926  
FXUS02 KWBC 230704  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
204 AM EST SAT NOV 23 2024  
 
VALID 12Z TUE NOV 26 2024 - 12Z SAT NOV 30 2024  
 
 
...MESSY THANKSGIVING TRAVEL POSSIBLE IN THE EASTERN HALF OF THE  
U.S., INCLUDING POTENTIAL CENTRAL/NORTHERN TIER WINTER WEATHER...  
 
   
..OVERVIEW
 
 
UPPER AND SURFACE LOW PRESSURE IS FORECAST TO TRAVERSE THE GREAT  
LAKES/NORTHEAST TUESDAY-WEDNESDAY WITH SOME MODEST RAIN/MAINLY  
HIGHER ELEVATION SNOW. QUASI-ZONAL UPPER FLOW SHOULD BE IN PLACE  
ATOP MOST OF THE LOWER 48 OTHER THAN THAT FEATURE AND A  
TROUGH/POSSIBLE SMALL UPPER LOW MOVING INTO THE WEST TUESDAY-  
WEDNESDAY. THE LATTER WILL HELP SPREAD PRECIPITATION ACROSS PARTS  
OF THE WEST AND PRODUCE HEAVY SNOW IN HIGHER ELEVATIONS THROUGH  
MIDWEEK. THE SHORTWAVE IS FORECAST TO COMBINE WITH NORTHERN STREAM  
ENERGIES AS IT MOVES EASTWARD, CREATING A DEEPENING UPPER TROUGH  
SETTING UP ATOP THE EAST-CENTRAL U.S. LATE WEEK. A SURFACE LOW  
PRESSURE/FRONTAL SYSTEM SHOULD DEVELOP IN RESPONSE TO THIS TROUGH  
BUT THERE REMAINS CONSIDERABLE UNCERTAINTY IN THE SURFACE PATTERN  
AND SENSIBLE WEATHER DETAILS. THERE IS SOME GENERAL CLUSTERING FOR  
AN AREA OF MODERATE TO HEAVY PRECIPITATION ACROSS THE TENNESSEE AND  
LOWER OHIO VALLEYS INTO THE APPALACHIANS AND MID-ATLANTIC BY  
WEDNESDAY AND THURSDAY, SPREADING INTO THE NORTHEAST THURSDAY-  
FRIDAY. SOME WINTRY PRECIPITATION MAY BE POSSIBLE IN THE NORTHERN  
PART OF THE MOISTURE SHIELD AND IN HIGHER ELEVATIONS. INCREASING  
COVERAGE OF COLDER THAN AVERAGE TEMPERATURES IS LIKELY AS THE WEEK  
PROGRESSES UNDERNEATH THE DEEPENING UPPER TROUGH AND COLD SURFACE  
HIGH PRESSURE.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL AGREEMENT APPEARS REASONABLE AT THE START OF THE MEDIUM  
RANGE PERIOD EARLY TUESDAY, WITH A MULTI-MODEL BLEND WORKING WELL  
FOR THE NORTHEAST UPPER/SURFACE LOW AND THE EXISTENCE OF SHORTWAVE  
TROUGHING/POSSIBLE SMALL CLOSED LOW JUST OFFSHORE OF OREGON.  
HOWEVER, MODELS DIVERGE AS SOON AS WEDNESDAY WITH THE TRACK AND  
TIMING OF THE WESTERN SHORTWAVE AS IT PUSHES EASTWARD. IN  
PARTICULAR, THE 18Z GFS DUG A STRONGER CLOSED LOW FARTHER SOUTH  
INTO CALIFORNIA WEDNESDAY BEFORE TRACKING IT EAST THURSDAY, CAUSING  
IT TO BE SLOWER THAN THE MODEL CONSENSUS. DOWNSTREAM THIS RESULTED  
IN SLOWER FORMATION OF A SOUTHERN HIGH PLAINS SURFACE LOW THAN  
OTHER GUIDANCE AND A MORE SUPPRESSED LOW TRACK ACROSS THE SOUTHEAST  
INTO LATER WEEK. THUS THE 18Z GFS APPEARED TO BE AN OUTLIER BUT IT  
DID HAVE SOME SUPPORT FROM A HANDFUL OF 12Z AI/ML MODELS  
(FOURCASTNET, GRAPHCAST, AND PANGU). MEANWHILE THE 12Z AIFS WAS  
WEAKER THAN ANYTHING ELSE WITH THE WESTERN SHORTWAVE, BUT THE 18Z  
WAS A BIT STRONGER. THE WPC FORECAST SUPPORTED MORE OF A MIDDLE  
GROUND APPROACH WITH THIS SHORTWAVE AND EVENTUAL SURFACE LOW,  
FAVORING THE 12Z RUN OF THE GFS AND THE 12Z ECMWF AND CMC. THE  
NEWER 00Z RUNS FORTUNATELY SEEM TO BE ALONG THE SAME LINES. SO  
DEVELOPING AND DEEPENING TROUGHING WITH AN AXIS ATOP THE EAST-  
CENTRAL U.S. BY LATER WEEK ALONG WITH A SURFACE LOW FORMING IN  
RESPONSE SHOWS SOME AGREEMENT IN PRINCIPLE. BUT EVEN WITHIN THIS  
GENERAL CONSENSUS THERE IS NOTABLE SPREAD INCLUDING IN THE ENSEMBLE  
MEMBERS. FOR EXAMPLE, THERE ARE STILL SOME MEMBERS ESPECIALLY IN  
THE EC ENSEMBLE THAT SHOW A SURFACE LOW TRACK IN THE OHIO VALLEY  
AROUND THURSDAY-FRIDAY LIKE SOME OLDER MODELS SHOWED. PLUS THE  
SENSIBLE WEATHER INCLUDING PRECIPITATION AMOUNT AND TYPE WILL BE  
VERY SENSITIVE TO SMALL DIFFERENCES. SO EXPECT CHANGES TO THE  
FORECAST IN FUTURE CYCLES.  
 
