736  
FXUS02 KWBC 201858  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
257 PM EDT SAT AUG 20 2022  
 
VALID 12Z TUE AUG 23 2022 - 12Z SAT AUG 27 2022  
 
...SIGNIFICANT EXCESSIVE RAINFALL/FLASH FLOODING THREAT LIKELY TO  
SETTLE OVER THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY NEXT  
WEEK...  
 
...HEAT OVER THE NORTHWEST MID-LATE WEEK MAY CHALLENGE SOME DAILY  
RECORDS...  
 
19Z UPDATE: SIMILAR TO THIS TIME YESTERDAY, THE MODELS STILL HAVE  
SOME DIFFERENCES REGARDING THE PLACEMENT OF THE HEAVY RAINFALL  
AXIS ACROSS THE GULF COAST REGION ON TUESDAY AND INTO WEDNESDAY,  
WITH AN OVERALL SLIGHTLY FARTHER EAST TREND. THE UKMET REMAINS  
MUCH MORE PROGRESSIVE WITH THE SURFACE LOW AND THE CORE OF  
HEAVIEST RAINFALL, AND ALTHOUGH IT'S MASS FIELDS WERE ABLE TO BE  
PART OF THE FORECAST BLEND FOR FRONTS AND PRESSURES ELSEWHERE  
ACROSS THE NATION, IT WAS NOT USED WITH THE QPF FORECAST. NO  
MAJOR CHANGES ARE WARRANTED FOR THE EXISTING SLIGHT RISK AREAS  
ACROSS THE SOUTH-CENTRAL U.S., EXCEPT FOR A SLIGHT EASTWARD  
EXTENSION ACROSS THE ARKLATEX REGION. DEWPOINTS WITH THE NBM  
APPEARED HIGH, SO THESE WERE SLIGHTLY REDUCED FOR THE ENTIRE  
FORECAST PERIOD. THE 12Z CMC TRENDED MORE IN LINE WITH THE MODEL  
CONSENSUS COMPARED TO ITS 00Z RUN THAT HAD SOME NOTABLE  
DIFFERENCES AS EARLY AS TUESDAY ACROSS THE NORTHEAST. BY THE END  
OF NEXT WEEK, FORECAST UNCERTAINTY INCREASES ACROSS THE NORTHWEST  
AND NORTHERN TIER STATES AS MODELS DIFFER CONSIDERABLY ON THE  
EVENTUAL BREAK-DOWN OF THE UPPER LEVEL RIDGE. THE WPC FRONTS AND  
PRESSURES FORECAST WAS PRIMARILY DERIVED FROM A  
GFS/ECMWF/GEFS/ECENS BLEND, WITH A LITTLE UKMET EARLY IN THE  
FORECAST PERIOD. /HAMRICK  
-----------------------  
   
