231  
FXUS02 KWBC 250700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT SUN SEP 25 2022  
 
VALID 12Z WED SEP 28 2022 - 12Z SUN OCT 02 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
 
 
LATEST MODELS CONTINUE TO SHOW A RAPID TRANSITION FROM AN  
AMPLIFIED ROCKIES-HIGH PLAINS UPPER RIDGE/EASTERN U.S. TROUGH  
TOWARD LESS AMPLIFIED MEAN FLOW DURING THE MID-LATE WEEK TIME  
FRAME, AS A SHORTWAVE TROUGH NEARING THE NORTHWEST COAST AS OF  
EARLY WEDNESDAY MOVES INLAND. ANOTHER HOLDOVER FROM PRIOR CYCLES  
IS INCREASING UNCERTAINTY IN FLOW DETAILS FROM THE PACIFIC INTO  
NORTH AMERICA AFTER LATE FRIDAY, WITH SOME INFLUENCE ON THE  
ULTIMATE EVOLUTION OF THE INITIAL NORTHWEST SHORTWAVE AS IT  
CONTINUES INTO THE CENTRAL PART OF THE CONTINENT. THE 03Z  
NATIONAL HURRICANE CENTER ADVISORY FOR TROPICAL STORM IAN HAS THE  
SYSTEM INTENSIFYING TO HURRICANE STATUS FAIRLY QUICKLY AND THEN  
TRACKING NORTHWARD OVER THE EASTERN GULF OF MEXICO, REACHING CLOSE  
TO FLORIDA BY LATE THURSDAY. THERE IS PERSISTENT TIMING/LONGITUDE  
SPREAD AS IAN REACHES NEAR FLORIDA. LATEST TRENDS SUGGEST THE  
DEPARTING EASTERN U.S. UPPER TROUGH MAY PLAY A MINIMAL ROLE IN  
IAN'S TRACK AND EVOLUTION BEYOND FLORIDA, BUT THE ISSUES WITH FLOW  
TO THE WEST COULD HAVE SOME INFLUENCE ON THE FORECAST BY NEXT  
WEEKEND. AWAY FROM THE HEAVY RAINFALL THREAT OVER PARTS OF THE  
SOUTHEAST AND PERHAPS FARTHER NORTHWARD, THE NORTHWEST SHOULD SEE  
A BRIEF PERIOD OF RAIN AROUND MIDWEEK AND THE SOUTHWEST INTO THE  
GREAT BASIN AND ROCKIES MAY SEE ONE OR MORE DAYS OF RAINFALL.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
INDIVIDUAL MODELS CONTINUE TO VARY WITH THE EXACT TRACK AND TIMING  
OF IAN AS THE SYSTEM PROGRESSES NORTHWARD OVER THE EASTERN GULF OF  
MEXICO TOWARD FLORIDA MID-LATE WEEK. THE OVERALL AVERAGE OF  
GUIDANCE HAS ADJUSTED NOTICEABLY WESTWARD OVER THE PAST DAY. GFS  
RUNS REMAIN ON THE WESTERN SIDE OF THE ENVELOPE WHILE OTHER MODELS  
ARE SHUFFLING AROUND FARTHER EAST THAN THE GFS TO VARYING DEGREES.  
AT LEAST RECENT ECMWF TRENDS TOWARD A FLATTER/MORE QUICKLY  
DEPARTING EASTERN U.S. UPPER TROUGH HAVE BROUGHT THAT MODEL INTO  
AGREEMENT WITH OTHER SOLUTIONS FOR THAT FEATURE, INCREASING  
CONFIDENCE THAT THE TROUGH SHOULD HAVE MINIMAL INFLUENCE ON IAN'S  
FORECAST. BY LATE IN THE PERIOD THE ADDED UNCERTAINTY WILL BE HOW  
THE WEAKENING SYSTEM WILL BECOME INCORPORATED INTO THE WESTERLIES.  
MOST MODELS SUGGEST THIS WILL HAPPEN BY AROUND SUNDAY, WITH  
SPECIFICS SOMEWHAT DEPENDENT ON FLOW DETAILS UPSTREAM.  
 
AS FOR THIS UPSTREAM FLOW, CLUSTERING IS GOOD FOR THE UPPER TROUGH  
MOVING INTO THE NORTHWEST DURING WEDNESDAY-THURSDAY. THEN DETAILS  
OVER THE PACIFIC FROM FRIDAY INTO THE WEEKEND HAVE BEEN AN ONGOING  
PROBLEM. LATEST GFS/ECMWF AND ENSEMBLE MEAN CONSENSUS SHOWS A  
PHASED UPPER TROUGH (THOUGH POSSIBLY CONTAINING AN EMBEDDED UPPER  
LOW FOR A BRIEF TIME) APPROACHING THE WEST COAST BY NEXT SUNDAY.  
HOWEVER RECENT CMC RUNS AND THE NEW 00Z UKMET PULL OFF A FARTHER  
SOUTHWEST CLOSED LOW, ULTIMATELY IMPACTING THE PATTERN FARTHER  
EAST. THIS INCLUDES THE CMC/00Z UKMET STRAYING SLOWER THAN THE  
MAJORITY WITH THE UPPER TROUGH/LOW CROSSING THE ROCKIES INTO THE  
PLAINS. PREFER THE MAJORITY GFS/ECMWF AND ENSEMBLE MEAN CLUSTER  
UNTIL THERE ARE COMPELLING TRENDS AWAY FROM THAT SCENARIO.  
 
