221
FXUS02 KWBC 140700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 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.
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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