506  
FXUS02 KWBC 122103  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
503 PM EDT MON SEP 12 2022  
 
VALID 12Z THU SEP 15 2022 - 12Z MON SEP 19 2022  
   
..OVERVIEW
 
 
A TROUGH-RIDGE-TROUGH UPPER-LEVEL PATTERN IS FORECAST TO PERSIST  
ACROSS THE U.S. MAINLAND THROUGH THE MEDIUM-RANGE PERIOD, WITH  
UNCERTAIN DETAILS ON THE VARIOUS SMALL SCALE SHORTWAVES MOVING  
INTO THE WESTERN U.S. ONE SUCH SHORTWAVE IS FORECAST TO WORK IN  
CONCERT WITH ANOTHER SHORTWAVE DIPPING ACROSS CENTRAL CANADA TO  
ORGANIZE AN AXIS OF HEAVY RAIN ACROSS THE ARROWHEADS OF MINNESOTA  
AND UPPER MICHIGAN THURSDAY TO FRIDAY. MEANWHILE, AMPLE MOISTURE  
ORIGINATING FROM WHAT WAS TROPICAL CYCLONE KAY IS FORECAST TO  
INTERACT WITH THE WESTERN U.S. UPPER TROUGH TO PROVIDE LOCALLY  
ENHANCED RAINFALL FROM THE GREAT BASIN TO THE NORTHERN ROCKIES  
LATE THIS WEEK. A MEANDERING FRONT WILL ALLOW UNSETTLED WEATHER  
TO CONTINUE ALONG THE GULF COAST INTO FLORIDA WITH AN OUTSIDE  
CHANCE OF TROPICAL CYCLOGENESIS OFF THE SOUTHEAST U.S. MEANWHILE,  
HEAT IS FORECAST TO GRADUALLY BUILD OVER THE CENTRAL PLAINS.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
MODEL GUIDANCE SHOWS REASONABLY GOOD CONSENSUS TO BEGIN THE MEDIUM  
RANGE PERIOD THURSDAY WITH THE PATTERN EVOLUTION MENTIONED ABOVE.  
TOWARD THE END OF THE FORECAST PERIOD, MODEL SOLUTIONS RAPIDLY  
DIVERGE ALONG THE WEST COAST. THE ECMWF IN PARTICULAR HAS  
ABANDONED YESTERDAY'S IDEA OF CLOSING OFF AN UPPER LOW IN FAVOR OF  
A MORE PROGRESSIVE TROUGH MOVING THROUGH THE NORTHWEST BY THIS  
WEEKEND. IN CONTRAST, THE GFS AND CMC ALONG WITH THEIR ENSEMBLE  
MEANS CONTINUE WITH A RATHER AMPLIFIED PATTERN OVER THE WESTERN  
U.S. THIS WEEKEND INTO EARLY NEXT WEEK. THE POOR MODEL AGREEMENT,  
POSSIBLY THE RESULT OF DIFFICULTIES IN RESOLVING TYPHOON MERBOK IN  
THE CENTRAL PACIFIC WILL LIKELY CONTINUE TO KEEP THE FORECAST  
PATTERN UNCERTAIN OVER THE WESTERN U.S. THE ECMWF HAS BEEN  
TRENDING FASTER WITH THE TRACK OF MERBOK IN BETTER AGREEMENT WITH  
THE GFS AND THE CMC WHICH HAVE BEEN ON THE FASTER SIDE OF THE  
GUIDANCE. THE ECMWF BECOMES NOTICEABLY FASTER THAN ITS ENSEMBLE  
MEAN WITH THE LOW PRESSURE WAVE DEVELOPING OVER THE NORTHERN  
PLAINS BY DAY 7. THUS, THE GFS/CMC CONSENSUS IS FAVORED FOR THE  
CONUS.  
 
THE WPC MEDIUM-RANGE FORECAST PACKAGE WAS BASED ON 40% FROM THE  
00Z ECMWF/EC MEAN, 40% FROM THE 06Z GFS/GEFS, AND 20% FROM THE 00Z  
CMC/CMC MEAN, TRANSITIONING TO MOSTLY THE CONSENSUS OF THE  
ENSEMBLE MEANS BY DAY 7, WITH LESS OF THE ECMWF BEING USED  
RELATIVE TO ITS ENSEMBLE MEAN.  
 
