865  
FXUS02 KWBC 250657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
257 AM EDT THU AUG 25 2022  
 
VALID 12Z SUN AUG 28 2022 - 12Z THU SEP 01 2022  
   
..OVERVIEW
 
 
MOST GUIDANCE OVER RECENT DAYS HAS BEEN ADVERTISING A TRANSITION  
FROM FAIRLY FLAT MEAN FLOW ALOFT ON SUNDAY TO AN AMPLIFIED REGIME  
CONSISTING OF AN EASTERN PACIFIC TROUGH, WESTERN NORTH AMERICA  
RIDGE, AND EASTERN CANADA/NORTHEASTERN U.S. TROUGH BY NEXT  
TUESDAY-THURSDAY. THERE HAVE BEEN SOME TYPICAL EMBEDDED  
DIFFERENCES WITH SPECIFICS OF ONE OR MORE OF THE LARGE SCALE  
FEATURES, WHILE CURRENT GULF OF ALASKA SHORTWAVE ENERGY FORECAST  
TO REACH THE NORTHWEST AS A COMPACT UPPER LOW AROUND SATURDAY (IN  
THE SHORT RANGE PERIOD) CONTINUES TO BE A PARTICULARLY TROUBLESOME  
FEATURE WITHIN OR AROUND THE MORE AGREEABLE MEAN PATTERN. THE  
PRIMARY AREAS OF FOCUS FOR RAINFALL DURING THE PERIOD SHOULD BE  
WITH A COLD FRONT THAT PUSHES FROM THE NORTHERN ROCKIES/PLAINS  
INTO THE EAST/SOUTH AS WELL AS WITH DIURNALLY FAVORED SHOWERS AND  
STORMS ACROSS THE SOUTHERN TIER. THE NORTHWEST UPPER LOW MAY  
SUPPORT SOME RAINFALL AS WELL DEPENDING ON ITS EVOLUTION/TRACK.  
WARMEST TEMPERATURE ANOMALIES SHOULD BE OVER NORTHERN AREAS EAST  
OF THE ROCKIES EARLY NEXT WEEK AND OVER THE NORTHWEST  
TUESDAY-THURSDAY.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
THE FORECAST OF THE COMPACT UPPER LOW REACHING THE NORTHWEST  
BEFORE THE START OF THE PERIOD CONTINUES TO BE ONE OF LOW  
CONFIDENCE/PREDICTABILITY AND HIGH SENSITIVITY TO EARLY  
DIFFERENCES IN TRACK. AMONG 12Z/18Z MODEL RUNS, THE POTENTIAL  
LOCATION OF THIS LOW RANGED BETWEEN NORTHERN CALIFORNIA (12Z  
ECMWF) AND THE NORTHERN ROCKIES (18Z GFS AND 12Z UKMET/CMC). FROM  
THERE GUIDANCE ULTIMATELY HAD IT DRIFTING INTO THE ROCKIES OR  
PLAINS. ENOUGH ENSEMBLE MEMBERS WERE ON THE SLOW SIDE FOR THE  
ENSEMBLE MEANS TO SUGGEST THE SLOW ECMWF SCENARIO. HIGHLIGHTING  
THE UNCERTAINTY OF THE FORECAST, THE NEW 00Z GFS HAS CHANGED TO A  
PROGRESSIVE NORTHERN TIER TRACK ALONG THE SOUTHERN FRINGE OF THE  
WESTERLIES (MORE LIKE WHAT CONSENSUS WAS DOING MULTIPLE DAYS AGO).  
THE 00Z UKMET/CMC DROP THE FEATURE INTO THE CENTRAL HIGH PLAINS BY  
TUESDAY. THE ECMWF HAS TRENDED WELL EAST OF ITS 12Z RUN CLOSER TO  
THE 00Z UKMET/CMC, LOWERING THE PROBABILITY OF THE WESTERN SIDE OF  
THE SPREAD. ANOTHER ASPECT OF THE FORECAST WITH LOW PREDICTABILITY  
DUE TO FAIRLY SMALL SCALE AND WEAK STRENGTH IS AN AREA OF  
SHORTWAVE ENERGY OVER THE CENTRAL U.S. AS OF SUNDAY AND POSSIBLY  
WEAK UPSTREAM IMPULSES. THUS FAR, AGREEMENT IS POOR WITH RESPECT  
TO WHAT PROPORTION PROGRESSES EASTWARD VERSUS LINGERING OVER THE  
PLAINS.  
 
