563  
FXUS02 KWBC 241859  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 PM EDT SAT SEP 24 2022  
 
VALID 12Z TUE SEP 27 2022 - 12Z SAT OCT 1 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...  
 
19Z UPDATE: THE 12Z MODEL CONSENSUS FOR THE EVENTUAL TRACK OF  
WHAT IS FORECAST TO BECOME HURRICANE IAN HAS NOW SHIFTED SOME TO  
THE WEST COMPARED TO EARLIER MODEL RUNS, AND THE MOST RECENT NHC  
FORECAST ADVISORY NOW REFLECTS THIS. HOWEVER, THE FORECAST BEYOND  
THREE DAYS STILL REMAINS RATHER UNCERTAIN AS THE STORM ENTERS A  
COL IN THE MIDDLE-UPPER TROPOSPHERE WITH COMPETING STEERING  
CURRENTS ALOFT WITH A BREAK IN THE RIDGE AXIS. ONE OF THE KEY  
PLAYERS IN THE EVENTUAL TRACK IS THE PROGRESSION AND AMPLITUDE OF  
A BUILDING UPPER LEVEL TROUGH ACROSS THE GREAT LAKES AND NORTHEAST  
U.S. FOR THE MIDDLE OF THE WEEK, AND HOW THAT AFFECTS THE OVERALL  
STEERING FLOW NORTHWARD FOR THE HURRICANE. SLIGHT RISKS FOR  
EXCESSIVE RAINFALL ARE IN THE EXPERIMENTAL DAY 4 AND DAY 5  
OUTLOOKS, BUT CHANGING MODEL GUIDANCE ON HIGHEST TOTALS PRECLUDES  
THE NEED FOR ISSUING A MODERATE RISK AREA FOR THIS FORECAST CYCLE.  
 
IN TERMS OF THE WPC FRONTS/PRESSURES FORECAST, A DETERMINISTIC  
BLEND OF THE 00Z NON-NCEP AND 6Z GFS WAS USED AS A BASELINE, AND  
THEN LESS OF THE 00Z CMC GOING FORWARD INTO THE END OF THE WEEK  
AND NEXT SATURDAY AS THIS MODEL INDICATES A MUCH STRONGER TROUGH  
OVER THE WESTERN U.S. COMPARED TO THE MODEL CONSENSUS. A GRADUAL  
INCREASE OF THE ENSEMBLE MEANS WAS INCORPORATED GOING INTO THE  
SECOND HALF OF THE FORECAST PERIOD. FOR QPF, THE DETERMINISTIC  
GUIDANCE HAD A GOOD SIGNAL FOR ADDITIONAL MONSOONAL SHOWERS/STORMS  
ACROSS THE FOUR CORNERS REGION FOR THE MIDDLE OF THE WEEK, SO  
AMOUNTS WERE RAISED ABOVE THE NBM ACROSS THIS REGION. THE  
PREVIOUS FORECAST DISCUSSION IS APPENDED BELOW FOR REFERENCE.  
/HAMRICK  
----------------------------  
   
