003  
FXUS02 KWBC 280700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT WED SEP 28 2022  
 
VALID 12Z SAT OCT 01 2022 - 12Z WED OCT 05 2022  
 
...IAN FORECAST TO BRING HEAVY/EXCESSIVE RAINFALL AND BRISK TO  
STRONG WINDS TO THE APPALACHIANS AND MID-ATLANTIC THIS WEEKEND AND  
POSSIBLY INTO EARLY NEXT WEEK...  
   
..OVERVIEW
 
 
THE 03Z NATIONAL HURRICANE CENTER ADVISORY HAS HURRICANE IAN  
WEAKENING TO A TROPICAL DEPRESSION NEAR NORTHWESTERN SOUTH  
CAROLINA BY THE START OF THE FORECAST EARLY SATURDAY AND BECOMING  
EXTRATROPICAL SHORTLY THEREAFTER. DUE TO A FAIRLY BLOCKY PATTERN  
TAKING SHAPE OVER THE LOWER 48, THIS SYSTEM MAY TAKE UNTIL EARLY  
NEXT WEEK TO TRACK OFF THE EAST COAST AND IN THE MEANTIME SHOULD  
PRODUCE AREAS OF SIGNIFICANT RAINFALL AND BRISK TO STRONG WINDS  
OVER THE CENTRAL-SOUTHERN PORTIONS OF THE APPALACHIANS INTO THE  
MID-ATLANTIC. AT THE SAME TIME A NORTHERN ROCKIES INTO HIGH  
PLAINS UPPER LOW (AND ACCOMPANYING LOCALLY ENHANCED PRECIPITATION)  
WILL DRIFT ALONG SLOWLY DUE TO ITS POSITION BETWEEN RIDGES NEAR  
THE WEST COAST AND OVER THE PLAINS/MIDWEST. RECENT GUIDANCE HAS  
VARIED WITH THE EXPECTED FLOW NEAR THE WEST COAST BY LATE IN THE  
PERIOD.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
FOR IAN AND ITS EXTRATROPICAL EVOLUTION, GUIDANCE CLUSTERS WELL  
FOR THE UPPER LOW INTO DAY 4 SUNDAY. AFTER THAT TIME THERE IS  
INCREASED SPREAD RELATED IN PART TO VARIED INFLUENCE OF NORTHERN  
STREAM ENERGY WHICH COULD EITHER PUSH THE INITIAL UPPER LOW AWAY  
OR REINFORCE IT IN PLACE. THE NEW 00Z CMC IS THE FIRST TO STRAY,  
BECOMING WEAKER/MORE OPEN THAN CONSENSUS. THEN THE 00Z GFS  
BECOMES SOMEWHAT PROGRESSIVE BY DAY 6 TUESDAY WITH THE GEFS MEAN  
RUNS SUPPORTING A SLOWER MOTION. MEANWHILE FOR THE ROCKIES/HIGH  
PLAINS UPPER LOW, GFS RUNS HAVE TENDED TO BE ON THE FASTER SIDE OF  
THE SPECTRUM. THE NEW 00Z GFS HAS MADE SOME PROGRESS IN BACKING  
UP CLOSER TO OTHER GUIDANCE. GUIDANCE HAS SHOWN SIGNIFICANT  
SPREAD AND VARIABILITY FOR THE SHORTWAVE EMERGING FROM THE GULF OF  
ALASKA AND TRACKING ACROSS CANADA, AS WELL AS FOR POSSIBLE  
UPSTREAM ENERGY THAT COULD EVENTUALLY AMPLIFY IN RESPONSE TO  
WESTERN CANADA RIDGING. THIS WILL AFFECT A COLD FRONT THAT MAY  
DROP INTO THE NORTHERN TIER STATES. 12Z/18Z GFS RUNS WERE ON THE  
SLOW/AMPLIFIED SIDE OF THE SPREAD BUT THE NEW 00Z RUN LOOKS MUCH  
MORE LIKE THE AVERAGE OF OTHER GUIDANCE. FINALLY, MODELS AND  
ENSEMBLE MEMBERS DIVERGE CONSIDERABLY OVER THE EASTERN  
PACIFIC/WEST COAST BY THE LATTER HALF OF THE PERIOD. THE 12Z  
ECMWF RUN WAS FASTEST TO BRING A SHORTWAVE TO THE WEST COAST (BUT  
WITHIN THE BROAD ENSEMBLE SPREAD, AND THE NEW 00Z ECMWF IS  
SIMILAR) WHILE THE 12Z/18Z GFS WERE FARTHEST WEST WITH THE WEST  
COAST RIDGE AND UPSTREAM TROUGH. THE ENSEMBLE MEANS OVERALL  
SUGGEST AN INTERMEDIATE SOLUTION.  
 