..OVERVIEW
 
 
MEAN TROUGHING OVER THE EAST WILL RELAX SOMEWHAT AFTER MIDWEEK  
WITH THE DEPARTURE OF INITIAL ENERGY, BUT STILL PERSIST ENOUGH TO  
SUPPORT A MEAN FRONTAL BOUNDARY THAT ULTIMATELY SETTLES NEAR THE  
SOUTHEAST/GULF COASTS. MEANWHILE FLOW AROUND THE SOUTHERN  
PERIPHERY OF A DEEPER EASTERN CANADA MEAN TROUGH SHOULD PUSH ONE  
OR MORE FRONTS INTO THE NORTHERN U.S. UPPER RIDGING OVER THE  
SOUTHERN HALF OF THE WEST SHOULD PERSIST INTO FRIDAY BEFORE  
BECOMING MORE SUPPRESSED. ENERGY FROM A NORTHWEST UPPER  
LOW/TROUGH SHOULD ULTIMATELY MAKE ITS WAY INTO THE NORTHERN PLAINS  
TOWARD THE END OF THE WEEK. CONTINUE TO EXPECT THE BEST FOCUS FOR  
HEAVY RAINFALL DURING THE PERIOD TO BE OVER THE SOUTHERN PLAINS  
AND VICINITY WITH THE INTERACTION OF SOUTHERN TIER SHORTWAVE  
ENERGY, A STATIONARY FRONT, AND ABUNDANT MOISTURE. THE NORTHEAST  
SHOULD TREND DRIER AFTER TUESDAY BUT SOUTHEASTERN AREAS WILL  
LIKELY CONTINUE TO SEE EPISODES OF RAINFALL THROUGH THE WEEK. THE  
COMBINATION OF UPPER TROUGH/LOW DRIFTING FROM THE NORTHWEST INTO  
THE PLAINS AND A FRONT SETTING UP OVER THE NORTHERN TIER MAY  
PRODUCE AREAS OF RAINFALL FROM THE NORTHERN ROCKIES EASTWARD.  
PERSISTENT CLOUDS AND RAINFALL WILL KEEP SOUTHERN TIER HIGH  
TEMPERATURES WELL BELOW NORMAL FOR MOST OF THE WEEK WHILE THE MAIN  
AREA OF ABOVE NORMAL HIGHS WILL BE OVER THE NORTHWEST  
WEDNESDAY-FRIDAY.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
WITHIN THE LARGE SCALE PATTERN EVOLUTION, THERE ARE A NUMBER OF  
EMBEDDED UNCERTAINTIES. THE MOST SIGNIFICANT ONE FROM THE HEAVY  
RAINFALL PERSPECTIVE IS THE HANDLING OF FAIRLY SMALL SCALE (AND  
THUS HAVING LOW PREDICTABILITY) SHORTWAVE ENERGY ACROSS THE  
SOUTHERN TIER. EVERY MODEL SHOWS DIFFERENT SPECIFICS WHILE  
MULTIPLE UKMET RUNS HAVE BEEN ON THE EXTREME SIDE OF THE SPREAD  
WITH MUCH FASTER EASTWARD PROGRESSION (AND FARTHER NORTH OVER THE  
EAST). GUIDANCE HAS ALSO VARIED WITH DETAILS OF THE EASTERN  
CANADA MEAN TROUGH, LEADING TO VARIABILITY IN DETAILS OF A FRONT  
THAT MAY PUSH INTO THE NORTHERN TIER BY LATE IN THE WEEK. THERE  
ARE TYPICAL DIFFERENCES FOR HOW NORTHWEST TROUGH/UPPER LOW ENERGY  
EVOLVES BUT AT LEAST THERE IS DECENT CLUSTERING IN PRINCIPLE FOR  
IT TO REACH THE NORTHERN PLAINS LATE IN THE PERIOD. TYPICALLY  
MORE SENSITIVE SPECIFICS OF THE SURFACE REFLECTION ARE A GREATER  
UNKNOWN THOUGH. FINALLY, TOWARD FRIDAY-SATURDAY THE OPERATIONAL  
MODEL RUNS AND INDIVIDUAL ENSEMBLE MEMBERS WILDLY DIVERGE FOR FLOW  
DETAILS OVER THE EASTERN PACIFIC INTO WESTERN NORTH AMERICA. AN  
AMPLIFIED OFFSHORE TROUGH IN 12Z/18Z GFS RUNS, A RIDGE IN THE NEW  
00Z RUN AND THE CMC, AND A PROGRESSIVE SHORTWAVE IN THE 12Z ECMWF  
ARE SOME OF THE POTENTIAL OPTIONS. PREFER THE FLAT FLOW SEEN IN  
THE ENSEMBLE MEANS UNTIL SOME DEGREE OF CLUSTERING AND CONTINUITY  
EMERGE.  
 