THE WPC FORECAST WAS BASED ON A BLEND OF DETERMINISTIC 12Z  
GUIDANCE EARLY IN THE PERIOD, AND REDUCED THE DETERMINISTIC  
PROPORTION IN FAVOR OF THE ENSEMBLE MEANS TO 60 PERCENT DAYS 6-7 TO  
MINIMIZE DETERMINISTIC DETAIL DIFFERENCES.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
 
A PACIFIC MOISTURE PLUME WILL SINK SLOWLY SOUTHWARD INTO TUESDAY  
WITH SOME CONTINUED MODEST RAIN INTO CALIFORNIA, MAINLY TO THE  
SOUTH OF AREAS THAT WERE MOST IMPACTED BY THE RECENT SIGNIFICANT  
ATMOSPHERIC RIVER. THE MOISTURE AND A SHORTWAVE PROVIDING DYNAMICAL  
SUPPORT WILL LEAD TO ACCUMULATING SNOW ACROSS THE SIERRA NEVADA  
INTO THE INTERMOUNTAIN WEST HIGHER ELEVATIONS INCLUDING THE WASATCH  
AND INTO THE CENTRAL ROCKIES. HEAVY SNOW AMOUNTS COULD PILE UP  
ESPECIALLY IN THE SIERRA (1 TO 3 FEET) AND IN THE COLORADO ROCKIES  
(1 TO 2 FEET). PRECIPITATION SHOULD FINALLY ABATE IN THE WEST FOR  
THE LATTER PART OF NEXT WEEK.  
 
