192  
FXUS02 KWBC 010659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT WED AUG 01 2018  
 
VALID 12Z SAT AUG 04 2018 - 12Z WED AUG 08 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
THE FORECAST OVER THE EASTERN PACIFIC AND WESTERN NORTH AMERICA  
AFTER DAY 4 SUN HAS BECOME DRAMATICALLY MORE UNCERTAIN OVER THE  
PAST 24-36 HOURS. DURING THE WEEKEND THERE IS RELATIVE AGREEMENT  
THAT AN UPPER TROUGH WITH EMBEDDED LOW WILL BE AMPLIFYING OVER THE  
EASTERN PACIFIC. AT THIS TIME FRAME THERE IS MODERATE SCATTER IN  
CLOSED LOW POSITION (00Z ECMWF A LITTLE NORTHEAST, GFS SOUTHWEST)  
BUT THESE DIFFERENCES COULD PROVE CRITICAL MID-LATE PERIOD.  
BEFORE TODAY THE PRIMARY QUESTION MARK WAS HOW QUICKLY THE INITIAL  
UPPER LOW/TROUGH ENERGY MIGHT EJECT IN RESPONSE TO UPSTREAM FLOW.  
NOW THE RECENT GFS RUNS AND 00Z CMC OFFER THE POTENTIAL FOR  
UPSTREAM FLOW TO BE SUFFICIENTLY FLAT THAT THE PACIFIC LOW WOULD  
LINGER FOR A LONGER TIME, WHILE WESTERN U.S. RIDGING BECOMES QUITE  
STRONG/WESTWARD INSTEAD OF SETTLING OVER THE SOUTHERN ROCKIES/HIGH  
PLAINS. AT THE SAME TIME THE GFS/00Z CMC SCENARIO WOULD HAVE A  
PROGRESSIVE NORTHERN STREAM SHORTWAVE REACHING WESTERN-CENTRAL  
CANADA BY DAYS 6-7 TUE-WED. ON THE OTHER HAND THE ENSEMBLE MEANS  
HAVE BEEN FAIRLY CONSISTENT IN ADVERTISING LESS FLOW SEPARATION  
OVER THE EASTERN PACIFIC, WITH THE ONLY QUESTION BEING THE EXACT  
LONGITUDE OF THE OVERALL TROUGH. YESTERDAY'S 00Z ECMWF APPEARS TO  
HAVE BEEN A RELATIVE HICCUP AS IT WAS FARTHER EAST/LESS AMPLIFIED  
WITH THE TROUGH VERSUS OTHER RUNS. THE 12Z ECMWF/CMC WERE NOT FAR  
FROM THE ENSEMBLE MEAN SCENARIO BUT DID STRAY A BIT FASTER WITH  
INLAND EJECTION LATE IN THE PERIOD. ADDING TO THE CONFUSION, THE  
12Z PARALLEL (FV3) GFS WAS VERY CLOSE TO THE MEANS BUT THE 18Z RUN  
CHANGED TO THE PATTERN OF RECENT OPERATIONAL GFS RUNS. THIS  
HIGHLIGHTS THE ALEUTIANS/GULF OF ALASKA AS AN AREA OF HIGH  
SENSITIVITY IN RELATION TO DETERMINING THE ULTIMATE FLOW EVOLUTION  
OVER THE EASTERN PACIFIC/WESTERN NORTH AMERICA. TELECONNECTIONS  
RELATIVE TO THE POSITIVE HEIGHT ANOMALY ASSOCIATED WITH THE STRONG  
NORTHERN PACIFIC/ALEUTIANS RIDGE EARLY-MID PERIOD HAVE THUS FAR  
BEEN SUPPORTIVE OF THE ENSEMBLE MEAN SCENARIO. FOR THIS REASON  
AND TO MAINTAIN CONTINUITY UNTIL THERE IS EMPHATIC SUPPORT FOR AN  
ALTERNATE SOLUTION--WHICH COULD BE FAIRLY SOON GIVEN 00Z GUIDANCE  
TRENDS--THE PREFERENCE BASED ON 12Z/18Z SOLUTIONS TRANSITIONED  
FROM AN OPERATIONAL MODEL BLEND EARLY IN THE PERIOD TOWARD A  
MAJORITY OF 18Z GEFS/12Z ECMWF ENSEMBLE MEANS WITH A SMALL  
LINGERING WEIGHT OF THE 12Z ECMWF/CMC BY DAYS 6-7 TUE-WED.  
 