..OVERVIEW
 
 
GUIDANCE IS CONSISTENT IN SHOWING A FAIRLY RAPID FLATTING OF THE  
INITIALLY AMPLIFIED UPPER PATTERN CONSISTING OF A WESTERN  
RIDGE/EASTERN TROUGH, AS A SHORTWAVE TROUGH APPROACHES FROM THE  
EASTERN PACIFIC AND MOVES INLAND. HOWEVER, FLOW DETAILS BECOME  
MORE UNCERTAIN FROM THE PACIFIC INTO NORTH AMERICA TOWARD THE END  
OF NEXT WEEK. THE 03Z NATIONAL HURRICANE CENTER ADVISORY SHOWS  
TROPICAL STORM IAN CURRENTLY IN THE CARIBBEAN AND INTENSIFYING TO  
HURRICANE STATUS AS IT TRACKS INTO THE FAR SOUTHEASTERN GULF OF  
MEXICO, REACHING THE FLORIDA PENINSULA BY WEDNESDAY. THERE IS  
STILL SOME TIMING/LONGITUDE SPREAD AS IAN REACHES NEAR FLORIDA,  
AND BY THE LATTER HALF OF THE PERIOD THE DETAILS OF THE WEAKENING  
EASTERN U.S. UPPER TROUGH WILL ALSO PLAY A ROLE IN HOW THIS SYSTEM  
AFFECTS AREAS TO THE NORTH. EXPECT THE GREAT LAKES REGION TO SEE  
SOME RAINFALL WITH THE CHILLY CYCLONIC FLOW IN PLACE DURING THE  
FIRST HALF OF THE WEEK, WHILE THE NORTHWEST MAY SEE MOSTLY LIGHT  
RAINFALL AROUND MIDWEEK. PARTS OF THE SOUTHWEST AND  
CENTRAL/SOUTHERN ROCKIES MAY ALSO SEE SOME POTENTIAL FOR SCATTERED  
RAINFALL.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
FOR TROPICAL STORM IAN, GUIDANCE IS CONTINUING TO SHOW MEANINGFUL  
DIVERGENCE BY 4-5 DAYS OUT IN TIME (THROUGH WEDNESDAY-THURSDAY),  
WITH THE ECMWF RUNS THROUGH 12Z/23 ON THE FAST/EASTERN SIDE OF THE  
SPREAD AND THE GFS ON THE WESTERN SIDE. IN THE NEW 00Z RUNS, THE  
GEFS MEAN IS EVEN WEST OF THE OPERATIONAL RUN WHILE THE CMC IS  
CLOSE TO THE GFS. THE 00Z UKMET IS A COMPROMISE. AFTER THURSDAY  
ECMWF RUNS HAVE BEEN MORE AMPLIFIED/PERSISTENT WITH LINGERING  
EASTERN UPPER TROUGH ENERGY THAT INTERACTS WITH IAN, VERSUS OTHER  
SOLUTIONS THAT LIFT THE TROUGH AWAY MORE QUICKLY (LEADING TO  
SLOWER MOTION FOR IAN). THE OFFICIAL NHC TRACK IS AN INTERMEDIATE  
SCENARIO WITHIN THIS ENVELOPE. THE NEW 00Z ECMWF HAS MADE NOTABLE  
TRENDS WITH BOTH ASPECTS OF THE FORECAST--TRENDING SLOWER/WESTWARD  
OVER FLORIDA AND THEN LESS AMPLIFIED WITH WHAT UPPER TROUGH ENERGY  
LINGERS OVER THE EAST.  
 
THERE IS DECENT CLUSTERING FOR THE UPPER TROUGH AND ASSOCIATED  
SURFACE SYSTEM/FRONT AFFECTING THE NORTHWEST DURING  
WEDNESDAY-THURSDAY. THEN MODELS AND ENSEMBLE MEMBERS DISPLAY  
RAPIDLY INCREASING SPREAD FOR UPSTREAM FLOW OVER THE PACIFIC LATE  
WEEK INTO THE WEEKEND. BY NEXT SATURDAY INDIVIDUAL SOLUTIONS  
RANGE BETWEEN PRONOUNCED FLOW SEPARATION (DEPOSITING A CLOSED LOW  
WEST OF 140W LONGITUDE AND ALLOWING A RIDGE TO BUILD NEAR THE WEST  
COAST BY SATURDAY) AND A PHASED TROUGH APPROACHING THE WEST COAST.  
HIGHLIGHTING THE UNCERTAINTY, THE NEW 00Z CMC/ECMWF HAVE BOTH  
FLIPPED SIGNIFICANTLY RELATIVE TO THEIR PREVIOUS RUNS. PREFER A  
SOLUTION CLOSER TO THE ENSEMBLE MEANS WITH A TROUGH OFFSHORE THE  
WEST COAST UNTIL THERE IS BETTER MODEL AGREEMENT. LATEST GFS RUNS  
ARE A DEEPER VERSION OF THIS IDEA. THESE ISSUES ULTIMATELY HAVE  
SOME EFFECT ON THE NORTHWEST SHORTWAVE AS IT CONTINUES INTO  
CENTRAL NORTH AMERICA.  
 