FARTHER EAST, LIGHT TO MODERATE PRECIPITATION IS LIKELY ACROSS THE  
NORTHEAST INTO TUESDAY UNDER THE LOW PRESSURE SYSTEM, AND BREEZY  
WESTERLY FLOW ON THE BACKSIDE OF THE LOW WILL ALSO PRODUCE SOME  
LAKE EFFECT AND UPSLOPE RAIN/SNOW SHOWERS. THEN AS ANOTHER LOW  
PRESSURE/FRONTAL SYSTEM DEVELOPS IN THE SOUTH-CENTRAL PLAINS AND  
TRACKS EASTWARD AS THE WEEK PROGRESSES, PRECIPITATION IS LIKELY FOR  
MUCH OF THE EASTERN HALF OF THE CONUS WEDNESDAY-FRIDAY. CURRENTLY  
THE HEAVIEST RAINFALL AXIS LOOKS TO BE AROUND THE MID-  
SOUTH/TENNESSEE VALLEY/LOWER OHIO VALLEY LATE WEDNESDAY-THURSDAY,  
THOUGH WITH UNCERTAINTY IN PLACEMENT. GAVE SOME CONSIDERATION TO A  
MARGINAL RISK FOR THE DAY 5/WEDNESDAY ERO AS CONVECTION IS FORECAST  
TO RAMP UP WEDNESDAY EVENING/NIGHT IN AN ENVIRONMENT WITH SOME  
INSTABILITY, AND THE WEST-EAST TRACK OF THE CONVECTION ALONG THE  
SURFACE LOW MAY LEAD TO SOME TRAINING OF STORMS. PLAN TO HOLD OFF  
ON ANY RISK AREA FOR NOW AWAITING BETTER TIMING AND PLACEMENT  
AGREEMENT IN THE MODEL GUIDANCE, PLUS THE THURSDAY/DAY 6 PERIOD MAY  
END UP BEING THE WETTER DAY. SNOW OR ICE MAY BE POSSIBLE NORTH OF  
THE LOW TRACK AND FRONTAL BOUNDARY, POSSIBLY IMPACTING TRAVEL. THE  
DAY 4-7 WINTER WEATHER OUTLOOK INDICATES SOME LOW PROBABILITIES FOR  
SNOW GREATER THAN 0.25" LIQUID EQUIVALENT ACROSS PARTS OF THE  
CENTRAL PLAINS/MIDDLE MISSISSIPPI VALLEY WEDNESDAY, THE UPPER OHIO  
VALLEY AND THE CENTRAL/NORTHERN APPALACHIANS INTO NORTHERN  
PENNSYLVANIA AND THE INTERIOR NORTHEAST BY THANKSGIVING DAY, AND  
LINGERING ACROSS THE NORTHEAST AND THE GREAT LAKES/APPALACHIANS ON  
THE BACKSIDE OF THE LOW BY NEXT FRIDAY. BUT THE AXIS OF SNOW (AND  
POSSIBLY ICE) WILL BE QUITE DEPENDENT ON THE UNCERTAIN SURFACE LOW  
TRACK, AND CONFIDENCE REMAINS BELOW AVERAGE REGARDING SPECIFICS OF  
PRECIPITATION COVERAGE/TYPE/AMOUNTS. CONTINUE TO MONITOR FORECASTS  
AS DETAILS BECOME BETTER REFINED OVER THE COMING DAYS.  
 
EXPECT TEMPERATURES TO BE WARMER THAN AVERAGE ACROSS THE EASTERN  
U.S. ON TUESDAY, BUT A COLD FRONTAL PASSAGE BY MIDWEEK WILL LEAD TO  
HIGHS NEAR OR A FEW DEGREES BELOW NORMAL. THE SOUTHWEST COULD ALSO  
SEE ABOVE AVERAGE TEMPERATURES TUESDAY THAT PUSH INTO THE SOUTHERN  
PLAINS WEDNESDAY. A COUPLE OF SURGES OF ARCTIC AIR LOOK TO IMPACT  
THE NORTHERN PLAINS THROUGH TUESDAY AND AGAIN LATER WEEK, WITH  
HIGHS IN THE TEENS AND LOWS BELOW 0F IN SOME AREAS, EQUATING TO AT  
LEAST 10-20F BELOW NORMAL. THE SECOND SURGE OF COLD AIR IS FORECAST  
TO MAKE IT THROUGH MUCH OF THE LOWER 48 (IN MODERATED FASHION) BY  
LATER NEXT WEEK. THIS WILL COOL THE WEST AND SOUTHERN TIER TO NEAR  
NORMAL, WHILE BELOW NORMAL TEMPERATURES ARE FORECAST ACROSS MUCH OF  
THE PLAINS/MISSISSIPPI VALLEY TO OHIO/TENNESSEE VALLEYS TO THE  
EASTERN SEABOARD BY THE END OF THE WEEK.  
 
TATE  
 
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 (QPF), EXCESSIVE RAINFALL  
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT  
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:  
 
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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