644  
FXUS02 KWBC 260701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT MON SEP 26 2022  
 
VALID 12Z THU SEP 29 2022 - 12Z MON OCT 03 2022  
 
...TROPICAL STORM IAN EXPECTED TO INTENSIFY AS IT TRACKS FROM THE  
CARIBBEAN TOWARD FLORIDA, WITH LIKELY ADVERSE EFFECTS OVER OTHER  
PARTS OF THE EASTERN U.S. AS WELL...  
 
   
..OVERVIEW
 
 
THE LATEST FORECAST OFFERS TWO PRIMARY CHALLENGES. THE FIRST IS  
THE PRECISE TRACK, AND TO SOME DEGREE TIMING, OF TROPICAL STORM  
IAN WHICH SHOULD INTENSIFY TO HURRICANE STATUS AS IT TRACKS FROM  
THE NORTHWESTERN CARIBBEAN INTO THE EASTERN GULF OF MEXICO TOWARD  
FLORIDA OVER THE COMING DAYS. THE SYSTEM SHOULD WEAKEN AS IT  
PROGRESSES INLAND BUT STILL PUSH SIGNIFICANT RAINFALL AND STRONG  
WINDS NORTHWARD OVER PARTS OF THE EAST. CHECK THE LATEST NATIONAL  
HURRICANE CENTER PRODUCTS FOR MORE INFORMATION ON IAN. THE OTHER  
SIGNIFICANT ISSUE INVOLVES DETAILS WITHIN AN EVOLVING EASTERN  
PACIFIC MEAN TROUGH AND THE INFLUENCE ON THE EVOLUTION OF A  
SHORTWAVE REACHING THE NORTHWEST BY EARLY THURSDAY.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
MODELS CONTINUE THE TENDENCY FROM RECENT DAYS TO SHOW SOME  
EAST-WEST OSCILLATION OVER SUCCESSIVE RUNS ALONG WITH SOME  
MODERATE TIMING DIFFERENCES. OVER THE PAST 24 HOURS THE AVERAGE  
OF GUIDANCE HAS BEEN GENERALLY NUDGING THE TRACK A LITTLE  
EASTWARD, REVERSING WHAT HAD BEEN A SLIGHT WESTWARD NUDGE OVER THE  
PRIOR DAY. VARYING PROPORTIONS/ADJUSTMENTS AMONG INDIVIDUAL  
MODELS REPRESENTED THE 03Z NHC TRACK FOR MASS FIELDS AND  
ASSOCIATED RAINFALL. MOST SOLUTIONS INDICATE AN EXTRATROPICAL  
TRANSITION BY THE LATTER HALF OF THE PERIOD, WITH THE SYSTEM  
ULTIMATELY TRACKING OFF THE EAST COAST BY EARLY NEXT WEEK. THE  
NEW 00Z ECMWF SUGGESTS A SOMEWHAT SLOWER EVOLUTION.  
 
WITH THE ARRIVAL OF THE 00Z GFS, GUIDANCE SEEMS TO BE MAKING  
FURTHER PROGRESS TOWARD A FAIRLY DRAMATIC ADJUSTMENT FROM WHAT HAD  
BEEN THE MAJORITY CONSENSUS 1-2 DAYS AGO. THE LATEST TRENDS  
SUGGEST MUCH GREATER POTENTIAL FOR PACIFIC ENERGY TO PULL OFF A  
CLOSE LOW WITH THE REMAINDER OF ENERGY PROGRESSING ALONG FARTHER  
NORTH. THEN DIGGING UPSTREAM FLOW COULD START TO PUSH THE LEADING  
EDGE OF HEIGHT FALLS TOWARD THE WEST COAST BY DAY 7 MONDAY.  
MEANWHILE THIS EVOLUTION LEADS TO A SLOWER AND MORE CLOSED  
DEPICTION OF THE SHORTWAVE ENERGY THAT STARTS OVER THE NORTHWEST.  
AS WOULD TYPICALLY BE THE CASE, THE ENSEMBLE MEANS ARE STILL  
PHASED WITH THE OVERALL TROUGH. HOWEVER THERE HAS BEEN A TREND  
TOWARD MORE SOUTHWEST ELONGATION, REFLECTING AN INCREASING NUMBER  
OF MEMBERS THAT PULL OFF A CLOSED LOW, AND KEEPING HIGHER HEIGHTS  
ALONG THE WEST COAST.  
 