   
..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. BY THE MEDIUM RANGE  
TIMEFRAME, THIS ANOMALOUS MOISTURE SHOULD BE IN PLACE ACROSS  
NORTHERN PARTS OF THE GREAT BASIN AND ROCKIES, LEADING TO RAIN AND  
SOME THUNDERSTORMS WITH POTENTIAL FOR LOCALLY MODERATE TO HEAVY  
RAINFALL. FLOODING/FLASH FLOODING IMPACTS OF THIS RAIN ARE  
UNCERTAIN SO THIS PRECLUDES A SLIGHT RISK IN THE EXPERIMENTAL DAY  
4 EXCESSIVE RAINFALL OUTLOOK AT THIS TIME. ANOTHER AREA OF FOCUS  
FOR HEAVY RAIN WILL BE PARTS OF THE UPPER MIDWEST, WHERE GOOD  
MOISTURE TRANSPORT WILL BE PRESENT NORTH OF A WARM FRONT WHERE A  
SHORTWAVE DIPPING SOUTHEAST ACROSS SOUTH-CENTRAL CANADA IS  
FORECAST TO MERGE WITH A DEVELOPING LOW PRESSURE WAVE OVER THE  
NORTHERN PLAINS THURSDAY AND INTO FRIDAY. GIVEN THAT QPF AMOUNTS  
FROM MODEL GUIDANCE HAVE GENERALLY INCREASED, A SLIGHT RISK OF  
EXCESSIVE RAINFALL HAS BEEN INTRODUCED FOR THIS AREA. BEHIND THIS  
AREA OF RAINFALL, CONFIDENCE IN DETAILS ACROSS THE NORTHERN HALF  
OF THE PLAINS AND VICINITY DECREASES CONSIDERABLY. THE PACIFIC  
NORTHWEST MAY START TO SEE AN INCREASE IN PRECIPITATION TOWARD THE  
END OF THE WEEK OR WEEKEND BUT AGAIN CONFIDENCE IS VERY LOW IN THE  
SPECIFICS AT THIS TIME.  
 
A QUASI-STATIONARY FRONT AND PERHAPS A WAVE OR TWO 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. THIS WAVY FRONT AND 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 HAS GENERALLY TRENDED OFFSHORE LATE THIS WEEK, WITH THE  
GFS INDICATING POSSIBE TROPICAL CYCLOGENESIS OFF THE COAST OF  
SOUTHEASTERN U.S. THIS WEEKEND. SOUTHERN TEXAS CAN EXPECT SHOWERS  
AND THUNDERSTORMS WITH THE BACK END OF THE FRONT.  
 
THE CENTRAL U.S. IS EXPECTED TO BE QUITE WARM DURING THE MEDIUM  
RANGE TIMEFRAME WITH PERIODS OF RIDGING ALOFT. HIGHS OF 5-15F  
ABOVE AVERAGE WILL BE WIDESPREAD ACROSS MUCH OF THE PLAINS AND  
MISSISSIPPI VALLEY, WITH THE CENTRAL PLAINS PERHAPS SEEING THE  
GREATEST ANOMALIES NEARING 20F ABOVE NORMAL WITH ACTUAL  
TEMPERATURES WELL INTO THE 90S. THE WEST MEANWHILE MAY SEE COOLER  
THAN NORMAL HIGHS BY 5-10F THOUGH LOWS COULD BE AROUND AVERAGE,  
BUT DETAILS DEPEND ON UNCERTAIN TROUGHING ALOFT. OVER THE EAST,  
EXPECT NEAR NORMAL TEMPERATURES ASIDE FROM SOME MODERATELY BELOW  
NORMAL HIGHS IN THE NORTHEAST THURSDAY-FRIDAY BEHIND A REINFORCING  
COLD FRONT.  
 
KONG/TATE/RAUSCH  
 
HAZARDS:  
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE CENTRAL  
GREAT BASIN, THE NORTHERN PLAINS, THE NORTHERN ROCKIES, THE  
CENTRAL ROCKIES, THE NORTHERN GREAT BASIN, THE UPPER MISSISSIPPI  
VALLEY, AND THE GREAT LAKES, THU-FRI, SEP 15-SEP 16.  
- EXCESSIVE HEAT ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE  
SOUTHERN PLAINS, SAT-MON, SEP 17-SEP 19.  
 
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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