BASED ON THE 12Z/18Z GUIDANCE, THE UPDATED FORECAST STARTED THE  
FIRST PART OF THE PERIOD WITH A BLEND OF THE 18Z GFS AND 12Z  
ECMWF/CMC SINCE THE UKMET DID NOT COMPARE WELL TO CONSENSUS FOR  
IAN. THE MID-LATE PART OF THE FORECAST INCORPORATED A MODEL/MEAN  
BLEND TO REFLECT AN INTERMEDIATE SCENARIO AND MAINTAIN DECENT  
CONTINUITY WHERE POSSIBLE, IN LIGHT OF THE DIVERGING GUIDANCE AT  
THAT TIME FRAME.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
WHILE HURRICANE IAN WILL LIKELY WEAKEN TO TROPICAL DEPRESSION  
STATUS BY EARLY SATURDAY AND BECOME EXTRATROPICAL SOON THEREAFTER,  
THE LINGERING SYSTEM SHOULD CONTINUE TO PRODUCE HEAVY RAINFALL AND  
WIND THREATS OVER PARTS OF THE APPALACHIANS AND MID-ATLANTIC  
THROUGH THE WEEKEND AND POSSIBLY INTO EARLY NEXT WEEK. HIGH  
PRESSURE WEAKENING IN PLACE OVER THE NORTHEAST ON SATURDAY SHOULD  
BE REINFORCED BY ANOTHER HIGH REACHING SOUTHEASTERN CANADA AND NEW  
ENGLAND BY SUNDAY-MONDAY, HELPING TO MAINTAIN A FAIRLY TIGHT  
GRADIENT TO THE NORTH OF EXTRATROPICAL IAN. EXPECT THE HEAVIEST  
RAINFALL TO BE OVER PARTS OF THE CENTRAL-SOUTHERN APPALACHIANS  
EAST INTO THE MID-ATLANTIC, WITH MUCH OF THIS AREA COVERED IN  
SLIGHT RISK AREAS IN THE EXPERIMENTAL DAYS 4-5 EXCESSIVE RAINFALL  
OUTLOOKS. PARTS OF WESTERN NORTH CAROLINA INTO SOUTHWEST VIRGINIA  
ARE IN AN EMBEDDED MODERATE RISK AREA, GIVEN THE FORECAST SURFACE  
LOW TRACK AND ITS SLOW MOVEMENT.  
 
THE COMBINATION OF ANOMALOUS MOISTURE AND THE UPPER LOW TRACKING  
OVER THE NORTHERN ROCKIES/HIGH PLAINS SHOULD PRODUCE SOME AREAS OF  
LOCALLY HEAVY RAINFALL WITHIN AN AREA FROM MONTANA THROUGH WYOMING  
AND PARTS OF COLORADO/EASTERN UTAH. BEST POTENTIAL FOR HEAVIER  
ACTIVITY SHOULD BE ON SATURDAY WITH A GRADUAL LIGHTER AND MORE  
SCATTERED TREND AFTER THEN. HIGHEST ELEVATIONS UNDER THE UPPER  
LOW COULD SEE SOME SNOW.  
 
CLOUDS/RAIN FROM IAN WILL SUPPORT COOL HIGHS FROM PARTS OF THE  
SOUTHEAST INTO THE MID-ATLANTIC, WITH SOME READINGS 10-15F BELOW  
NORMAL FROM SATURDAY INTO MONDAY. TEMPERATURES SHOULD FINALLY  
REBOUND CLOSER TO NORMAL BY THE MIDDLE OF NEXT WEEK. THE PLAINS  
WILL SEE MODERATELY WARM ANOMALIES FROM THE WEEKEND INTO MONDAY,  
WITH THE WARMTH GETTING SUPPRESSED SOUTHWARD THEREAFTER. THE  
UPPER LOW DRIFTING OVER THE ROCKIES/HIGH PLAINS WILL BRING COOL  
TEMPERATURES ALONG ITS PATH WITH A FEW LOCATIONS AT LEAST 10F  
BELOW NORMAL FOR HIGHS ON SATURDAY. THE UPPER RIDGE NEAR THE WEST  
COAST WILL PROMOTE A GRADUALLY EXPANDING AREA OF ABOVE NORMAL  
TEMPERATURES, WITH SOME LOCATIONS IN THE NORTHWEST SEEING HIGHS UP  
TO 10-15F ABOVE NORMAL.  
 
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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