BASED ON GUIDANCE AVAILABLE THROUGH THE 18Z CYCLE, THE UPDATED  
FORECAST STARTED WITH A 12Z/18Z OPERATIONAL MODEL BLEND EARLY IN  
THE PERIOD AND THEN TRANSITIONED TOWARD 60 PERCENT TOTAL 18Z  
GEFS/12Z ECMWF MEANS AND THE REST 18Z GFS/12Z ECMWF BY DAY 7  
MONDAY. THIS PROVIDED AT LEAST MODEST TRENDING TO REFLECT THE  
INCREASING PROPORTION OF MODELS SHOWING FLOW SEPARATION OVER THE  
EASTERN PACIFIC AND SLOWER/CLOSED ROCKIES INTO HIGH PLAINS UPPER  
SYSTEM. NEW 00Z RUNS SUGGEST ADDITIONAL TRENDING MAY BE REQUIRED.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
TROPICAL CYCLONE IAN, EXPECTED TO BE A HURRICANE AS IT TRACKS OVER  
THE EASTERN GULF OF MEXICO TOWARD FLORIDA, WILL PRODUCE THE MOST  
PROMINENT RAINFALL AND WIND THREATS DURING THE PERIOD. A FRONT  
LIKELY TO BE STALLED OVER FLORIDA BEFORE ARRIVAL OF THE SYSTEM  
COULD PROVIDE AN ADDED FOCUS FOR RAINFALL. STRONG HIGH PRESSURE  
TRACKING ACROSS THE GREAT LAKES AND NORTHEAST WILL HELP TO ENHANCE  
THE WINDS TO THE NORTH OF IAN LATE THIS WEEK. MEANINGFUL SPREAD  
FOR THE EXACT TRACK/TIMING OF THE SYSTEM PERSISTS BUT THERE IS  
STILL A FAIRLY STRONG SIGNAL FOR A HEAVY RAINFALL THREAT OVER  
FLORIDA AND OTHER PARTS OF THE SOUTHEAST DURING THE LATTER HALF OF  
THE WEEK WITH SOME OF THIS MOISTURE EXTENDING FARTHER NORTH DURING  
THE WEEKEND. THE EXPERIMENTAL DAY 4 EXCESSIVE RAINFALL OUTLOOK  
DEPICTS SLIGHT/MODERATE RISK AREAS OVER THE FAR SOUTHEAST AND  
PARTS OF FLORIDA, WITH THE DAY 5 SLIGHT RISK AREA ENCOMPASSING  
MUCH OF THE CAROLINAS AND GEORGIA. CONTINUE TO MONITOR NATIONAL  
HURRICANE CENTER PRODUCTS FOR THE LATEST INFORMATION REGARDING  
THIS SYSTEM.  
 
THE COMBINATION OF ANOMALOUS MOISTURE AND THE UPPER TROUGH/LOW  
EMERGING FROM THE NORTHWEST SHOULD PRODUCE AREAS OF RAINFALL THAT  
SHOULD TRACK FROM THE SOUTHWEST/GREAT BASIN INTO THE ROCKIES AND  
HIGH PLAINS. RECENT TREND FOR THE UPPER SYSTEM TO BE SLOWER AND  
MORE CLOSED RAISES THE POTENTIAL FOR GREATER COVERAGE OF RAINFALL  
AND FOR SOME OF THIS RAIN TO BE LOCALLY MODERATE TO HEAVY.  
DEPENDING ON THE DEPTH/TRACK OF THE UPPER LOW, THERE MAY ALSO BE A  
SLIGHT CHANCE THAT HIGHEST ELEVATIONS IN PARTS OF THE ROCKIES  
COULD SEE A LITTLE SNOW.  
 
A MAJORITY OF THE EAST WILL SEE COOL TEMPERATURES LATE THIS WEEK  
WITH SOME HIGHS 10-15F BELOW NORMAL. NORTHERN AREAS WILL SEE THE  
LINGERING EFFECTS OF A DEPARTING UPPER TROUGH ON THURSDAY WHILE  
THE SOUTHEAST WILL SEE BELOW NORMAL HIGHS DUE TO CLOUDS/RAINFALL  
FROM IAN. READINGS SHOULD GET CLOSER TO NORMAL BY LATE  
WEEKEND/EARLY NEXT WEEK. THE NORTHERN-CENTRAL HIGH PLAINS WILL BE  
QUITE WARM ON THURSDAY WITH SOME NORTHERN AREAS SEEING HIGHS AT  
LEAST 15F ABOVE NORMAL. THEREAFTER, PLAINS WARMTH WILL MODERATE  
AND SETTLE OVER THE SOUTHERN HALF OF THE REGION BY  
SATURDAY-MONDAY. THE UPPER TROUGH/LOW DRIFTING INTO THE  
ROCKIES/HIGH PLAINS WILL BRING MODERATELY BELOW NORMAL HIGHS IN  
ITS VICINITY.  
 
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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