782  
FXUS02 KWBC 161559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT THU AUG 16 2018  
 
VALID 12Z SUN AUG 19 2018 - 12Z THU AUG 23 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
GUIDANCE IS CONSISTENT IN SHOWING AN EVOLUTION TOWARD AN AMPLIFIED  
EASTERN NORTH AMERICAN TROUGH ALOFT WHILE SOME DEGREE OF MEAN  
RIDGING PERSISTS OVER OR NEAR THE ROCKIES/WEST-CENTRAL U.S.,  
ANCHORED BY AN UPPER HIGH TENDING TO BE OVER NEW MEXICO OR  
ARIZONA. GRADUAL OPENING/DEAMPLIFICATION OF THE PATTERN OVER THE  
NORTHEASTERN PACIFIC SHOULD ULTIMATELY LEAD TO SHEARING AND  
EJECTION OF A TROUGH (LIKELY WITH AN EMBEDDED LOW) EVOLVING NEAR  
THE WEST COAST FROM THE WEEKEND INTO EARLY NEXT WEEK.  
 
ONE OF THE MORE INTRIGUING ASPECTS OF THE FORECAST CONTINUES TO BE  
THE SPECIFICS OF PLAINS INTO EASTERN CANADA LOW PRESSURE IN  
RESPONSE TO THE COMBINATION OF THE SHORTWAVE REACHING THE PLAINS  
AS OF SUN AND POSSIBLE INTERACTION WITH UPSTREAM ENERGY FLOWING  
FROM WESTERN CANADA. OPERATIONAL ECMWF RUNS HAVE TENDED TO BE ON  
THE DEEPER SIDE OF THE ENVELOPE WITH THE SURFACE LOW. THE DEPTH  
IN SOME OF THOSE RUNS, AS WELL AS IN A VERY SMALL NUMBER OF ECMWF  
ENSEMBLE MEMBERS, BY THE TIME THE LOW CROSSES THE GREAT LAKES DAY  
5 TUE WOULD BE SUFFICIENT TO EQUAL OR EXCEED LOWEST SEA LEVEL  
PRESSURE RECORDS FOR THE MONTH OF AUGUST. ON THE OTHER EXTREME  
OPERATIONAL GFS RUNS HAVE GENERALLY BEEN ON THE WEAKER SIDE OF THE  
SPREAD. THE 00Z GFS IS PARTICULARLY WEAK UNTIL AFTER TUE DUE TO  
BEING MORE SHEARED WITH THE INITIAL PLAINS ENERGY AND LACK OF  
INFLUENCE FROM CANADIAN FLOW. SOMEWHAT IN DEFERENCE TO  
OPERATIONAL RUNS THE GEFS MEAN HAS BEEN FADING IN AND OUT WITH ITS  
SURFACE LOW DEPICTION. ECMWF MEANS HAVE BEEN MORE STEADY AND  
GENERALLY TRENDING A LITTLE STRONGER OVER THE PAST COUPLE DAYS.  
00Z UKMET/CMC RUNS ARE AS DEEP OR DEEPER THAN THE 00Z ECMWF.  
OVERALL FAVOR AN OPERATIONAL MODEL COMPROMISE AS CONSENSUS  
SUGGESTS ENOUGH STREAM INTERACTION TO YIELD A FAIRLY STRONG  
SURFACE LOW RELATIVE TO THE TIME OF YEAR--BUT NOT AS DEEP AS THE  
ECMWF/UKMET/CMC GIVEN THE POSSIBILITY THAT OCCASIONAL WARM-SEASON  
BIASES OF HOLDING CONVECTION TOO CLOSE TO SURFACE LOWS VERSUS  
EXTENDING OUT INTO THE WARM SECTOR MAY BE PLAYING A ROLE IN THE  
DEEPEST SURFACE SOLUTIONS. THE PAST TWO DAYS OF 00Z PARALLEL  
(FV3) GFS RUNS SUPPORT THIS INTERMEDIATE APPROACH AS THEY HAVE  
BEEN NEAR THE ECMWF MEAN IN DEPTH/POSITION OVER THE GREAT LAKES AS  
OF EARLY TUE. REGARDLESS OF SURFACE LOW DIFFERENCES, THERE HAS  
BEEN FAIRLY GOOD AGREEMENT/CONTINUITY REGARDING THE TRAILING FRONT  
REACHING THE EAST COAST/GULF COAST BY AROUND MIDWEEK.  
 
