647  
FXUS02 KWBC 140709  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
309 AM EDT WED SEP 14 2022  
 
VALID 12Z SAT SEP 17 2022 - 12Z WED SEP 21 2022  
   
..OVERVIEW
 
 
THE UPPER LEVEL FLOW PATTERN DURING THE MEDIUM RANGE PERIOD WILL  
BE CHARACTERIZED BY TROUGHING DEEPENING IN THE WEST AND EVENTUALLY  
SHIFTING SLIGHTLY EASTWARD NEXT WEEK, WHILE RIDGING AMPLIFIES  
ACROSS MUCH OF THE CENTRAL AND EASTERN U.S. AHEAD OF IT. THIS WILL  
PROMOTE COOL TEMPERATURES AND SOME PRECIPITATION COMING INTO PARTS  
OF CALIFORNIA AND THE NORTHWEST BEGINNING THIS WEEKEND, AND THEN  
MOISTURE INCREASING AHEAD OF A POSSIBLY FORMING TROPICAL SYSTEM IN  
THE EASTERN PACIFIC COULD LEAD TO SHOWERS AND STORMS IN THE FOUR  
CORNERS STATES BY TUESDAY-WEDNESDAY OF NEXT WEEK. MEANWHILE WARMER  
THAN AVERAGE TEMPERATURES WILL BE THE NORM ACROSS THE CENTRAL AND  
EASTERN U.S. INTO THE FIRST HALF OF NEXT WEEK, THOUGH A COOLDOWN  
MAY REACH THE NORTH-CENTRAL U.S. BY AROUND WEDNESDAY AS THE TROUGH  
AND COLD FRONT EDGE EAST. ROUNDS OF RAIN ARE POSSIBLE ALONG THE  
NORTHERN TIER WITH PROGRESSIVE SHORTWAVES AND FRONTAL BOUNDARIES  
PASSING THROUGH, WHILE A MEANDERING FRONT SHOULD CONTINUE TO ALLOW  
FOR SCATTERED THUNDERSTORMS ACROSS FLORIDA AND THE GULF COAST.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
THE MAIN AREA OF MODEL DISCREPANCIES ON THE LARGER SCALE CONTINUES  
TO BE WITH THE TROUGH IN THE EASTERN PACIFIC/WEST, THOUGH MODELS  
HAVE CONVERGED REASONABLY WELL ON TIMING OF THE TROUGH DIGGING  
THROUGH THE WEEKEND. RECENT MODEL GUIDANCE INCLUDING THE ECMWF  
RUNS (WHICH HAD BEEN A HOLDOUT) ARE NOW ALSO AGREEABLE FOR CLOSING  
OFF A LOW IN THE SOUTHERN END OF THE TROUGH THROUGH MONDAY. BY  
TUESDAY-WEDNESDAY GREATER TIMING DIFFERENCES ARISE WITH THE  
TROUGH'S SLOW MOVEMENT EASTWARD, DEPENDING IN PART ON HOW LONG THE  
CLOSED LOW HOLDS ON AND HOW PHASED THE SOUTHERN/NORTHERN STREAMS  
MAY BE. THE 12Z AND NOW 00Z CMC RUNS ARE ON THE MORE PHASED AND  
THUS FASTER SIDE WITH THE OVERALL TROUGH. WITH THE GFS, THE 18Z  
RUN IN PARTICULAR KEPT THE SOUTHERN STREAM CLOSED LOW CUT OFF AND  
LINGERING IN THE GREAT BASIN WHILE THE NORTHERN STREAM FLOW RUSHED  
A LOW AMPLITUDE TROUGH EASTWARD INTO THE GREAT LAKES NEXT  
WEDNESDAY. THE 12Z ECMWF SEEMED TO BE A MIDDLE GROUND APPROACH AND  
WAS SOMEWHAT SIMILAR TO THE ENSEMBLE MEANS--THOUGH EACH ENSEMBLE  
MEAN SHOWED SIMILAR CHARACTERISTICS TO THEIR DETERMINISTIC RUNS SO  
THE MEANS WERE NOT PERFECTLY AGREEABLE EITHER. HOWEVER, THE 00Z  
ECMWF HAS A SOLUTION WHERE IT LINGERS THE CLOSED LOW CENTERED  
OFFSHORE IN THE PACIFIC EVEN INTO MIDWEEK--UNLIKE ANY OF ITS  
PREVIOUS RUNS AND FARTHER WEST THAN OTHER DETERMINISTIC GUIDANCE  
TOO.  
 
