018
FXUS02 KWBC 141859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM 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 PLUS
HIGHER ELEVATION SNOW. SOME RAIN MAY SPREAD INTO THE PLAINS AS
WELL, POSSIBLY BECOMING MODERATE TO HEAVY AT SOME LOCATIONS, BUT
WITH WARMER THAN AVERAGE TEMPERATURES. MEANWHILE IN THE EAST, A
PERSISTENT UPPER LOW WILL MEANDER WHILE POTENTIALLY ACCOMPANIED BY
SURFACE LOW PRESSURE SYSTEMS. IN PARTICULAR, NHC AND WPC ARE
MONITORING POSSIBLE SURFACE LOW DEVELOPMENT IN THE SHORT TERM OFF
THE SOUTHEAST COAST (MAY BECOME TROPICAL OR SUBTROPICAL) WITH A
TRACK REACHING INTO NORTH CAROLINA OR VICINITY BY THE START OF THE
PERIOD TUESDAY. HOWEVER THERE HAS BEEN CONSIDERABLE UNCERTAINTY
WITH THIS SYSTEM'S EVOLUTION AND PATH ALONG WITH THE
COVERAGE/INTENSITY OF ASSOCIATED RAINFALL.
..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 AN
EAST-WEST ORIENTED RIDGE THAT SHOULD BE ERODING OVER THE NORTHEAST
TUESDAY. THIS UPPER LOW MAY OPEN UP, WITH UNCERTAINTY OVER HOW MUCH
NORTHERN STREAM TROUGHING OVER THE CANADIAN MARITIMES/NEW ENGLAND
COULD INTERACT BY LATE WEEK. HOWEVER, DETAILS OF LOWER
PREDICTABILITY SMALL-SCALE 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 SUBTROPICAL FOR A TIME 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. AI MODELS THROUGH 00Z HAVE FAVORED
A NEAR-COASTAL OR MORE OF AN OFFSHORE TRACK, BUT NEW 12Z GUIDANCE
IS SHOWING BETTER CLUSTERING A MORE INLAND TRACK (BUT STILL UP FOR
DEBATE HOW FAR NORTHWEST, WITH THE GFS/CMC/UKMET ULTIMATELY
REACHING THE UPPER OHIO VALLEY BUT THE ECMWF REMAINING SLOWER AND
FARTHER EAST/SOUTHEAST). IN THE WAKE OF THIS SYSTEM, WEAK
WAVINESS/FRONTS MAY MAY PERSIST OR REFORM NEAR THE COAST OR OVER
THE WESTERN ATLANTIC INTO LATE WEEK ON THE LEADING SIDE OF THE
LINGERING UPPER TROUGH.
IN THE WEST, THERE IS GENERALLY BETTER MODEL AGREEMENT FOR AT LEAST
THE FIRST THREE DAYS OF THE PERIOD, AS AN UPPER LOW OVER THE GREAT
BASIN EARLY TUESDAY PIVOTS NORTHEASTWARD WEDNESDAY-THURSDAY ALONG
WITH A SURFACE LOW FOLLOWED BY TRAILING ENERGY DIGGING INTO
CALIFORNIA/NEVADA WITH ANOTHER POTENTIAL EMBEDDED LOW. BY FRIDAY-
SATURDAY THE DYNAMICAL/ML MODELS ALONG WITH ENSEMBLES SHOW
INCREASING SPREAD REGARDING NORTH PACIFIC FLOW ENTERING THE WESTERN
NORTH AMERICA MEAN TROUGH AND ITS INFLUENCE ON THURSDAY'S
CALIFORNIA/NEVADA TROUGH OR UPPER LOW. THE MOST COMMON THEME IS FOR
MORE NORTHERN STREAM INFLUENCE AND FASTER UPPER LOW EJECTION THAN
DEPICTED BY THE 00Z CMC, AND THE NEW 12Z CMC HAS ADJUSTED MORE
TOWARD CONSENSUS. ON THE OTHER HAND, THE NEW 12Z GFS TRENDS
SOMEWHAT FAST/FLAT WITH THE NORTHERN STREAM BY NEXT SATURDAY.
FOR ABOUT THE FIRST HALF OF THE PERIOD, THE UPDATED FORECAST BASED
ON THE 00Z/06Z GUIDANCE STARTED WITH A 70 PERCENT MODEL/30 PERCENT
ENSEMBLE MEAN (06Z GEFS/00Z ECENS) TO ACCOUNT FOR A PREFERRED
INTERMEDIATE SOLUTION NEAR THE EAST COAST WHILE REFLECTING
CONSENSUS ELSEWHERE. WEIGHTING SHIFTED TOWARD A 50/50 MIX LATER IN
THE PERIOD AND REMOVED THE CMC GIVEN INCREASING DETAIL UNCERTAINTY
WITHIN THE MORE AGREEABLE LARGER SCALE PATTERN. LATE ADJUSTMENTS
WERE MADE ALONG THE EAST COAST TO ACCOMMODATE A COMPROMISE OF NEW
12Z GUIDANCE AVAILABLE FOR REVIEW IN THE PROCESS OF WPC/NHC MEDIUM
RANGE COORDINATION.
..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 RISK AREAS S 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.
RAUSCH/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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