987  
FXUS02 KWBC 171807  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
206 PM EDT SAT SEP 17 2022  
 
VALID 12Z TUE SEP 20 2022 - 12Z SAT SEP 24 2022  
 
..TROPICAL STORM FIONA FORECAST TO SLOWLY GAIN STRENGTH WHILE  
LIFTING FROM THE CARIBBEAN TOWARD/EAST OF THE BAHAMAS INTO  
MIDWEEK...  
 
...MODERATE TO LOCALLY HEAVY RAINFALL THREAT FROM THE FOUR CORNERS  
REGION INTO THE ROCKIES TUESDAY-THURSDAY...  
 
   
..PATTERN OVERVIEW
 
 
THE UPPER PATTERN DURING THE MEDIUM RANGE PERIOD WILL FEATURE A  
VERY AMPLIFIED TROUGH/CLOSED LOW SLATED TO FORM OFF THE CALIFORNIA  
COAST THAT MAY SLOWLY EJECT NORTHEASTWARD OVER THE NORTH-CENTRAL  
GREAT BASIN/ROCKIES BY THE LATTER PART OF NEXT WEEK. DOWNSTREAM,  
HOT SUMMERTIME RIDGING WILL BUILD OVER THE CENTRAL-EASTERN U.S.  
AND AMPLE NORTHERN STREAM CANADIAN SHORTWAVE ENERGY SEEMS SLATED  
TO DIG SOUTHEASTWARD ACROSS THE NORTHERN TIER TO AMPLIFY OVER THE  
GREAT LAKES AND THE NORTHEAST AROUND THURSDAY-SATURDAY OF NEXT  
WEEK. THIS SYSTEM HAS SOME ENERGY CONNECTION OF THE CURRENT  
EXTREMELY DEEP BERING SEA LOW ASSOCIATED WITH THE EXTRATROPICAL  
REMNANTS OF TYPHOON MERBOK. THE EVOLUTION OF THIS TROUGH ALSO HAS  
STRONG IMPLICATIONS FOR EVENTUAL STEERING FLOW OF TROPICAL STORM  
FIONA BEYOND DAY 5, FORECAST TO REACH HURRICANE STRENGTH WITH A  
TRACK NEAR/EAST OF THE BAHAMAS WEDNESDAY INTO THURSDAY.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE OVERALL REMAINS IN GOOD ENOUGH AGREEMENT FOR A  
MULTI-MODEL BLEND THROUGH DAY 4. AFTER ABOUT WEDNESDAY, TIMING  
DIFFERENCES CONTINUE TO BE MORE APPARENT THURSDAY INTO NEXT  
WEEKEND AS THE UPPER LOW OVER CALIFORNIA UNCERTAINLY EJECTS INLAND  
ACROSS THE NORTH-CENTRAL GREAT BASIN/ROCKIES. RECENT RUNS OF THE  
GFS AND TO A LESSER EXTENT THE CANADIAN REMAIN ON THE FASTER SIDE  
OF THE GUIDANCE, WHILE RECENT RUNS OF THE ECMWF AND UKMET HANG THE  
LOW BACK OVER CALIFORNIA A LITTLE LONGER. THE ENSEMBLE MEANS  
GENERALLY SUPPORT THEIR DETERMINISTIC COUNTERPARTS, THOUGH WITH  
MAYBE SLIGHTLY LESS SPREAD. WPC OVERALL PREFERS THE SLOWER SIDE OF  
THE ENVELOPE OF SOLUTIONS CLOSEST TO THE ECMWF/CMC/ECMWF ENSEMBLE  
MEAN GIVEN THE NATURE OF CLOSED SYSTEMS IN SEPARATED FLOW AND AS  
PER FAVORABLE LATEST RUN TO RUN GUIDANCE TRENDS.  
 