FARTHER EAST, OVERALL RIDGING IS FORECAST EAST OF THE TROUGH  
ACROSS THE CENTRAL AND EASTERN U.S. WITH GOOD AGREEMENT, BUT  
GUIDANCE VARIES WITH WHERE THE WESTERN SIDE OF THE RIDGE WILL BE  
(NECESSARILY DUE TO WHERE THE EASTERN SIDE OF THE TROUGH WILL BE).  
MODELS ALSO VARY SOME WITH SHORTWAVE ENERGY/TROUGHING THAT  
SUPPRESSES THE NORTHERN SIDE OF THE RIDGE IN THE NORTHERN TIER OF  
THE U.S. AROUND SUNDAY-TUESDAY, AFFECTING THE EXACT FRONTAL  
POSITION AND PLACEMENT OF RAINFALL, BUT A MULTI-MODEL BLEND SEEMED  
REASONABLE.  
 
THUS THE WPC FORECAST BLEND WAS BASED ON MULTI-MODEL DETERMINISTIC  
BLEND OF THE 12/18Z GFS AND 12Z ECMWF, CMC, AND UKMET EARLY ON,  
ADDING IN AND INCREASING THE WEIGHTING OF THE GEFS AND EC MEANS TO  
OVER HALF AS THE PERIOD PROGRESSED, WHILE KEEPING IN SOME OF THE  
12Z ECMWF FOR ADDED DETAIL DEFINITION LATE IN THE PERIOD. THIS  
PROVIDED REASONABLE CONTINUITY FROM THE PREVIOUS FORECAST.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
UPPER TROUGHING ACROSS THE EASTERN PACIFIC AND WESTERN U.S. WILL  
PROMOTE A COOL SEASON TYPE PRECIPITATION PATTERN THIS WEEKEND INTO  
EARLY NEXT WEEK, WITH RAIN AND EVEN SOME SNOW IN THE HIGHEST  
ELEVATIONS ACROSS CALIFORNIA INTO THE NORTHWEST. SOME SNOW IS MOST  
LIKELY IN THE NORTHERN ROCKIES BUT MAY REACH THE SIERRA NEVADA AS  
WELL. THEN TROPICAL MOISTURE FROM A DISTURBANCE IN THE EASTERN  
PACIFIC THAT IS LIKELY TO BECOME TROPICAL (PER THE NATIONAL  
HURRICANE CENTER) IS FORECAST TO STREAM INTO THE FOUR CORNERS  
STATES AS EARLY AS MONDAY BUT ESPECIALLY BY TUESDAY-WEDNESDAY,  
INCREASING CHANCES FOR SHOWERS AND THUNDERSTORMS THERE. A FRONTAL  
SYSTEM OR TWO AND A SHORTWAVE TROUGH ROUNDING THE NORTHERN SIDE OF  
THE RIDGE SHOULD CAUSE A ROUND OF RAINFALL TRACKING EAST FROM THE  
UPPER MIDWEST AND UPPER GREAT LAKES REGION THIS WEEKEND INTO THE  
NORTHEAST MONDAY. WHILE THERE COULD BE SOME LOCALIZED HEAVY RAIN  
WITH THIS ACTIVITY, CONFIDENCE IN PLACEMENT OF HEAVY RAIN AND THE  
POTENTIAL FOR ANY FLOODING/FLASH FLOODING IMPACTS REMAINS LOW.  
ADDITIONALLY, A STALLED FRONT ACROSS THE FLORIDA PENINSULA AND  
GULF OF MEXICO COULD CONTINUE TO SPARK SCATTERED THUNDERSTORMS  
ACROSS THE SUNSHINE STATE, WITH SOME RAIN SNEAKING INTO THE  
WESTERN/CENTRAL GULF COAST AS WELL.  
 
COOL WEATHER WEST OF THE ROCKIES AND WARM WEATHER EAST OF THE  
ROCKIES WILL BE THE MAIN WEATHER STORY IN TERMS OF TEMPERATURES  
GIVEN THE WESTERN TROUGH/CENTRAL-EASTERN RIDGE UPPER PATTERN.  
HIGHS IN PARTICULAR ARE FORECAST TO BE 10-20F BELOW NORMAL IN THE  
WEST THIS WEEKEND, WITH BELOW AVERAGE HIGHS GRADUALLY SHIFTING  
EASTWARD INTO THE NORTHERN ROCKIES AND NORTHERN HIGH PLAINS AS  
NEXT WEEK PROGRESSES, WHILE LOWS REMAIN CLOSER TO NORMAL.  
MEANWHILE THE CENTRAL PLAINS INTO THE MIDDLE MISSISSIPPI VALLEY  
CAN EXPECT HIGH TEMPERATURES 10-20F ABOVE AVERAGE THROUGH TUESDAY,  
WITH ACTUAL TEMPERATURES WELL INTO THE 90S. WARMER THAN NORMAL  
TEMPERATURES WILL EXTEND EASTWARD ALL THE WAY TO THE EAST COAST  
FOR THE FIRST HALF OF NEXT WEEK, THOUGH WITH SLIGHTLY LESSER  
ANOMALIES.  
 
TATE  
 
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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