BASED ON GUIDANCE COMPARISONS, THE EARLY PART OF THE FORECAST  
EMPHASIZED A COMPOSITE OF THE 12Z/18Z OPERATIONAL MODELS BUT STILL  
INCLUDED A SMALL 18Z GEFS/12Z ECMWF MEAN COMPONENT IN LIGHT OF  
SOME DETAIL QUESTIONS. ISSUES WITH EASTERN PACIFIC INTO WESTERN  
NORTH AMERICA FLOW LATE IN THE PERIOD AS WELL AS OTHER DETAIL  
UNCERTAINTIES LED TO TRENDING THE TOTAL ENSEMBLE WEIGHT IN THE  
FORECAST UP TO 70 PERCENT BY DAY 7 SATURDAY.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE EXPECTED SOUTHERN PLAINS HEAVY RAINFALL EVENT SHOULD BE IN  
PROGRESS AT THE START OF THE PERIOD EARLY TUESDAY AND CONTINUE  
THROUGH A DECENT PORTION OF WEEK, POSSIBLY EXTENDING AS FAR EAST  
AS THE LOWER MISSISSIPPI VALLEY. CONTRIBUTING MOISTURE SHOULD  
ORIGINATE FROM THE SOUTHWEST, AS WELL AS WHAT MAY BE LEFT OVER  
FROM POTENTIAL TROPICAL CYCLONE FOUR (FORECAST TO BE DISSIPATED BY  
THE START OF THE MEDIUM RANGE PERIOD, CHECK LATEST NATIONAL  
HURRICANE CENTER PRODUCTS). MULTIPLE SHORTWAVE FEATURES ALOFT AND  
A WAVY STATIONARY SURFACE FRONT WILL BE ADDED INGREDIENTS TO  
PROMOTE THE HEAVY RAINFALL POTENTIAL. PORTIONS OF TEXAS SHOULD SEE  
THE HIGHEST RAINFALL TOTALS IN THIS EVENT. WPC EXPERIMENTAL DAYS  
4-5 OUTLOOKS HIGHLIGHT A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER  
PARTS OF TEXAS/LOUISIANA FROM TUESDAY INTO EARLY THURSDAY.  
PERSISTENT GUIDANCE SPREAD SUGGESTS THERE IS STILL AT LEAST A  
MODERATE DEGREE OF UNCERTAINTY IN SPECIFICS, WITH THE RELATIVELY  
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.  
 
MONSOON ACTIVITY OVER THE WEST SHOULD PERSIST BUT IN LESS EXTREME  
FASHION THAN IN THE SHORT TERM. STILL, THERE MAY BE A WESTWARD  
PUSH OF MOISTURE FOR A TIME AROUND MIDWEEK. THE NORTHEAST SHOULD  
SEE ORGANIZED RAINFALL OF VARYING INTENSITY TUESDAY INTO WEDNESDAY  
WITH ONE OR MORE FRONTAL WAVES, FOLLOWED BY A DRIER TREND. THE  
TRAILING FRONT WILL BE MORE PERSISTENT OVER SOUTHERN PARTS OF THE  
EAST, MAINTAINING A DAILY FOCUS FOR DIURNALLY FAVORED SHOWERS AND  
THUNDERSTORMS. FLORIDA WILL SEE MULTIPLE EPISODES OF  
SHOWERS/STORMS AS WELL. EXPECT RAINFALL TO EXPAND BETWEEN THE  
NORTHERN ROCKIES AND GREAT LAKES, EVENTUALLY SPREADING INTO THE  
NORTHEAST, AS A COUPLE FRONTS SETTLE OVER THIS AREA. THE  
COMBINATION OF THE UPPER TROUGH/LOW EMERGING FROM THE NORTHWEST  
AND SOME LOW LEVEL UPSLOPE FLOW MAY ENHANCE RAINFALL OVER AND NEAR  
THE NORTHERN HIGH PLAINS. OTHER AREAS OF LOCALLY HEAVY RAINFALL  
COULD BE POSSIBLE WITHIN THE BROADER MOISTURE SHIELD BUT WITH LOW  
CONFIDENCE IN SPECIFICS AT THIS TIME.  
 
THE PRIMARY FOCUS FOR ANOMALOUS HEAT NEXT WEEK WILL BE OVER THE  
NORTHWEST DURING WEDNESDAY-FRIDAY WHEN HIGHS MAY REACH 10-20F  
ABOVE NORMAL, WITH MORNING LOWS ALSO WELL ABOVE NORMAL. SOME  
LOCATIONS COULD SEE TEMPERATURES CHALLENGE DAILY RECORDS FOR HIGHS  
AND/OR WARM LOWS. CLOUDS AND RAINFALL WILL KEEP HIGH TEMPERATURES  
BELOW NORMAL OVER THE SOUTHERN STATES THROUGH THE PERIOD. SOME  
LOCATIONS MAY SEE ONE OR MORE DAYS WITH HIGHS 10-15F BELOW NORMAL  
ESPECIALLY DURING TUESDAY-WEDNESDAY. SLIGHTLY BELOW NORMAL HIGHS  
SHOULD SETTLE INTO NORTHERN PARTS OF THE PLAINS LATER IN THE WEEK  
AFTER THE REGION SEES ABOVE NORMAL READINGS EARLY IN THE WEEK.  
THE NORTHEAST MAY SEE HIGHS REACH UP TO 5-10F ABOVE NORMAL AFTER  
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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