373
FXUS02 KWBC 211901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST THU DEC 21 2023
VALID 12Z SUN DEC 24 2023 - 12Z THU DEC 28 2023
...HEAVY SNOW THREAT SPREADS TO THE NORTH-CENTRAL PLAINS CHRISTMAS
EVE...
...HEAVY RAIN THREAT FROM THE LOWER MISSISSIPPI VALLEY TO THE
SOUTHEAST CHRISTMAS EVE AND CHRISTMAS AND THEN THE MID-ATLANTIC
TOWARD MIDWEEK...
..OVERVIEW
A CLOSED SOUTHERN STREAM UPPER LOW OFF SOUTHERN CALIFORNIA WILL
OPEN AND EJECT NORTHEAST TO THE COLORADO ROCKIES THIS WEEKEND
BEFORE PHASING WITH A DIGGING NORTHERN STREAM TROUGH OVER COLORADO
TO BECOME A MULTI-PHASE AND SLOW MOVING LOW THAT SHIFTS SLOWLY
DOWNSTREAM THROUGH LATE NEXT WEEK. THE WEEKEND TRANSLATION OF THE
DYNAMIC SYSTEM WILL SPAWN DOWNSTREAM SOUTH-CENTRAL PLAINS
CYCLOGENESIS/FRONTOGENESIS WITH PHASING/OCCLUSIONS KEEPING
EASTWARD MOTION SLOW THROUGH MONDAY/CHRISTMAS. RETURN GULF OF
MEXICO MOISTURE AND INSTABILITY WILL FUEL AN EMERGING AREA OF
ENHANCED RAINS AND CONVECTION OVER THE EASTERN PORTIONS OF THE
SOUTHERN PLAINS/LOWER MS VALLEY THEN SOUTHEAST SUNDAY THOUGH
MONDAY/CHRISTMAS THAT THEN SHIFTS MAIN FOCUS TO THE MID-ATLANTIC
AND ONWARD 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
PLENTY OF UNCERTAINTY DURING THE MEDIUM RANGE PERIOD. A GENERAL
MODEL BLEND CONSISTING OF THE 00Z EC/CMC/UKMET AND 06Z GFS WERE
USED THROUGH DAY 5. THE LAST FEW RUNS OF THE GFS HAVE BEEN TOO
FAST WITH THE EVOLUTION OF THE UPPER TROUGH/LOW THROUGH THE
PLAINS, MISSISSIPPI VALLEY AND MIDWEST THROUGH THE PERIOD COMPARED
TO THE REST OF THE GUIDANCE, BUT ESPECIALLY ON DAY 3. IT'S FOR
THAT REASON THAT IT WAS WEIGHTED LESS IN THE DAY 3 BLEND. THE
CANADIAN AND UKMET DIVERGE A BIT FROM THE REST OF THE GUIDANCE
WITH RESPECT TO A POTENTIAL SHORTWAVE PROPAGATING THROUGH THE
NORTHWEST ON DAY 4 SO THEY HAD THEIR WEIGHTING REDUCED A BIT FOR
THAT. THE ENSEMBLES WERE INTRODUCED ON DAY 5 AND WEIGHTED HEAVILY
THROUGH THE REMAINDER OF THE PERIOD TO ACCOUNT FOR A LOT OF
DISPERSION IN THE GUIDANCE.
..WEATHER/HAZARDS HIGHLIGHTS
COMPLEX UPPER TROUGH PROGRESSION AND PLAINS SURFACE SYSTEM
ORGANIZATION SUPPORTS A HEAVY SNOW THREAT FROM THE SOUTHERN
ROCKIES THROUGH THE NORTH-CENTRAL PLAINS IN TIME FOR CHRISTMAS EVE
AND CHRISTMAS. THE WPC WINTER WEATHER OUTLOOK (WWO) SHOWS
RISING/ENHANCED PLOWABLE COMMAHEAD SNOW PROBABILITIES.
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 DIFLUENCE/RIGHT
ENTRANCE REGION UPPER JET DYNAMICS AND ALSO FROM A STRONG/SURGING
SUBTROPICAL JET. MAINTAINED THE DAY 4/CHRISTMAS EVE SLIGHT RISK
ERO.
EXPECT A HEAVY RAINFALL SIGNATURE FROM THE SOUTHEAST/SOUTHERN
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. IN THIS PERIOD, 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 WELL UPSTREAM, MEAN UPPER TROUGH AMPLITUDE SET 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/LONG
FETCH SOUTHERLY MOISTURE FLOW SHOULD ALLOW FOR SOME PERIODS OF
MODERATE TO HEAVY PRECIPITATION INTO MAINLY COASTAL AREAS AND
ESPECIALLY SOUTH FACING FAVORED TERRAIN FROM THE PACIFIC NORTHWEST
TO NORTHERN CALIFORNIA SUNDAY NIGHT THROUGH TUESDAY, WITH
ADDITIONAL ACTIVITY INTO WEDNESDAY/NEXT THURSDAY. A WPC ERO
MARGINAL RISK AREA WAS INTRODUCED FOR DAY5/CHRISTMAS WHEN GUIDANCE
SHOWS THE BEST DEFINED ATMOSPHERIC RIVER SIGNAL INTO COASTAL
WASHINGTON AND IN PARTICULAR THE OLYMPICS.
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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