531  
FXUS02 KWBC 110717  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
316 AM EDT SUN SEP 11 2022  
 
VALID 12Z WED SEP 14 2022 - 12Z SUN SEP 18 2022  
   
..OVERVIEW
 
 
AS THE MEDIUM RANGE PERIOD BEGINS WEDNESDAY, AN UPPER RIDGE WILL  
BE IN PLACE OVER THE CENTRAL U.S. CAUSING WARM TEMPERATURES FOR  
MUCH OF THE PLAINS/MISSISSIPPI VALLEY AND SEPARATING TROUGHING IN  
THE NORTHEAST AND MEAN TROUGHING IN THE WEST. THE WARM RIDGE IS  
FORECAST TO TRACK SLIGHTLY EASTWARD AND FLATTEN SOMEWHAT THROUGH  
LATE WEEK WHILE ENERGY STALLS IN THE SOUTHEAST, WHICH ALONG WITH A  
MEANDERING FRONTAL BOUNDARY WILL CAUSE RAIN AND THUNDERSTORMS AT  
LEAST FOR FLORIDA AND PERHAPS REACHING THE GEORGIA/CAROLINA  
COASTS. MEANWHILE PERSISTENT SOUTHWESTERLY FLOW COMING INTO THE  
WEST THROUGH WEDNESDAY-THURSDAY WILL CONTINUE TO DIRECT AMPLE  
MOISTURE ORIGINATING FROM WHAT WAS TROPICAL CYCLONE KAY INTO THE  
INTERMOUNTAIN WEST AND PARTS OF THE ROCKIES, LEADING TO SOME  
LOCALLY ENHANCED RAINFALL FROM SHOWERS AND THUNDERSTORMS. THEN  
ROUNDS OF RAIN ARE POSSIBLE LATE WEEK INTO THE WEEKEND FOR THE  
NORTH-CENTRAL U.S. WITH FRONTAL SYSTEMS PASSING THROUGH.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
RECENT MODEL GUIDANCE CONTINUES TO SHOW GENERALLY GOOD AGREEMENT  
WITH THE PATTERN AT THE START OF THE MEDIUM RANGE PERIOD, WITH TWO  
SHORTWAVES WITHIN THE TROUGH IN THE NORTHEASTERN U.S., A COMMON  
RIDGE AXIS OVER THE PLAINS/MISSISSIPPI VALLEY, AND TROUGHING IN  
THE WEST WITH EMBEDDED SHORTWAVES. MODELS ARE ALSO REASONABLY  
SIMILAR WITH ENERGY AND A WEAKNESS/TROUGH/POSSIBLE CLOSED LOW AT  
TIMES IN THE SOUTHEAST THURSDAY-FRIDAY AND TRACKING SLIGHTLY  
NORTHEASTWARD THROUGH THE WEEKEND, CONSIDERING THE SMALL SCALE OF  
THE FEATURE. HOWEVER EVEN SMALL DIFFERENCES IN THIS ENERGY COULD  
LEAD TO QPF DIFFERENCES, SPECIFICALLY HOW MUCH RAIN MAKES ITS WAY  
ONSHORE FROM THE HEAVIER OFFSHORE WESTERN ATLANTIC TOTALS. THE 18Z  
GEFS MEAN THAT WAS AVAILABLE AT THE TIME OF FORECAST PRODUCTION  
SHOWED MORE QPF ONSHORE THAN THE BULK OF OTHER GUIDANCE, LEADING  
TO THE WPC IN HOUSE ENSEMBLE BIAS CORRECTED DATA AND THE 01Z NBM  
TO SHOW SOME HIGHER TOTALS. OTHER THAN THOSE, THE TREND WAS  
PERHAPS FOR LOWER QPF ONSHORE FOR THIS FORECAST. BUT CONFIDENCE IS  
LOW CONSIDERING THE MODEL WAFFLING AND THAT THESE  
GEORGIA/CAROLINAS COASTAL AREAS ARE RIGHT ON THE QPF GRADIENT,  
WHERE SMALL SHIFTS LEAD TO BIG DIFFERENCES LOCALLY. THE 18Z  
YESTERDAY AND 00Z TODAY GFS RUNS SHOW A STRONGER SURFACE LOW  
DEVELOPING NEAR THE COAST NEXT WEEKEND COMPARED TO CONSENSUS.  
 
