464  
FXUS02 KWBC 140708  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
308 AM EDT SAT SEP 14 2024  
 
VALID 12Z TUE SEP 17 2024 - 12Z SAT SEP 21 2024  
 
...POSSIBLE SUBTROPICAL OR TROPICAL LOW COULD SPREAD HEAVY RAIN TO  
THE CAROLINAS AND MID-ATLANTIC INTO TUESDAY...  
   
..OVERVIEW  
 
RELOADING UPPER TROUGHING WILL BE IN PLACE ACROSS THE WESTERN U.S.  
THROUGH MUCH OF NEXT WEEK, PROMOTING COOLER WEATHER AND RAIN AND  
HIGHER ELEVATION SNOW. SOME RAIN MAY SPREAD INTO THE PLAINS AS  
WELL, BUT WITH WARMER THAN AVERAGE TEMPERATURES. MEANWHILE IN THE  
EAST, A PERSISTENT UPPER LOW WILL MEANDER WHILE POTENTIALLY  
ACCOMPANIED BY SURFACE LOW PRESSURE SYSTEMS. NHC AND WPC ARE  
MONITORING ONE SURFACE LOW OFF THE CAROLINA COAST FOR POSSIBLE  
TROPICAL OR SUBTROPICAL AS THE PERIOD BEGINS TUESDAY, BUT WITH  
AMPLE UNCERTAINTY REMAINING IN ITS EVOLUTION AND THUS THE COASTAL  
AND INLAND RAINFALL FORECAST.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT  
 
THE EASTERN U.S. IS THE AREA OF GREATEST UNCERTAINTY DURING THE  
PERIOD, DESPITE GENERAL AGREEMENT IN AN UPPER LOW SOUTH OF A EAST-  
WEST ORIENTED RIDGE THAT SHOULD BE ERODING OVER THE NORTHEAST  
TUESDAY. THIS UPPER LOW IS FORECAST TO EXPAND NORTHWARD AND  
EVENTUALLY JOIN WITH A NORTHERN STREAM UPPER TROUGH OVER THE  
NORTHEAST LATE WEEK. HOWEVER, DETAILS OF ENERGY WITHIN THE UPPER  
LOW AND TROUGH WILL IMPACT THE PATTERN AT THE SURFACE, INCLUDING A  
LOW PRESSURE SYSTEM THAT COULD SHOW TROPICAL OR SUBTROPICAL  
DEVELOPMENT EARLY TO MIDWEEK. THE LOW MAY BE MOST LIKELY TO BE  
SUBTROPICAL AS CYCLONE PHASE SPACE DIAGRAMS FROM VARIOUS MODELS ARE  
GENERALLY SHOWING A SYMMETRIC WARM CORE LOW, BUT BORDERLINE AT  
TIMES, WHILE THERE SEEM TO BE SOME MOISTURE/TEMPERATURE GRADIENTS  
THAT WOULD SUGGEST FRONTS NEARBY. THERE IS NOTABLE UNCERTAINTY WITH  
THE LOW TYPE AND ALSO THE LOW TRACK. RECENT MODEL RUNS/CYCLES HAVE  
STARTED TO SHOW A BIT MORE PERSISTENCE FOR THE LOW TO MOVE INLAND  
INTO NORTH CAROLINA BY TUESDAY, WHILE GUIDANCE DIVERGES IN HOW FAR  
INLAND IT COULD MAINTAIN A LOW. HOWEVER, AI MODELS FOR A WHILE NOW  
HAVE FAVORED MORE OF AN OFFSHORE TRACK, SO CANNOT RULE THAT OUT.  
THE UKMET ALSO HAS AN OFFSHORE TRACK. THESE ENERGY/LOW DIFFERENCES  
DO CREATE SIGNIFICANT SENSIBLE WEATHER DIFFERENCES IN TERMS OF QPF,  
AND THE FORECAST MAY CONTINUE TO FLUCTUATE. WEAK LOWS THAT MAY  
STAY FRONTAL MAY FORM OVER THE WESTERN ATLANTIC INTO LATER IN THE  
WEEK AS WELL.  
 
IN THE WEST, FORTUNATELY THERE IS GOOD MODEL AGREEMENT IN  
INDICATING AN UPPER LOW ATOP THE GREAT BASIN EARLY TUESDAY PIVOTING  
NORTHEASTWARD WEDNESDAY-THURSDAY ALONG WITH A SURFACE LOW.  
MEANWHILE ADDITIONAL ENERGY DIGS THROUGH THE WEST SIDE OF THE  
TROUGH NEAR THE WEST COAST MIDWEEK AND REINFORCES THE TROUGH,  
POSSIBLY DEVELOPING ANOTHER EMBEDDED UPPER LOW AS WELL. SOME  
UNCERTAINTY ARISES AS THE BASE OF THE TROUGH STARTS TO SHIFT  
EASTWARD LATE WEEK. THE 00Z CMC IS THE SLOWEST OF RECENT SOLUTIONS  
IN PROGRESSING IT EAST.  
 