THE COMBINATION OF INCREASING MODEL DIVERGENCE FOR IAN LATER IN  
THE PERIOD AS WELL AS THE DEVELOPING UNCERTAINTY WITH THE PACIFIC  
INTO NORTH AMERICA PATTERN LED TO TRANSITIONING THE MANUAL  
FORECAST FROM AN EARLY-PERIOD MODEL BLEND (12Z/18Z RUNS) TOWARD 70  
PERCENT EMPHASIS OF THE GEFS/ECWMF/CMC MEANS BY DAY 7 SATURDAY.  
THE STARTING BLEND REQUIRED ADJUSTMENT TO ACCOMMODATE THE 03Z NHC  
FORECAST FOR IAN.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE DOMINANT RAINFALL AND WIND THREAT DURING THE PERIOD WILL BE  
WITH TROPICAL CYCLONE IAN THAT SHOULD AFFECT FLORIDA AND  
POTENTIALLY OTHER PARTS OF THE EAST. A FRONT EXPECTED TO STALL  
OVER FLORIDA BEFORE ARRIVAL OF THE SYSTEM COULD PROVIDE AN ADDED  
FOCUS FOR RAINFALL. WHILE THERE IS STILL MEANINGFUL SPREAD  
REGARDING THE ULTIMATE TRACK OF THIS SYSTEM, THERE IS A STRONG  
ENOUGH SIGNAL FOR A HEAVY RAINFALL THREAT OVER THE FLORIDA  
PENINSULA TUESDAY-WEDNESDAY TO SUPPORT A SLIGHT RISK AREA OVER  
PARTS OF THE REGION IN THE EXPERIMENTAL DAYS 4-5 EXCESSIVE  
RAINFALL OUTLOOKS. CONTINUE TO MONITOR NATIONAL HURRICANE CENTER  
PRODUCTS FOR THE LATEST INFORMATION REGARDING THIS SYSTEM. IT  
APPEARS LIKELY THAT IAN WILL SPREAD RAIN AND WIND FARTHER NORTH  
OVER THE EAST LATER IN THE WEEK INTO THE WEEKEND BUT WITH FAIRLY  
LOW CONFIDENCE IN THE COVERAGE AND TIMING. HIGH PRESSURE TRACKING  
ACROSS THE GREAT LAKES AND NORTHEAST MAY SERVE TO STRENGTHEN THE  
SURFACE GRADIENT TO THE NORTH OF IAN AND THUS ENHANCE THE WINDS  
OVER SOME AREAS.  
 
THE GREAT LAKES AND VICINITY WILL SEE PERIODS OF RAIN THROUGH THE  
FIRST HALF OF THE WEEK WITH CHILLY AND BRISK CYCLONIC FLOW IN  
PLACE AT LOW LEVELS AND THE UPPER TROUGH OVERHEAD. A FEW FOCUSED  
BANDS TO THE LEE OF THE GREAT LAKES COULD PRODUCE SOME ENHANCED  
TOTALS. THE PACIFIC NORTHWEST SHOULD SEE A PERIOD OF LIGHT TO  
PERHAPS ISOLATED MODERATE RAINFALL AROUND WEDNESDAY-EARLY THURSDAY  
WITH THE FRONTAL SYSTEM FORECAST TO PUSH INTO THE REGION.  
INCREASING MOISTURE MAY PROMOTE SCATTERED RAIN OVER PORTIONS OF  
THE SOUTHWEST AND INTO THE CENTRAL/SOUTHERN ROCKIES, WITH BEST  
COVERAGE PERHAPS AROUND THURSDAY.  
 
THE EVOLUTION OF THE UPPER PATTERN WILL FAVOR CHILLY TEMPERATURES  
OVER A MAJORITY OF THE EASTERN U.S. WITH BEST POTENTIAL FOR HIGHS  
UP TO 10-15F BELOW NORMAL OVER AND NEAR THE GREAT LAKES/OHIO  
VALLEY, ESPECIALLY TUESDAY-THURSDAY. TEMPERATURES WILL LIKELY  
START TO MODERATE BY THE END OF THE WEEK WITH THE TRACK OF IAN  
DETERMINING SOME OF THE SPECIFICS. WARMTH OVER THE WEST ON  
TUESDAY WILL PROGRESS INTO THE PLAINS WITH TIME, WITH ANOMALIES  
MODERATING SLIGHTLY BY FRIDAY-SATURDAY. AREAS FROM THE  
NORTHWEST/NORTHERN GREAT BASIN INTO THE NORTHERN HIGH PLAINS MAY  
SEE ONE OR MORE DAYS WITH HIGHS 10-15F ABOVE NORMAL. THE FRONT  
PUSHING INTO THE WEST AFTER EARLY WEDNESDAY WILL BRING  
TEMPERATURES OVER THAT REGION CLOSER TO NORMAL, WITH LOWER  
CONFIDENCE IN TEMPERATURE SPECIFICS BY NEXT WEEKEND.  
 
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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