774  
FXUS02 KWBC 181857  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
257 PM EDT WED OCT 18 2023  
 
VALID 12Z SAT OCT 21 2023 - 12Z WED OCT 25 2023  
   
..OVERVIEW
 
 
LATEST GUIDANCE CONTINUES TO ADVERTISE A SIGNIFICANT PATTERN  
CHANGE OVER THE LOWER 48 DURING THE PERIOD. A VIGOROUS UPPER  
TROUGH IN THE EASTERN U.S. WILL SUPPORT A STRENGTHENING LOW  
PRESSURE SYSTEM THAT MAY BRING LOCALLY HEAVY RAIN AND GUSTY WINDS  
TO THE NORTHEAST THIS WEEKEND. SOME DEGREE OF UPPER TROUGHING WILL  
LINGER NEAR THE EAST COAST INTO EARLY NEXT WEEK BEFORE A RIDGE  
BUILDS INTO THE EAST. ON THE OTHER SIDE OF THE NATION, EXPECT MEAN  
TROUGHING TO REPLACE THE INITIAL RIDGE OVER THE WEST. A LEADING  
SHORTWAVE WITH POSSIBLE EMBEDDED LOW SHOULD MOVE INTO THE REGION  
AROUND SUNDAY WHILE STRONGER UPSTREAM ENERGY SHOULD FEED INTO THE  
DEVELOPING MEAN TROUGH BY TUESDAY-WEDNESDAY, BUT WITH INCREASING  
UNCERTAINTY FOR DETAILS. THIS PATTERN TRANSITION WOULD BRING  
COOLER AND WETTER WEATHER TO THE WEST, WITH THE PLAINS TRENDING  
WETTER AS WELL BY MIDWEEK, WHILE THE EAST WILL TREND DRIER AFTER  
THE WEEKEND WITH COOL TEMPERATURES EVENTUALLY MODERATING.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
MODEL GUIDANCE STILL SHOWS UNCERTAINTY IN THE FINER SCALE DETAILS  
OF THE LOW PRESSURE SYSTEM FORECAST TO IMPACT THE NORTHEAST LATE  
THIS WEEK INTO THE WEEKEND, WHICH WILL HAVE IMPACTS ON THE  
PRECIPITATION FORECAST. THE GFS/ECMWF RUNS AND THEIR ENSEMBLE  
MEANS HAVE HELD UP THE BEST FOR THE OVERALL EVOLUTION OF THE UPPER  
WAVE THAT WILL FORM THE NORTHEAST COASTAL LOW. THE LATEST GFS  
STILL BRINGS THE LOW UP THE COAST SLOWER THAN THE ENSEMBLE MEANS  
AND OTHER DETERMINISTIC GUIDANCE, WHICH WOULD RESULT IN HEAVIER  
RAINFALL SPREADING FURTHER WEST AS WELL. SINCE THIS SOLUTION SEEMS  
THE LEAST LIKELY, HEAVIER WEIGHT WAS PUT ON THE OTHER GUIDANCE FOR  
THIS FORECAST. LATEST GFS RUNS ARE ALSO SOMEWHAT SLOWER THAN THE  
MAJORITY FOR THE WEAK TRAILING SYSTEM REACHING THE MIDWEST EARLY  
SATURDAY.  
 