THE WPC FORECAST STARTED WITH A BLEND OF THE DETERMINISTIC 18Z GFS  
AND 12Z ECMWF/CMC/UKMET ALONG WITH SMALL PROPORTIONS OF THE GEFS  
AND EC ENSEMBLE MEANS TO REDUCE DEPENDENCY ON ANY INDIVIDUAL MODEL  
RUN GIVEN THE VARIATIONS ESPECIALLY IN THE EAST. AS THE PERIOD  
PROGRESSED, INCREASED THE PROPORTION OF THE ENSEMBLE MEANS TO ABOUT  
HALF BY DAYS 6-7.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS  
 
THE SURFACE LOW THAT MAY BE TROPICAL OR SUBTROPICAL PER NHC  
REMAINS UNCERTAIN, BUT IS CURRENTLY FORECAST TO COME INTO THE  
CAROLINAS BY TUESDAY, SPREADING RAIN THERE AND INTO THE MID-  
ATLANTIC AND APPALACHIANS. MARGINAL RISKS ARE IN PLACE FOR NORTH  
CAROLINA INTO MUCH OF THE MID-ATLANTIC DAY 4/TUESDAY AND SHIFTING A  
BIT FARTHER NORTH ACROSS THE MID-ATLANTIC TOWARD SOUTHERN PORTIONS  
OF THE NORTHEAST DAY 5/WEDNESDAY. IF THE FORECAST PANS OUT WITH A  
RELATIVELY DEEP LOW COMING INTO THESE AREAS, A RISK LEVEL HIGHER  
THAN A MARGINAL WOULD LIKELY BE WARRANTED PARTICULARLY ON TUESDAY,  
BUT GIVEN THE LINGERING UNCERTAINTY IN THE LOW TRACK MAINTAINED THE  
MARGINAL FOR NOW. SHOWERS COULD BE PERSISTENT INTO THE LATTER HALF  
OF THE WEEK AS UPPER TROUGHING REMAINS OVERHEAD. FRONTAL  
BOUNDARIES COULD ALSO HELP FOCUS MOISTURE ACROSS PARTS OF THE  
SOUTHEAST FOR RAIN AND THUNDERSTORM CHANCES. A MARGINAL RISK  
REMAINS IN EFFECT FOR DAY 4/TUESDAY FOR THE FLORIDA PANHANDLE  
PARTICULARLY DUE TO THEIR WET ANTECEDENT CONDITIONS. INTO  
WEDNESDAY-THURSDAY, THE URBAN AREAS OF SOUTHEAST FLORIDA MAY BE A  
FOCUS FOR CONVECTION, POTENTIALLY PROMPTING MARGINAL RISK AREAS IN  
FUTURE FORECAST CYCLES IF GUIDANCE AGREES.  
 
THE UPPER LOW/TROUGH ACROSS THE WEST WILL PROVIDE AMPLE  
LIFT/FORCING TO SUPPORT ROUNDS OF PRECIPITATION THERE NEXT WEEK.  
FOR DAY 4/TUESDAY, A MARGINAL RISK ERO IS IN PLACE FOR PARTS OF  
NORTHERN IDAHO TO THE WESTERN HALF OF MONTANA AS THE REGION COULD  
SEE HEAVY RAIN RATES NEAR THE LOW TRACK THAT PERSIST OVER THE SAME  
AREAS FOR MULTIPLE HOURS, AND THERE COULD BE SOME INSTABILITY IN  
PLACE. THE HIGHER ELEVATIONS CAN EXPECT SNOW WITH THIS SYSTEM GIVEN  
THE DEPTH OF THE UPPER LOW. ANOTHER ROUND OF ENERGY WILL CAUSE THE  
PACIFIC NORTHWEST TO SEE PRECIPITATION ON TUESDAY, SHIFTING INTO  
MUCH OF THE GREAT BASIN MIDWEEK AND THEN INTO THE NORTHERN ROCKIES.  
FARTHER EAST, SCATTERED RAIN AND THUNDERSTORMS ARE LIKELY IN THE  
PLAINS FOR MULTIPLE DAYS NEXT WEEK NEAR AND AHEAD OF FRONTAL  
SYSTEMS. RAINFALL AMOUNTS MAY INCREASE IN THE NORTH-CENTRAL U.S. BY  
LATER NEXT WEEK.  
 
THE WEST CAN EXPECT MUCH COOLER THAN NORMAL TEMPERATURES NEXT WEEK  
UNDER THE UPPER TROUGH. HIGH TEMPERATURES WILL BE 15-20F BELOW  
NORMAL ACROSS MUCH OF THE WEST, GRADUALLY MODERATING CLOSER TO  
NORMAL ACROSS THE WEST COAST STATES BUT PERSISTING IN THE INTERIOR  
WEST AS THE WEEK PROGRESSES. SOME AREAS MAY REACH THE FREEZING MARK  
IN THE HIGHER ELEVATIONS OF THE GREAT BASIN/ROCKIES. MEANWHILE,  
THE CENTRAL U.S. SHOULD SEE WARMER THAN NORMAL HIGHS BY 10-15F AND  
LOWS BY 15-20F, INCLUDING HIGHS IN THE 90S IN THE SOUTHERN HALF OF  
THE PLAINS. WARMER THAN AVERAGE TEMPERATURES WILL STRETCH INTO THE  
GREAT LAKES AND NORTHEAST (UNTIL A COLD FRONTAL PASSAGE IN THE  
NORTHEAST LATE WEEK), WITH MORE TYPICAL TEMPERATURES SOUTH OF  
THERE.  
 
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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