686
FXUS02 KWBC 200836
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
335 AM EST WED DEC 20 2023
VALID 12Z SAT DEC 23 2023 - 12Z WED DEC 27 2023
...HEAVY RAIN THREAT FOR THE SOUTH-CENTRAL U.S. INTO THE HOLIDAYS
THEN WORKS TO THE SOUTHEAST/MID-ATLANTIC...
...HEAVY SNOW THREAT FROM THE GREAT BASIN/ROCKIES TO THE
NORTH-CENTRAL PLAINS THIS WEEKEND INTO CHRISTMAS...
..OVERVIEW
A CLOSED UPPER TROUGH/LOW WILL DIG SOUTHEASTWARD OFF CALIFORNIA
BEFORE WORKING INLAND THROUGH BAJA TO THE SOUTHWEST BY SATURDAY.
THIS WILL BRING PERIODS OF LEADING/WRAPPING ENHANCED MOISTURE AND
PRECIPITATION INTO THE SOUTHWEST. THE EJECTION OF THE DYNAMIC
SYSTEM WILL SPAWN DOWNSTREAM SOUTH-CENTRAL PLAINS HOLIDAY
CYCLOGENESIS/FRONTOGENESIS. RETURN MOISTURE AND INSTABILITY WILL
FUEL AN EMERGING AREA OF ENHANCED RAINS AND CONVECTION WITH BROAD
FOCUS OVER THE SOUTH-CENTRAL U.S. STATES. MEANWHILE, A THREAT OF
HEAVY SNOW IS EXPECTED TO DEVELOP ON THE BACKSIDE OF THE SYSTEM
FROM THE GREAT BASIN/ROCKIES INTO SATURDAY TO THE NORTH-CENTRAL
PLAINS IN TIME FOR CHRISTMAS EVE AND CHRISTMAS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM A
COMPOSITE BLEND OF BEST CLUSTERED GUIDANCE FROM THE 18 UTC GFS AND
12 UTC ECMWF/CANADIAN ALONG WITH VARIANCE SMOOTHING GEFS/ECMWF
ENSEMBLE MEANS AND INPUT FROM THE 01 UTC NATIONAL BLEND OF MODELS
(NBM) AND WPC CONTINUITY IN A PATTERN WITH AVERAGE TO SLIGHTLY
ABOVE NORMAL PREDICTABILITY. NEWER 00 UTC GUIDANCE GENERALLY
REMAINS IN LINE WITH THE MID-UPPER LEVEL FLOW EVOLUTION, BUT STILL
OFFERS EMBEDDED SYSTEM DETAIL DIFFERENCES TO VARY LOCAL FOCUS
INCREASINGLY OVER TIME.
..WEATHER/HAZARDS HIGHLIGHTS
LINGERING ANOMOLOUS MOISTURE TO FUEL RAINS A TERRAIN/MOUNTAIN SNOW
FOCUS AND SOME LOCALLY HIGHER WINDS WILL WORK OUT THROUGH THE
SOUTHWEST AND THE GREAT BASIN/ROCKIES SATURDAY WITH CLOSED
SOUTHERN STREAM SYSTEM PASSAGE AND INTERACTION WITH THE DIGGING OF
LESS CERTAIN NORTHERN STREAM UPPER TROUGH ENERGIES. HOWEVER, NO
DAY 4/SATURDAY EXCESSIVE RAINFALL OUTLOOK (ERO) THREAT AREA HAS
BEEN INTRODUCED. HOWVER, THE WPC WINTER WEATHER OUTLOOK (WWO)
SHOWS BROAD ENHANCED SNOW PROBABILITIES. UPPER TROUGH PROGRESSION
AND PLAINS SURFACE SYSTEM ORGANIZATION DOES THEN LOOK TO SUPPORT A
HEAVY SNOW THREAT FROM THE CENTRAL ROCKIES THROUGH THE
NORTH-CENTRAL PLAINS IN TIME FOR CHRISTMAS EVE AND CHRISTMAS.