OVER THE WEST COAST/NORTHWEST THERE IS STILL A MODERATE DEGREE OF  
UNCERTAINTY OVER THE DETAILS 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. MULTI-DAY MEANS HAVE BEEN SHOWING A CORE  
OF NEGATIVE HEIGHT ANOMALIES NEAR THE CENTRAL ALEUTIANS AND  
POSITIVE HEIGHT ANOMALIES EAST OF THE CANADIAN MARITIMES AS THE  
MOST STABLE AND BEST DEFINED FEATURES, WITH TELECONNECTIONS  
RELATIVE TO BOTH FAVORING A MEAN TROUGH AXIS NEAR OR A FRACTION  
OFFSHORE THE WEST COAST AND RIDGING SOMEWHAT INLAND AS LATEST  
CONSENSUS SHOWS. THIS RELATIONSHIP SUPPORTS ENSEMBLE TRENDS OVER  
THE PAST DAY, TRIMMING AWAY PRIOR MEMBERS THAT HAD LOWER HEIGHTS  
OVER PORTIONS OF THE WEST. THUS EXPECT EJECTING WEST COAST ENERGY  
TO BE FAIRLY WEAK BY DAYS 6-7 WED-THU. APPROACHING TROUGH ENERGY  
LATE IN THE PERIOD MAY BE A LITTLE FAST IN THE 00Z ECMWF, WITH  
REMAINING MODELS/MEANS A BIT SLOWER TO VARYING DEGREES.  
 
A BLEND AMONG THE 06Z GFS AND 00Z ECMWF/UKMET/CMC PROVIDED A GOOD  
STARTING POINT FOR THE DESIRED INTERMEDIATE EVOLUTION OF THE  
PLAINS-GREAT LAKES SYSTEM FOR DAYS 3-5 SUN-TUE. FAVORABLE TRENDS  
OF THE 06Z GEFS MEAN VERSUS THE 00Z CYCLE, RELATIVE AGREEMENT OF  
THE MEANS FOR THE OVERALL PATTERN LATE IN THE PERIOD, AND VARYING  
QUESTION MARKS WITH SOME OPERATIONAL DETAILS AT THAT TIME FRAME,  
LED TO A TRANSITION TOWARD EVEN OR GREATER WEIGHT OF ENSEMBLE MEAN  
GUIDANCE BY DAYS 6-7 WED-THU.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE DEVELOPING SYSTEM TRACKING FROM THE PLAINS THROUGH GREAT LAKES  
INTO EASTERN CANADA WILL LIKELY PRODUCE A BROAD AREA OF ACTIVE  
WEATHER FROM SUN ONWARD, WITH BEST POTENTIAL FOR HEAVY RAINFALL  
AND/OR STRONG CONVECTION FROM THE PLAINS INTO THE GREAT LAKES/OHIO  
VALLEY. EXPECT THE FRONT TRAILING FROM THE SURFACE LOW TO  
GENERATE SOME SHOWERS/THUNDERSTORMS TO THE SOUTH AND EAST AS WELL.  
MEANWHILE ALSO EXPECT RAINFALL OF VARYING INTENSITY NEAR AND  
SOUTH OF A LEADING WAVY FRONT INITIALLY EXTENDING EAST FROM THE  
PLAINS THROUGH THE MID-ATLANTIC. THIS FRONT WILL LIFT NORTH AS A  
WARM FRONT AHEAD OF THE PLAINS/GREAT LAKES SYSTEM. THE FOUR  
CORNERS STATES WILL LIKELY SEE DIURNALLY FAVORED CONVECTION WITH  
AT LEAST SOME POCKETS OF MODERATE OR HEAVIER ACTIVITY, WHILE THE  
UPPER TROUGH EVOLVING ALONG THE NORTHERN HALF OF THE WEST COAST  
AND THEN EVENTUALLY EJECTING OUT COULD SUPPORT SOME SCATTERED  
RAINFALL OVER INLAND LOCATIONS ACROSS NORTHERN PARTS OF THE WEST.  
 
PORTIONS OF THE NORTHERN/CENTRAL PLAINS SHOULD BE WITHIN THE CORE  
OF COOLEST DAYTIME TEMPERATURES VERSUS NORMAL (MINUS 10-15F  
ANOMALIES) IN THE SUN-TUE TIME FRAME. 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 LESS EXTREME ANOMALIES. EXPECT WARMEST HIGHS  
VERSUS NORMAL TO BE OVER PARTS OF THE PACIFIC NORTHWEST WITH SOME  
READINGS LOCALLY 10-15F ABOVE NORMAL MON-TUE. MORNING LOWS SHOULD  
BE ABOVE NORMAL MOST DAYS OVER MUCH OF THE WEST.  
 
RAUSCH  
 
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