THE MOST NOTABLE LARGE SCALE ISSUE IN THE 12Z/18Z GUIDANCE  
INVOLVED THE GFS CLOSING OFF AN UPPER LOW WITHIN THE PACIFIC  
TROUGH BY DAYS 6-7 WEDNESDAY-THURSDAY, ULTIMATELY LEADING TO A  
FLATTENING OF THE CANADIAN UPPER RIDGE SEEN IN OTHER MODELS AND  
ALL THREE OF THE ENSEMBLE MEANS. THE NEW 00Z GFS HAS TRENDED MUCH  
CLOSER TO CONSENSUS FOR THE PACIFIC TROUGH BUT STILL STRAYS A BIT  
FOR DOWNSTREAM DETAILS BY DAY 7 THURSDAY. OUTSIDE OF THE  
QUESTIONABLE 12Z/18Z GFS, THE DIFFERENCES FOR HOW QUICKLY THE  
LEADING EDGE OF THE PACIFIC TROUGH REACHES THE NORTHWEST ARE  
WITHIN TYPICAL GUIDANCE SPREAD/ERROR RANGES.  
 
BASED ON 12Z/18Z GUIDANCE, THE UPDATED FORECAST EMPHASIZED  
OPERATIONAL RUNS FOR ABOUT THE FIRST HALF OF THE PERIOD ALONG WITH  
SOME EDITING TO REFLECT A MORE TRUE COMPROMISE AMONG THE MODEL  
RUNS FOR THE WESTERN UPPER LOW. THEN THE FORECAST TRANSITIONED  
TOWARD A HALF MODELS/HALF MEANS BLEND BY THE END OF THE PERIOD TO  
BALANCE DETAIL UNCERTAINTY WITHIN THE MORE CONFIDENT LARGE SCALE  
PATTERN.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE BRIEF PERIOD OF MORE PROGRESSIVE FLOW ALOFT OVER THE WEST  
THROUGH THE WEEKEND SHOULD PROMOTE A LIGHTER AND MORE SCATTERED  
TREND FOR MONSOON RAINFALL OVER THE SOUTHWEST/FOUR CORNERS STATES  
INTO THE INTERMOUNTAIN WEST. SOME OF THE WESTERN U.S. DEEP  
MOISTURE THAT IS PUSHED EASTWARD MAY CONTRIBUTE TO THE AREA OF  
RAINFALL FORECAST TO SPREAD ACROSS PORTIONS OF THE CENTRAL/EASTERN  
U.S. AHEAD OF A HIGH PLAINS SURFACE TROUGH AND COLD FRONT  
PROGRESSING SOUTHEASTWARD FROM THE NORTHERN TIER. THERE MAY BE  
MULTIPLE AREAS OF LOCALLY MODERATE TO HEAVY RAINFALL WITHIN THE  
BROAD MOISTURE SHIELD BUT WITH LOWER THAN AVERAGE CONFIDENCE IN  
SPECIFICS. ONE POTENTIAL AREA OF FOCUS COULD BE OVER AND JUST EAST  
OF THE CENTRAL/SOUTH-CENTRAL PLAINS WHERE THE COLD FRONT COULD  
INTERACT WITH SOME GULF MOISTURE. THE UPPER LOW EMERGING FROM THE  
NORTHWEST COULD PRODUCE SOME AREAS OF RAINFALL BETWEEN THE ROCKIES  
AND PLAINS BUT AGAIN WITH LOW CONFIDENCE ON DETAILS AT THIS TIME.  
FARTHER EAST, SHOWERS/THUNDERSTORMS WILL LIKELY CONTINUE OVER THE  
SOUTH AND FLORIDA PENINSULA THROUGH THE PERIOD BUT WITH ANY AREAS  
OF ENHANCED TOTALS LIKELY TO BE FAIRLY LOCALIZED AND UNCERTAIN ON  
A DAY-TO-DAY BASIS, INCLUDING SOME MODEL VARIABILITY WITH WHETHER  
THE BULK OF RAINFALL WILL BE OFFSHORE OR ONSHORE IN ANY GIVEN  
PERIOD.  
 
THE WARMEST TEMPERATURE ANOMALIES DURING THE PERIOD SHOULD BE OVER  
PARTS OF THE NORTHWEST DURING TUESDAY-THURSDAY AS MEAN RIDGING  
ALOFT BUILDS OVER THE WEST, WITH SOME LOCATIONS 10-15F ABOVE  
NORMAL. A FEW PLACES COULD CHALLENGE DAILY RECORDS FOR HIGHS/WARM  
LOWS. THERE IS STILL SOME UNCERTAINTY DUE TO A POTENTIAL EMBEDDED  
SHORTWAVE/LOW BUT LATEST TRENDS ARE TAKING THIS FEATURE FARTHER  
EAST. EXPECT HIGHS OF 5-10F ABOVE NORMAL FROM THE NORTHERN PLAINS  
INTO THE NORTHEAST EARLY IN THE WEEK FOLLOWED BY A COOLER TREND  
AFTER FRONTAL PASSAGE. THE SOUTHERN TIER WILL SEE NEAR TO SLIGHTLY  
BELOW NORMAL HIGHS THROUGH THE PERIOD, THOUGH CALIFORNIA MAY TREND  
A LITTLE ABOVE NORMAL BY MIDWEEK.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, EXPERIMENTAL EXCESSIVE RAINFALL  
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES ARE  
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/#PAGE=ERO  
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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