951  
FXUS02 KWBC 210700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT SUN AUG 21 2022  
 
VALID 12Z WED AUG 24 2022 - 12Z SUN AUG 28 2022  
 
...SOUTHERN TIER HEAVY RAINFALL THREAT LIKELY TO CONTINUE THROUGH  
AT LEAST WEDNESDAY...  
 
...HEAT OVER THE NORTHWEST MID-LATE WEEK MAY CHALLENGE SOME DAILY  
RECORDS...  
 
   
..OVERVIEW
 
 
MOST GUIDANCE IS ADVERTISING A TRANSITION IN THE PATTERN FROM WEAK  
EASTERN U.S. MEAN TROUGHING WITH A STRONGER TROUGH OVER THE  
EASTERN HALF OF CANADA WEDNESDAY-FRIDAY TOWARD A MODEST MEAN  
TROUGH OVER THE CENTRAL U.S./SOUTHERN CANADA BY NEXT WEEKEND. THE  
DETAILS OF THIS EVOLUTION INVOLVE LIFTING OUT OF EASTERN NORTH  
AMERICA TROUGH ENERGY WHILE A MODEST TROUGH INITIALLY ANCHORED BY  
A COMPACT NORTHWEST UPPER LOW HEADS INTO THE NORTHERN PLAINS WHILE  
UPSTREAM NORTH PACIFIC TROUGH ENERGY FEEDS INTO THE OVERALL  
TROUGH. THE WESTERN PERIPHERY OF ATLANTIC UPPER RIDGING MAY ALSO  
PUSH A LITTLE WESTWARD OVER THE EASTERN U.S. NEXT WEEKEND. MEAN  
RIDGING OVER THE SOUTHERN HALF OF THE WEST SHOULD WEAKEN AND/OR  
DROP SOUTHWARD BY THE WEEKEND BUT WITH UNCERTAIN DETAILS. INITIAL  
SOUTHERN TIER HEAVY RAINFALL SHOULD TREND LIGHTER AND EASTWARD  
WITH TIME AS THE SUPPORTING FRONT SETTLES TOWARD THE SOUTHEASTERN  
COAST. THE COMBINATION OF THE UPPER TROUGH/LOW DRIFTING FROM THE  
NORTHWEST INTO THE PLAINS AND A COUPLE NORTHERN TIER FRONTS MAY  
FOCUS RAINFALL OVER THE NORTHERN HIGH PLAINS WITH SOME RAIN ALSO  
EXTENDING EAST ALONG THE ONE FRONT THAT REACHES THE NORTHEAST.  
THE MOST PROMINENT AREA OF ABOVE NORMAL TEMPERATURES WILL BE IN  
THE NORTHWEST WEDNESDAY-FRIDAY WHILE LESS EXTREME ANOMALIES ARE  
LIKELY OVER THE NORTHEAST LATE THIS WEEK. SOUTHERN TIER HIGH  
TEMPERATURES WILL BE BELOW NORMAL FOR MOST OF THE PERIOD THOUGH  
WITH SOME MODERATION AFTER MIDWEEK.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE COMPARISONS DURING THE FIRST HALF OF THE PERIOD FAVORED  
INCORPORATION OF THE 12Z/18Z GFS AND 12Z ECMWF/CMC. THE 12Z UKMET  
WAS NOT USABLE DUE TO HAVING A MORE PRONOUNCED AND  
FASTER/NORTHWARD WAVE TRACKING OUT OF THE SOUTH. THE NEW 00Z  
UKMET HAS ADJUSTED BACK SOME BUT REMAINS RATHER EXTREME COMPARED  
TO OTHER SOLUTIONS. MODELS CONTINUE TO VARY WITH SPECIFICS OF THE  
SOUTHERN PERIPHERY OF THE EASTERN CANADA UPPER TROUGH THAT PUSHES  
A COLD FRONT INTO GREAT LAKES/NORTHEAST, WITH CURRENT CONSENSUS  
SHOWING A FASTER ADJUSTMENT VERSUS PRIOR FORECAST (CONTINUING  
THROUGH THE END OF THE PERIOD). ALSO, AFTER EARLY THURSDAY THERE  
IS QUICKER PROGRESSION/WEAKENING OF THE WESTERN PART OF THE  
SOUTHERN TIER SURFACE FRONT.  
 
