339  
FXUS02 KWBC 241858  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 PM EDT WED AUG 24 2022  
 
VALID 12Z SAT AUG 27 2022 - 12Z WED AUG 31 2022  
   
..OVERVIEW
 
 
THIS WEEKEND INTO EARLY NEXT WEEK, REASONABLY FLAT UPPER FLOW BUT  
WITH MULTIPLE EMBEDDED SHORTWAVE IMPULSES OVER THE LOWER 48 IS  
FORECAST, BUT THE FLOW WILL AMPLIFY THROUGH NEXT WEEK IN RESPONSE  
TO THE SHARP DIGGING OF AN AMPLIFIED GULF OF ALASKA UPPER TROUGH.  
THIS SHOULD PRODUCE A RIDGE OVERALL IN WESTERN NORTH AMERICA, BUT  
PERHAPS WITH AN EMBEDDED SHORTWAVE OR TWO, AND BROAD TROUGHING  
OVER EASTERN PORTIONS OF THE CONTINENT.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
MODEL GUIDANCE HAS CONTINUED TO WAFFLE CONSIDERABLY WITH THE  
DETAILS OF THE UPPER-LEVEL PATTERN ESPECIALLY EARLY IN THE PERIOD.  
THE SMALL SCALE OF SHORTWAVES DOMINATING THE FLOW AND THEIR  
ORIGINS FROM THE HIGH LATITUDE PACIFIC/ALASKA LEADS TO LOWER THAN  
NORMAL CONFIDENCE WITH THE DETAILS. MOST MODELS SHOW ONE SUCH  
SHORTWAVE ENTERING THE PACIFIC NORTHWEST AROUND SATURDAY, THOUGH  
THE 00Z CMC WAS WEAKER THAN CONSENSUS, AND THE 00Z UKMET  
MAINTAINED PHASED SHORTWAVE ENERGY ACROSS SOUTHWESTERN CANADA TO  
CAUSE A DIFFERENT PATTERN AND SEEMED TO BE AN OUTLIER. BY EARLY  
NEXT WEEK, THERE WERE ALREADY CONSIDERABLE DIFFERENCES IN THE  
POSITION OF THIS SHORTWAVE (POSSIBLY COMPACT CLOSED LOW) IN THE  
00Z/06Z MODEL GUIDANCE. THE 12Z GUIDANCE HAS EMERGED LOOKING MOST  
SIMILAR TO THE 00Z ECMWF, WITH THIS SHORTWAVE TRACKING SLOWLY  
THROUGH THE NORTHERN PORTIONS OF THE GREAT BASIN, ROCKIES, AND  
HIGH PLAINS THROUGH WEDNESDAY, WHICH COULD DISRUPT THE OVERALL  
RIDGE PATTERN ACROSS THE WEST. EARLIER RUNS OF THE GFS FOR EXAMPLE  
TRACKED THE FEATURE FARTHER EAST AND BECOMING EMBEDDED IN THE  
CENTRAL-EASTERN U.S. FLOW. CONFIDENCE REMAINS LOW WITH THE  
SHORTWAVE TRACK GIVEN THE RUN-TO-RUN DIFFERENCES AND THE VARIOUS  
SOLUTIONS SHOWN BY THE GEFS/EC/CMCE ENSEMBLE MEMBERS. BY  
TUESDAY-WEDNESDAY, AT LEAST MODELS ARE MORE AGREEABLE WITH  
AMPLIFYING FLOW AHEAD OF THE DIGGING PACIFIC TROUGH, LEADING TO  
GENERAL RIDGING IN THE WESTERN OR PERHAPS CENTRAL U.S. (DEPENDING  
ON THE AFOREMENTIONED SHORTWAVE PLACEMENT) AND TROUGHING IN THE  
EAST.  
 
GIVEN THE SHORTWAVE DIFFERENCES, THE WPC FORECAST USED A HIGHER  
PROPORTION OF ENSEMBLE MEANS COMPARED TO NORMAL EARLY IN THE  
PERIOD, STARTING AROUND 30 PERCENT WHILE THE 06Z GFS, 00Z ECMWF,  
AND 00Z CMC COMPRISED THE REST OF THE BLEND. GRADUALLY INCREASED  
THE PROPORTION OF ENSEMBLE MEANS TO OVER HALF DAY 6-7. THIS  
MAINTAINED REASONABLE CONTINUITY WITH THE MASS FIELDS. THE QPF DID  
GENERALLY TREND DOWN COMPARED TO THE PREVIOUS FORECAST GIVEN AN  
OVERALL MODEL TREND DOWNWARD BUT ALSO THE LACK OF CONFIDENCE IN  
THE SPECIFICS OF ANY HEAVY TOTALS.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE BRIEF TRANSITION TO MORE PROGRESSIVE FLOW ALOFT OVER THE WEST  
DURING 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 EXPANDING  
AREA OF RAINFALL FORECAST TO SPREAD ACROSS THE PLAINS AND  
EVENTUALLY INTO THE MISSISSIPPI VALLEY/GREAT LAKES. A LEADING WARM  
FRONT FOLLOWED BY A SURFACE TROUGH AND COLD FRONT, AS WELL AS  
NORTHERN TIER UPPER DYNAMICS, WILL ALL PROVIDE VARYING DEGREES OF  
FOCUS FOR WHAT COULD BE MULTIPLE AREAS OF LOCALLY MODERATE TO  
HEAVY RAINFALL WITHIN THE BROAD MOISTURE SHIELD. CURRENT FORECASTS  
SHOW ONE AREA OF RAINFALL FOCUS AROUND OKLAHOMA/MISSOURI/ARKANSAS  
WHERE THE FRONT MAY BE LESS PROGRESSIVE THAN THE NORTHERN PART OF  
THE FRONT AND AS ENHANCED GULF MOISTURE RETURNS INLAND ACROSS THE  
SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY. 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 MORE 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.  
 
TEMPERATURES AROUND 5-10 DEGREES WARMER THAN AVERAGE ARE FORECAST  
TO SHIFT FROM THE NORTHERN PLAINS THIS WEEKEND INTO THE GREAT  
LAKES/OHIO VALLEY AND INTO THE NORTHEAST EARLY NEXT WEEK. FARTHER  
SOUTH, TEMPERATURES SHOULD BE SEASONABLE OR EVEN A BIT BELOW  
AVERAGE FOR LATE SUMMER BUT STILL MAY FEEL WARM IN THE UPPER  
80S-90S. WITH THE BUILDING RIDGE IN THE NORTHWEST, THERE IS A  
POSSIBILITY FOR ANOMALOUS HEAT THERE WITH TEMPERATURES 10-15F  
ABOVE NORMAL, BUT WITH UNCERTAINTY AT THIS TIME GIVEN DIFFERENCES  
WITH AN EMBEDDED SHORTWAVE.  
 
TATE/SCHICHTEL  
 
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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