640
FXUS02 KWBC 141931
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
331 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 DYNAMICAL
GUIDANCE IS SHOWING BETTER CLUSTERING WITH 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 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/WPC
COORDINATION REMAINS MORE UNCERTAIN THAN DESIRED, BUT FOR NOW IS
MOST LIKELY TO REACH NORTH CAROLINA AND VIRGINIA BY TUESDAY-
WEDNESDAY, SPREADING RAIN INTO THE MID-ATLANTIC AND APPALACHIANS.
MARGINAL RISK AREAS REMAIN IN PLACE OVER THIS REGION DURING THE
DAYS 4-5 PERIOD, WITH DAY 5 REFLECTING SOMEWHAT OF A NORTHWARD
SHIFT RELATIVE TO DAY 4. IF A RELATIVELY DEEP LOW ULTIMATELY
DEVELOPS WITH STRONGER MOISTURE FOCUS, A RISK LEVEL HIGHER THAN A
MARGINAL WOULD LIKELY BE WARRANTED PARTICULARLY ON TUESDAY, BUT
LINGERING UNCERTAINTY IN THE LOW TRACK FAVORS HOLDING ONTO THE
MARGINAL FOR NOW. SOME 12Z GUIDANCE OFFERS POTENTIAL FOR MEANINGFUL
RAINFALL TO EXTEND FARTHER NORTHWARD INTO SOUTHERN NEW ENGLAND AND
VICINITY BUT WITH RELATIVELY DRY ANTECEDENT CONDITIONS PLUS THE
GUIDANCE SPREAD/VARIABILITY PRECLUDING INCLUSION IN THE MARGINAL
RISK AREA FOR NOW. SHOWERS MAY PERSIST 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 MEAN TROUGH ALOFT OVER THE WEST WILL PROVIDE AMPLE
LIFT/FORCING TO SUPPORT ROUNDS OF PRECIPITATION THERE NEXT WEEK.
THE DAY 4/TUESDAY ERO SHOWS A MARGINAL RISK AREA OVER PARTS OF
NORTHERN IDAHO TO THE WESTERN HALF OF MONTANA AS THE REGION COULD
SEE HEAVY RAIN RATES NEAR THE TRACK OF A LEADING UPPER LOW EJECTING
THROUGH THE NORTHERN ROCKIES, WITH POTENTIAL FOR SOME INSTABILITY
TO BE 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, AS THE FRONT TRAILING FROM THE
INITIAL MONTANA INTO CANADA SYSTEM STALLS FOR A PERIOD OF TIME
WHILE AWAITING SOME COMBINATION OF NORTHERN STREAM AND WESTERN U.S.
DYNAMICS (WITH POSSIBLE WAVE DEVELOPMENT).
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 WITH SOME MINUS 10-15F
ANOMALIES PERSISTING IN THE INTERIOR WEST THROUGH AT LEAST FRIDAY
AND REACHING THE NORTHERN HIGH PLAINS BY SATURDAY. 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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