NORTHERN STREAM FLOW ACROSS THE NORTHERN TIER AND EVENTUALLY INTO  
THE NORTHEAST CONTINUES TO PROVIDE PLENTY OF UNCERTAINTIES ON  
TIMING AND AMPLITUDE. LIKELY A RESULT OF THE ISSUES FARTHER  
UPSTREAM, THE GFS IS AGAIN THE FASTEST TO EJECT THE TROUGH OUT OF  
THE NORTHEAST, WHILE THE CMC AND ECMWF (AND THE MEANS) ARE SLOWER.  
THIS WOULD HAVE STRONG IMPLICATIONS ON HOW CLOSE TO THE US EAST  
COAST FIONA EVENTUALLY TRACKS IN THE LATE PERIODS. HOWEVER, RECENT  
GUIDANCE CONTINUES TO BOAST CONFIDENCE THAT THIS UPPER TROUGH  
SHOULD HELP STEER FIONA SUFFICIENTLY OUT TO SEA WITH ONLY MINIMAL  
IMPACTS TO THE U.S. EAST COAST. A SLOWED EJECTION OF THE  
CALIFORNIAN CLOSED LOW ALSO LEAVES MORE ROOM FOR THE NORTHERN  
STREAM UPPER TROUGH TO AMPLIFY A BIT MORE DOWNSTREAM THAN PREVIOUS  
FORECASTS. IMPORTANT TO NOTE THOUGH THAT WHILE ODDS ARE INCREASING  
FOR A WELL OFFSHORE TRACK, THERE IS STILL SOME LINGERING POTENTIAL  
FIONA COULD COME CLOSE ENOUGH TO THE NORTHEAST COAST FOR SOME  
GREATER IMPACTS. PLEASE REFER TO THE LATEST NHC FORECAST FOR THE  
UPDATED INFORMATION REGARDING FIONA.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE EXPECTED SLOWER INLAND EJECTION OF AN INITIALLY QUITE  
AMPLIFIED UPPER LEVEL TROUGH FROM CALIFORNIA THROUGH THE COOLING  
NORTH-CENTRAL GREAT BASIN/ROCKIES NEXT WEEK WILL SUPPORT COOLER  
TEMPERATURES THAT MAY LEAD TO SOME MIXED PRECIPITATION, OR SNOW  
FOR THE HIGHEST PEAKS ACROSS CALIFORNIA AND THE NORTH-CENTRAL  
GREAT BASIN/ROCKIES. THE DAILY TEMPERATURES ARE FORECAST TO BE 10  
TO 15 DEGREES F BELOW SEASONAL AVERAGE FOR MID SEPTEMBER FOR THE  
WEST WITH THE COOL AIRMASS SPREADING TO THE ROCKIES/HIGH PLAINS  
EARLY IN THE WEEK AND THEN TO THE UPPER MISSISSIPPI VALLEY BY THE  
MIDDLE/END OF THE WEEK AS THE COLD FRONT MAKES ITS WAY EAST. THE  
LINGERING ONSHORE RAIN MAY BE HEAVY AT TIMES ACROSS PORTIONS OF  
NORTHERN CALIFORNIA INTO OREGON INTO MONDAY, AND BURN SCARS WOULD  
BE PARTICULARLY VULNERABLE TO FLOODING CONCERNS AND DEBRIS FLOWS.  
THE SIERRA NEVADA COULD SEE SNOW EARLY NEXT WEEK, THOUGH THE BEST  
CHANCES FOR SNOW WITH THIS ACTIVITY ARE IN THE NORTHERN ROCKIES  
TUESDAY-THURSDAY. ANOMALOUS MOISTURE WITH SOME EAST PACIFIC  
SUBTROPICAL/TROPICAL CONNECTION IS FORECAST TO LIFT ACROSS THE  
FOUR CORNERS REGION AND NORTHWARD EARLY-MID NEXT WEEK AHEAD OF THE  
EJECTING WESTERN U.S. UPPER TROUGH, THAT WITH UPPER SUPPORT MAY  
INCREASE CHANCES FOR MODERATE TO LOCALLY HEAVY OR EXCESSIVE  
RAINFALL AND THUNDERSTORMS. THERE WAS ENOUGH SUPPORT IN THE LATEST  
GUIDANCE TO CONTINUE SMALL SLIGHT RISK AREAS ON THE DAY 4 AND 5  
EXPERIMENTAL WPC EXCESSIVE RAINFALL OUTLOOK ACROSS PARTS OF  
SOUTHWEST COLORADO AND NORTHERN NEW MEXICO IN THE VICINITY OF THE  
SAN JUAN MOUNTAINS.  
 
WITH LEAD RIDGING BUILDING INTO THE CENTRAL U.S. INTO THE START OF  
THIS PERIOD, WARMER THAN AVERAGE TEMPERATURES BY 10-20F+ ARE  
FORECAST FOR THE CENTRAL PLAINS AND MIDDLE MISSISSIPPI VALLEY  
THROUGH WEDNESDAY WITH ACTUAL TEMPERATURES WELL INTO THE 90S INTO  
MIDWEEK THREATENING RECORD VALUES FOR THIS TIME OF THE YEAR.  
WARMER THAN NORMAL TEMPERATURES WILL EXTEND TO THE EAST COAST INTO  
AROUND THURSDAY, THOUGH WITH SLIGHTLY LESSER ANOMALIES. A STRONG  
COLD FRONT IS FORECAST TO THEN ADVANCE ACROSS THE NORTHERN TIER  
FROM CANADA WHICH WOULD BRING SHOWERS/THUNDERSTORMS WITH SOME  
LOCALLY ENHANCED DOWNPOURS INTO THE MIDWEST/GREAT LAKES THEN THE  
NORTHEAST WEDNESDAY INTO THURSDAY. THE NBM STILL APPEARS TOO LIGHT  
WITH QPF AND SO SOME MODEST INCREASE IN AMOUNTS WAS APPLIED GIVEN  
POTENTIAL FOR AMPLIFICATION. THE LEADING CENTRAL U.S. HEAT WILL  
GET SUPPRESSED WITH THIS FRONTAL PASSAGE WITH QUITE COOLED  
CANADIAN POST-FRONTAL HIGH PRESSURE MOVING IN BEHIND. MEANWHILE, A  
STALLED FRONT IS FORECAST TO LINGER ACROSS CENTRAL TO SOUTHERN  
FLORIDA AND COULD TRIGGER SHOWERS AND THUNDERSTORMS THERE AS FIONA  
LIFTS WELL OFFSHORE THE U.S. MAINLAND AS PER THE LATEST NHC TRACK.  
 
SANTORELLI/SCHICHTEL  
 
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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