FORECAST DETAILS HAVE BECOME MORE UNCERTAIN OVER THE WESTERN AND  
EVENTUALLY CENTRAL U.S. AFTER MOST GUIDANCE HAD BEEN SHOWING A  
FAIRLY DIFFUSE LEADING UPPER TROUGH MOVING INTO THE WEST SUNDAY  
ONWARD OVER RECENT RUNS, THE PAST DAY HAS TRENDED TOWARD A BETTER  
DEFINED EMBEDDED LOW THAT MAY REACH THE SOUTHWEST BY EARLY NEXT  
WEEK. BEYOND THIS TREND, INDIVIDUAL MODELS AND ENSEMBLES ARE  
SUGGESTING THAT PATTERN COMPLEXITIES NEAR AND SOUTH OF ALASKA FROM  
THE WEEKEND INTO NEXT WEEK ARE LEADING TO HIGH SENSITIVITY FOR THE  
ULTIMATE DETAILS OF NORTHERN STREAM ENERGY EXPECTED TO DROP INTO  
THE WESTERN U.S. BY TUESDAY-WEDNESDAY. DIFFERENCES OVER THAT  
UPSTREAM AREA OF SENSITIVITY LEAD TO THE RECENT CMC RUNS BEING THE  
MOST AGGRESSIVE WITH DIGGING ENERGY BY MIDWEEK. ENSEMBLE MEMBERS  
ARE EXPECTEDLY DIVERSE BUT THE MEANS AVERAGE OUT TO A SOLUTION  
FALLING BETWEEN THE TO THE 00Z ECMWF AND 12Z GFS. CORRESPONDING TO  
THESE UPSTREAM ISSUES, A WIDE SPREAD DEVELOPS FOR HOW QUICKLY THE  
LEADING TROUGH/LOW MAY EJECT NORTHEASTWARD, LEADING TO LOW  
FORECAST CONFIDENCE FOR MID-NEXT WEEK. FINALLY, THE GUIDANCE STILL  
SIGNALS THAT THE OVERALL TROUGH MAY INCORPORATE MOISTURE FROM  
TROPICAL CYCLONE NORMA, WHICH MAY CONTRIBUTE TO POTENTIAL HEAVY  
RAINFALL IN THE SOUTHERN PLAINS TUESDAY INTO WEDNESDAY.  
 
PREFERENCES BASED ON THE LATEST GUIDANCE LED TO STARTING THE EARLY  
PART OF THE FORECAST WITH A DETERMINISTIC BLEND WITH HEAVIER  
WEIGHTS ON THE GFS/ECMWF THAN THE CMC/UKMET. THE GFS AND EC  
ENSEMBLE MEANS WERE SLOWLY INCORPORATED INTO THE BLEND IN  
INCREASING AMOUNTS THROUGH THE SECOND HALF OF THE FORECAST PERIOD.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
DURING THE DAY 4 EXCESSIVE RAINFALL OUTLOOK THAT COVERS SATURDAY  
AND SATURDAY NIGHT, GUIDANCE CONTINUES TO SHOW THE POTENTIAL FOR  
SOME AREAS OF SIGNIFICANT RAINFALL OVER PARTS OF THE NORTHEAST AS  
NORTHWARD MOVING LOW PRESSURE DEEPENS. SPECIFICS ARE STILL  
UNCERTAIN AND DEPEND ON FINE-SCALE DETAILS AND EXACT TIMING THAT  
WOULD AFFECT HOW LONG THE SYSTEM COULD BRING IN MOIST INFLOW FROM  
THE ATLANTIC TO ENHANCE RAINFALL AMOUNTS. THE SLOW GFS SOLUTION  
REMAINS THE OUTLIER WITH HEAVIER RAINFALL SPREADING FURTHER WEST  
THAN THE OTHER DETERMINISTIC GUIDANCE AND ENSEMBLE MEANS. FOR THE  
AFTERNOON FORECAST, CONFIDENCE WAS NOT HIGH ENOUGH TO MAKE ANY  
SIGNIFICANT CHANGES, AND THE MARGINAL RISK AREA REMAINS IN PLACE.  
SURFACE LOW INTENSIFICATION WILL ALSO SUPPORT THE POTENTIAL FOR A  
PERIOD OF BRISK TO STRONG WINDS AT LEAST THROUGH SUNDAY. DEPENDING  
ON SYSTEM STRENGTH/TRACK, HIGHEST ELEVATIONS COULD SEE A LITTLE  
SNOW. NEXT WEEK WILL BRING A DRIER TREND OVER THE EAST AS THIS  
SYSTEM DEPARTS.  
 