A BLEND OF THE OPERATIONAL MODELS AND 18Z GEFS/12Z ECMWF MEANS  
PROVIDED A REASONABLE INTERMEDIATE/MOST LIKELY SCENARIO BY THE  
LATTER HALF OF THE PERIOD. THERE IS STILL SIGNIFICANT MODEL AND  
ENSEMBLE MEMBER SPREAD FOR EASTERN PACIFIC INTO WESTERN NORTH  
AMERICA FLOW FROM LATE WEEK THROUGH THE WEEKEND, ORIGINATING FROM  
DIFFERENCES IN HOW FLOW SEPARATES WITHIN AN UPPER TROUGH TO THE  
SOUTH OF ALASKA. THE 00Z GFS IS STARTING TO COME INTO BETTER  
AGREEMENT WITH THE MAJORITY CLUSTER OVER THE PACIFIC, TRIMMING THE  
MODEL'S DIFFERENCES VERSUS OTHER GUIDANCE IN SOME RESPECTS. THERE  
IS STILL CONSIDERABLE SPREAD FOR AMPLITUDE/TIMING OF SHORTWAVE  
ENERGY REACHING WESTERN NORTH AMERICA THOUGH. THE GULF OF MEXICO  
PROVIDES ANOTHER AREA OF UNCERTAINTY BY THE WEEKEND. SOME GFS  
RUNS HAVE BEEN REFLECTING A WEAK SURFACE FEATURE OVER THE EASTERN  
GULF (DAMPENED IN THE NEW 00Z RUN) WHILE THE CMC HAS BEEN FAVORING  
THE FAR SOUTHWESTERN GULF. 12Z GEFS/ECMWF ENSEMBLE MEMBERS FOR LOW  
LEVEL VORTICITY CENTERS ARE DISPERSED ACROSS THE GULF IN NO  
DEFINED PATTERN THOUGH AT THE SAME TIME ENSEMBLES SHOW A LITTLE  
GREATER PROBABILITY FOR HIGHER MOISTURE/LOW SHEAR OVER THE EASTERN  
HALF OF THE GULF. THE MANUAL FORECAST VALID ON DAY 6 SATURDAY  
MAINTAINS THE EASTERN GULF SURFACE TROUGH FROM YESTERDAY'S NHC/WPC  
COORDINATION WHILE DAY 7 SUNDAY EXTRAPOLATES THIS TROUGH  
NORTHWESTWARD.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
EXPECT AREAS OF HEAVY RAINFALL OVER THE SOUTHERN TIER TO CONTINUE  
AT LEAST INTO WEDNESDAY, WITH LINGERING MOISTURE AND A WAVY FRONT  
CONTRIBUTING TO ENHANCED TOTALS. THE WPC EXPERIMENTAL DAY 4  
OUTLOOK VALID WEDNESDAY INTO EARLY THURSDAY HIGHLIGHTS A SLIGHT  
RISK FOR EXCESSIVE RAINFALL OVER SOME AREAS FROM TEXAS THROUGH THE  
LOWER MISSISSIPPI VALLEY. AS HAS BEEN THE CASE IN PREVIOUS  
CYCLES, GUIDANCE SPREAD STILL SUGGESTS AT LEAST A MODERATE DEGREE  
OF UNCERTAINTY IN SPECIFICS. LOW PREDICTABILITY OF IMPORTANT  
SMALL-SCALE FEATURES ALOFT AS WELL AS DEPENDENCE ON MESOSCALE  
CONVECTIVE ASPECTS THAT MAY NOT BECOME APPARENT UNTIL CLOSE TO THE  
EVENT COMPLICATE THE MEDIUM RANGE FORECAST. RECENT TRENDS HAVE  
BEEN TOWARD A GRADUALLY MORE PROGRESSIVE/SUPPRESSED EVOLUTION FOR  
RAINFALL, LEADING TO LESS CONFIDENCE IN WHERE ANY ORGANIZED AREAS  
OF SIGNIFICANT RAIN MAY REMAIN BY LATE IN THE WEEK. THE SOUTHEAST  
SHOULD CONTINUE TO SEE PERIODS OF SHOWERS AND THUNDERSTORMS  
THROUGH THE WEEKEND WITH A FRONT LINGERING NEAR THE SOUTHEASTERN  
COAST, WHILE FLORIDA SEES PERIODS OF SHOWERS/STORMS AS WELL.  
MONSOONAL MOISTURE OVER THE WEST SHOULD PUSH WESTWARD SOMEWHAT  
AROUND MIDWEEK, WITH SOME DEGREE OF RAINFALL ENHANCEMENT POSSIBLE.  
THEN THE MOISTURE WILL LIKELY RETURN BACK TO THE EAST AS THE  
UPPER RIDGE OVER THE SOUTHERN HALF OF THE WEST WEAKENS. THE  
FORECAST IS CONSISTENT IN SHOWING POTENTIAL FOR SOME ENHANCED  
RAINFALL OVER THE NORTHERN HIGH PLAINS MID-LATE WEEK WITH THE  
COMBINATION OF A COUPLE SURFACE FRONTS AND THE APPROACHING  
NORTHWEST UPPER TROUGH/LOW, PLUS SOME LOW LEVEL UPSLOPE FLOW.  
AREAS FARTHER EAST ALL THE WAY TO THE EAST COAST MAY ALSO SEE SOME  
RAINFALL WITH THE MAIN FRONT EXPECTED TO REACH THE  
NORTHEAST/MID-ATLANTIC. A BROADER AREA OF RAINFALL MAY SPREAD  
OVER THE CENTRAL U.S. NEXT WEEKEND AS PACIFIC ENERGY ALOFT  
APPROACHES.  
 
THE PRIMARY FOCUS FOR ANOMALOUS HEAT WILL BE OVER THE NORTHWEST  
DURING WEDNESDAY-FRIDAY WHEN HIGHS MAY REACH 10-15F ABOVE NORMAL,  
WITH MORNING LOWS ALSO WELL ABOVE NORMAL AND COVERING A LARGER  
PORTION OF THE WEST. SOME LOCATIONS COULD SEE TEMPERATURES  
CHALLENGE DAILY RECORDS FOR HIGHS AND/OR WARM LOWS. THE REGION  
SHOULD REMAIN ABOVE NORMAL NEXT WEEKEND BUT WITH LOWER ANOMALIES.  
HIGH TEMPERATURES SHOULD BE BELOW NORMAL OVER THE SOUTHERN STATES  
THROUGH THE PERIOD BUT WITH MOST AREAS MODERATING TO SINGLE-DIGIT  
ANOMALIES. SLIGHTLY BELOW NORMAL HIGHS SHOULD SETTLE INTO  
NORTHERN PARTS OF THE PLAINS LATER IN THE WEEK. THE NORTHEAST MAY  
SEE HIGHS REACH UP TO 5-10F ABOVE NORMAL THURSDAY-FRIDAY.  
 
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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