MEANWHILE THE FORECAST LOOKS LESS SENSITIVE OVER THE EASTERN HALF  
OF THE CONTINENT. REGARDLESS OF DIFFERENCES FOR THE EASTERN  
PACIFIC INTO WESTERN NORTH AMERICA EVOLUTION MID-LATE PERIOD,  
NEARLY ALL GUIDANCE TRANSITIONS FROM LOW AMPLITUDE  
WEST-SOUTHWESTERLY NORTHERN TIER FLOW TO AN EASTERN NORTH AMERICA  
MEAN TROUGH THAT ENDS UP SUPPRESSING ATLANTIC RIDGING THAT BUILDS  
INTO THE SOUTHEAST EARLY-MID PERIOD. THE UNCERTAINTY FOR THIS  
PART OF THE FORECAST WILL LIE WITH INDIVIDUAL SHORTWAVE IMPULSES  
THAT WILL AFFECT SURFACE FRONTS/WAVES. RELATIVELY SMALL SCALE OF  
THE SHORTWAVES WILL LEAD TO LOW PREDICTABILITY AT EXTENDED TIME  
FRAMES. THE GUIDANCE SELECTION MADE FOR THE UPSTREAM FORECAST  
PROVIDED A REASONABLE STARTING POINT FOR DEPICTING THE MOST COMMON  
ELEMENTS OF THE PATTERN EVOLUTION OVER CENTRAL/EASTERN PARTS OF  
THE COUNTRY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
INCREASING SPREAD AND CONTINUITY CHANGES IN RECENT GUIDANCE ARE  
LOWERING CONFIDENCE IN EXACTLY HOW MUCH MOISTURE REACHES THE  
PACIFIC NORTHWEST AHEAD OF THE UPPER TROUGH/LOW FORECAST TO EVOLVE  
OVER THE EASTERN PACIFIC. THE CURRENTLY PREFERRED SOLUTION WOULD  
HAVE SOME RAINFALL ARRIVING TOWARD THE END OF THE WEEKEND/EARLY  
NEXT WEEK BUT LATEST ADJUSTMENTS IN 00Z RUNS ARE INCREASING THE  
POTENTIAL FOR LATER ARRIVAL WITH LOWER AMOUNTS. LIKEWISE THE  
TEMPERATURE FORECAST FOR PORTIONS OF THE WEST NEXT WEEK HAS BECOME  
MORE UNCERTAIN. ESTABLISHED CONSENSUS HAS SUGGESTED AREAS WEST OF  
THE ROCKIES SHOULD TEND TO BE NEAR TO BELOW NORMAL FOR DAYTIME  
HIGHS AND COOLEST VERSUS NORMAL OVER THE NORTHWEST. HOWEVER THERE  
IS THE POSSIBILITY OF WARMER TEMPERATURES.  
 
ELSEWHERE, EXPECT DIURNALLY DRIVEN SHOWERS/THUNDERSTORMS ALONG THE  
TERRAIN IN THE SOUTHWEST/FOUR CORNERS REGION THROUGH MOST OF THE  
PERIOD. FROM THE WEEKEND INTO EARLY NEXT WEEK A MEAN FRONTAL  
BOUNDARY WITH EMBEDDED WAVES WILL BE A FOCUS FOR PERIODS OF  
LOCALLY HEAVY/STRONG CONVECTION FROM THE NORTHERN PLAINS INTO THE  
GREAT LAKES. TOWARD MIDWEEK THE GRADUAL DEVELOPMENT OF UPPER  
TROUGHING OVER THE EAST SHOULD PUSH THE BEST ORGANIZED ACTIVITY  
INTO THE APPALACHIANS AND VICINITY. BEFORE THAT OCCURS, WESTWARD  
PROTRUSION OF THE BERMUDA HIGH INTO THE SOUTHEAST WILL PROMOTE A  
DRIER TREND OVER THE EAST AFTER A WET SHORT RANGE PERIOD. THERE  
WILL STILL BE SOME SCATTERED SHOWERS/STORMS ENCOURAGED BY DAYTIME  
HEATING. AREAS ALONG THE GULF COAST MAY SEE ONE OR MORE DAYS OF  
RAINFALL AS WELL. FOR TEMPERATURES, ABOVE NORMAL ANOMALIES WILL  
BE FAIRLY WIDESPREAD EAST OF THE ROCKIES. WARMEST READINGS VERSUS  
NORMAL SHOULD EXIST FOR MORNING LOWS FROM THE UPPER MS VALLEY INTO  
NEW ENGLAND, WITH SOME POCKETS OF VERY WARM HIGHS FROM PORTIONS OF  
THE PLAINS INTO NORTHEAST ON ONE OR MORE DAYS.  
 
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