MODEL GUIDANCE IS ALSO REASONABLY AGREEABLE IN THE WESTERN U.S.  
WEDNESDAY-THURSDAY BEFORE DIFFERENCES GROW LATE WEEK. SOME OF THIS  
IS LIKELY DUE TO DIFFERENCES THAT DEVELOP FOR THE HANDING OF A  
POTENTIALLY STRONG BERING SEA STORM AS WELL AS FOR A RECURVING  
WEST-CENTRAL PACIFIC TROPICAL SYSTEM. CMC RUNS OVER THE PAST  
COUPLE OF DAYS HAVE BEEN PERSISTENT IN SHOWING STRONG UPSTREAM  
ENERGY CUTTING OFF OVER THE PACIFIC FRIDAY-SUNDAY. THIS HAS  
APPEARED TO BE AN OUTLIER SOLUTION AS IT WAS EVEN MORE AGGRESSIVE  
WITH CONTAINING THIS ENERGY OFFSHORE THAN BASICALLY ALL 12Z  
ENSEMBLE MEMBERS FROM THE EC, CMC, AND GEFS. THE NEW 00Z CMC HAS  
ALSO GONE AWAY FROM THIS SOLUTION...HOWEVER THE 00Z GFS HAS  
STARTED TO SHOW A SIMILAR CLOSED LOW FORMING IN THE PACIFIC LIKE  
THE OLDER CMC RUNS. THERE IS STILL AMPLE UNCERTAINTY ESPECIALLY  
CONSIDERING THE ORIGINS OF THE ENERGY. THE ECMWF RUNS HAVE  
APPEARED TO BE THE MOST CONSISTENT WITH THEIR SPLITTING OF THE FAR  
NORTHERN STREAM ENERGY OVER CANADA AND THUS A COUPLE OF WEAKER  
TROUGHS EXTENDING INTO THE CONUS NEXT WEEKEND. THIS IS ALSO  
SOMEWHAT SIMILAR TO THE ENSEMBLE MEANS.  
 
THUS THE WPC FORECAST BEGAN WITH BLEND OF DETERMINISTIC GUIDANCE  
FAVORING THE ECMWF AND GFS RUNS WITH SOME INCORPORATION OF THE CMC  
AND UKMET INITIALLY, BUT DROPPING THOSE IN FAVOR OF THE GEFS AND  
EC ENSEMBLE MEANS (TO 50-60 PERCENT BY DAYS 6-7) ALONG WITH THE  
OPERATIONAL ECMWF IN PARTICULAR CONSIDERING ITS RELATIVE  
CONSISTENCY.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
WHILE THE SURFACE LOW ASSOCIATED WITH FORMER TROPICAL CYCLONE KAY  
SHOULD CONTINUE TO DISSIPATE OVER THE EASTERN PACIFIC DURING THE  
SHORT RANGE PERIOD, DEEP LAYER SOUTHWESTERLY FLOW WILL LIKELY  
CARRY THE LINGERING MOISTURE FROM KAY INTO THE INTERIOR  
WEST/ROCKIES THIS WEEK, LEADING TO PRECIPITABLE WATER VALUES OF AT  
LEAST 2-3 STANDARD DEVIATIONS ABOVE NORMAL. EXPECT RAIN AND SOME  
THUNDERSTORMS TO SPREAD ACROSS THE GREAT BASIN AND NORTHERN  
ROCKIES WEDNESDAY-THURSDAY, WITH POTENTIAL FOR LOCALLY MODERATE TO  
HEAVY RAINFALL. REFINING THE AREAS OF BEST FOCUS FOR THIS  
POTENTIAL WILL DEPEND ON EXACT SHORTWAVE DETAILS/TIMING ALONG WITH  
OTHER INGREDIENTS, SO CONTINUE TO MONITOR FORECASTS OVER THE  
COMING DAYS. RAINFALL SHOULD SHIFT EASTWARD INTO THE NORTHERN  
PLAINS AND MIDWEST IN THE VICINITY OF A COUPLE OF FRONTAL  
BOUNDARIES FOR THE END OF THE WEEK WITH GENERALLY LIGHT AMOUNTS  
CURRENTLY FORECAST BUT WITH POTENTIAL FOR EMBEDDED HIGHER AMOUNTS,  
WHILE AN EARLIER EVENT WEDNESDAY-THURSDAY IN THE UPPER MIDWEST  
COULD CONTAIN SOME MODERATE TO HEAVY AMOUNTS.  
 
