHORE  
NEXT THU 23 AUG-FRI 24 AUG THE NORTHWEST COOLS WHILE DOWNSTREAM  
WARM ADVECTION PRODUCES A SURGE OF WARMTH INTO THE NORTHERN  
PLAINS, RESULTING IN AN AREA OF TEMPERATURES SEVERAL DEGREES ABOVE  
NORMAL.  
 
PETERSEN  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page