THE WEST WILL SEE EXPANDING COVERAGE OF PRECIPITATION FROM THE  
WEEKEND INTO NEXT WEEK. A LEADING SYSTEM SHOULD BRING LIGHT TO  
MODERATE TOTALS TO NORTHERN AND CENTRAL LATITUDES AFTER DAYTIME  
SATURDAY, WITH THIS MOISTURE SHIFTING ACROSS THE INTERMOUNTAIN  
WEST ON MONDAY. ADDITIONAL TROUGH ENERGY SHOULD BRING MORE  
PRECIPITATION INTO THE NORTHWEST NEXT WEEK BUT DETAILS ARE  
BECOMING MORE UNCERTAIN. LEADING MOISTURE SHOULD SPREAD ACROSS THE  
ROCKIES AND REACH THE CENTRAL U.S. BY TUESDAY-WEDNESDAY. GUIDANCE  
CONTINUES TO SUGGEST THAT RAINFALL OVER THE PLAINS COULD BE  
ENHANCED BY MOISTURE ORIGINATING FROM NORMA IN THE EAST PACIFIC,  
BUT THERE IS STILL SIGNIFICANT UNCERTAINTY IN THE TIMING AND  
LOCATION OF ANY POTENTIAL HEAVY RAINFALL. SOME OF THE  
PRECIPITATION OVER THE WEST SHOULD FALL AS SNOW IN THE HIGHER  
NORTHERN ROCKIES, WITH SNOW LEVELS GRADUALLY DECLINING WITH TIME.  
 
A BROAD AREA OF TEMPERATURES 10-20F ABOVE NORMAL WILL EXTEND FROM  
THE INTERIOR WEST INTO THE PLAINS ON SATURDAY. WITH TIME THIS  
WARMTH WILL MODERATE AND SHRINK IN COVERAGE AS IT REACHES PORTIONS  
OF THE PLAINS AND MISSISSIPPI VALLEY BY TUESDAY. ABOVE NORMAL  
HIGHS MAY EXPAND INTO THE OHIO VALLEY/GREAT LAKES BY WEDNESDAY BUT  
MOST ANOMALIES AS OF MIDWEEK SHOULD BE WITHIN 10 DEGREES OF  
AVERAGE. POTENTIAL FOR SOME RECORD WARM LOWS MAY EXTEND INTO  
SATURDAY ESPECIALLY OVER CALIFORNIA, WHILE A FEW RECORD HIGHS MAY  
BE POSSIBLE FROM ARIZONA INTO TEXAS DURING THE WEEKEND. STILL  
EXPECT THE WEST TO SEE BELOW NORMAL HIGHS EXPAND IN COVERAGE FROM  
WEST TO EAST, THOUGH LATEST TRENDS HAVE SLIGHTLY DELAYED THE  
ARRIVAL OF COLDER AIR (WEDNESDAY INSTEAD OF TUESDAY) THAT WOULD  
BRING HIGHS AT SOME LOCATIONS DOWN TO 10-20F BELOW NORMAL. THE  
NORTHEAST SHOULD SEE ABOVE NORMAL TEMPERATURES (ESPECIALLY FOR  
MORNING LOWS) INTO THE WEEKEND. OTHERWISE THE AMPLIFIED UPPER  
TROUGH CROSSING THE EAST WILL SUPPORT HIGHS UP TO 5-10F BELOW  
NORMAL FROM SATURDAY INTO EARLY NEXT WEEK. THE TROUGH'S DEPARTURE  
WILL PROMOTE A REBOUND CLOSER TO OR SLIGHTLY ABOVE NORMAL BY  
MIDWEEK.  
 
RAUSCH/DOLAN  
 
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 FORECAST, EXCESSIVE RAINFALL OUTLOOK,  
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY  
MESSAGES 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  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW  
 

 
 

 
 
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