WHILE NO DAY 4/SATURDAY ERO AREA WAS DEPICTED FOR AN EMERGENING
DOWNSTEAM RAIN/CONVECTION PATTERN OVER THE SOUTHERN PLAINS WITH
INITIAL CYCLO/FRONTOGENESIS AND RETURN MOISTURE/INSTABILITY FLOW,
THERE IS SOME POTENTIAL FOR SOME LOCALLY HEAVIER DOWNPOURS TO
MONITOR AS SUPPORTED BY VARIED MODELS AND ENSEMBLE PROBABILITIES.
HOWEVER, SYSTEM/ENERGY DOWNSTREAM PROPAGATION WILL GENERATE AN
EMERGING WET PATTERN WITH POTENTIAL FOR STRONG CONVECTION SET TO
ERUPT CHRISTMAS EVE WITH FOCUS FROM EASTERN PORTIONS OF THE
SOUTHERN PLAINS THROUGH THE LOWER MISSISSIPPI VALLEY AND CENTRAL
GULF COAST. INCREASING INSTABILITY WITH UPWARDS OF A 50KT LLJ/2 SD
ABOVE NORMAL PWS WILL COMBINE WITH FAVORABLE UPPER
DIFFLUENCE/RIGHT ENTRANCE REGION UPPER JET DYNAMICS AND ALSO FROM
A STRONG/SURGING SUBTROPICAL JET. THIS PROMPTED A DAY 5/CHRISTMAS
EVE SLIGHT RISK ERO GIVEN RUNOFF RISKS. EXPECT A HEAVY RAINFALL
SIGNATURE TO MONITOR INTO LONGER TIME FRAMES MAY EXTEND FROM THE
SOUTHEAST/APPALACHIANS TO THE MID-ATLANTIC CHRISTMAS INTO TUESDAY
GIVEN SYSTEM TRANSLATION AND ADDED ATLANTIC MOISTURE. THE
NORTHWARD EXPANSION OF THE BROAD PRECIPITATION SHIELD WITH FRONTAL
WAVE PROPAGATION IS ALSO LIKELY TO MEANWHILE SPREAD MODERATE
PRECIPITATION AMOUNTS UP FROM THE MIDWEST THROUGH THE NORTHEAST
WITH STEADY EROSION OF A LEADING/COOLED EASTERN U.S. SURFACE
RIDGE. THIS SHOULD EQUATE TO SOME NORTHERN TIER WINTRY
PRECIPITATION FROM THE GREAT LAKES TO NORTHERN NEW ENGLAND.
MEANWHILE UPSTREAM, MEAN UPPER TROUGH AMPLITUDE TO PERSIST OVER
THE EASTERN PACIFIC SHOULD PROVE SLOW TO TRANSLATE TOWARD THE WEST
COAST, BUT SYSTEM INTENSITIES AND GRADUALLY CLOSER PROXIMMITY
SUGGESTS THAT LEAD FLOW IMPULSES AND DEEP LAYERED SOUTHERLY FLOW
SHOULD ALLOW FOR PERIODS OF LIGHT TO MODERATE PRECIPITATION INTO
MAINLY COASTAL AREAS AND FAVORED TERRAIN FROM THE PACIFIC
NORTHWEST TO NORTHERN CALIFORNIA THROUGH THE HOLIDAY WEEKEND INTO
NEXT WEEK.
TEMPERATURES SHOULD TREND QUITE WARM ACROSS THE CENTRAL U.S.,
ESPECIALLY INTO THE NORTHERN PLAINS AND UPPER MIDWEST THIS
WEEKEND. DAYTIME HIGHS COULD BE 20 TO 25 DEGREES ABOVE NORMAL IN
THIS REGION, WHILE LOWS OF 25-35 DEGREES ABOVE AVERAGE ARE
EXPECTED. A FEW RECORD HIGHS ARE FORECAST WHILE RECORD WARM
MINIMUM TEMPERATURES MAY BE WIDESPREAD. ELSEWHERE, DAYTIME HIGH
ANOMALIES RETURN BACK TO NORMAL ACROSS THE EAST AS UPPER RIDGING
BUILDS AND UPPER TROUGHING INTO THE WEST SHOULD RESULT IN A
COOLING TREND BY THE WEEKEND.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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