502
FXUS02 KWBC 031853
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
153 PM EST TUE DEC 3 2024
VALID 12Z FRI DEC 6 2024 - 12Z TUE DEC 10 2024
...RETURN FLOW MAY FUEL EMERGING RAINFALL FROM THE WEST-CENTRAL
GULF COAST TO THE APPALACHIANS THIS WEEKEND TO EARLY NEXT WEEK...
19Z UPDATE: THE MODELS ARE INITIALLY IN GOOD AGREEMENT ACROSS THE
DOMAIN FOR FRIDAY WITH THE TROUGH OVER THE EASTERN U.S., THE CUT-
OFF LOW OVER THE SOUTHWEST, AND THE RIDGE OVER THE PACIFIC
NORTHWEST. GOING INTO THE WEEKEND, THE UKMET BECOMES NOTABLY FASTER
WITH THE NEXT SHORTWAVE TROUGH MOVING INLAND ACROSS THE PACIFIC
NORTHWEST AND QUICKLY LOSES ENSEMBLE SUPPORT, SO A NON-UKMET
CONSENSUS BEST REPRESENTS THE OVERALL WEATHER PATTERN BY SATURDAY.
THE 12Z CMC HAS TRENDED MORE IN LINE WITH THE CONSENSUS COMPARED TO
ITS FASTER 00Z RUN FOR THE WEST COAST REGION. LOOKING AHEAD TO
EARLY NEXT WEEK, THERE IS GOOD OVERALL AGREEMENT WITH A DEEP
TROUGH DEVELOPING OVER THE PLAINS AND BACK TO THE SOUTHERN ROCKIES,
BUT GREATER DIFFERENCES ACROSS THE EASTERN PACIFIC AND ALSO WITH
EXTENT OF RETURN FLOW AND QPF ACROSS THE GULF COAST REGION, SO THE
ENSEMBLE MEANS WERE GRADUALLY INCREASED TO ABOUT 40% BY THE END OF
THE FORECAST PERIOD NEXT TUESDAY. THE PREVIOUS FORECAST DISCUSSION
IS APPENDED BELOW. /HAMRICK
----------------------------
..GUIDANCE/PREDICTABILITY ASSESSMENT
LEANED ON RECENT ECMWF/UKMET AND TRENDS FROM GEFS/ECMWF ENSEMBLES
THAT HOLD MAIN RAINFALL DEVELOPMENT CLOSER TO THE WEST-CENTRAL
GULF COASTAL REGIONS LATE WEEK UNTIL LATER WEEKEND/EARLY NEXT WEEK
BREAKOUT UP THROUGH THE APPALACHIANS. INLAND QPF DEVELOPMENT IS
DELAYED COMPARED TO RECENT GFS/CANADIAN RUNS AND THE NBM. ACROSS
THE NATION, IT IS NOTABLE THAT RECENT DETERMINISTIC MODEL GUIDANCE
FORECAST SPREAD AND CYCLE TO CYCLE CONTINUITY ISSUES ARISE EARLIER
THAN NORMAL INTO THE MEDIUM RANGE TIME SCALES, LOWERING FORECAST
CONFIDENCE FROM THE WEEKEND INTO NEXT WEEK DESPITE SOME COMMON
TRANSITION OF THE LARGER SCALE PATTERN. ACCORDINGLY, OPTED TO PIVOT
BROADLY TOWARD A MORE COMPATIBLE AND RUN STABLE GEFS/ECMWF
ENSEMBLE MEAN BLEND ALONG WITH THE NATIONAL BLEND OF MODELS TO
MAINTAIN FEASIBLE WPC CONTINUITY AMID UNCERTAINTY.
..WEATHER/HAZARDS HIGHLIGHTS
ARCTIC AIR THAT HAS PERSISTED OVER THE EAST-CENTRAL U.S. WILL
MODERATE THIS WEEKEND AS AMPLIFIED UPPER TROUGHING FINALLY LIFTS
OUT AFTER A PROLONGED TIME PERIOD. NEVERTHELESS, LAKE EFFECT SNOW
IS LIKELY TO BE ONGOING ON FRIDAY AND INTO EARLY SATURDAY IN THE
WAKE OF A STRONG CLIPPER-TYPE SYSTEM. BY EARLY NEXT WEEK,
TEMPERATURES SHOULD BE NEAR TO EVEN A LITTLE ABOVE AVERAGE ACROSS
MUCH OF THE EASTERN U.S.
MEANWHILE, MOIST RETURN FLOW FROM THE GULF OF MEXICO WITH
RETREATING HIGH PRESSURE TO THE EAST WILL SUPPORT INCREASING
COVERAGE OF SHOWERS AND THUNDERSTORMS, SOME OF WHICH COULD BE
HEAVY, FROM THE WEST-CENTRAL GULF COAST STATES UP THROUGH THE
SOUTHERN APPALACHIANS THIS WEEKEND INTO EARLY NEXT WEEK, ALBEIT
WITH UNCERTAINTY ON THE TIMING AND INLAND EXTENT OF THE RAIN. IN
TERMS OF THE EXCESSIVE RAINFALL OUTLOOK, DAY 4 (FRIDAY) WILL BE
KEPT VOID OF ANY RISK AREAS, ALTHOUGH SOME AREAS OF MODERATE
RAINFALL ARE POSSIBLE ACROSS TEXAS BY THIS TIME. THE LATEST GFS AND
CMC MODELS, ALONG WITH THE BIASED CORRECTED QPF GUIDANCE, SUGGEST
THE POTENTIAL FOR HEAVIER SWATHS OF RAIN AND EMBEDDED STORMS TO
DEVELOP DURING THE DAY 5 (SATURDAY) PERIOD, AND THEREFORE A
MARGINAL RISK AREA IS NOW PLANNED FOR PORTIONS OF EASTERN TEXAS TO
WESTERN LOUISIANA TO ACCOUNT FOR THIS. ADDITIONAL HEAVY RAINFALL IS
EXPECTED BEYOND THE DAY 5 PERIOD ACROSS THE DEEP SOUTH AS STRONG
MOISTURE FLUX FROM THE CENTRAL GULF CONTINUES AHEAD OF AN
APPROACHING COLD FRONT.
UPSTREAM, MODERATE TERRAIN ENHANCED PRECIPITATION IS EXPECTED TO
DEVELOP THIS WEEKEND OVER THE PACIFIC NORTHWEST WITH ENERGETIC
UPPER TROUGH PASSAGE AND LEAD MOISTURE FEED. TERRAIN ENHANCED SNOWS
WILL LIKELY SPREAD INLAND ACROSS THE NORTHWEST TO THE NORTH-
CENTRAL ROCKIES WITH COLD FRONTAL PASSAGE OVER THE WEST INTO EARLY
NEXT WEEK.
HAMRICK/SCHICHTEL
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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