A QUASI-STATIONARY FRONT AND PERHAPS A LOW ALONG IT WILL BE  
STALLED ACROSS THE FLORIDA PENINSULA AND BACK ACROSS THE GULF OF  
MEXICO AND WESTERN ATLANTIC THROUGH THE LATTER HALF OF THE WEEK  
INTO THE WEEKEND, AND COMBINED WITH A POSSIBLE DEVELOPING UPPER  
FEATURE OVER THE SOUTHEAST COULD FOCUS RAIN CHANCES ACROSS THESE  
AREAS. FLORIDA IS MOST LIKELY TO SEE ROUNDS OF CONVECTION WITH  
POSSIBLY HEAVY RAINFALL TOTALS. RAIN AMOUNTS ACROSS AREAS LIKE  
COASTAL GEORGIA AND THE CAROLINAS ARE MUCH LESS CERTAIN. MODEL  
GUIDANCE AGREES THAT THE HEAVIEST QPF SHOULD BE OFFSHORE BUT  
DIFFERS IN TERMS OF HOW MUCH RAIN COULD SNEAK ONSHORE. KEEP  
WATCHING FOR UPDATES TO THE FORECAST AS SLIGHT ADJUSTMENTS IN  
SPECIFICS COULD SIGNIFICANTLY INFLUENCE RAINFALL COVERAGE AND  
AMOUNTS OVER SOME LOCATIONS. SOUTHERN TEXAS CAN EXPECT SHOWERS AND  
THUNDERSTORMS WITH THE BACK END OF THE FRONT.  
 
THE MOST NOTABLE TEMPERATURE ANOMALIES THIS WEEK WILL BE OVER THE  
PLAINS, WITH ABOVE NORMAL TEMPERATURES BY 5-15F LEADING TO HIGHS  
IN THE 90S FOR THE SOUTHERN HALF OF THE PLAINS. SOME ABOVE AVERAGE  
TEMPERATURES COULD ALSO STRETCH INTO THE MIDWEST AT TIMES.  
MEANWHILE THE WESTERN U.S. WILL SEE FAIRLY BROAD COVERAGE OF BELOW  
NORMAL HIGHS (BY UP TO 5-10F) WEDNESDAY-THURSDAY, LIKELY A WELCOME  
CHANGE FROM THE RECENT HEAT. HIGHS OVER THE WEST MAY RETURN CLOSER  
TO NORMAL FRIDAY-SATURDAY WHILE STAYING A LITTLE ON THE COOL SIDE  
OVER THE PACIFIC NORTHWEST. OVERNIGHT/MORNING LOWS SHOULD REMAIN  
NEAR TO ABOVE AVERAGE OVER THE WEST THROUGH THE PERIOD. OVER THE  
EAST, NEAR NORMAL TEMPERATURES ARE MOSTLY FORECAST, THOUGH SOME  
MODERATELY BELOW NORMAL HIGHS ARE POSSIBLE IN THE NORTHEAST BEHIND  
A REINFORCING FRONT THURSDAY-FRIDAY.  
 
TATE/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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