584
FXUS02 KWBC 201902
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
201 PM EST WED DEC 20 2023
VALID 12Z SAT DEC 23 2023 - 12Z WED DEC 27 2023
...HEAVY SNOW THREAT SPREADS ONTO THE NORTH-CENTRAL PLAINS
SUNDAY...
...HEAVY RAIN THREAT FOR THE LOWER MISSISSIPPI VALLEY INTO THE
SOUTHEAST SUNDAY AND CHRISTMAS MONDAY AND THE MID-ATLANTIC TOWARD
MIDWEEK...
..OVERVIEW
A CLOSED SOUTHERN STREAM UPPER LOW OFF SOUTHERN CALIFORNIA WILL
OPEN AND EJECT NORTHEAST TO THE COLORADO ROCKIES SATURDAY BEFORE
PHASING WITH A DIGGING NORTHERN STREAM TROUGH OVER COLORADO SUNDAY
THAT BECOMES A MULTI-PHASE, SLOW MOVING LOW THAT SHIFTS EAST
THROUGH LATE NEXT WEEK. THE EJECTION OF THE DYNAMIC SYSTEM WILL
SPAWN DOWNSTREAM SOUTH-CENTRAL PLAINS CYCLOGENESIS/FRONTOGENESIS
SATURDAY NIGHT WITH PHASING/OCCLUSIONS KEEPING EASTWARD MOTION
SLOW THROUGH MONDAY/CHRISTMAS. RETURN GULF MOISTURE AND
INSTABILITY WILL FUEL AN EMERGING AREA OF ENHANCED RAINS AND
CONVECTION OVER THE EASTERN PORTIONS OF THE SOUTHERN PLAINS/LOWER
MS VALLEY SUNDAY THOUGH MONDAY/CHRISTMAS THAT THEN SHIFTS TO THE
MIDWEST/MID-ATLANTIC THROUGH MIDWEEK. MEANWHILE, THE THREAT OF
HEAVY SNOW IS EXPECTED TO SHIFT FROM THE SOUTHERN ROCKIES TO THE
NORTH-CENTRAL PLAINS SATURDAY NIGHT INTO MONDAY/CHRISTMAS WITH THE
LINGERING/RE-PHASING LOW CAUSING MORE SNOW CONCERNS ON THE BACK
SIDE OVER THE CENTRAL PLAINS TO UPPER MIDWEST INTO MIDWEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL PREFERENCES WERE BASED ON THE EVOLUTION OF THE
WAVE/REDEVELOPING MULTI-PHASE LOW SPREADING FROM THE SOUTHWEST AND
EAST THROUGH THE MEDIUM RANGE FORECAST TIME. THE ECMWF/CMC/UKMET
FAVOR A SLOWER, DEEPER LOW CROSSING THE PLAINS EARLY NEXT WEEK AS
OPPOSED TO THE MORE PROGRESSIVE GFS. THIS IS TRUE FOR BOTH THE 00Z
AND 12Z RUNS OF THESE MODELS. PREFERENCE IS HEAVIEST TOWARD THE
ECMWF WITH SOME CMC/UKMET INCLUSION AND LITTLE WITH THE GFS
THROUGH DAY 5. BEYOND, THE ECENS MEAN IS PREFERRED WITH SOME CMC
ENSEMBLE MEANS INCLUDED. QPF IN THE MEDIUM RANGE INCLUDES
ENHANCEMENTS TO THE 13Z NBM FROM THE 00Z ECMWF WITH SOME UKMET/CMC
AS AVAILABLE. MUCH UNCERTAINTY REMAINS WITH THE STRENGTH AND
POSITION OF COMMA HEAD PRECIP OVER THE CENTRAL US, SO FOR THIS
SHIFT THE 00Z ECMWF WAS CHOSEN AS THE BEST/MIDDLE OF THE ROAD
SOLUTION.
..WEATHER/HAZARDS HIGHLIGHTS
ANOMALOUS MOISTURE AND FORCING SHIFTS FROM THE DESERT SOUTHWEST
SATURDAY WITH ADDITIONAL LOWER ELEVATION RAINFALL GENERALLY ENDING
SHORTLY AFTER 12Z SAT, SO THERE CONTINUES TO BE NO DAY 4/SATURDAY
EXCESSIVE RAINFALL OUTLOOK (ERO). HOWEVER, THE WPC WINTER WEATHER
OUTLOOK (WWO) SHOWS BROAD ENHANCED SNOW PROBABILITIES. UPPER
TROUGH PROGRESSION AND PLAINS SURFACE SYSTEM ORGANIZATION THAT
SUPPORTS A HEAVY SNOW THREAT FROM THE SOUTHERN ROCKIES THROUGH THE
NORTH-CENTRAL PLAINS IN TIME FOR CHRISTMAS EVE AND CHRISTMAS.
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 AND 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. WITH THE ECMWF FOCUS EXPANDING
NORTH, THE DAY 5/CHRISTMAS EVE SLIGHT RISK ERO IS EXPANDED NORTH A
BIT THROUGH MUCH OF ARKANSAS/THE OZARKS GIVEN RUNOFF RISKS.
EXPECT A HEAVY RAINFALL SIGNATURE FROM THE SOUTHEAST/APPALACHIANS
TO THE MID-ATLANTIC CHRISTMAS INTO TUESDAY GIVEN SYSTEM
TRANSLATION/RE-PHASING 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 CENTRAL PLAINS TO GREAT LAKES TO NORTHERN
NEW ENGLAND THOUGH UNCERTAINTY REMAINS GIVEN THE VARIED TIMING OF
THE DETERMINISTIC SOLUTIONS.
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 PROXIMITY
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 SUNDAY NIGHT THROUGH TUESDAY.
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.
JACKSON
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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