THE UPDATED FORECAST STARTED WITH THE 18Z GFS/12Z ECMWF AND LESSER  
INPUT FROM THE UKMET/CMC FOR DAYS 3-4 WEDNESDAY-THURSDAY. THEN  
THE BLEND PHASED OUT THE CMC AND ULTIMATELY TRANSITIONED TO A HALF  
GFS/ECMWF AND HALF GEFS/ECENS SOLUTION BY DAY 7 SUNDAY.  
SUBSEQUENT ADJUSTMENTS WERE MADE TO REFLECT THE 03Z NATIONAL  
HURRICANE CENTER ADVISORY TRACK FOR IAN.  
 
   
..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. 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 FAR SOUTHEAST MID-LATE WEEK. THUS THE  
EXPERIMENTAL DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS DEPICT SLIGHT  
RISK AREAS OVER THIS REGION, WITH SOME IMPROVEMENT IN DETAIL  
CLUSTERING NEEDED TO IMPROVE CONFIDENCE BEFORE INTRODUCING ANY  
EMBEDDED MODERATE RISK AREAS. CONTINUE TO MONITOR NATIONAL  
HURRICANE CENTER PRODUCTS FOR THE LATEST INFORMATION REGARDING  
THIS SYSTEM. FROM FRIDAY THROUGH THE WEEKEND IAN SHOULD CONTINUE  
NORTHWARD, SPREADING WIND AND AREAS OF LOCALLY HEAVY RAINFALL OVER  
OTHER PORTIONS OF THE EAST. STRONG HIGH PRESSURE TRACKING ACROSS  
THE GREAT LAKES AND NORTHEAST WILL HELP TO ENHANCE THE WINDS TO  
THE NORTH OF IAN.  
 
RAPID WEAKENING/DEPARTURE OF THE EASTERN U.S. UPPER TROUGH AND THE  
APPROACH OF HIGH PRESSURE SHOULD BRING A DRIER TREND TO THE LEE OF  
THE GREAT LAKES AFTER SOME LINGERING LIGHT RAINFALL ON WEDNESDAY.  
THE PACIFIC NORTHWEST WILL SEE A PERIOD OF LIGHT TO ISOLATED  
MODERATE RAINFALL AROUND WEDNESDAY-EARLY THURSDAY WITH THE FRONTAL  
SYSTEM FORECAST TO PUSH INTO THE REGION. THE NORTHWEST START  
ANOTHER EPISODE OF LIGHT RAIN WITH A FRONT NEXT WEEKEND. THE  
COMBINATION OF ANOMALOUS MOISTURE AND SOME SHORTWAVE ENERGY  
(INCLUDING THE SOUTHERN PERIPHERY OF THE SHORTWAVE CROSSING THE  
NORTHWEST) SHOULD PRODUCE SCATTERED RAINFALL FROM THE  
SOUTHWEST/GREAT BASIN INTO THE ROCKIES, WITH THE GREATEST COVERAGE  
LIKELY TO BE AROUND THURSDAY. SOME OF THIS ACTIVITY COULD BE  
LOCALLY MODERATE OR HEAVY.  
 
THE UPPER TROUGH INITIALLY OVER THE EAST WILL SUPPORT CHILLY  
TEMPERATURES, WITH HIGHS UP TO 10-15F BELOW NORMAL OVER AND NEAR  
THE GREAT LAKES/OHIO VALLEY EAST INTO THE APPALACHIANS  
WEDNESDAY-THURSDAY. THEN CLOUDS/RAIN ASSOCIATED WITH IAN WILL  
KEEP DAYTIME HIGHS SIMILARLY COOL VERSUS NORMAL OVER THE SOUTHEAST  
THURSDAY-FRIDAY. EASTERN U.S. TEMPERATURES SHOULD MODERATE CLOSER  
TO NORMAL BY NEXT SUNDAY. WARMTH OVER THE GREAT BASIN AND  
NORTHERN ROCKIES/HIGH PLAINS ON WEDNESDAY (UP TO PLUS 15F OR SO  
ANOMALIES FOR HIGHS) SHOULD BECOME LESS PRONOUNCED AS IT MOVES  
FARTHER INTO THE PLAINS FRIDAY-SUNDAY, WITH MOST AREAS SEEING ONLY  
SINGLE-DIGIT POSITIVE ANOMALIES. THE FRONT PUSHING THROUGH THE  
NORTHWEST WEDNESDAY-THURSDAY WILL BRING A BRIEF PERIOD OF SLIGHTLY  
BELOW NORMAL HIGHS, FOLLOWED BY SOME OSCILLATION CORRESPONDING TO  
UPPER RIDGE/TROUGH PROGRESSION.  
 
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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